Ronomar - This Course Has Been Developed Under Romanian Norwegian Maritime Project

Transcript

This course has been developed under RoNoMar - Romanian Norwegian Maritime Project (2008/111922) Supported by a grant from Norway through the Norwegian Cooperation Programme for Economic Growth and Sustainable Development with Romania. INSTRUCTOR MANUAL CONSTANTA 2010 1. DESCRIBE THE BASIC THEORY AND OPERATION OF A MARINE RADAR SYSTEM 1.1. Basic Radar Principles and General Characteristics 1.1.1 Introduction The word radar is an acronym derived from the phrase RAdio Detection And Ranging and applies to electronic equipment designed for detecting and tracking objects (targets) at considerable distances. The basic principle behind radar is simple - extremely short bursts of radio energy (traveling at the speed of light) are transmitted, reflected off a target and then returned as an echo. Radar makes use of a phenomenon we have all observed, that of the ECHO PRINCIPLE. To illustrate this principle, if a ship’s whistle were sounded in the middle of the ocean, the sound waves would dissipate their energy as they traveled outward and at some point would disappear entirely. If, however the whistle sounded near an object such as a cliff some of the radiated sound waves would be reflected back to the ship as an echo. The form of electromagnetic signal radiated by the radar depends upon the type of information needed about the target. Radar, as designed for marine navigation applications, is pulse modulated. Pulse-modulated radar can determine the distance to a target by measuring the time required for an extremely short burst of radio-frequency (rf) energy to travel to the target and return to its source as a reflected echo. Directional antennas are used for transmitting the pulse and receiving the reflected echo, thereby allowing determination of the direction or bearing of the target echo. Once time and bearing are measured, these targets or echoes are calculated and displayed on the radar display. The radar display provides the operator a birds eye view of where other targets are relative to own ship. Radar is an active device. It utilizes its own radio energy to detect and track the target. It does not depend on energy radiated by the target itself. The ability to detect a target at great distances and to locate its position with high accuracy are two of the chief attributes of radar. There are two groups of radio frequencies allocated by international standards for use by civil marine radar systems. The first group lies in the X band which corresponds to a wavelength of 3 cm. and has a frequency range between 9300 and 9500 MHz. The second group lies in the S-band with a wavelength of 10 cm. and has a frequency range of 2900 to 3100 MHz. It is sometimes more convenient to speak in terms of wavelength rather than frequency because of the high values associated with the latter. A fundamental requirement of marine radar is that of directional transmission and reception, which is achieved by producing a narrow horizontal beam. In order to focus the radio energy into a narrow beam the laws of physics prevail and the wavelength must be within the few centimeters range. The radio-frequency energy transmitted by pulse-modulated radars consists of a series of equally spaced pulses, frequently having durations of about 1 microsecond or less, separated by very short but relatively long periods during which no energy is transmitted. The terms PULSEMODULATED RADAR and PULSE MODULATION are derived from this method of transmission of radio-frequency energy. 1 If the distance to a target is to be determined by measuring the time required for one pulse to travel to the target and return as a reflected echo, it is necessary that this cycle be completed before the pulse immediately following is transmitted. This is the reason why the transmitted pulses must be separated by relatively long non-transmitting time periods. Otherwise, transmission would occur during reception of the reflected echo of the preceding pulse. Using the same antenna for both transmitting and receiving, the relatively weak reflected echo would be blocked by the relatively strong transmitted pulse. 1.1.2 A Brief History Radar, the device which is used for detection and ranging of contacts, independent of time and weather conditions, was one of the most important scientific discoveries and technological developments that emerged from WWII. It’s development, like that of most great inventions was mothered by necessity. Behind the development of radar lay more than a century of radio development. The basic idea of radar can be traced back to the classical experiments on electromagnetic radiation conducted by the scientific community in the 19th century. In the early 1800s, an English physicist, Michael Faraday, demonstrated that electric current produces a magnetic field and that the energy in this field returns to the circuit when the current is stopped. In 1864 the Scottish physicist, James Maxwell, had formulated the general equations of the electromagnetic field, determining that both light and radio waves are actually electromagnetic waves governed by the same fundamental laws but having different frequencies. He proved mathematically that any electrical disturbance could produce an effect at a considerable distance from the point of origin and that this electromagnetic energy travels outward from the source in the form of waves moving at the speed of light. At the time of Maxwell’s conclusions there was no available means to propagate or detect electromagnetic waves. It was not until 1886 that Maxwell’s theories were tested. The German physicist, Heinrich Hertz, set out to validate Maxwell’s general equations. Hertz was able to show that electromagnetic waves travelled in straight lines and that they can be reflected from a metal object just as light waves are reflected by a mirror. In 1904 the German engineer, Christian Hulsmeyer obtained a patent for a device capable of detecting ships. This device was demonstrated to the German navy, but failed to arouse interest probably due in part to its very limited range. In 1922, Guglielmo Marconi drew attention to the work of Hertz and repeated Hertz’s experiments and eventually proposed in principle what we know now as marine radar. The first observation of the radar effect was made in 1922 by Dr. Albert Taylor of the Naval Research Laboratory (NRL) in Washington, D.C. Dr. Taylor observed that a ship passing between a radio transmitter and receiver reflected some of the waves back to the transmitter. In 1930 further tests at the NRL observed that a plane flying through a beam from a transmitting antenna caused a fluctuation in the signal. The importance of radar for the purposes of tracking aircraft and ships finally became recognized when scientists and engineers learned how to use a single antenna for transmitting and receiving. Due to the prevailing political and military conditions at the time, the United States, Great Britain, Soviet Union, France, Italy, Germany and Japan all began experimenting with radar, with varying degrees of success. During the 1930s, efforts were made by several countries to use radio echo for aircraft detection. Most of these countries were 2 able to produce some form of operational radar equipment for use by the military at the start of the war in 1939. At the beginning of WWII, Germany had progressed further in radar development and employed radar units on the ground and in the air for defense against allied aircraft. The ability of radar to serve as an early warning device proved valuable as a defensive tool for the British and the Germans. Although radar was employed at the start of the war as a defensive weapon, as the war progressed, it came to be used for offensive purposes too. By the middle of 1941 radar had been employed to track aircraft automatically in azimuth and elevation and later to track targets automatically in range. All of the proven radar systems developed prior to the war were in the VHF band. These low frequency radar signals are subject to several limitations, but despite the drawbacks, VHF represented the frontier of radar technology. Late in 1939, British physicists created the cavity magnetron oscillator which operated at higher frequencies. It was the magnetron that made microwave radar a reality. It was this technological advance that marks the beginning of modern radar. Following the war, progress in radar technology slowed as post war priorities were directed elsewhere. In the 1950s new and better radar systems began to emerge and the benefits to the civil mariner became more important. Although radar technology has been advanced primarily by the military, the benefits have spilled over into many important civilian applications, of which a principal example is the safety of marine navigation. The same fundamental principles discovered nearly a century ago and the basic data they provide, namely target range and bearing, still apply to today’s modern marine radar units. 1.1.3 Radar Propagation Characteristics 1.1.3.1 The radio wave To appreciate the capabilities and limitations of a marine radar and to be able to use it to full advantage, it is necessary to comprehend the characteristics and behavior of radio waves and to grasp the principles of their generation and reception, including the echo display as seen by the observer. Understanding the theory behind the target presentation on the radar scope will provide the radar observer a better understanding of the art and science of radar interpretation. Radar (radio) waves, emitted in pulses of electromagnetic energy in the radio-frequency band 3,000 to 10,000 MHz used for shipborne navigational radar, have many characteristics similar to those of other waves. Like light waves of much higher frequency, radar waves tend to travel in straight lines or rays at speeds approximating that of light. Also, like light waves, radar waves are subject to refraction or bending in the atmosphere. Radio-frequency energy travels at the speed of light, approximately 162,000 nautical miles per second; therefore, the time required for a pulse to travel to the target and return to its source is a measure of the distance to the target. Since the radio-frequency energy makes a round trip, only half the time of travel determines the distance to the target. The round trip time is accounted for in the calibration of the radar. The speed of a pulse of radio-frequency energy is so fast that the pulse can circumnavigate the earth at the equator more than 7 times in 1 second. It should be 3 obvious that in measuring the time of travel of a radar pulse or signal from one ship to a target ship, the measurement must be an extremely short time interval. For this reason, the MICROSECOND (msec) is used as a measure of time for radar applications. The microsecond is one-millionth part of 1 second, i.e., there are 1,000,000 microseconds in 1 second of time. Radio waves have characteristics common to other forms of wave motion such as ocean waves. Wave motion consists of a succession of crests and troughs which follow one another at equal intervals and move along at a constant speed. Like waves in the sea, radar waves have energy, frequency, amplitude, wavelength, and rate of travel. Whereas waves in the sea have mechanical energy, radar waves have electromagnetic energy, usually expressed in watt units of power. An important characteristic of radio waves in connection with radar is polarization. This electromagnetic energy has associated electric and magnetic fields, the directions of which are at right angles to each other. The orientation of the ELECTRIC AXIS in space establishes what is known as the POLARIZATION of the wave. Horizontal polarization is normally used with navigational radars, i.e., the direction of the electric axis is horizontal in space. Horizontal polarization has been found to be the most satisfactory type of polarization for navigational radars in that stronger echoes are received from the targets normally used with these radars when the electric axis is horizontal. Each pulse of energy transmitted during a few tenths of a microsecond or a few microseconds contains hundreds of complete oscillations. A CYCLE is one complete oscillation or one complete wave, i.e., that part of the wave motion passing zero in one direction until it next passes zero in the same direction (see figure 1.1). The FREQUENCY is the number of cycles completed per second. The unit now being used for frequency in cycles per second is the HERTZ. One hertz is one cycle per second; one kilohertz (kHz) is one thousand cycles Figure 1.1 - Wave. The WAVELENGTH is the distance along the direction of propagation between successive crests or troughs. When one cycle has been completed, the wave has traveled one wavelength. The AMPLITUDE is the maximum displacement of the wave from its mean or zero value. Since the speed of radar waves is constant at 300,000 kilometers per second, there is a definite relationship between frequency and wavelength. The CYCLE is a complete alternation or oscillation from one crest through a trough to the next crest. 4 Frequency=speed of radar waves/wavelength EXAMPLE: When the wavelength is 3.2 centimeters (0.000032 km), the frequency= 300, 000km/s + 0.000032km/cycle. Frequency = 9375 megahertz 1.1.3.2 The radar beam The pulses of r-f energy emitted from the feedhorn at the focal point of a reflector or emitted and radiated directly from the slots of a slotted waveguide antenna would, for the most part, form a single lobe-shaped pattern of radiation if emitted in free space. Figure 1.2 illustrates this free space radiation pattern, including the undesirable minor lobes or SIDE LOBES associated with practical antenna design. Because of the large differences in the various dimensions of the radiation pattern, figure 1.2 is necessarily distorted. Figure 1.2 - Free space radiation pattern. Although the radiated energy is concentrated or focused into a relatively narrow main beam by the antenna, similar to a beam of light from a flashlight, there is no clearly defined envelope of the energy radiated. While the energy is concentrated along the axis of the beam, its strength decreases with distance along the axis. The strength of the energy decreases rapidly in directions away from the beam axis. The power in watts at points in the beam is inversely proportional to the square of the distance. Therefore, the power at 3 miles is only 1/9th of the power at 1 mile in a given direction. The field intensity in volts at points in the beam is inversely proportional to the distance. Therefore, the voltage at 2 miles is only one-half the voltage at 1 mile in a given direction. With the rapid decrease in the amount of radiated energy in directions away from the axis and in conjunction with the rapid decreases of this energy with distance, it follows that practical limits of power or voltage may be used to define the dimensions of the radar beam or to establish its envelope of useful energy. 5 1.1.3.3 Beam Width The three-dimensional radar beam is normally defined by its horizontal and vertical beam widths. Beam width is the angular width of a radar beam between points within which the field strength or power is greater than arbitrarily selected lower limits of field strength or power. There are two limiting values, expressed either in terms of field intensity or power ratios, used conventionally to define beam width. One convention defines beam width as the angular width between points at which the field strength is 71 percent of its maximum value. Expressed in terms of power ratio, this convention defines beam width as the angular width between HALF-POWER POINTS. The other convention defines beam width as the angular width between points at which the field strength is 50 percent of its maximum value. Expressed in terms of power ratio, the latter convention defines beam width as the angular width between QUARTERPOWER POINTS. The half-power ratio is the most frequently used convention. Which convention has been used in stating the beam width may be identified from the decibel (dB) figure normally included with the specifications of a radar set. Half power and 71 percent field strength correspond to -3 dB; quarter power and 50 percent field strength correspond to -6 dB. The radiation diagram illustrated in figure 1.3 depicts relative values of power in the same plane existing at the same distances from the antenna or the origin of the radar beam. Maximum power is in the direction of the axis of the beam. Power values diminish rapidly in directions away from the axis. The beam width in this case is taken as the angle between the half-power points. Fig.1.3. Radiation diagram For a given amount of transmitted power, the main lobe of the radar beam extends to a greater distance at a given power level with greater concentration of power in narrower beam widths. To increase maximum detection range capabilities, the energy is concentrated into as narrow a beam as is feasible. Because of practical considerations related to target detection and discrimination, only the horizontal beam width is quite narrow, typical values being between about 0.65° to 2.0°. The vertical beam width is relatively broad, typical values being between about 15° to 30°. The beam width is dependent upon the frequency or wavelength of the transmitted energy, antenna design, and the dimensions of the antenna. For a given antenna size (antenna aperture), narrower beam widths are obtained when using shorter wavelengths. For a given 6 wavelength, narrower beam widths are obtained when using larger antennas. The slotted waveguide antenna has largely eliminated the side-lobe problem. 1.1.3.4 Effect of sea surface on radar beam With radar waves being propagated in the vicinity of the surface of the sea, the main lobe of the radar beam, as a whole, is composed of a number of separate lobes as opposed to the single lobe-shaped pattern of radiation as emitted in free space. This phenomenon is the result of interference between radar waves directly transmitted and those waves which are reflected from the surface of the sea. The vertical beam widths of navigational radars are such that during normal transmission, radar waves will strike the surface of the sea at points from near the antenna (depending upon antenna height and vertical beam width) to the radar horizon. The indirect waves (see figure 1.4) reflected from the surface of the sea may, on rejoining the direct waves, either reinforce or cancel the direct waves depending upon whether they are in phase or out of phase with the direct waves, respectively. Where the direct and indirect waves are exactly in phase, i.e., the crests and troughs of the waves coincide, hyperbolic lines of maximum radiation known as LINES OF MAXIMA are produced. Where the direct and indirect waves are exactly of opposite phase, i.e., the trough of one wave coincides with the crest of the other wave, hyperbolic lines of minimum radiation known as LINES OF MINIMA are produced. Along directions away from the antenna, the direct and indirect waves will gradually come into and pass out of phase, producing lobes of useful radiation separated by regions within which, for practical purposes, there is no useful radiation. Figure 1.4 - Direct and indirect waves. Figure 1.5 illustrates the lower region of the INTERFERENCE PATTERN of a representative navigational radar. Since the first line of minima is at the surface of the sea, the first region of minimum radiation or energy is adjacent to the sea’s surface. From figure 1.5 it should be obvious that if r-f energy is to be reflected from a target, the target must extend somewhat above the radar horizon, the amount of extension being dependent upon the reflecting properties of the target. A vertical-plane coverage diagram as illustrated in figure 1.5 is used by radar designers and analysts to predict regions in which targets will and will not be detected. Of course, on the small page of a book it would be impossible to illustrate the coverage of a radar beam to scale with antenna height being in feet and the lengths of the various lobes of the interference pattern being in miles. In providing greater clarity of the presentation of the lobes, non-linear graduations of the arc of the vertical beam width are used. 7 1.1.4 Radar System Constants Before describing the functions of the components of a marine radar, there are certain constants associated with any radar system that will be discussed. These are carrier frequency, pulse repetition frequency, pulse length, and power relation. The choice of these constants for a particular system is determined by its operational use, the accuracy required, the range to be covered, the practical physical size, and the problems of generating and receiving the signals. 1.1.4.1 Carrier Frequency The carrier frequency is the frequency at which the radio-frequency energy is generated. The principal factors influencing the selection of the carrier frequency are the desired directivity and the generation and reception of the necessary microwave radio-frequency energy. For the determination of direction and for the concentration of the transmitted energy so that a greater portion of it is useful, the antenna should be highly directive. The higher the carrier frequency, the shorter the wavelength and hence the smaller is the antenna required for a given sharpness of the pattern of radiated energy. The problem of generating and amplifying reasonable amounts of radiofrequency energy at extremely high frequencies is complicated by the physical construction of the tubes to be used. The common tube becomes impractical for certain functions and must be replaced by tubes of special design. Among these are the klystron and magnetron. Since it is very difficult to amplify the radio-frequency echoes of the carrier wave, radio-frequency amplifiers are not used. Instead, the frequency of the incoming signals (echoes) is mixed (heterodyned) with that of a local oscillator in a crystal mixer to produce a difference frequency called the intermediate frequency. This intermediate frequency is low enough to be amplified in suitable intermediate frequency amplifier stages in the receiver. 1.1.4.2 Pulse Repetition Frequency The Pulse Repetition Frequency (PRF), sometimes referred to as Pulse Repetition Rate (PRR) is the number of pulses transmitted per second. Some characteristic values may be 600, 1000, 1500, 2200 and 3000 pulses per second. The majority of modern marine radars operate within a range of 400 to 4000 pulses per second. If the distance to a target is to be determined by measuring the time required for one pulse to travel to the target and return as a reflected echo, it is necessary that this cycle be completed before the pulse immediately following is transmitted. This is the reason why the transmitted pulses must be separated by relatively long non-transmitting time periods. Otherwise, transmission would occur during reception of the reflected echo of the preceding pulse. Using the same antenna for both transmitting and receiving, the relatively weak reflected echo would be blocked by the relatively strong transmitted pulse. Sufficient time must be allowed between each transmitted pulse for an echo to return from any target located within the maximum workable range of the system. Otherwise, the reception of the echoes from the more distant targets would be blocked by succeeding transmitted pulses. The maximum measurable range of a radar set depends upon the peak power in relation to the pulse repetition rate. Assuming sufficient power 8 is radiated, the maximum range at which echoes can be received may be increased through lowering the pulse repetition rate to provide more time between transmitted pulses. The PRR must be high enough so that sufficient pulses hit the target and enough are returned to detect the target. The maximum measurable range, assuming that the echoes are strong enough for detection, can be determined by dividing 80,915 (radar nautical miles per second) by the PRR. With the antenna being rotated, the beam of energy strikes a target for a relatively short time. During this time, a sufficient number of pulses must be transmitted in order to receive sufficient echoes to produce the necessary indication on the radarscope. With the antenna rotating at 15 revolutions per minute, a radar set having a PRR of 800 pulses per second will produce approximately 9 pulses for each degree of antenna rotation. The PERSISTENCE of the radarscope, i.e., a measure of the time it retains images of echoes, and the rotational speed of the antenna, therefore, determine the lowest PRR that can be used. 1.1.4.4 Pulse Length Pulse length is defined as the duration of the transmitted radar pulse and is usually measured in microseconds. The minimum range at which a target can be detected is determined largely by the pulse length. If a target is so close to the transmitter that the echo is returned to the receiver before the transmission stops, the reception of the echo, obviously, will be masked by the transmitted pulse. For example, a radar set having a pulse length of 1 microsecond will have a minimum range of 164 yards. This means that the echo of a target within this range will not be seen on the radarscope because of being masked by the transmitted pulse. Since the radio-frequency energy travels at a speed of 161,829 nautical miles per second or 161,829 nautical miles in one million microseconds, the distance the energy travels in 1 microsecond is approximately 0.162 nautical mile or 328 yards. Because the energy must make a round trip, the target cannot be closer than 164 yards if its echo is to be seen on the radarscope while using a pulse length of 1 microsecond. Consequently, relatively short pulse lengths, about 0.1 microseconds, must be used for close-in ranging. Many radar sets are designed for operation with both short and long pulse lengths. Many of these radar sets are shifted automatically to the shorter pulse length on selecting the shorter range scales. On the other radar sets, the operator must select the radar pulse length in accordance with the operating conditions. Radar sets have greater range capabilities while functioning with the longer pulse length because a greater amount of energy is transmitted in each pulse. While maximum detection range capability is sacrificed when using the shorter pulse length, better range accuracy and range resolution are obtained. With the shorter pulse, better definition of the target on the radar-scope is obtained; therefore, range accuracy is better. RANGE RESOLUTION is a measure of the capability of radar set to detect the separation between those targets on the same bearing but having small differences in range. If the leading edge of a pulse strikes a target at a slightly greater range while the trailing part of the pulse is still striking a closer target, it is obvious that the reflected echoes of the two targets will appear as a single elongated image on the radarscope. 9 1.1.4.5 Power Relation The useful power of the transmitter is that contained in the radiated pulses and is called the PEAK POWER of the system. Power is normally measured as an average value over a relatively long period of time. Because the radar transmitter is resting for a time that is long with respect to the operating time, the average power delivered during one cycle of operation is relatively low compared with the peak power available during the pulse time. A definite relationship exists between the average power dissipated over an extended period of time and the peak power developed during the pulse time. The PULSE REPETITION TIME, or the overall time of one cycle of operation, is the reciprocal of the pulse repetition rate (PRR). Other factors remaining constant, the longer the pulse length, the higher will be the average power; the longer the pulse repetition time, the lower will be the average power. average power/ peak power= pulse length/ pulse repetition time These general relationships are shown in figure 1.4 Figure 1.4 - Relationship of peak and average power. The operating cycle of the radar transmitter can be described in terms of the fraction of the total time that radio-frequency energy is radiated. This time relationship is called the DUTY CYCLE and may be represented as follows: duty cycle= pulse length/ pulse repetition time For a radar having a pulse length of 2 microseconds and a pulse repetition rate of 500 cycles per second (pulse repetition time = 2,000 microseconds), the duty cycle= 2 μsec/ 2,000 μsec=0.001 Likewise, the ratio between the average power and peak power may be expressed in terms of the duty cycle. duty cycle =average power/ peak power In the foregoing example assume that the peak power is 200 kilowatts. Therefore, for a period of 2 microseconds a peak power of 200 kilowatts is supplied to the antenna, while for the remaining 1998 microseconds the transmitter output is zero. Because average power is equal to peak power times the duty cycle, 10 average power=200 kw x 0.001=0.2 kilowatt High peak power is desirable in order to produce a strong echo over the maximum range of the equipment. Low average power enables the transmitter tubes and circuit components to be made smaller and more compact. Thus, it is advantageous to have a low duty cycle. The peak power that can be developed is dependent upon the interrelation between peak and average power, pulse length, and pulse repetition time, or duty cycle. 1.1.5 Components and Summary of Functions While pulse-modulated radar systems vary greatly in detail, the principles of operation are essentially the same for all systems. Thus, a single basic radar system can be visualized in which the functional requirements are essentially the same as for all specific equipments. The functional breakdown of a basic pulse-modulated radar system usually includes six major components, as shown in the block diagram, figure 1.13. The functions of the components may be summarized as follows: The power supply furnishes all AC and DC voltages necessary for the operation of the system components. The modulator produces the synchronizing signals that trigger the transmitter the required number of times per second. It also triggers the indicator sweep and coordinates the other associated circuits. The transmitter generates the radio-frequency energy in the form of short powerful pulses. The antenna system takes the radio-frequency energy from the transmitter, radiates it in a highly directional beam, receives any returning echoes, and passes these echoes to the receiver. The receiver amplifies the weak radio-frequency pulses (echoes) returned by a target and reproduces them as video pulses passed to the indicator. The indicator produces a visual indication of the echo pulses in a manner that furnishes the desired information. 11 Figure 1.5 - Block diagram of a basic pulse-modulated radar system 1.1.5.1 Functions of components Power supply In figure 1.4 the power supply is represented as a single block. Functionally, this block is representative. However, it is unlikely that any one supply source could meet all the power requirements of a radar set. The distribution of the physical components of a system may be such as to make it impractical to group the power-supply circuits into a single physical unit. Different supplies are needed to meet the varying requirements of a system and must be designed accordingly. The power supply function is performed by various types of power supplies distributed among the circuit components of a radar set. 12 In figure 1.5 the modulator, transmitter, and receiver are contained in the same chassis. In this arrangement, the group of components is called a TRANSCEIVER. (The term transceiver is an acronym composed from the words TRANSmitter and reCEIVER.) Modulator The function of the modulator is to insure that all circuits connected with the radar system operate in a definite time relationship with each other and that the time interval between pulses is of the proper length. The modulator simultaneously sends a synchronizing signal to trigger the transmitter and the indicator sweep. This establishes a control for the pulse repetition rate (PRR) and provides a reference for the timing of the travel of a transmitted pulse to a target and its return as an echo. Transmitter The transmitter is basically an oscillator which generates radio-frequency (r-f) energy in the form of short powerful pulses as a result of being turned on and off by the triggering signals from the modulator. Because of the frequencies and power outputs required, the transmitter oscillator is a special type known as a MAGNETRON. Transmitting and receiving antenna system The function of the antenna system is to take the r-f energy from the transmitter, radiate this energy in a highly directional beam, receive any echoes or reflections of transmitted pulses from targets, and pass these echoes to the receiver. In carrying out this function the r-f pulses generated in the transmitter are conducted to a FEEDHORN at the focal point of a directional reflector, from which the energy is radiated in a highly directional pattern. The transmitted and reflected energy (returned by the same dual purpose reflector) are conducted by a common path. This common path is an electrical conductor known as a WAVEGUIDE. A waveguide is hollow copper tubing, usually rectangular in cross section, having dimensions according to the wavelength or the carrier frequency, i.e., the frequency of the oscillations within the transmitted pulse or echo. Because of this use of a common waveguide, an electronic switch, a TRANSMITRECEIVE (TR) TUBE capable of rapidly switching from transmit to receive functions, and vice versa, must be utilized to protect the receiver from damage by the potent energy generated by the transmitter. The TR tube, as shown in figure 1.14 blocks the transmitter pulses from the receiver. During the relatively long periods when the transmitter is inactive, the TR tube permits the returning echoes to pass to the receiver. To prevent any of the very weak echoes from being absorbed by the transmitter, another device known as an ANTI-TR (A-TR) TUBE is used to block the passage of these echoes to the transmitter. 13 Figure 1.6 - A basic radar system. The feedhorn at the upper extremity of the waveguide directs the transmitted energy towards the reflector, which focuses this energy into a narrow beam. Any returning echoes are focused by the reflector and directed toward the feedhorn. The echoes pass through both the feedhorn and waveguide enroute to the receiver. The principles of a parabolic reflector are illustrated in figure 1.15. Figure 1.7 - Principles of a parabolic reflector. 14 Indicator The primary function of the indicator is to provide a visual display of the ranges and bearings of radar targets from which echoes are received. In this basic radar system, the type of display used is the PLAN POSITION INDICATOR (PPI), which is essentially a polar diagram, with the transmitting ship’s position at the center. Images of target echoes are received and displayed at either their relative or true bearings, and at their distances from the PPI center. With a continuous display of the images of the targets, the motion of the target relative to the motion of the transmitting ship is also displayed. The secondary function of the indicator is to provide the means for operating various controls of the radar system. The CATHODE-RAY TUBE (CRT), illustrated in figure 1.17, is the heart of the indicator. The CRT face or screen, which is coated with a film of phosphorescent material, is the PPI. The ELECTRON GUN at the opposite end of the tube (see figure 1.18) emits a very narrow beam of electrons which impinges upon the center of the PPI unless deflected by electrostatic or electromagnetic means. Since the inside face of the PPI is coated with phosphorescent material, a small bright spot is formed at the center of the PPI. Figure 1.8 - Electromagnetic cathode-ray tube. If the electron beam is rapidly and repeatedly deflected radially from the center, a bright line called a TRACE is formed on the PPI. Should the flow of electrons be stopped, this trace will continue to glow for a short period following the stoppage of the electron beam because of the phosphorescent coating. The slow decay of the brightness is known as PERSISTENCE; the slower the decay the higher the persistence. At the instant the modulator triggers the transmitter; it sends a TIMING TRIGGER signal to the indicator. The latter signal acts to deflect the electron beam radially from the center of the CRT screen (PPI) to form a trace of the radial movement of the electron beam. This radial movement of the electron beam is called the SWEEP or TIME BASE. While the terms trace and sweep are frequently used interchangeably, the term trace is descriptive only of the visible evidence of the sweep movement. Since the electron beam is deflected from the center of the CRT screen with each pulse of the transmitter, the sweep must be repeated very rapidly even when the lower pulse repetition rates are used. With a pulse repetition rate of 750 pulses per second, the 15 sweep must be repeated 750 times per second. Thus, it should be quite obvious why the sweep appears as a solid luminous line on the PPI. The speed of movement of the point of impingement of the electron beam is far in excess of the capability of detection by the human eye. While the sweep must be repeated in accordance with the PRR, the actual rate of radial movement of the electron beam is governed by the size of the CRT screen and the distance represented by the radius of this screen according to the range scale being used. If the 20-mile range scale is selected, the electron beam must be deflected radially from the center of the CRT screen having a particular radius at a rate corresponding to the time required for radi o-frequency energy to travel twice the distance of the range scale or 40 nautical miles. When using the 20-mile range scale, the electron beam must move radially from the center of the CRT screen to its periphery in 247 microseconds. Speed of radio frequency - frequency energy = 0.161829 nm per microsecond Distance = Speed X Time 40 nm ¸ 0.161829 nm per microsecond = 247 microseconds The objective of regulating the rate of travel of the electron beam in this manner is to establish a time base on the PPI which may be used for direct measurements of distances to targets without further need to take into account the fact that the transmitted pulse and its reflected echo make a round trip to and from the target. With the periphery of the PPI representing a distance of 20 miles from the center of the PPI at the 20-mile range scale setting, the time required for the electron beam to move radially from the center to the periphery is the same as the time required for the transmitted pulse to travel to a target at 20 miles and return to the antenna as a reflected echo or the time to travel 40 miles in this case. It follows that a point on the sweep or time base halfway between the center of the PPI and its periphery represents a distance of 10 miles from the center of the PPI. Figure 1.9 - The sweep on the plan position indicator. 16 The foregoing assumes that the rate of travel of the electron beam is constant, which is the usual case in the design of indicators for navigational radar. If the antenna is trained on a target at 10 miles while using the 20-mile range scale, the time for the 20-mile round trip of the transmitted pulse and the returning echo is 123.5 microseconds. At 123.5 microseconds, following the instant of triggering the transmitter and sending the timing trigger pulse to the indicator to deflect the electron beam radially, the electron beam will have moved a distance of 10 miles in its sweep or on the time base. On receiving the echo at 123.5 microseconds after the instant of the pulse, the receiver sends a video signal to the indicator which in turn acts to intensify or brighten the electron beam at the point in its sweep at 123.5 microseconds, i.e., at 10 miles on the time base. This brightening of the trace produced by the sweep at the point corresponding to the distance to the target in conjunction with the persistence of the PPI produces a visible image of the target. Because of the pulse repetition rate, this painting of an image on the PPI is repeated many times in a short period, resulting in a steady glow of the target image if the target is a reasonably good reflector. In navigational and collision avoidance applications of radar, the antenna and the beam of r-f energy radiated from it are rotated at a constant rate, usually about 10 to 20 revolutions per minute in order to detect targets all around the observer’s ship. In the preceding discussion of how a target image is painted on the PPI, reference is made only to radial deflection of the electron beam to produce the sweep or time base. If target images are to be painted at their relative bearings as well as distances from the center of the PPI, the sweep must be rotated in synchronization with the rotation of the antenna. Just as the electron beam may be deflected radially by electrostatic or electromagnetic means, the sweep may be rotated by the same means. The sweep is usually rotated electromagnetically in modern radars. As the antenna is rotated past the ship’s heading, the sweep, in synchronization with the antenna, is rotated past the 0° graduation on the relative bearing dial of the PPI. The image of any target detected ahead is painted on the PPI at its relative bearing and distance from the center of the PPI. As targets are detected in other directions, their images are painted on the PPI at their relative bearings and distances from the center of the PPI. Up to this point the discussion of how target information is displayed on the PPI has been limited to how the target images are painted, virtually instantaneously, at their distances and relative bearings from the reference ship at the center of the PPI. It follows that through continuous display (continuous because of the persistence of the CRT screen and the pulse repetition rate) of the positions of targets on the PPI, their motions relative to the motion of the reference ship are also displayed. In summary, the indicator of this basic radar system provides the means for measuring and displaying, in a useful form, the relative bearings and distances to targets from which reflected echoes may be received. In displaying the positions of the targets relative to the reference ship continuously, the motions of the targets relative to the motion of the reference ship are evident. 17 1.2 Safe Distances Safe Distances are Explained Correctly “Action taken to avoid collision with another vessel shall be such as to result in passing at a safe distance. The effectiveness of the action shall be carefully checked until the other vessel is finally past and clear” (Colregs Rule 8(d)) The requirement that action taken to avoid collision shall be such as to result in passing at a safe distance is introduced for the first time in the 1972 Regulations. When vessels are in sight of one another and one of the two vessels is required to keep out of the way, the obligation to take action which results in passing at a safe distance will obviously apply almost exclusively to the give way vessel. The stand on vessel must initially keep her course and speed, and is only permitted to take action by rule 17 a when it becomes apparent that the give way vessel will not take action. The first moment for such permitted action may not be at a sufficiently early stage to ensue that her manoeuvre alone will achieve a sufficiently really safe passing distance Rule 8 makes a mention of "close-quarters situation" and "safe passing distance". These concepts cannot be concisely formulated. For obvious that their extent depends on many factors, such as weather conditions, state of visibility, type of vessel, manoeuvrability and observations are carried out by visual means or by radar - which is far less discriminating than the naked eye in discerning changes in aspect. But even, when considering radar navigation in fog only, formulation does not come easily. For example, to pass another vessel at half a mile at three knots could be considered just as safe as passing a vessel four miles off at 15 knots. One must also take into account, when deciding what is a safe distance to pass, the direction in which vessels are shaping to pass. For instance, it could be quite reasonable and safe when overtaking a vessel to pass two miles off, whereas this could be unwise when passing a vessel on a reciprocal course with speed. In other words, it is really the relative speed and the direction of the target which should be considered when judging what is a close-quarters situation. In thick fog, however, when there is plenty of sea room, it is practical to keep the minimum radius of the close-quarters situation at about three miles in order to allow for bearing errors, unsuspected manoeuvres of the target and to keep out the range of audibility of the other ship's sound signals so that delays owing to the application of Rule 19 (e) can be avoided. Really, in order to assess the radius of the close-quarters situation, the master must rely on intuition based on experience to give him the right answer (radar simulator courses are generally useful to accelerate this experience). CPA and TCPA data should be set on ARPA so that sufficient warning can be given to the O.O.W. when a close-quarters situation is approaching. In restricted visibility every vessel which detects the presence of another vessel by radar is required to take avoiding action if a close quarter’s situation is developing and/or risk of collision exists (Rule 19(d)), but the circumstances may not permit actions which permit action to be taken which will result in passing at a safe distance. If, for instance in open sea, a ship is detected ahead of fine on the bow and careful plotting or equivalent method of assessment indicates that other vessel is proceeding at a relatively high speed, and that if no action is taken the two vessels will pass starboard to starboard at too close a distance, of the order of a mile, it may be dangerous to alter your course either to starboard or port. A reduction of speed may be the safest form of action in such circumstances but this may not appreciably affect the passing distance. 18 1.3 Radiation hazards and precaution Irradiation is exposure to radiation but not radioactive material (ie, no contamination is involved). Radiation exposure can occur without the source of radiation (eg radioactive material, x-ray machine) being in contact with the person. When the source of the radiation is removed or turned off, exposure ends. Irradiation can involve the whole body, which, if the dose is high enough, can result in systemic symptoms and radiation syndromes, or a small part of the body (eg, from radiation therapy), which can result in local effects. People do not emit radiation (ie, become radioactive) following irradiation. Sources of exposure: Sources may be naturally occurring or man-made. People are constantly exposed to low levels of naturally occurring radiation called background radiation. Background radiation comes from cosmic radiation and from radioactive elements in the air, water, and earth. Cosmic radiation is concentrated at the poles by the earth's magnetic field and attenuated by the atmosphere. Thus, exposure is greater for people living at high latitudes, at high altitudes, or both and during airplane flights. Radioactive elements, particularly uranium and its radioactive progeny and potassium40, are present in many rocks and minerals. These elements end up in various substances, including food, water, and construction materials. Radon, a radioactive gas resulting from the decay of uranium, typically accounts for about 2/3 of naturally occurring radiation dose to the US population. In the US, people receive an average effective dose of about 3 millisieverts (mSv)/yr from natural sources. However, in some parts of the world, people receive between 5 and 10 mSv/yr. The doses from natural background radiation are far too low to cause radiation injuries, although they may increase the risk of cancer. People receive on the average about 3 mSv/yr from man-made sources, the vast majority of which involve medical imaging. Imaging exposure tends to be highest from CT scans and nuclear cardiology procedures. However, medical diagnostic procedures rarely impart doses sufficient to cause radiation injury, although they may increase the risk of cancer. Exceptions may include certain prolonged fluoroscopically guided interventional procedures (eg, endovascular reconstruction, vascular embolization, cardiac radiofrequency ablation); these procedures have caused injuries to skin and underlying tissues. Radiation therapy commonly causes injury to some normal tissues near the target tissue. A small portion of public exposure results from radiation accidents and fallout from nuclear weapons testing. Accidents may involve industrial irradiators, industrial radiography sources, and nuclear reactors. These accidents commonly result from failure to follow safety procedures (eg, interlocks being bypassed). Radiation injuries have also been caused by lost or stolen medical or industrial sources containing radionuclides. People seeking medical care for these injuries may be unaware that they were exposed to radiation. Radioactive material has escaped from nuclear power plants, including the Three Mile Island plant in Pennsylvania in 1979 and at Chernobyl in Ukraine in 1986. Exposure from Three Mile Island was minimal; people living within 1.6 km of the plant received only about 0.08 mSv. However, people living in 2 villages near the Chernobyl plant received an average dose of about 300 mSv, and people at the Chernobyl plant itself received significantly higher doses. More than 30 workers and emergency responders died, and many more were injured. Low-level contamination from that accident was 19 detected as far away as Europe, Asia, and even the US. The average cumulative exposure for the general population in various affected regions of Belarus, Russia, and Ukraine over a 20-yr period after the accident is estimated to be between 10 and 30 mSv. Another significant radiation event was the detonation of 2 atomic bombs over Japan in August 1945, which caused about 110,000 deaths from the immediate trauma of the blast and heat. A much smaller number of deaths resulted later from radiation-induced illnesses. While several criminal cases of intentional contamination of individuals have been reported, radiation exposure to a population through terrorist activities has not occurred but is a concern. A possible scenario involves the use of a device to contaminate an area by dispersing radioactive material (a radiation dispersal device that uses conventional explosives is referred to as a dirty bomb). Other terrorist scenarios include using a hidden radiation source to expose unsuspecting people to large doses of radiation, attacking a nuclear reactor or radioactive material storage facility, and detonating a nuclear weapon. Pathophysiology Ionizing radiation can damage DNA, RNA, and proteins directly, but more often the damage to these molecules is indirect, caused by highly reactive free radicals generated by radiation's interaction with intracellular water molecules. Large doses of radiation can cause cell death, and lower doses may interfere with cellular proliferation. Damage to other cellular components can result in progressive tissue hypoplasia, atrophy, and eventually fibrosis. Types of exposure: Radiation exposure may involve: - Contamination - Irradiation - Radioactive contamination is the unintended contact with and retention of radioactive material, usually as a dust or liquid. Contamination may be: o External o Internal External contamination is that on skin or clothing, from which some can fall or be rubbed off, contaminating other people and objects. Internal contamination is unintended radioactive material within the body, which it may enter by ingestion, inhalation, or through breaks in the skin. Once in the body, radioactive material may be transported to various sites (eg, bone marrow), where it continues to emit radiation until it is removed or decays. Internal contamination is more difficult to remove. Although internal contamination with any radionuclide is possible, historically, most cases in which contamination posed a significant risk to the patient involved a relatively small number of radionuclides: hydrogen-3, cobalt-60, strontium-90, cesium-137, iodine-131, radium-226, uranium-235, uranium-238, plutonium-238, plutonium-239, polonium-210, and americium-241. Factors affecting response: Biologic response to radiation varies with: - Tissue radiosensitivity - Dose 20 - Duration of exposure Cells and tissues differ in their radiosensitivity. In general, cells that are undifferentiated and those that have high mitotic rates (eg, stem cells) are particularly vulnerable to radiation. Because radiation preferentially depletes rapidly dividing stem cells over the more resistant mature cells, there is typically a latent period between radiation exposure and overt radiation injury. Injury does not manifest until a significant fraction of the mature cells die of natural senescence and, due to loss of stem cells, are not replaced. Cellular sensitivities in approximate descending order from most to least sensitive are: - Lymphoid cells - Germ cells - Proliferating bone marrow cells - Intestinal epithelial cells - Epidermal stem cells - Hepatic cells - Epithelium of lung alveoli and biliary passages - Kidney epithelial cells - Endothelial cells (pleura and peritoneum) - Nerve cells - Bone cells - Muscle and connective tissue cells The severity of radiation injury depends on the dose and the length of time over which it is delivered. A single rapid dose is more damaging than the same dose given over weeks or months. Dose response also depends on the fraction of the body exposed. Significant illness is certain, and death is possible, after a whole-body dose > 4.5 Gy delivered over a short time interval; however, 10s of Gy can be well tolerated when delivered over a long period to a small area of tissue (eg, for cancer therapy). Other factors can increase the sensitivity to radiation injury. Children are more susceptible to radiation injury because they have a higher rate of cellular proliferation. People who are homozygous for the ataxia-telangiectasia gene exhibit greatly increased sensitivity to radiation injury. Disorders, such as connective tissue disorders and diabetes, may increase the sensitivity to radiation injury. Chemotherapeutic agents also increase the sensitivity to radiation injury. Definition of Radiation sickness: Radiation sickness is illness and symptoms resulting from excessive exposure to radiation. Exposure may be accidental or intentional (as in radiation therapy). Considerations: There are two basic types of radiation: ionizing and nonionizing. - Nonionizing radiation comes in the form of light, radio waves, microwaves and radar. This kind of radiation usually does not cause tissue damage. - Ionizing radiation is radiation that produces immediate chemical effects on human tissue . X-rays, gamma rays, and particle bombardment (neutron beam, electron beam, 21 protons, mesons, and others) give off ionizing radiation. This type of radiation can be used for medical testing and treatment, industrial and manufacturing purposes, weapons and weapons development, and more. Radiation sickness results when humans are exposed to very large doses of ionizing radiation. Radiation exposure can occur as a single large exposure (acute), or a series of small exposures spread over time (chronic).Radiation sickness is generally associated with acute exposure and has a characteristic set of symptoms that appear in an orderly fashion. Chronic exposure is usually associated with delayed medical problems such as cancer and premature aging, which may happen over a long period of time. The risk of cancer depends on the dose and begins to build up even with very low doses. There is no "minimum threshhold." Exposure from x-rays or gamma rays is measured in units of roentgens. For example: - Total body exposure of 100 roentgens (or 1 Gy) causes radiation sickness. - Total body exposure of 400 roentgens (or 4 Gy) causes radiation sickness and death in half the individuals. Without medical treatment, nearly everyone who receives more than this amount of radiation will die within 30 days. - 100,000 rads causes almost immediate unconsciousness and death within an hour The severity of symptoms and illness (acute radiation sickness) depends on the type and amount of radiation, how long you were exposed, and which part of the body was exposed. Symptoms of radiation sickness may occur immediately after exposure, or over the next few days, weeks, or months. Because it is difficult to determine the amount of radiation exposure from nuclear accidents, the best signs of the severity of the exposure are: the length of time between the exposure and the onset of symptoms, the severity of symptoms, and severity of changes in white blood cells. If a person vomits less than an hour after being exposed, that usually means the radiation dose received is very high and death may be expected. Children who receive radiation treatments or who are accidentally exposed to radiation will be treated based on their symptoms and their blood cell counts. Frequent blood studies are necessary and require a small puncture through the skin into a vein to obtain blood samples. Causes: - Accidental exposure to high doses of radiation such as a nuclear power plant accidents - Exposure to excessive radiation for medical treatments Radiation sickness is a harmful effect produced on body tissues by exposure to radioactive substances. The biological action of radiation is not fully understood, but it is believed that a disturbance in cellular activity results from the chemical changes caused by ionization (see ion ). Some body tissues are more sensitive to radiation than others and are more easily affected; the cells in the blood-forming tissues (bone marrow, spleen, and lymph nodes) are extremely sensitive. Radiation sickness may occur from exposure to a single massive emanation such as a nuclear explosion (such as Hiroshima and Nagasaki), or it may occur after repeated large exposure or to even very small doses in a plant or laboratory, since radiation effects are cumulative. Moreover, exposure to the ultraviolet radiation of the sun can cause tissue destruction and trigger mutations that can lead to skin cancer . 22 There is no treatment for radiation sickness, although it is sometimes possible for persons to survive otherwise lethal doses of radiation if bone marrow transplants are performed. Potassium iodide is to protect against thyroid cancer from radiation exposure, but the drug should ideally be taken four hours prior to the exposure. Exposure to radiation can cause genetic mutation; the progeny of those subjected to excessive radiation tend to show deleterious genetic changes. The genetic damage from the atomic bombs dropped on Japan is still evident and such damage will continue to surface in people directly affected by the nuclear diasaster at Chernobyl. Persons working with radioactive materials or X rays protect themselves from excessive exposure to radiation by shields and special clothing usually containing lead. Processes involving radioactive substances are observed through thick plates of specially prepared glass that exclude the harmful rays. A dosimeter, a device measuring the amount of radiation to which an individual has been exposed, is always worn by persons working in radioactive areas Ionizing radiation injures tissues variably, depending on factors such as radiation dose, rate of exposure, type of radiation, and part of the body exposed. Symptoms may be local (eg, burns) or systemic (eg, acute radiation sickness). Diagnosis is by history of exposure, symptoms and signs, and sometimes use of radiation detection equipment to localize and identify radionuclide contamination. Management focuses on associated traumatic injuries, decontamination, supportive measures, and minimizing exposure of health care workers. Patients with severe acute radiation sickness receive reverse isolation and bone marrow support. Patients internally contaminated with certain specific radionuclides may receive uptake inhibitors or chelating agents. Prognosis is initially estimated by the time between exposure and symptoms, the severity of those symptoms, and by the lymphocyte count during the initial 24 to 72 h. Ionizing radiation is emitted by radioactive elements and by equipment such as x-ray and radiation therapy machines. Acute Radiation Syndromes [ARS]: After the whole body, or a large portion of the body, receives a high dose of radiation, several distinct syndromes may occur: - Cerebrovascular syndrome - GI syndrome - Hematopoietic syndrome These syndromes have 3 different phases: 1. Prodromal phase (0 to 2 days after exposure): Lethargy and GI symptoms (nausea, anorexia, vomiting, diarrhea) are possible. 2. Latent asymptomatic phase (0 to 31 days after exposure) 3. Overt systemic illness phase: Illness is classified by the main organ system affected. Which syndrome develops, its severity, and rate of progression depends on radiation dose. The symptoms and time course are fairly consistent for a given dose of radiation and thus can help estimate radiation exposure. The cerebrovascular syndrome, the dominant manifestation of extremely high wholebody doses of radiation (> 30 Gy), is always fatal. The prodrome develops within minutes to 1 h of exposure. There is little or no latent phase. Patients develop tremors, seizures, ataxia, and cerebral edema and die within hours to 1 or 2 days. 23 The GI syndrome is the dominant manifestation after whole-body doses of about 6 to 30 Gy. Prodromal symptoms, often marked, develop within about 1 h and resolve within 2 days. During the latent period of 4 to 5 days, GI mucosal cells die. Cell death is followed by intractable nausea, vomiting, and diarrhea, which lead to severe dehydration and electrolyte imbalances, diminished plasma volume, and vascular collapse. Necrosis of intestine may also occur, predisposing to bacteremia and sepsis. Death is common. Patients receiving > 10 Gy may have cerebrovascular symptoms (suggesting a lethal dose). Survivors also have the hematopoietic syndrome. The hematopoietic syndrome is the dominant manifestation after whole-body doses of about 1 to 6 Gy and consists of a generalized pancytopenia. A mild prodrome may begin after 1 to 6 h, lasting 24 to 48 h. Bone marrow stem cells are significantly depleted, but mature blood cells in circulation are largely unaffected (circulating lymphocytes are an exception, and lymphopenia may be evident within hours to days after exposure). As the cells in circulation die by senescence, they are not replaced in sufficient numbers, resulting in pancytopenia. Thus, patients remain asymptomatic during a latent period of up to 4 ½ wk after a 1-Gy dose as marrow production falls. Risk of various infections is increased as a result of the neutropenia (most prominent at 2 to 4 wk) and decreased antibody production. Petechiae and mucosal bleeding result from thrombocytopenia, which develops within 3 to 4 wk and may persist for months. Anemia develops slowly, because preexisting RBCs have a longer life span than WBCs and platelets. Survivors have an increased incidence of radiation-induced cancer, including leukaemia. 1.4 Characteristics performance of radar sets and factors affecting 1.4.1 Factors affecting maximum range Frequency The higher the frequency of a radar (radio) wave, the greater is the attenuation (loss in power), regardless of weather. Lower radar frequencies (longer wavelengths) have, therefore, been generally superior for longer detection ranges. Peak Power The peak power of a radar is its useful power. Range capabilities of the radar increase with peak power. Doubling the peak power increases the range capabilities by about 25 percent. Pulse Length The longer the pulse length, the greater is the range capability of the radar because of the greater amount of energy transmitted. Pulse Repetition Rate The pulse repetition rate (PRR) determines the maximum measurable range of the radar. Ample time must be allowed between pulses for an echo to return from any target located within the maximum workable range of the system. Otherwise, echoes returning from the more distant targets are blocked by succeeding transmitted pulses. This necessary time interval determines the highest PRR that can be used. The PRR must be high enough, however, that sufficient pulses hit the target and enough echoes are 24 returned to the radar. The maximum measurable range can be determined approximately by dividing 81,000 by the PRR. Beam Width The more concentrated the beam, the greater is the detection range of the radar. Target Characteristics Targets that are large can be seen on the scope at greater ranges, provided line-of-sight exists between the radar antenna and the target. Conducting materials (a ship’s steel hull, for example) return relatively strong echoes while non conducting materials (a wood hull of a fishing boat, for example) return much weaker echoes. Receiver Sensitivity The more sensitive receivers provide greater detection ranges but are more subject to jamming. Antenna Rotation Rate The more slowly the antenna rotates, the greater is the detection range of the radar. For a radar set having a PRR of 1,000 pulses per second, a horizontal beam width of 2.0°, and an antenna rotation rate of 6 RPM (1 revolution in 10 seconds or 36 scanning degrees per second), there is 1 pulse transmitted each 0.036° of rotation. There are 56 pulses transmitted during the time required for the antenna to rotate through its beam width. Beam Width/Degrees per Pulse=2.0°/0.036°= 56 pluses With an antenna rotation rate of 15 RPM (1 revolution in 4 seconds or 90 scanning degrees per second), there is only 1 pulse transmitted each 0.090° of rotation. There are only 22 pulses transmitted during the time required for the antenna to rotate through its beam width. Beam Width/Degrees per Pulse=2.0°/0.090°= 22 pluses From the foregoing it is apparent that at the higher antenna rotation rates, the maximum ranges at which targets, particularly small targets, may be detected are reduced. 1.4.2 Factors affecting minimum range Pulse Length The minimum range capability of a radar is determined primarily by the pulse length. It is equal to half the pulse length of the radar (164 yards per microsecond of pulse length). Electronic considerations such as the recovery time of the receiver and the duplexer (TR and anti-TR tubes assembly) extend the minimum range at which a target can be detected beyond the range determined by the pulse length. Sea Return Sea return or echoes received from waves may clutter the indicator within and beyond the minimum range established by the pulse length and recovery time. Side-Lobe Echoes Targets detected by the side-lobes of the antenna beam pattern are called side-lobe echoes. When operating near land or large targets, side-lobe echoes may clutter the 25 indicator and prevent detection of close targets, without regard to the direction in which the antenna is trained. Vertical Beam Width Small surface targets may escape the lower edge of the vertical beam when close. 1.4.3 Factors affecting range accuracy The range accuracy of radar depends upon the exactness with which the time interval between the instants of transmitting a pulse and receiving the echo can be measured. Fixed Error A fixed range error is caused by the starting of the sweep on the indicator before the r-f energy leaves the antenna. The zero reference for all range measurements must be the leading edge of the transmitted pulse as it appears on the indicator. Inasmuch as part of the transmitted pulse leaks directly into the receiver without going to the antenna, a fixed error results from the time required for r-f energy to go up to the antenna and return to the receiver. This error causes the indicated ranges to be greater than their true values. A device called a trigger delay circuit is used to eliminate the fixed error. By this means the trigger pulse to the indicator can be delayed a small amount. Such a delay results in the sweep starting at the instant an echo would return to the indicator from a flat plate right at the antenna not at the instant that the pulse is generated in the transmitter. Line Voltage Accuracy of range measurement depends on the constancy of the line voltage supplied to the radar equipment. If supply voltage varies from its nominal value, ranges indicated on the radar may be unreliable. This fluctuation usually happens only momentarily, however, and after a short wait ranges normally are accurate. Frequency Drift Errors in ranging also can be caused by slight variations in the frequency of the oscillator used to divide the sweep (time base) into equal range intervals. If such a frequency error exists, the ranges read from the radar generally are in error by some small percentage of the range. To reduce range errors caused by frequency drift, precision oscillators in radars usually are placed in a constant temperature oven. The oven is always heated, so there is no drift of range accuracy while the rest of the set is warming up. Calibration The range to a target can be measured most accurately on the PPI when the leading edge of its pip just touches a fixed range ring. The accuracy of this measurement is dependent upon the maximum range of the scale in use. Representative maximum error in the calibration of the fixed range rings is 75 yards or 11/2 percent of the maximum range of the range scale in use, whichever is greater. With the indicator set on the 6-mile range scale, the error in the range of a pip just touching a range ring may be about 180 yards or about 0.1 nautical mile because of calibration error alone when the range calibration is within acceptable limits. On some PPI’s, range can only be estimated by reference to the fixed range rings. When the pip lies between the range rings, the estimate is usually in error by 2 to 3 percent of 26 the maximum range of the range scale setting plus any error in the calibration of the range rings. Radar indicators usually have a variable range marker (VRM) or adjustable range ring which is the normal means for range measurements. With the VRM calibrated with respect to the fixed range rings within a tolerance of 1 percent of the maximum range of the scale in use, ranges as measured by the VRM may be in error by as much as 21/2 percent of the maximum range of the scale in use. With the indicator set on the 8-mile range scale, the error in a range as measured by the VRM may be in error by as much as 0.2 nautical mile. Pip and VRM Alignment The accuracy of measuring ranges with the VRM is dependent upon the ability of the radar observer to align the VRM with the leading edge of the pip on the PPI. On the longer range scales it is more difficult to align the VRM with the pip because small changes in the reading of the VRM range counter do not result in appreciable changes in the position of the VRM on the PPI. Range Scale The higher range scale settings result in reduced accuracy of fixed range ring and VRM measurements because of greater calibration errors and the greater difficulty of pip and VRM alignment associated with the higher settings. PPI Curvature Because of the curvature of the PPI, particularly in the area near its periphery, range measurements of pips near the edge are of lesser accuracy than the measurements nearer the center of the PPI. Radarscope Interpretation Relatively large range errors can result from incorrect interpretation of a landmass image on the PPI. The difficulty of radarscope interpretation can be reduced through more extensive use of height contours on charts. For reliable interpretation it is essential that the radar operating controls be adjusted properly. If the receiver gain is too low, features at or near the shoreline, which would reflect echoes at a higher gain setting, will not appear as part of the landmass image. If the receiver gain is too high, the landmass image on the PPI will “bloom”. With blooming the shoreline will appear closer than it actually is. A fine focus adjustment is necessary to obtain a sharp landmass image on the PPI. Because of the various factors introducing errors in radar range measurements, one should not expect the accuracy of navigational radar to be better than + or - 50 yards under the best conditions. 1.4.4 Factors affecting range resolution Range resolution is a measure of the capability of a radar to display as separate pips the echoes received from two targets which are on the same bearing and are close together. The principal factors that affect the range resolution of a radar are the length of the transmitted pulse, receiver gain, CRT spot size, and the range scale. A high degree of range resolution requires a short pulse, low receiver gain, and a short range scale. 27 Pulse Length Two targets on the same bearing, close together, cannot be seen as two distinct pips on the PPI unless they are separated by a distance greater than one-half the pulse length (164 yards per microsecond of pulse length). If radar has a pulse length of 1microsecond duration, the targets would have to be separated by more than 164 yards before they would appear as two pips on the PPI. Radio-frequency energy travels through space at the rate of approximately 328 yards per microsecond. Thus, the end of a 1-microsecond pulse traveling through the air is 328 yards behind the leading edge, or start, of the pulse. If a 1-microsecond pulse is sent toward two objects on the same bearing, separated by 164 yards, the leading edge of the echo from the distant target coincides in space with the trailing edge of the echo from the near target. As a result the echoes from the two objects blend into a single pip, and range can be measured only to the nearest object. The reason for this blending is illustrated in figure 1.10. In part A of figure 1.10, the transmitted pulse is just striking the near target. Part B shows energy being reflected from the near target, while the leading edge of the transmitted pulse continues toward the far target. In part C, 1/2 microsecond later, the transmitted pulse is just striking the far target; the echo from the near target has traveled 164 yards back toward the antenna. The reflection process at the near target is only half completed. In part D echoes are traveling back toward the antenna from both targets. In part E reflection is completed at the near target. At this time the leading edge of the echo from the far target coincides with the trailing edge of the first echo. When the echoes reach the antenna, energy is delivered to the set during a period of 2 microseconds so that a single pip appears on the PPI. Figure 1.10 – Pulse length and range resolution 28 1.5 Factors External to The Radar Set Affecting Detection 1.5.1 Factors affecting bearing accuracy Horizontal Beam Width Bearing measurements can be made more accurately with the narrower horizontal beam widths. The narrower beam widths afford better definition of the target and, thus, more accurate identification of the center of the target. Several targets close together may return echoes which produce pips on the PPI which merge, thus preventing accurate determination of the bearing of a single target within the group. The effective beam width can be reduced through lowering the receiver gain setting. In reducing the sensitivity of the receiver, the maximum detection range is reduced, but the narrower effective beam width provides better bearing accuracy. Target Size For a specific beam width, bearing measurements of small targets are more accurate than large targets because the centers of the smaller pips of the small targets can be identified more accurately. Target Rate of Movement The bearings of stationary or slowly moving targets can be measured more accurately than the bearings of faster moving targets. Stabilization of Display Stabilized PPI displays provide higher bearing accuracies than unstabilized displays because they are not affected by yawing of the ship. Sweep Centering Error If the origin of the sweep is not accurately centered on the PPI, bearing measurements will be in error. Greater bearing errors are incurred when the pip is near the center of the PPI than when the pip is near the edge of the PPI. Since there is normally some centering error, more accurate bearing measurements can be made by changing the range scale to shift the pip position away from the center of the PPI. Parallax Error Improper use of the mechanical bearing cursor will introduce bearing errors. On setting the cursor to bisect the pip, the cursor should be viewed from a position directly in front of it. Electronic bearing cursors used with some stabilized displays provide more accurate bearing measurements than mechanical bearing cursors because measurements made with the electronic cursor are not affected by parallax or centering errors. Heading Flash Alignment For accurate bearing measurements, the alignment of the heading flash with the PPI display must be such that radar bearings are in close agreement with relatively accurate visual bearings observed from near the radar antenna. 1.5.2 Factors affecting bearing resolution Bearing resolution is a measure of the capability of a radar to display as separate pips the echoes received from two targets which are at the same range and are close together. The principal factors that affect the bearing resolution of a radar are horizontal beam width, the range to the targets, and CRT spot size. 29 Horizontal Beam Width As the radar beam is rotated, the painting of a pip on the PPI begins as soon as the leading edge of the radar beam strikes the target. The painting of the pip is continued until the trailing edge of the beam is rotated beyond the target. Therefore, the pip is distorted angularly by an amount equal to the effective horizontal beam width. As illustrated in figure 1.11, in which a horizontal beam width of 10° is used for graphical clarity only, the actual bearing of a small target having good reflecting properties is 090°, but the pip as painted on the PPI extends from 095° to 085°. The left 5° and the right 5° are painted while the antenna is not pointed directly towards the target. The bearing must be read at the center of the pip. Figure 1.11 – Angular distorsion 30 Range of Targets Assuming a more representative horizontal beam width of 2°, the pip of a ship 400 feet long observed beam on at a distance of 10 nautical miles on a bearing of 090° would be painted on the PPI between 091.2° and 088.8°, the actual angular width of the target being 0.4°. The pip of a ship 900 feet long observed beam on at the same distance and bearing would be painted on the PPI between 091.4° and 088.6°, the angular width of the target being 0.8°. Since the angular widths of the pips painted for the 400 and 900-foot targets are 1.4° and 1.8°, respectively, any attempt to estimate target size by the angular width of the pip is not practical, generally. Since the pip of a single target as painted on the PPI is elongated angularly an amount equal to beam width, two targets at the same range must be separated by more than one beam width to appear as separate pips. The required distance separation depends upon range. Assuming a 2° beam width, targets at 10 miles must be separated by over 0.35 nautical miles or 700 yards to appear as separate pips on the PPI. At 5 miles the targets must be separated by over 350 yards to appear as separate pips if the beam width is 2°. In as much as bearing resolution is determined primarily by horizontal beam width, a radar with a narrow horizontal beam width provides better bearing resolution than one with a wide beam. CRT Spot Size The bearing separation required for resolution is increased because the spot formed by the electron beam on the screen of the CRT cannot be focused into a point of light. The increase in the pip width because of CRT spot size varies with the size of the CRT and the range scale in use. 1.5.3 Wavelength Generally, radars transmitting at the shorter wavelengths are more subject to the effects of weather than radars transmitting at the longer wavelengths. Without use of anti-rain and anti-sea clutter controls, the clutter is more massive on the PPI of the radar having the shorter wavelength. Also, three targets, which can be detected on the PPI of the radar having the longer wavelength, cannot be detected on the PPI of the radar having the shorter wavelength. Following use of the anti-rain and antisea clutter controls, the three targets still cannot be detected on the PPI of the radar having the shorter wavelength because too much of the energy has been absorbed or attenuated by the rain. 1.5.4 Target characteristics There are several target characteristics which will enable one target to be detected at a greater range than another, or for one target to produce a stronger echo than another target of similar size. Height Since radar wave propagation is almost line of sight, the height of the target is of prime importance. If the target does not rise above the radar horizon, the radar beam cannot be reflected from the target. Because of the interference pattern, the target must rise somewhat above the radar horizon. 31 Size Up to certain limits, targets having larger reflecting areas will return stronger echoes than targets having smaller reflecting areas. Should a target be wider than the horizontal beam width, the strength of the echoes will not be increased on account of the greater width of the target because the area not exposed to the radar beam at any instant cannot, of course, reflect an echo. Since the vertical dimensions of most targets are small compared to the vertical beam width of marine navigational radars, the beam width limitation is not normally applicable to the vertical dimensions. However, there is a vertical dimension limitation in the case of sloping surfaces or stepped surfaces. In this case, only the projected vertical area lying within the distance equivalent of the pulse length can return echoes at any instant. Aspect The aspect of a target is its orientation to the axis of the radar beam. With change in aspect, the effective reflecting area may change, depending upon the shape of the target. The nearer the angle between the reflecting area and the beam axis is to 90°, the greater is the strength of the echo returned to the antenna. Shape Targets of identical shape may give echoes of varying strength, depending on aspect. Thus a flat surface at right angles to the radar beam, such as the side of a steel ship or a steep cliff along the shore, will reflect very strong echoes. As the aspect changes, this flat surface will tend to reflect more of the energy of the beam away from the antenna, and may give rather weak echoes. A concave surface will tend to focus the radar beam back to the antenna while a convex surface will tend to scatter the energy. A smooth conical surface will not reflect energy back to the antenna. However, echoes may be reflected to the antenna if the conical surface is rough. Texture The texture of the target may modify the effects of shape and aspect. A smooth texture tends to increase the reflection qualities, and will increase the strength of the reflection, but unless the aspect and shape of the target are such that the reflection is focused directly back to the antenna, the smooth surface will give a poor radar echo because most of the energy is reflected in another direction. On the other hand, a rough surface will tend to break up the reflection, and will improve the strength of echoes returned from those targets whose shape and aspect normally give weak echoes. Composition The ability of various substances to reflect radar pulses depends on the intrinsic electrical properties of those substances. Thus metal and water are good reflectors. Ice is a fair reflector, depending on aspect. Land areas vary in their reflection qualities depending on the amount and type of vegetation and the rock and mineral content. Wood and fiber glass boats are poor reflectors. It must be remembered that all of the characteristics interact with each other to determine the strength of the radar echo, and no factor can be singled out without considering the effects of the others. 32 1.6 Factors Which Might Cause Faulty Interpretation 1.6.1 Interpretation of display Sometimes unwanted echoes or signals appear on the screen. The most common are: (i) Multiple echoes; (ii) Indirect echoes; (iii) Side Echoes; (iv) Interference pips or spokes. It often happens that a target is recorded by several echoes shown in different positions on the screen. In such cases the echoes which are shown in positions which do not truly represent the position of the target are known as either Fake echoes or Spurious echoes, and the other echo which is shown in the correct representative position of the target is known as the True Echo. 1.6.2 Multiple echoes Multiple echoes are caused by the reflection of the signal between own ship and the target before its energy is finally collected by the scanner. The effect on the screen is that, besides the original echo, there are seen, on the same line of bearing, one or more echoes, equidistantly spaced and having ranges of multiples of the true range. They generally happen at short ranges up to about one mile, often when another vessel is passing closely, beam-on to own vessel (Fig. 1.16). The shapes of multiple echoes are less defined than that of the original echo and they are weakening in intensity outwards. Fig. 1.16 - Multiple echoes Reduction of gain or clutter will remove them before the true echo. For those who have been sailing with an electronic sounding apparatus, the phenomenon should ring a familiar note. These multiple reflections are also recorded when a ship is sailing in shallow waters. Multiple echoes are a nuisance when a ship is approaching an anchor road where several ships are riding at anchor. The picture on the screen can become confused due to several multiple echoes of different ships, and it is difficult to distinguish between true and false echoes. 33 Multiple echoes can sometimes be observed in the dark sectors on the screen caused by the shadow sectors of large sheds, etc., nearby. 1.6.3 Indirect echoes Indirect echoes are caused by the reflection of the outgoing pulse against part of the superstructure (funnel or foremast) or against a building cliff river bank bridge or ship etc. nearby. The echo pulse returns the same way, and due to reflection, it paints its echo in the wrong direction on the screen. What really happens is that we see the target via a mirror. The most offending parts are the cross-trees of the foremast. Their mirror effect is worse than that of the funnel because the pulse reflected by the cross-trees contains more concentrated energy than that reflected by the funnel. Containers on deck can also be the cause of strong indirect echoes. An excellent remedy is to cover the cross-trees with angle bars or a piece of corrugated metal, so that the reflected energy returns directly back to the scanner when the receiver is paralyzed or is scattered in all directions, thereby losing all its energy. Radar absorbent material (RAM) is also available for this purpose. In some radar transmission is switched off when the radar beam passes through a ship's blind sector. Owing to its curvature, the funnel may distort the echo. An indirect echo, from a ship approaching from ahead, reflected via the funnel, will show up on the screen as an overtaking vessel. An indirect echo reflected via the cross-trees from a receding cliff or building appears on the screen as a target ahead of own ship, moving away and possessing, twice her speed. Large ships passing close by in confined waters may give rise to part of the picture being re-produced in reverse on the screen. These strong indirect echoes move very fast in a circular path over the display. A ship dead astern sometimes gives rise to four echoes on the screen. Three of them nearly in one line and perpendicular to the course, show up forward and one shows up abaft the beam. The three indirect echoes forward' of the beam are caused by reflection via the samson posts on the foreship and the foremast, the fourth one is the true echo. When there are many echoes on the screen, indirect echoes are likely to escape notice. It is when the screen is reasonably clear that one starts wondering about some echo, which one knows, from experience or visual observations should not be there. Indirect echoes can appear from targets at quite large ranges provided the reflecting surface has an excellent aspect. They can be wiped off the display by reduction of gain or clutter. Characteristics of indirect echoes can be summed up as follows: (a) They usually appear in blind or shadow sectors and areas because the obstructions producing these regions of reduced response often they act as "mirrors". See Fig. 1.17. (b) When caused by the ship's obstructions, they will appear on the same relative bearing (foremast or funnel) although the bearing of the target may change. This is also the case when the ship is stationary and the indirect echo is caused by shore object (Fig. 1.17 (a). If the ship is moving, in the latter case indirect echoes will appear only for a very short time. (c) When caused by an obstruction on own ship the true echo and its image, the false echo, appear at about the same range. This is because the range discrimination is not 34 sufficient to measure the difference in distance between the two routes of the pulse (Fig. 1.17 (a) and (b). (d) When caused by an obstruction not on own ship the range of the false echo will be the range of the obstruction from own ship plus the range of the true echo from the obstruction. See Fig. 1.17 (c). (e) Movements of false echoes are abnormal if they are compared with the movements on the screen of the true echoes. (f) There is a distortion in shape (via the funnel) and in presentation on the screen as only the best reflecting surfaces of a large target can produce a false echo (for example, a false echo of a river bank could be presented on the screen as a single spot). (g) When caused by obstructions ("mirrors” on own ship an alteration of course will make the false echo disappear although another false echo may appear on the same relative bearing. Fig 1.17 Indirect echoes 35 1.6.4 Side echoes Side echoes are caused by the side lobes, which are beams of electromagnetic energy reradiated by the scanner plate in a different direction to that of the much stronger main lobe. It is impossible to design a scanner without side lobes although the construction of an aerial affects the magnitude of the side lobes. Nearby targets are picked up by the side lobes as well as with the main lobes. See Fig 1.18. As the side lobes are far less powerful than the main lobe, the effect of side echoes will only be observed at short ranges. Large buildings near the river bank, piers or harbor entrances, etc., give rise to side echoes. When another vessel alters course, side echoes may suddenly flash over the screen. It happens when the other vessel presents her hull beam-on and can be observed up to a distance of maximum 5 miles. Side echoes appear as concentric arcs round the centre of the screen. Their radius represent the true ranges of the targets. Near the centre of the screen, the possibility of side-echoes naturally is great and this is the reason why, in narrow waters, the centre of the display is often masked by a white circular patch. By reducing the gain judiciously, or increasing the anti-clutter, the less powerful side echoes will disappear. However, as remarked before in this book, it is not always good practice to reduce the side echoes too much. We do not want to have a tidy picture, but a good navigational picture-and that may simply have to include side echoes so that other important small echoes can be observed. Many scanners at present-especially slotted waveguide aerials-are for all practical purposes unidirectional and the visible effect of side echoes is almost eliminated. Fig. 1.18 - Side echoes 36 1.6.5 Interference This word is used for many unwanted effects which reveal themselves on the screen. These can be caused by internal disturbances in the radar set it, by disturbances from electrical apparatus on board or by radar sets of other vessels. The first two disturbances may cause an effect known as spoking. The interference from other radar sets is due to the recording of the other ship's pulses and their echo pulses on the screen of own ship. This can only happen when the other vessel transmits in the same frequency band as that of own ship. If the p.d. of own ship's radar pulses and the other ship's radar pulses are the same, the interference pips will be displayed in circular patterns, if the p.r.f.'s differ, interference will reveal itself in spiraling pips or traces towards the centre of the display. When there is a large difference between the p.d.'s the pips will be distributed haphazardly. Interference can be most markedly seen on the longer range scales. On the short range scales the trace or sweep is so quick that the pips are pulled out into thin lines which are liable to escape notice, unless the interference source is very close. If very strong interference makes it difficult for the observer to keep an eye on important navigational echoes, he might try, just temporarily, to off-tune the set slightly. Indirectly, all radar operators would make proper use of the Stand-by Switch in clear weather, and then there would be a great decrease in external radar interference. N.B. Observation of the presence or absence of external interference on the display is of no use in determining whether a vessel, whose echo is displayed on the screen, is or is not using radar. Interference could be shown which is caused by the radar of a vessel not yet detected. On the other hand, the vessel which has been detected may be using radar which is transmitting in. a different frequency band to that of our own radar, in which case no interference from her radar would be shown on the display. Some radar sets have quite effective suppressor units for this type of external interference 1.7 Performance standards – IMO Resolution A.477(Xll) 1.7.1 Detection Where a separate facility is provided for detection of targets, other than by the radar observer, it should have a performance not inferior to that which could be obtained by use of the radar display. 1.7.2 Acquisition Target acquisition may be manual or automatic. However, there should always be a facility to provide for manual acquisition and cancellation. ARPAs with automatic acquisition should have a facility to suppress acquisition in certain areas. On any range scale where acquisition suppressed over a certain area, the area of acquisition should be indicated on the display. Automatic or manual acquisition should have a performance not inferior to that which could be obtained by the user of the radar display. 1.7.3 Tracking The ARPA should be able to automatically track, process, simultaneously display and continuously update the information on at least: 37 1. 20 targets, if automatic acquisition is provided, whether automatically or manually acquired; 2. 10 targets. if only manual acquisition is provided. If automatic acquisition is provided, description of the criteria of selection of targets for tracking should be provided to the user. If the ARPA does not track all targets visible on the display, targets which are being tracked should be clearly indicated on the display. The reliability of tracking should not be less than those obtainable using manual recordings of successive target positions obtained from the radar display. Provided the target is not subject to target swoop, the ARPA should continue to track an acquired target which is clearly distinguishable on the display for 5 out of 10 consecutive scans. The possibility of tracking errors, including target swoop, should be minimized by ARPA design. A qualitative description of the effects of error sources on the automatic tracking and corresponding errors should be provided to the user, including the effects of low signal lo noise and low signal lo clutter ratios caused by sea returns, rain, snow, low clouds and non-synchronous emissions. The ARPA should be able to display on request al least four equally time-spaced past positions of any targets being tracked over a period of at least eight minutes. 1.7.4 Display The display may be a separate or integral part of the ship's radar. However, the ARPA display should include all the data required to be provided by a radar display in accordance with the performance standards for navigational radar equipment. The design should be such that any malfunction of ARPA parts producing data additional to information to be produced by the radar. should not affect the integrity of the basic radar presentation. The size of the display on which ARPA information is presented should have an effective display diameter of at least 340 mm. The ARPA facilities should be available on at least the following range scales: 1. 12 or 16 miles. 2. 3 or 4 miles. There should be a positive indication of the range scale in use. The ARPA should be capable of operating with a relative motion display with "north-up" and either "headUp" or "course-up" azimuth stabilization. In addition the ARPA may also provide for a true motion display. If true motion is provided, the operator should be able to .select for his display either true or relative motion. There should be a positive indication of the display mode and orientation in use. The course and speed information generated by the ARPA for acquired targets should be displayed in a vector or graphic form which clearly indicates the target's predicted motion. In this regard: 1. ARPA presenting predicted information in vector form only should have the option of both true and relative vectors; 2. an ARPA which is capable of presenting target course and speed information in graphic form, should also, on request, provide the target's true and/or relative vector; 3. vectors displayed should be either time adjustable or have a fixed time-scale; 4. a positive indication of the time-scale of the vector in use should be given. 38 The ARPA information should not obscure radar information in such a manner as to degrade the process of detecting targets. The display of ARPA data should be under the control of the radar observer. It should be possible to cancel the display of unwanted ARPA data. Means should be provided to adjust independently the brilliance of the ARPA data and radar data, including complete elimination of the ARPA data. The method of presentation should ensure that the ARPA data is clearly visible in general to more than one observer in the conditions of light normally experienced on the bridge of a ship by day and by night. Screening may be provided to shade the display from sunlight but not to the extent that it will impair the observers' ability to maintain a proper lookout. Facilities to adjust the brightness should be provided. Provision should be made lo obtain quickly the range and bearing of any object which appears on the ARPA display. When a target appears on the radar display and, in the case of automatic acquisition, enters within the acquisition area chosen by the observer or, in the case of manual acquisition, has been acquired by the observer, the ARPA should present in a period of not more than one minute an indication of the target's motion trend and display within three minutes. After changing range scales on which the ARPA facilities are available or re-setting the display, full plotting information should be displayed within a period of lime not exceeding four scans. 1.7.5 Operational warnings The ARPA should have the capability to warn the observer with a visual and/0or audible signal of any distinguishable target which closes to a range or transits a zone chosen by the observer. The target causing the warning should be clearly indicated on the display. The ARPA should have the capability to warn the observer with a visual and/or audible signal of any tracked target which is predicted to close to within a minimum range and time chosen by the observer. The target causing the warning should be clearly indicated on the display. The ARPA should clearly indicate If a tracked target is lost, other than out of range and the target's last tracked position should be clearly indicated on the display. It should be possible to activate or de-activate the operational warnings. 1.7.6 Date requirements At the request of the observer the following information should be immediately available from the ARPA in alphanumeric form in regard to any tracked target: 1. present range to the target; 2. present bearing of the target; 3. predicted target range at the closest point of approach (CPA); 4. predicted time of CPA (TCPA); 5. calculated true course of target; 6. calculated true speed of target 1.7.7 Trial Maneuver The ARPA shou1d be capable of simulating the effect on all tracked targets if an own ship maneuvers without interrupting the updating of target information. The simulation 39 should be initiated by the depression either of a spring-loaded switch, or of a function key, with a positive identification on the display. 1.7.8 Accuracy The ARPA should provide accuracies not less than those given in paragraphs 3.8.2 and 3.8.3, or the four scenarios defined in Annex 2. With the sensor errors specified 10 Annex 3, the values given relate to the best possible manual plotting performance under environmental conditions of plus and minus ten degrees of roll. An ARPA should present within one minute of steady state tracking the relative motion trend of a target with the following accuracy values (95 per cent probability values). When a tracked target, or own ship, has completed a maneuver, the system should present in a period of not more than one minute an indication of the target's motion trend, and display within three minutes the target's predicted motion. The ARPA should be designed in such a manner that under the most favorable conditions of own ship motion the error contribution from the ARPA should remain insignificant compared to the errors associated with the input scenarios of Annex 2. 1.7.9 Connections with other equipment The ARPA should not degrade the performance of any equipment providing sensor inputs. The connection of the ARPA to any other equipment should not degrade the performance of that equipment. 1.7.10 Performance tests and warnings The ARPA should provide suitable warnings of ARPA malfunction to enable the observer to monitor the proper operation of the system. Additionally test programs should be available so that the overall performance of ARPA can be assessed periodically against a known solution. 1.7.11 Equipment used with ARPA Log and speed indicators providing inputs to ARPA equipment should be capable of providing the ship's speed through the water. 1.7.12 Characteristics of Set Echoing area is the projected area of an equivalent sphere which possesses the same echoing strength as one unit of area of the specific type of target. Its magnitude depends on the aspect. If, for example, the echoing area of a target is 10 sq. meters, then for a given direction, one sq. meter of the target will yield the same echoing strength as a sphere, 10 sq. meters in cross-section (about l.8 meters radius). It is interesting to read through the specification and obtain information about the standards required for a Marine Radar Set. For the observer, however, it is much more important to get fully acquainted with the characteristics and the limitations of the radar set he is using. These characteristics will now be discussed 1.7.13 Maximum range The maximum range of a radar set depends on: (a) The number of pulses transmitted per second or p.r.f. 40 There is, however, an upper limit to the p.r.f. for longer range scales as the trace on the PPI has to be completed between two successive pulse transmissions. In other words the length of the time base for these longer range scales must be such that echo signals from good ranges can be recorded (b) The peak R.F. power. c) The shape and size of the aerial. These determine the aerial gain. d) The wavelength used. Both (c) and (d) determine the beam-width. The narrower the beam-widths in both the vertical and horizontal planes the greater will be the maximum range. (e) The receiver sensitivity. This is limited by "noise" arising from thermal agitation in conductors and by the irregular nature of electronic emission in vacuum tubes. In general an echo can only be distinguished if its power is of the same order as that due to the noise. The noise power output varies linearly with the bandwidth. On the other hand, the bandwidth cannot be made too narrow, otherwise severe pulse distortion would take place. Hence a compromise must be made, but for certain long range warning sets it is possible to work with a smaller bandwidths and less noise-thereby obtaining a longer pulse-length and sacrificing the range discrimination. (f) The height of the scanner. 1.7.14 Minimum range The minimum range is governed by: (a) The pulse-length. Generally the set is not ready for reception before the total pulse has left the scanner opening. The shorter the pulse, the earlier the set is ready to receive and the shorter the ranges which can be recorded. (b) The position of the scanner. The fore castle head may cause an unfavorable shadow sector if the scanner is placed too low and the cross-trees may cause a shadow downwards if the scanner is placed too high. (c) The vertical beam-width and the wavelength used. Sea surface reflection breaks up the vertical coverage pattern. It must be noted, however, that the minimum range is not governed solely by the number of degrees of the vertical beam-width stated in the Manual. Strongly reflecting objects nearby can be detected well outside the half-power points (see: Vertical Beamwidth. (d) The "Change-over" lime of the T/R cell. This extends the minimum range and gives a ragged edge to the dark transmission circle which can be seen on the short range scale. The radius of the dark spot can be varied internally so it does not accurately indicate the extent of the minimum range. The introduction of solid state protection devices have reduced this time considerably. 1.7.15 Range accuracy The range accuracy depends on: (a) The uniformity and rectilinearity of the time-base or sweep. (b) The size of the spot, especially on long range scales. 41 (c) The curvature of the screen (especially near the tube edge). d) The height of the scanner which can introduce parallax when the scanner is high and objects are near and low-lying. The range accuracy of the fixed range rings is generally such that the maximum error does not exceed 1.5% (1982 Specification) of the maximum range of the scale in use. The range accuracy of the rings should be determined in port or when anchored in a roadstead by comparing the true range with the recorded radar range of a radar conspicuous object. It can be done sometimes on a moving vessel by measuring the difference in radar ranges between two radar conspicuous objects which lie on the same or opposite bearings and comparing them with the chart distances. Alternatively parallel index lines engraved on a rotating bearing mask at intervals of the fixed range rings can be used to measure the radar range between two objects. Even a pair of rubber-tipped dividers could be employed to measure the radar range between two fixed objects. After measurement take the radar range off along the radius of the rings and compare with the chart. For interpolation between the fixed range rings it may be advisable to use the variable range marker or strobe. Generally the accuracy of the variable range marker is slightly less than that of the range rings. The error may vary for different range scales and also at different ranges on the same range scale. The variable range marker must be checked and calibrated against the fixed range rings. It should be done fairly frequently because it sometimes happens that small drifts are introduced in the error. The method of calibration is as follows: Bring range rings and range marker simultaneously on the screen. Let the range marker coincide with the range ring concerned. Then take the reading of the variable range marker and compare it with the range indicated by the particular fixed range ring. Always measure the range of the leading edge of the echo. A radar pulse cannot "feel" the off-side of a target. When taking a range with the range marker, the outer edge of the range marker should touch the inner edge of the echo. Always measure the range of the leading edge of the echo. A radar pulse cannot "feel" the off-side of a target. When taking a range with the range marker, the outer edge of the range marker should touch the inner edge of the echo. 1.7.16 Bearing accuracy Bearing accuracy is governed by: (a) The horizontal beam width (which depends on the wave-length used in relation to the scanner width). A mm. waveband radar set used in connection with an ordinary sized scanner yields a very fine horizontal beam-width. (b) Rectilinearity of the time-base. (c) Correct synchronization between the revolving scanner and the rotation of the trace on the PPI. (d) The thickness and the correct alignment of the heading marker. (e) The type of bearing marker employed. It should be pointed out here that bearing accuracy can be affected by other factors such as faulty centering, parallax of the cursor and yaw of own ship but generally these can be reduced and perhaps eliminated by a careful operator. They do not fall under the characteristics of the set and these errors will be discussed fully in the next chapter. It 42 may happen, however, that even after careful centering, the rest position of the spot wanders slightly round the centre of the PPI. In such a case, as will be seen later, slight errors may be introduced when a bearing is taken by means of the mechanical cursor of an object giving an echo at the edge of the display, but far greater errors may occur ;n the bearing of a target which has its echo situated near the centre of the screen. It is therefore always advisable to take a bearing of an object whose echo appears in the outer half of the display. On most radar sets, the range scales are multiples of the shortest range scale, for example, the scales represent, 3/4, 1/2, 3, 6, 12 etc. nautical miles, so that with the exception of the shortest range scale it will always be possible to keep the echo of a target in the outer half of the display. Let us now look closer at factors (a) and (d). Suppose that the horizontal beam-width is 28 degrees and the radar beam sweeps over a thin vertical pole (Fig. 3.1 (a)). At any time when the pole is in the beam it will return radiation. During that time the centre of the beam sweeps through an angle of 28 degrees and an arc 2& degrees in width will be painted on the display. The middle of the arc is the correct bearing. The same thing happens when the beam strikes the corner of a coastline which is at right angles to the beam (Fig. 3.1 (b). The picture on the screen shows the coastline echoes being extended to an amount of 8 degrees. This effect is known as beam-width distortion. In practice, the radiation intensity is neither uniform within the beam, nor sharply limited to exact boundaries, as shown in Fig. 3.1. Strongly reflecting targets and Objects close-by may well be detected outside the half-power points and such targets may give rise to a greater angular extent of the echo. Hence beam-width distortion depends on the reflecting property and the distance of the target. It is therefore more accurate to say that for a given distance beamwidth distortion is proportional to the stated beam-width. For example, a radar set which has a beam-width of two degrees might produce a beamwidth distortion with a particular target of three degrees, whereas a radar with a beamwidth of one degree will produce a distortion of only 1.5 degrees with the same target at the same range. Fig. 1.12 - Bearing accuracy and beam width distortion 43 Beam-width distortion should not be confused with "tube-edge” distortion. The latter distortion also increases the size of the echo at the circumference of the screen but this is due to a certain amount of de-focusing which takes place at the tube edge. It is not difficult to eliminate an error due to beam-width distortion for isolated targets. The correct bearing corresponds to the middle of the arc. The comer of a coastline, however, or targets on the coast itself, have, on the PPI, their arcs merged with arcs of other echoes and exact determination of the middle of the arc is difficult. In such a case one can try to reduce the sensitivity or gain and a sharp echo, showing the correct bearing of the object, may come out on the screen. The outline, for example, of the echo of a comer of a coast seems to contract on the display if we reduce the sensitivity. This is because most of the returning radiation comes back when the scanner points exactly at the target. The re-radiation which enters the aerial just before and just after that moment is weaker in power (due to the directional properties of the aerial) and their echoes on the screen become fainter when we reduce the sensitivity below saturation brilliance of the echo. The divergent parts of the returning signal are, as it were, "cut away". Another way of looking at this is by means of the horizontal coverage diagram for reception. Consider a target well inside detection range. Reduction of gain will result in contraction of the pattern (Fig. 1.12). The contours each side of the target will close up towards it. These contours show the limit of detection; hence bearing accuracy (and discrimination) will be increased. Targets near the detection range in such a case, of course, will be 'lost because they move outside the area bounded by the contours. During each revolution of the scanner, a line named heading marker is painted on the display which should represent the fore and aft line of the vessel and hence the course, not allowing for current. Owing to incorrect positioning of a contact inside the scanner housing or pedestal when the radar was installed, the heading marker will differ from the correct position, sometimes by several degrees. When this is the case, errors in bearings will occur. With a "Head-Up" picture, i.e., the heading marker indicates zero degrees, all relatives bearings will be out. But this will also be the case with "true" bearings taken on a stabilized display-connected to a gyro-compass transmitter and the heading marker acting as a gyro repeater-when the heading marker, painting an incorrect position of the fore and aft line, indicates the true course reading on the bearing scale. Hence the position of the heading marker is related to bearing accuracy. . . Figure 1.13 - Angle of squint It might sometimes happen with end-fed slotted waveguide aerials, that, although the heading marker contact originally was aligned correctly, owing to a slight change in the frequency of the cm. wave, the lobe pattern of the aerial is slewed round through a small 44 angle-the angle of squint-and the axis of the main lobe is no longer perpendicular to the longitudinal axis of the aerial. This is so because the distance between the slots is based on one particular frequency and when this frequency is changed, a re-distribution of radiation from the slots will take place. As a result of this the picture on the display will slew round with respect to the heading marker, through an angle equal to the angle of squint and the heading marker should correspondingly be re-aligned (see figure. 1.13). The error caused due to the incorrect position of the heading marker contact or electronic sensor in the scanner housing, is constant. All bearings will be wrong by an equal amount. It can be removed mechanically by re-adjustment of the said contact/ sensor. The maintenance manual can be consulted or an expert called in. Still, it is the operator's duty to find out if there is an error. This can be done in port or at anchor or anywhere when there is a target at a distance. Use the "Head-Up" orientation and set the heading marker to zero degrees. Check that the centre of the rotation of the trace coincides with the centre of the PPI. If there is a discrepancy between the two centers, make an adjustment as described in the next chapter. Select an object with yields a distinct echo near the edge of the display. Check the radar bearing with respect to the heading marker against the visual bearing-taken from near the scanner position-with respect to the fore and aft line of the ship. Record the difference between the bearings. It is good practice to repeat the procedure for different bearings of different objects, if possible, in succession about 90 degrees in azimuth apart. If there is an error and it is due to the placing of the heading marker only, then the results for the different bearings should be practically constant. If a variable error is found then it could be because there is centering error or incorrect synchronism, or a combination of errors. For example, if the visual bearing and the radar bearing are respectively 2800 and 2820 relative, then the heading marker is displaced two degrees to port or Red 2°. At sea one could bring a small object or a ship, at a distance of not less than two miles' dead ahead and at the same time take a radar bearing. Suppose that the relative bearing is 351° on the bearing scale, then the fore and aft line is three degrees to the left of the heading marker, or in other words, the heading marker in fact represents 30 Green. A rough idea about the position of the heading marker, when the scanner is sited in the centre-line, can often be obtained by observing the heading flash against the dark sector (caused by the shadow sector of the foremast) projected on rain or sea clutter. If a heading marker error is found to exist and is not corrected by re-adjustment of the contacts, a notice indicating the error should be displayed prominently near the radar display. Several ferry-boats, by the way, have stem markers on their equipment. With a good radar set and an experienced operator the bearing accuracy can be within one degree. 1.7.17 Range discrimination Range discrimination is the ability of the set to discriminate between two small objects close together on the same bearing, i.e. to produce separate echoes. The further object must not be in the shadow of the nearer one (Fig. 1.13 (a). 45 Fig. 1.13 - Range and bearing discrimination The range discrimination depends on: (a) The pulse-length. If, for example, the duration of transmission is '25 micro-second, then the length of the pulse leaving the scanner is '25 X 300, i.e. 75 meters long. The theoretical range discrimination is a minimum of half the pulse-length, ie. 37.5 meters. A shorter separation between the two targets will make the echo pulse from the further target join or overlap the echo pulse from the nearer one. The two targets would be shown as one echo on the display. This is illustrated in Fig. 1.15. Here the distance between two small objects 'A' .and 'B' is 32 meters while the range discrimination amounts to 75/2 i.e. 31.5 meters. The combined reflected pulse-length equals 75+ 32+32 i.e. 139 meters shown on the radar displays one echo 139/2 or 69.5 meters in length. In practice the range discrimination amounts to more than half the pulse-length due to: (b) The size of the spot. On a short range scale (i.e. a large scale picture) the influence of the spot-size will be less than when a longer range scale is employed. Range discrimination can be improved by correct focusing and brightness, by selecting a short pulse-length and by differentiating the echoes. 1.7.18 Bearing discrimination Bearing discrimination is the ability of the set to discriminate between two small objects close together at the same range, i.e. the ability to produce separate echoes (Fig. 1.13(b). The bearing discrimination is governed by: (a) The horizontal beam-width (which depends on the horizontal dimensions of the scanner and the wave-length used). The narrower the beam-width, the better the bearing discrimination. (b) The spot size. Again, the influence of the spot-size is less when a lower range scale is employed. Here again, as with bearing accuracy, the horizontal beam width plays an important role. If the beam-width (between half-power points) were too wide, there would be a short moment when the two objects are contained in the same beam and send out returning pulses simultaneously. Conversely, the scanner picks up signal pulses with the same divergence in bearing as the beam-width and the echo-paints on the display coming from the two objects at different bearings but at the same range, will join up or overlap. 46 Bearing discrimination can be improved by correct focusing and brightness, by reducing the sensitivity or gain and by off-centering on a lower range scale. Radar sets in the mm. waveband have excellent bearing discrimination. 1.7.19 Further considerations relating to beam-width and pulse-length. When a ship is sailing very near to a coast, or is maneuvering near a jetty or a quay, her echo on the observing vessel will merge together with echo of the coast jetty or quay. This is due, of course, to the limitations in bearing and range discrimination. It is one of the reasons why a radar cannot be used for berthing unless a wide aerial is employed. Knowledge of this limitation was used during the Second World War. Taking station near a cliff made radar detection by enemy surface craft very difficult. Let us now consider the shape of an echo on the display in relation to the shape of the target. The shape of the leading edge (towards the scanner) of a target can only be determined from the screen if the width of the target is large in comparison with the horizontal beam-width. Movements of ships, for the same reason, can only be observed without any appreciable time interval from the screen when they are at a very short range (we are referring to cm. radar). Only in such cases is the beam able to "feel" the individual parts out of which the target is made up, one after the other, and to paint their correct positions relative to each other on the screen. Let us go back to our example (Fig. 1.12) where we illustrated a beam sweeping over a thin vertical pole. The echo on the display is shaped something like a rectangle whose width is determined by the horizontal beam-width and whose radial length is a function of the pulse-length. What we really see on the display is the image of the horizontal cross-section of half the pulse. Besides this, the dimensions of the echo will be increased in all directions by half the spot-width. Small isolated targets or targets which are small in comparison with the horizontal beam-width, whatever their real shape, are recorded on the PPI as echoes which possess dimensions which are mainly functions of the angular horizontal beam-width and the pulse length. Such targets are, as it were, "drowned" in the beam. For wide and deep targets such successive echoes may overlap and a rough outline is displayed on the screen. In order to make some deeper study of what the echo of a small isolated target looks like at a short and a long range wing the same range scale. Let us assume that the scanner has a radiation pattern as shown in Fig. 1.7. Suppose that the target is at a distance of 7 miles and produces an echo 6 db above the minimum detectable, then the scanner will pick up the signal over an arc of 6°. If a 30 cm. PPI is used with a spot diameter of 0.5 mm. and the range scale is 12 M then the width of the echo on the display represents 7X* 6/57*3 (arc of signal)+(l/20)/15 X 12 (spot diameter)=·07 +04, i.e. 11nautical mile or 204 meters. The same target at a distance of 1M produces an echo 40 db (Le. (10 log 74+6) db) above the minimum detectable (the echoing strength is inversely proportional to the fourth power of the distance) and consulting Fig. 1.7 it is seen that the scanner picks up the signal over an arc of 1'50 , representing a width of the echo on the screen of I X 1'5/57'3 (arc of signal)+(1/20)/15 X 12 (spot diameter) =·025+ '04, i.e. '065 nautical mile or 120 meters. If the pulse-length used with this range scale is '5 micro-second, then this corresponds to a length of 150 meters of the radiated pulse. The echo pulse is also 150 meters long but the spot registers an echo representing 75 meters. This is so because the scale on the PPI indicates half the distance traveled by the radar pulse (out 47 and back). The echo therefore depicts the target, both at the 7 M and I M position as having a length of 75 meters+ spot diameter =75 + 74 meters, ie. 149 meters. Hence at 7 M the apparent dimensions of the target seems to be 204X 149 meters while the same target at a distance of 1M seems to measure 120 X 149 meters (see Fig. 1.15). Figure 1.15 - Echo distortion This consideration explains why, on the same range scale, ships at the shorter ranges appear to be end-an and at the longer ranges beam-an. Tube edge distortion may also increase the "beam-an" effect at the circumference of a traditional CRT screen. Thus no inferences should be drawn from the shape of an echo about the relative or true movement of the ship's echo over the screen. Neither can the aspect be read off directly (at least not for cm. sets). It is one of the reasons why plotting is necessary in the observance of motion of other vessels. 1.7.20 Vertical beam-width. The vertical beam-width for different sets can vary between 15 and 30 degrees (between half-power points). It is wide in order to allow for rolling and in order to pick up lowlying objects at a minimum distance of 50 meters. Close objects however, can be detected well outside the boundaries of the strong centre radiation. Hence the minimum range is not limited to the main beam 1.7.21 Aerial gain An omni-directional aerial radiates energy in all directions, but a scanner has its radiation concentrated into a beam. If one could compare the field strength within the beam at a given distance from the scanner with the field strength the same distance away from an omni-directional aerial which has the same power output, then it should be clear that the ratio between these should be a number greater than the unity. This ratio is known as "Aerial Gain" and is expressed in decibels. It obviously depend on the solid angle of the beam, Le. on the horizontal and vertical angular beam-widths. 1.7.22 Scanner rotation British marine radar sets have a continuously clockwise rotating scanner, maintained at a uniform speed. The speed for different types varies between 20 and 80 r.p.m. This rate 48 must be maintained in relative high wind speeds and sometimes for this purpose balancing fins are fitted to the sides of the reflector. In some American sets, working in the to cm. band and designed for navigational purposes primarily, the rate of rotation is much lower. 1.7.23 Pulse-repetition frequency As already remarked, most radar sets use short pulses for the short range scales, but the p.d. is high. Longer range scales employ long pulses combined with a lower p.r.f. The brightness of the echo on the screen is also determined to a certain extent by the number of pulses striking the target when the beam sweeps over it, and also upon the number of pulses returning. There is an upper limit to the storing of these pulses on the screen and hence to the brightness. Most radar sets are designed so that at least 10 pulses are returned from the target (having a suitable aspect) during a single sweep. 49 2. SET UP AND OPERATE RADAR IN ACCORDANCE WITH MANUFACTURER'S INSTRUCTIONS 2.1. Set Up and Maintain Radar Display Modes and Orientations and the Dangers of Misinterpretation There are two basic displays used to portray target position and motion on the PPI’s of navigational radars. The relative motion display portrays the motion of a target relative to the motion of the observing ship. The true motion display portrays the actual or true motions of the target and the observing ship. Depending upon the type of PPI display used, navigational radars are classified as either relative motion or true motion radars. However, true motion radars can be operated with a relative motion display. In fact, radars classified as true motion radars must be operated in their relative motion mode at the longer range scale settings. Some radars classified as relative motion radars are fitted with special adapters enabling operation with a true motion display. These radars do not have certain features normally associated with true motion radars, such as high persistence CRT screens. 2.1.1 Relative motion radar Through continuous display of target pips at their measured ranges and bearings from a fixed position of own ship on the PPI, relative motion radar displays the motion of a target relative to the motion of the observing (own) ship. With own ship and the target in motion, the successive pips of the target do not indicate the actual or true movement of the target. A graphical solution is required in order to determine the rate and direction of the actual movement of the target. If own ship is in motion, the pips of fixed objects, such as landmasses, move on the PPI at a rate equal to and in a direction opposite to the motion of own ship. If own ship is stopped or motionless, target pips move on the PPI in accordance with their true motion. 2.1.1.1 Orientations of Relative Motion Display There are two basic orientations used for the display of relative motion on PPI’s. In the HEADING-UPWARD display, the target pips are painted at their measured distances in direction relative to own ship’s heading. In the NORTH-UPWARD display, target pips are painted at their measured distances in true directions from own ship, north being upward or at the top of the PPI. 50 Figure 2.1 - Unstabilized Heading-Upward display. In figure 2.1 own ship on a heading of 270° detects a target bearing 315° true. The target pip is painted 045° relative to ship’s heading on this Heading-Upward display. In figure 2.2 the same target is painted at 315° true on a North-Upward display. While the target pip is painted 045° relative to the heading flash on each display, the HeadingUpward display provides a more immediate indication as to whether the target lies to port or starboard. 2.1.1.2 Stabilization The North-Upward display in which the orientation of the display is fixed to an unchanging reference (north) is called a STABILIZED display. The Heading-Upward display in which the orientation changes with changes in own ship’s heading is called an UNSTABILIZED display. Some radar indicator designs have displays which are both stabilized and Heading- Upward. In these displays, the cathode-ray tubes must be rotated as own ship changes heading in order to maintain ship’s heading upward or at the top of the PPI. 51 Figure 2.2 - Stabilized North-Upward display. 2.1.2 True motion radar True motion radar displays own ship and moving objects in their true motion. Unlike relative motion radar, own ship’s position is not fixed on the PPI. Own ship and other moving objects move on the PPI in accordance with their true courses and speeds. Also unlike relative motion radar, fixed objects such as landmasses are stationary, or nearly so, on the PPI. Thus, one observes own ship and other ships moving with respect to landmasses. True motion is displayed on modern indicators through the use of a microprocessor computing target true motion rather than depending on an extremely long persistence phosphor to leave “trails”. 2.1.2.1 Stabilization Usually, the true motion radar display is stabilized with North-Upward. With this stabilization, the display is similar to a plot on the navigational chart. On some models the display orientation is Heading-Upward. Because the true motion display must be stabilized to an unchanging reference, the cathode-ray tube must be rotated to place the heading at the top or upward. 52 2.1.2.2 Radarscope Persistence and Echo Trails High persistence radarscopes are used to obtain maximum benefit from the true motion display. As the radar images of the targets are painted successively by the rotating sweep on the high persistence scope, the images continue to glow for a relatively longer period than the images on other scopes of lesser persistence. Depending upon the rates of movement, range scale, and degree of persistence, this afterglow may leave a visible echo trail or tail indicating the true motion of each target. If the afterglow of the moving sweep origin leaves a visible trail indicating the true motion of own ship, estimates of the true speeds of the radar targets can be made by comparing the lengths of their echo trails or tails with that of own ship. Because of the requirement for resetting own ship’s position on the PPI, there is a practical limit to the degree of persistence (see figure 2.3). 2.1.2.3 Reset Requirements and Methods Because own ship travels across the PPI, the position of own ship must be reset periodically. Depending upon design, own ship’s position may be reset manually, automatically, or by manually overriding any automatic method. Usually, the design includes a signal (buzzer or indicator light) to warn the observer when resetting is required. Figure 2.3 – True Motion, Stabilized North-Upward display. 53 A design may include North-South and East-West reset controls to enable the observer to place own ship’s position at the most suitable place on the PPI. Other designs may be more limited as to where own ship’s position can be reset on the PPI, being limited to a point from which the heading flash passes through the center of the PPI. The radar observer must be alert with respect to reset requirements. To avoid either a manual or automatic reset at the most inopportune time, the radar observer should include in his evaluation of the situation a determination of the best time to reset own ship’s position. Range setting examples for Radio marine true motion radar sets having double stabilization are as follows: Type CRM-NID-75 (3.2cm) and True motion range settings 1, 2, 6, and 16 miles Type CRM-N2D-30 (10cm) Relative motion range settings ½ , 1, 2, 6, 16, and 40 miles 2.1.2.4 Modes of Operation True motion radars can be operated with either true motion or relative motion displays, with true motion operation being limited to the short and intermediate range settings. In the relative motion mode, the sweep origin can be off-centered to extend the view ahead. With the view ahead extended, requirements for changing the range scale are reduced. Also, the off-center position of the fixed sweep origin can permit observation of a radar target on a shorter range scale than would be the case with the sweep origin fixed at the center of the PPI. Through use of the shorter range scale, the relative motion of the radar target is more clearly indicated. 2.1.2.5 Types of True Motion Display While fixed objects such as landmasses are stationary, or nearly so, on true motion displays, fixed objects will be stationary on the PPI only if there is no current or if the set and drift are compensated for by controls for this purpose. Dependent upon set design, current compensation may be effected through set and drift controls or by speed and course-made-good controls. When using true motion radar primarily for collision avoidance purposes, the sea-stabilized display is preferred generally. The latter type of display differs from the ground-stabilized display only in that there is no compensation for current. Assuming that own ship and a radar contact are affected by the same current, the sea-stabilized display indicates true courses and speeds through the water. If own ship has leeway or is being affected by current, the echoes of stationary objects will move on the sea-stabilized display. Small echo trails will be formed in a direction opposite to the leeway or set. If the echo from a small rock appears to move due north at 2 knots, then the ship is being set due south at 2 knots. The usable afterglow of the CRT screen, which lasts from about 11/2 to 3 minutes, determines the minimum rate of movement which can be detected on the display. The minimum rate of movement has been found to be about 11/2 knots on the 6- mile range scale and proportional on other scales. The ground-stabilized display provides the means for stopping the small movements of the echoes from stationary objects. This display may be used to obtain a clearer PPI presentation or to determine leeway or the effects of current on own ship. In the ground-stabilized display own ship moves on the display in accordance with its course and speed over the ground. Thus, the movements of target echoes on the display 54 indicate the true courses and speeds of the targets over the ground. Ground-stabilization is affected as follows: (1) The speed control is adjusted to eliminate any movements of the echoes from stationary targets dead ahead or dead astern. If the echoes from stationary targets dead ahead are moving towards own ship, the speed setting is increased; otherwise the speed setting is decreased. (2) The course-made-good control is adjusted to eliminate any remaining movement at right angles to own ship’s heading. The course-made-good control should be adjusted in a direction counter to the echo movement. Therefore, by trial and error procedures, the display can be ground stabilized rapidly. However, the display should be considered only as an approximation of the course and speed made good over the ground. Among other factors, the accuracy of the groundstabilization is dependent upon the minimum amount of movement which can be detected on the display. Small errors in speed and compass course inputs and other effects associated with any radar set may cause small false movements to appear on the true motion display. The information displayed should be interpreted with due regard to these factors. During a turn when compass errors will be greater and when speed estimation is more difficult, the radar observer should recognize that the accuracy of the ground stabilization may be degraded appreciably. The varying effects of current, wind and other factors make it unlikely that the display will remain ground stabilized for long periods. Consequently, the display must be readjusted periodically. Such readjustments should be carried out only when they do not detract from the primary duties of the radar observer. While in rivers or estuaries, the only detectable movement may be the movement along own ship’s heading. The movements of echoes of stationary objects at right angles to own ship’s heading are usually small in these circumstances. Thus, in rivers and estuaries adjustment of the speed control is the only adjustment normally required to obtain ground stabilization of reasonable accuracy in these confined waters. 2.2. Basic Knowledge of the Physical, Atmospheric and Inherent Errors and Limitations in a Marine Radar System 2.1.1 Attenuation by rain, fog, clouds, hail, snow, and dust The amount of attenuation caused by these weather factors is dependent upon the amount of water, liquid or frozen, present in a unit volume of air and upon the temperature. Therefore, as one would expect, the affects can differ widely. The further the radar wave and returning echo must travel through this medium then the greater will be the attenuation and subsequent decrease in detection range. This is the case whether the target is in or outside the precipitation. A certain amount of attenuation takes place even when radar waves travel through a clear atmosphere. The affect will not be noticeable to the radar observer. The effect of precipitation starts to become of practical significance at wavelengths shorter than 10cm. In any given set of precipitation conditions, the (S-band) or 10cm will suffer less attenuation than the (X-band) or 3cm. Rain In the case of rain the particles which affect the scattering and attenuation take the form of water droplets. It is possible to relate the amount of attenuation to the rate of precipitation. If the size of the droplet is an appreciable proportion of the 3cm 55 wavelength, strong clutter echoes will be produced and there will be serious loss of energy due to scattering and attenuation. If the target is within the area of rainfall, any echoes from raindrops will further decrease its detection range. Weaker target responses, as from small vessels and buoys, will be undetectable if their echoes are not stronger than that of the rain. A very heavy rainstorm, like those sometimes encountered in the tropics, can obliterate most of the (X-band) radar picture. Continuous rainfall over a large area will make the center part of the screen brighter than the rest and the rain clutter, moving along with the ship, looks similar to sea clutter. It can be clearly seen on long range scales. This is due to a gradual decrease in returning power as the pulse penetrates further into the rain area. Fog In most cases fog does not actually produce echoes on the radar display, but a very dense fogbank which arises in polar regions may produce a significant reduction in detection range. A vessel encountering areas known for industrial pollution in the form of smog may find a somewhat higher degree of attenuation than sea fog. Clouds The water droplets which form clouds are too small to produce a detectable response at the 3cm wavelength. If there is precipitation in the cloud then the operator can expect a detectable echo. Hail With respect to water, hail which is essentially frozen rain reflects radar energy less effectively than water. Therefore, in general the clutter and attenuation from hail are likely to prove less detectable than that from rain. Snow Similar to the effects of hail, the overall effect of clutter on the picture is less than that due to rain. Falling snow will only be observed on the displays of 3cm except during heavy snowfall where attenuation can be observed on a 10cm set. The strength of echoes from snow depends upon the size of the snowflake and the rate of precipitation. For practical purposes, however, the significant factor is the rate of precipitation, because the water content of the heaviest snowfall will very rarely equal that of even moderate rain. It is important to keep in mind those in areas receiving and collecting snowfall and where the snow is collecting on possible danger targets it may render them less detectable. Accumulation of snow produces a limited absorption characteristic and reduces the detection range of an otherwise strong target. Dust There is a general reduction in radar detection in the presence of dust and sandstorms. On the basis of particle size, detectable responses are extremely unlikely and the operator can expect a low level of attenuation. 56 Unusual Propagation Conditions Similar to light waves, radar waves going through the earth’s atmosphere are subject to refraction and normally bend slightly with the curvature of the earth. Certain atmospheric conditions will produce a modification of this normal refraction. 2.1.2 Range Accuracy The range accuracy depends on: (a) The uniformity and rectilinearity of the time-base or sweep. (b) The size of the spot, especially on long range scales. (c) The curvature of the screen (especially near the tube edge). d) The height of the scanner which can introduce parallax when the scanner is high and objects are near and low-lying. The range accuracy of the fixed range rings is generally such that the maximum error does not exceed 1.5% (1982 Specification) of the maximum range of the scale in use. The range accuracy of the rings should be determined in port or when anchored in a roadstead by comparing the true range with the recorded radar range of a radar conspicuous object. It can be done sometimes on a moving vessel by measuring the difference in radar ranges between two radar conspicuous objects which lie on the same or opposite bearings and comparing them with the chart distances. Alternatively parallel index lines engraved on a rotating bearing mask at intervals of the fixed range rings can be used to measure the radar range between two objects. Even a pair of rubber-tipped dividers could be employed to measure the radar range between two fixed objects. After measurement take the radar range off along the radius of the rings and compare with the chart. For interpolation between the fixed range rings it may be advisable to use the variable range marker or strobe. Generally the accuracy of the variable range marker is slightly less than that of the range rings. The error may vary for different range scales and also at different ranges on the same range scale. The variable range marker must be checked and calibrated against the fixed range rings. It should be done fairly frequently because it sometimes happens that small drifts are introduced in the error. The method of calibration is as follows: Bring range rings and range marker simultaneously on the screen. Let the range marker coincide with the range ring concerned. Then take the reading of the variable range marker and compare it with the range indicated by the particular fixed range ring. Always measure the range of the leading edge of the echo. A radar pulse cannot "feel" the off-side of a target. When taking a range with the range marker, the outer edge of the range marker should touch the inner edge of the echo. 2.1.3 Bearing Accuracy Bearing accuracy is governed by: (a) The horizontal bea m-width (which depends on the wave-length used in relation to the scanner width). A mm. waveband radar set used in connection with an ordinary sized scanner yields a very fine horizontal beam-width. (b) Rectilinearity of the time-base. 57 (c) Correct synchronization between the revolving scanner and the rotation of the trace on the PPI. (d) The thickness and the correct alignment of the heading marker. (e) The type of bearing marker employed. It should be pointed out here that bearing accuracy can be affected by other factors such as faulty centering, parallax of the cursor and yaw of own ship but generally these can be reduced and perhaps eliminated by a careful operator. They do not fall under the characteristics of the set and these errors will be discussed fully in the next chapter. It may happen, however, that even after careful centering, the rest position of the spot wanders slightly round the centre of the PPI. In such a case, as will be seen later, slight errors may be introduced when a bearing is taken by means of the mechanical cursor of an object giving an echo at the edge of the display, but far greater errors may occur in the bearing of a target which has its echo situated near the centre of the screen. It is therefore always advisable to take a bearing of an object whose echo appears in the outer half of the display. On most radar sets, the range scales are multiples of the shortest range scale, for example, the scales represent,3/4, 1/2, 3, 6, 12 etc. nautical miles, so that with the exception of the shortest range scale it will always be possible to keep the echo of a target in the outer half of the display. Let us now look closer at factors (a) and (d). Suppose that the horizontal beam-width is 28 degrees and the radar beam sweeps over a thin vertical pole (Fig. 2.4 (a)). At any time when the pole is in the beam it will return radiation. During that time the centre of the beam sweeps through an angle of 28 degrees and an arc 2& degrees in width will be painted on the display. The middle of the arc is the correct bearing. The same thing happens when the beam strikes the corner of a coastline which is at right angles to the beam (Fig. 2.4 (b). The picture on the screen shows the coastline echoes being extended to an amount of 8 degrees. This effect is known as beam-width distortion. In practice, the radiation intensity is neither uniform within the beam, nor sharply limited to exact boundaries, as shown in Fig. 3.1. Strongly reflecting targets and Objects close-by may well be detected outside the half-power points and such targets may give rise to a greater angular extent of the echo. Hence beam-width distortion depends on the reflecting property and the distance of the target. It is therefore more accurate to say that for a given distance beam width distortion is proportional to the stated beam-width. For example, a radar set which has a beam-width of two degrees might produce a beam-width distortion with a particular target of three degrees, whereas a radar with a beam-width of one degree will produce a distortion of only 1~ degrees with the same target at the same range. 58 Fugure 2.4 - Radar beam sweeps over a thin vertical pole Beam-width distortion should not be confused with "tube-edge” distortion. The latter distortion also increases the size of the echo at the circumference of the screen but this is due to a certain amount of de-focusing which takes place at the tube edge. It is not difficult to eliminate an error due to beam-width distortion for isolated targets. The correct bearing corresponds to the middle of the arc. The comer of a coastline, however, or targets on the coast itself, have, on the PPI, their arcs merged with arcs of other echoes and exact determination of the middle of the arc is difficult. In such a case one can try to reduce the sensitivity or gain and a sharp echo, showing the correct bearing of the object, may come out on the screen. The outline, for example, of the echo of a comer of a coast seems to contract on the display if we reduce the sensitivity. This is because most of the returning radiation comes back when the scanner points exactly at the target. The re-radiation which enters the aerial just before and just after that moment is weaker in power (due to the directional properties of the aerial) and their echoes on the screen become fainter when we reduce the sensitivity below saturation brilliance of the echo. The divergent parts of the returning signal are, as it were, "cut away". Another way of looking at this is by means of the horizontal coverage diagram for reception. Consider a target well inside detection range. Reduction of gain will result in contraction of the pattern (Fig. 2.5). The contours each side of the target will close up towards it. These contours show the limit of detection; hence bearing accuracy (and discrimination) will be increased. Targets near the detection range in such a case, of course, will be 'lost because they move outside the area bounded by the contours. During each revolution of the scanner, a line named heading marker is painted on the display which should represent the fore and aft line of the vessel and hence the course, not allowing for current. Owing to incorrect positioning of a contact inside the scanner housing or pedestal when the radar was installed, the heading marker will differ from the correct position, sometimes by several degrees. When this is the case, errors in bearings will occur. With a "Head-Up" 59 picture, i.e., the heading marker indicates zero degrees, all relatives bearings will be out. But this will also be the case with "true" bearings taken on a stabilized displayconnected to a gyro-compass transmitter and the heading marker acting as a gyro repeater-when the heading marker, painting an incorrect position of the fore and aft line, indicates the true course reading on the bearing scale. Hence the position of the heading marker is related to bearing accuracy. It might sometimes happen with end-fed slotted waveguide aerials, that, although the heading marker contact originally was aligned correctly, owing to a slight change in the frequency of the cm. wave, the lobe pattern of the aerial is slewed round through a small angle-the angle of squint-and the axis of the main lobe is no longer perpendicular to the longitudinal axis of the aerial. Figure.2.5 - Angle of squint This is so because the distance between the slots is based on one particular frequency and when this frequency is change, a redistribution of radiation from the slots will take place. As a result of this the picture on the display will slew round with respect to the heading marker, through an angle equal to the angle of squint and the heading marker should correspondingly be re-aligned. See Fig. 3.2. The error caused due to the incorrect position of the heading marker contact or electronic sensor in the scanner housing, is constant. All bearings will be wrong by an equal amount. It can be removed mechanically by re-adjustment of the said contact/ sensor. The maintenance manual can be consulted or an expert called in. Still, it is the operator's duty to find out if there is an error. This can be done in port or at anchor or anywhere when there is a target at a distance. Use the "Head-Up" orientation and set the heading marker to zero degrees. Check that the centre of the rotation of the trace coincides with the centre of the PPI. If there is a discrepancy between the two centers, make an adjustment as described in the next chapter. Select an object which yields a distinct echo near the edge of the display. Check the radar bearing. With respect to the heading marker against the visual bearing-taken from near the scanner position-with respect to the fore and aft line of the ship. Record the difference between the bearings. It is good practice to repeat the procedure for different bearings of different objects, if possible, in succession about 90 degrees in azimuth apart. If there is an error and it is due to the placing of the heading marker only, then the results for the different bearings should be practically constant. If a variable error is found then it could be because there is centring error or incorrect synchronism, or a combination of errors. For example, if the visual bearing and the radar bearing are respectively 2800 and 2820 relative, then the heading marker is displaced two degrees to port or Red 2° 60 At sea one could bring a small object or a ship, at a distance of not less than two miles' dead ahead and at the same time take a radar bearing. Suppose that the relative bearing is 351° on the bearing scale, then the fore and aft line is three degrees to the left of the heading marker, or in other words, the heading marker in fact represents 3° Green. A rough idea about the position oft he heading marker, when the scanner is sited in the centre-line, can often be obtained by observing the heading flash against the dark sector (caused by the shadow sector of the foremast) projected on rain or sea clutter. If a heading marker error is found to exist and is not corrected by re-adjustment of the contacts, a notice indicating the error should be displayed prominently near the radar display. With a good radar set and an experienced operator the bearing accuracy can be within one degree. See M-Notice 1158. 2.1.4 Range Discrimination Range discrimination is the ability of the set to discriminate between two small objects close together on the same bearing, i.e. to produce separate echoes. The further object must not be in the shadow of the nearer one (Fig. 3.3 (a). Figure 2.6 - Range and bearing discrimination The range discrimination depends on: (a) The pulse-length. If, for example. the duration of transmission is '25 micro-second, then the length of the pulse leaving the scanner is '25 X 300, i.e. 75 meters long. The theoretical range discrimination is a minimum of half the pulse-length, i.e. 37.5 meters. A shorter separation between the two targets will make the echo pulse from the further target join or overlap the echo pulse from the nearer one. The two targets would be shown as one echo on the display. This is illustrated in Fig. 3.4. Here the distance between two small objects 'A' .and 'B' is 32 meters while the range discrimination amounts to 75/2 i.e. 31.5 meters. The combined reflected pulse-length equals 75+ 32+32 Le. 139 meters shown on the radar display as one echo 139/2 or 69.5 meters in length. In practice the range discrimination amounts to more than half the pulse-length due to: 61 (b) The size of the spot. On a short range scale (Le. a large scale picture) the influence of the spot-size will be less than when a longer range scale is employed. Range discrimination can be improved by correct focusing and brightness, by selecting a short pulse-length and by differentiating the echoes. 2.1.5 Bearing Discrimination Bearing discrimination is the ability of the set to discriminate between two small objects close together at the same range, i.e. the ability to produce separate echoes (Fig. 2.6) (b).The bearing discrimination is governed by: (a) The horizontal beam-width (which depends on the horizontal dimensions of the scanner and the wave-length used). The narrower the beam-width, the better the bearing discrimination. (b) The spot-size. Again, the influence of the spot-size is less when a lower range scale is employed. Here again, as with bearing accuracy, the horizontal beam-width plays an important role. If the beam-width (between half-power points) were too wide, there would be a short moment when the two objects are contained in ·the same beam and send out returning pulses simultaneously. Conversely, the scanner picks up signal pulses with the same divergence in bearing as the beam-width and the echo-paints on the display coming from the two objects at different bearings but at the same range, will join up or overlap. Bearing discrimination can be improved by correct focusing and brightness, by reducing the sensitivity or gain and by off-centering on a lower range scale. Radar sets in the mm. waveband have excellent bearing discrimination. Small isolated targets or targets which are small in comparison with the horizontal beam-width, whatever their real shape, are recorded on the PPI as echoes which possess dimensions which are mainly functions of the angular horizontal beam-width and the pulse length. Such targets are, as it were, "drowned" in the beam. For wide and deep targets such successive echoes may overlap and a rough outline is displayed on the screen. In order to make some deeper study of what the echo of a small isolated target looks like at a short and a long range wing the same range scale, let us assume that the scanner has a radiation pattern. Suppose that the target is at a distance of 7 miles and produces an echo 6 db above the minimum detectable, then the scanner will pick up the signal over an arc of '6° If a 30 mm. PPI is used with a spot diameter of 0.5 mm and the range scale is 12 M then the width of the echo on the display represents 7X '6/57'3 (arc of signal)+(l/20)j15 X 12 (spot diameter)=·07 +04, ie. nautical miles or 204 meters. The same target at a distance of 1M produces an echo 40 db (Le. (10 log 74+6) db) above the minimum detectable (the echoing strength is inversely proportional to the fourth power of the distance) and it is seen that the scanner picks up the signal over an arc of 1'50 , representing a width of the echo on the screen of I X 1'5/57'3 (arc of signal)+(1/20)/15 X 12 (spot diameter) =·025+ '04, i.e. '065 nautical mile or 120 meters. If the pulse-length used with this range scale is '5 micro--second, then this corresponds to a length of 150 meters of the radiated pulse. The echo pulse is also 150 meters long but the spot registers an echo representing 75 meters. This is so because the scale on the PPI indicates half the distance traveled by the radar pulse (out and back). The echo therefore depicts the target, both at the 7 M and I M position as having a length of 75 meters+ spot diameter =75 + 74 meters, Le. 149 meters. Hence at 7 M the apparent 62 dimensions of the target seems to be 204X 149 meters while the same target at a distance of 1M seems to measure 120 X 149 meters (see Fig. 2.7). Figure 2.7 - Echo distortion This consideration explains why, on the same range scale, ships at the shorter ranges appear to be end-an and at the longer ranges beam-an. Tube edge distortion may also increase the "beam-an" effect at the circumference of a traditional CRT screen. Thus no inferences should be drawn from the shape of an echo about the relative or true movement of the ship's echo over the screen. Neither can the aspect be read off directly (at least not for cm. sets). It is one of the reasons why plotting is necessary in the observance of motion of other vessels. 2.1.6 Recognition of unwanted echoes and effects The navigator must be able to recognize various abnormal echoes and effects on the radarscope so as not to be confused by their presence. Indirect (False) Echoes Indirect or false echoes are caused by reflection of the main lobe of the radar beam off ship’s structures such as stacks and kingposts. When such reflection does occur, the echo will return from a legitimate radar contact to the antenna by the same indirect path. Consequently, the echo will appear on the PPI at the bearing of the reflecting surface. This indirect echo will appear on the PPI at the same range as the direct echo received, assuming that the additional distance by the indirect path is negligible (see figure 2.8). 63 Figure 2.8 - Indirect echo. Characteristics by which indirect echoes may be recognized are summarized as follows: (1) The indirect echoes will usually occur in shadow sectors. (2) They are received on substantially constant bearings although the true bearing of the radar contact may change appreciably. (3) They appear at the same ranges as the corresponding direct echoes. (4) When plotted, their movements are usually abnormal. (5) Their shapes may indicate that they are not direct echoes. Side-lobe Effects Side-lobe effects are readily recognized in that they produce a series of echoes on each side of the main lobe echo at the same range as the latter. Semi-circles or even complete circles may be produced. Because of the low energy of the side-lobes, these effects will normally occur only at the shorter ranges. The effects may be minimized or eliminated through use of the gain and anticlutter controls. Slotted wave guide antennas have largely eliminated the side-lobe problem. Multiple Echoes Multiple echoes may occur when a strong echo is received from another ship at close range. A second or third or more echoes may be observed on the radarscope at double, triple, or other multiples of the actual range of the radar contact (Second-Trace (Multiple-Trace) Echoes Second-trace echoes (multiple-trace echoes) are echoes received from a contact at an actual range greater than the radar range setting. If an echo from a distant target is received after the following pulse has been transmitted, the echo will appear on the 64 radarscope at the correct bearing but not at the true range. Second-trace echoes are unusual except under abnormal atmospheric conditions, or conditions under which super-refraction is present. Second-trace echoes may be recognized through changes in their positions on the radarscope on changing the pulse repetition rate (PRR); their hazy, streaky, or distorted shape; and their erratic movements on plotting. 2.3. Measure ranges and bearings 2.3.1 Range and bearing measurement 2.3.1.1 Mechanical Bearing Cursor The mechanical bearing cursor is a radial line or cross hair inscribed on a transparent disk which can be rotated manually about its axis coincident with the center of the PPI. This cursor is used for bearing determination. Frequently, the disk is inscribed with a series of lines parallel to the line inscribed through the center of the disk, in which case the bearing cursor is known as a PARALLEL-LINE CURSOR or PARALLEL INDEX (see figure 2.9.) To avoid parallax when reading the bearing, the lines are inscribed on each side of the disk. When the sweep origin is at the center of the PPI, the usual case for relative motion displays, the bearing of a small, well defined target pip is determined by placing the radial line or one of the radial lines of the cross hair over the center of the pip. The true or relative bearing of the pip can be read from the respective bearing dial. Figure 2.9 - Measuring bearing with parallel-line cursor. 65 2.3.1.1 Variable Range Marker (Range Strobe) The variable range marker (VRM) is used primarily to determine the ranges to target pips on the PPI. Among its secondary uses is that of providing a visual indication of a limiting range about the position of the observer’s ship, within which targets should not enter for reasons of safety. The VRM is actually a small rotating luminous spot. The distance of the spot from the sweep origin corresponds to range; in effect, it is a variable range ring. The distance to a target pip is measured by adjusting the circle described by the VRM so that it just touches the leading (inside) edge of the pip. The VRM is adjusted by means of a range crank. The distance is read on a range counter. For better range accuracy, the VRM should be just bright enough to see and should be focused as sharply as possible. 2.3.1.2 Electronic Bearing Cursor The designs of some radar indicators may include an electronic bearing cursor in addition to the mechanical bearing cursor. This electronic cursor is a luminous line on the PPI usually originating at the sweep origin. It is particularly useful when the sweep origin is not at the center of the PPI. Bearings are determined by placing the cursor in a position to bisect the pip. In setting the electronic cursor in this manner, there are no parallax problems such as are encountered in the use of the mechanical bearing cursor. The bearings to the pips or targets are read on an associated bearing indicator. The electronic bearing cursor may have the same appearance as the heading flash. To avoid confusion between these two luminous lines originating at the sweep origin on the PPI, the design may be such that the electronic cursor appears as a dashed or dotted luminous line. Another design approach used to avoid confusion limits the painting of the cursor to that part of the radial beyond the setting of the VRM. Without special provision for differentiating between the two luminous lines, their brightness may be made different to serve as an aid in identification. In the simpler designs of electronic bearing cursors, the cursor are independent of the VRM, i.e., the bearing is read by cursor and range is read by the rotating VRM. In more advanced designs, the VRM (range strobe) moves radially along the electronic bearing as the range crank is turned. This serves to expedite the reading of the range and bearing to a pip. 2.3.1.3 Interscan The term INTERSCAN is descriptive of various designs of electronic bearing cursors, the lengths of which can be varied for determining the range to a pip. Interscans are painted continuously on the PPI; the paintings of the other electronic bearing cursors are limited to one painting for each rotation of the antenna. Thus, the luminous lines of the latter cursors tend to fade between paintings. The continuously luminous line of the interscan serves to expedite measurements. In some designs the interscan may be positioned at desired locations on the PPI; the length and direction of the luminous line may be adjusted to serve various requirements, including the determination of the bearing and distance between two pips. 66 2.3.2 Off - Center Display While the design of most relative motion radar indicators places the sweep origin only at the center of the PPI, some indicators may have the capability for off-centering the sweep origin (see figure 2.10). The primary advantage of the off-center display is that for any particular range scale setting, the view ahead can be extended. This lessens the requirement for changing range scale settings. The off-centering feature is particularly advantageous in river navigation. With the sweep origin off-centered, the bearing dials concentric with the PPI cannot be used directly for bearing measurements. If the indicator does not have an electronic bearing cursor (interscan), the parallel-line cursor may be used for bearing measurements. By placing the cursor so that one of the parallel lines passes through both the observer’s position on the PPI (sweep origin) and the pip, the bearing to the pip can be read on the bearing dial. Generally, the parallel lines inscribed on the disk are so spaced that it would be improbable that one of the parallel lines could be positioned to pass through the sweep origin and pip. This necessitates placing the cursor so that the inscribed lines are parallel to a line passing through the sweep origin and the pip. Figure 2.10 - Off-center display. 2.3.3 Expanded Center Display Some radar indicator designs have the capability for expanding the center of the PPI on the shortest range scale, 1 mile for instance. While using an expanded center display, zero range is at one-half inch, for instance, from the center of the PPI rather than at its center. With sweep rotation the center of the PPI is dark out to the zero range circle. 67 Figure 2.11 - Normal display. Ranges must be measured from the zero range circle rather than the center of the PPI. While the display is distorted, the bearings of pips from the center of the PPI are not changed. Through shifting close target pips radially away from the PPI center, better resolution or discrimination between the pips is afforded. Also because of the normal small centering errors of the PPI display, the radial shifting of the target pips permits more accurate bearing determinations. Figure 2.11 illustrates a normal display in which range is measured from the center of the PPI. Figure 2.12 illustrates an expanded center display of the same situation. Figure 2.12 - Expanded center display. 68 3. PERFORM MANUAL RADAR PLOTTING 3.1 The need for radar plotting in collision avoidance Radar for the Merchant Service is designed for what is known as "Surface Warning" and for anti-collision purposes its main use was during reduced visibility. With the faster ships, radar, however, is also now extensively used in clear weather as an extra aid for the look-out man. With the range-scale on 12 miles and the electronic centre offset, strong echoes of ships can be detected up to 16 miles for an average bridge height and at a time when the ship has not yet been sighted visually. Another reason for earlier detection by radar is that the white echo pip against the dark background is often more conspicuous than the appearance of a. ship against a grey sky and seas. By placing the cursor over the echo, a timely check can be kept on the bearing change. When fog-banks are expected the radar set should be at least on "Stand-by" during daytime, making it ready for immediate use; but at night the set should be left on "Transmit", as the vessel could well be streaming along near a fog bank which is giving no visual indication on its precise. When approaching a fog-bank Rule 35 (Sound Signals must be used Visibility) must be adhered to and radar must be used to see what is inside the fog-bank. Failure to employ the radar in such a case contravenes Rule 2 and blame accordingly has been attached to ships which did not comply with this rule near a fog-bank. Upon approach of the fogbank, radar watch routine should be started, and inside the fog-bank, the observer should realize that some echoes on the screen might represent ships which are not in fog and may not exercise the same caution as his own ship. Unexplained manoeuvres by other vessels as observed from the radar screen might indicate the existence of a small vessel or vessels undetected by the ship's own radar, and a close watch should be kept on the suspected area. Shipmasters have been blamed for not keeping a proper "look-out" because they were not using their radar on clear nights to detect the presence of the unlit oil-rigs with which their vessels collided. It may, therefore, be said that it is always good practice, especially for fast ships, to keep the radar working.. This also offers an opportunity to the officer of watch to maintain his plotting expertise, which are so important 18 cases when the visibility deteriorates and plotting becomes really essential. At night, when in a region where fog-banks and/or unlit obstruction can be expected, the radar must be in continuous operation. Previous Court Cases, by the way, have stressed that a shipmaster is considered to be at fault for not using radar provided for his ship and also for allowing the radar installation on his ship to remain in a defective condition for a prolonged period. At present the U.K. Government has made radar compulsory for all British ships of 500 gross tons or more, following an IMO recommendation that at least one radar must be fitted in ships of 500 g.r.t. or more (300 g.r.t. after 1st February 1995), and at least two radars must be fitted to all ships of 10,000 g.r.t. or more, each 69 capable of operating independently of the other. Some important points to be kept in mind when using radar in reduced visibility are the following: (a) The setting of the anti-clutter control on raw radar displays. Adjust, if possible, in such a way that echoes can be traced near the spot representing own ship. Be aware of over-suppression, as this will wipe off most of the echoes of ships nearby. (b) The existence of blind and shadow sectors caused by objects on the ship itself. A slight "weaving" around the course is recommendable in such a case. (c) The selection of range scale, taking account of: (i) The speed of own ship (the faster the ship, the greater the range scale). (ii) The accuracy of bearing and range observations (shorter range scales with the echo in the outer half of the screen yields an increased accuracy). (iii) The length of the "tadpole" tails (the shorter the range scale, the longer the tails). If a True Motion Display is available, this might entail off-centring the time-base and use of the Zero Speed switch. (iv) The possibility of encountering small craft or ice growlers (easier discernable on the shorter range scales and, if possible, with long pulse selected). (v) The number of ships in the vicinity of own ship (a long-range scale can produce a confusing array of closely-packed echoes). (vi) The range at which most merchant ships are first detected (generally about 10-15 miles). In addition to the above considerations, it should be remembered that a plot on a reflection plotter mounted on a True Motion Display will become distorted if the range scale is altered during the plotting interval. This problem does not arise with Relative Motion radar. Summarizing on this problem of selecting a range scale, it is generally best to relate the range-scale used most of the time to the vessel's speed changing to shorter range scales now and again to obtain more accurate observations of bearings and ranges of any nearby objects and also to conduct a search for smaller objects. (d) The obedience of Rule 35 (Sound signals in restricted visibility) even if the screen is free from echoes on the longer range scales and one knows that the set is fully efficient. (e) The obedience of Rule 34 (0) (Manoeuvring signals) only when the other vessel is in sight. 3.1.1 Radar and the Collision Regulations Under Part B (Steering and Sailing Rules) there are two sections which have a special bearing on this Chapter. These are Section I and Section m. The former deals with the conduct of vessels in any condition of visibility (Rules 4, 5, 6, 7. 8 and 10); the latter (Rule 19) discusses the conduct of vessels in restricted visibility. Turning our attention to Section 1 fist, it will be seen that Rule 5 specifically deals with the importance of maintaining "a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision". The word "specifically" is stressed because in previous Regulations this came under "the ordinary practice of seamen". 70 The inclusion of "as well as by all available means" refers obviously to a radar watch (the use of guard rings on the radar display will be helpful in this connection), but it also incorporates a V.H.F. R/T watch and the words "full appraisal" may be taken to include proper radar plotting procedures and active V.H.F. Radio-Telephony communications. Although this section deals with clear weather conditions and conditions of restricted visibility, the master is given some latitude in making use of radar and R/T information by the addition "appropriate in the prevailing circumstances and conditions". Rule 6 introduces a new concept, namely Safe Speed. When, about 45 years ago radar was introduced on board ships, one of the greatest difficulties with which Mariners were confronted, was the term "Moderate Speed". What. in fact, was a moderate speed using radar? A concise answer was not possible. It could be argued that a moderate speed, using radar, could in some cases mean "Full speed with engines on Stand-by", but in other cases could mean a slower speed than a Mariner without radar might consider "moderate". From the legal and philosophical point of view these arguments are quite correct but in the literary sense they are unsatisfactory. A Safe Speed as defined in the 1972 Rules is not based only on the state of visibility (as in the 1960 Rules) but "Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions". Besides the state of visibility (i) the following factors should be taken into account in determining a safe speed: (ii) "the traffic density, including concentrations of fishing-vessels or any other vessels"; (iii) "the manoeuvrability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions"; The manoeuvrability depends on the stern power of the vessel, the number and type of screws, the provision of a bow-thrusters. the size of the ship and her loaded condition while the prevailing conditions are mainly governed by the wind and wave directions, wind force and wave height, and current and tidal conditions. (iv) "at night the presence of background light such as from shore lights or from back scatter of her own lights"; (v) "the state of wind, sea and current, and the proximity of navigational hazards"; (vi) "the draught in relation to the available depth of water. These factors, which determine a safe speed in general, are applicable to all ships. Vessels which use their radar need, in addition, take the following conditions into account:(i) "the characteristics, efficiency and limitations of the radar equipment"; The age and reliability of the equipment, the number of radars and displays, inter switching facilities, types of display presentations, plotting devices and facilities for automatic plotting etc., are all factors to consider. (ii) "any constraints imposed by the radar range scale in use"; A constraint may be imposed on a particular range scale owing to strong radar or electrical interference, or for a very fast vessel the use of the l2--mile range scale (the 24-mile range scale is. too small for effective plotting on a reflection plotter) for echo observation, might compel her to reduce speed. (iii) "the effect on radar detection of the sea state, weather and other sources of interference"; Excessive "noise" due to wave, sea, ram-drops, snow crystals, other ships' 71 radar pulses or electrical interference may swamp the signal and essential information .may be lost. . . (iv) "the possibility that small vessels, Ice and other floating objects may not be detected by radar at an adequate range". This "possibility" can be a result of atmospheric conditions such as sub-refraction or it might be caused by a small reflection coefficient of the object. (v) "the number, location and movement of vessels detected by radar". (vi) "the more exact assessment of the visibility that may be possible when radar is used to determine the range of vessels or other objects 10 the vicinity". In addition, we may say that the number of men for keeping radar watch and a plot, and their efficiency could influence the master’s opinion about what is, or what is not a safe speed. . Rule 7 deals with the "Risk of Collision". The Rule stresses again the use of "all the available means appropriate to the prevailing circumstances and conditions to determine if the risk of collision exists". This includes the 'listening to V.H.F. R/T messages of other ships and shore radar stations, but no guidance is give about actual active participation. There is a very important last sentence in the first paragraph: "If there IS any doubt such risk shall be deemed to exist". This might remove the possible element of indecision in a radar encounter. Rule 7 (b) stresses the importance of making proper use of radar equipment, including early warning of collision risk on the long-range scales. It furthermore emphasizes the practice of radar plotting or "equivalent systematic observation of detected objects" (recording in writing and tabulation, automatic plotting aids). Rule 7 (e) states: "''Assumptions shall not be made on the basis of scanty information, especially scanty radar information". The omission of a plot, an incomplete plot or a plot based on an insufficient number of observations, in short the determination of the position of another vessel without finding her movement, might be termed as "scanty". ARPA provides a solution here. The last paragraph of Rule 7 states how risk of collision can be obtained from compass bearings and gives a warning that an appreciable change in bearing does not always indicate a safe passing (large vessel, Or a tow, or a ship at close range; see also Fig. 14.4). Bearings should be recorded as compass bearings and not as relative bearings as is so easily done on an unstabilized display. It is not possible to compare relative bearings when own ship is subject to yaw or makes alterations of course, and often the Master has been led to believe, that. By making a small alteration of course, the situation improved because the relative bearing changed and he did not realize that the change in the relative bearing was mainly due to own ship's alteration of course. If he had converted the relative bearings to compass bearings, he would have noticed that danger of collision after the alteration had become greater instead of less. Rule 8 is headed "Action to avoid Collision". Paragraph (a) states that, if the circumstances of the case admit, any action shall "be positive, made in ample time and with due regard to the observance of good seamanship". The word "positive" in this connection means "effective" and bears no relationship to the conventional adaptations "'positive and negative actions", mentioned in certain papers about collision-avoidance (more about these later). 72 Paragraph (b) is an extension of paragraph (a), stating that "Any alteration of course and! or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and (or speed should be avoided". The Rule requires substantial action in order to make clear one's intention to all vessels in the neighbourhood ("another vessel" is not necessarily the vessel for which avoiding action was taken) both in clear weather as well as in fog. This requirement should be kept in mind when an agreement is reached about collision-avoiding tactics between two vessels via V.H.F. R/T and also when using the 'Trial Manoeuvre' facility on ARPA. The remainder of the Rule (paragraphs (c), (d) and (e» emphasizes that an alteration of course, provided there is sufficient sea room, may be the most effective action to avoid a close quarters situation on condition that it is made in good time, is substantial and does not result in another close-quarters situation. It stresses the safe passing distance and warns that effectiveness of the action shall be carefully checked until the other vessel is finally past and clear. If necessary, or to allow more time to assess the situation, a vessel shall slacken her speed or take out way off by stopping Or reversing her means of propulsion. In short, what this Rule is saying is that if avoiding action for another vessel is going to be taken such action should be bold both in clear weather and in conditions of restricted visibility so that the intention of the vessel taking the action becomes readily apparent to other vessels in the vicinity. Seen in this light, an alteration of course is generally more effective than an alteration in speed. There are some contributory factors for substantial action to be taken where radar navigation in fog is concerned. The first factor is that for a collision encounter between two vessels meeting ends on or crossing – and each forward of the other's beam - an alteration of course or speed by one of the vessels shows up far less pronounced in the relative track (direction or rate) on the other vessel's display or plot when a relative presentation is used than if a true motion presentation were used. This is understandable when one remembers that the relative motion line is produced as the result of two vectors, of which only one is changed in this case. The reverse is also true. If our own ship, for example makes an alteration of course of 30 degrees, another crossing vessel, with approximately the same speed, forward of the beam, involving risk of collision, will observe a change in her relative motion line of about IS degrees. To make, therefore, a course alteration - and this holds also for alterations in speed - readily apparent and on the assumption that other vessels in the vicinity use a relative motion display presentation, a substantial alteration is required by own ship. The second reason for making substantial alterations is that errors in plotting and a wrong estimation of the direction of the relative motion line can easily take place especially when the display is unstabilized. The observer may, for example, conclude that the other vessel is on a collision course or will be passing on her port side while, in fact, the other ship, if she maintains her course and speed will be passing on her starboard side. If, in this case, own ship makes a small alteration to starboard, then, instead of improving the situation, the nearest approach between the two vessels will become even smaller. If later on, own ship makes a second alteration to starboard, and perhaps even a third one, then this may lead to collision. This type of action whereby one ship makes a succession of small alterations of course has become known as The Cumulative Turn and the majority of collisions in fog have been caused by this type of action. 73 In many of these collision cases, while one ship carried out the cumulative turn, the other vessel maintained her course and speed, simply because she had not detected the effect of the turn on her display. Although, for her, the bearing opened out it did not open out sufficiently (Rule 7 (d) (ii)) and in the final stages of the encounter it became steady. Hence the warning in Rule 8 (b) against a succession of small alterations of course and/ or speed. Rule 8 (c) states that if there is sufficient sea room, alterations of course alone may be the most effective action to avoid a close-quarters situation. The sea room, however, could be restricted by navigational dangers or a fair amount of traffic with ships crossing from different directions. In such cases substantial alterations of course may not be possible and may make the situation even more dangerous when ships which, before the alteration, were not on collision courses, may after the alteration, involve a risk of collision. If it becomes necessary to avoid collision, a substantial reduction in speed must be made (Rule g (e)). Paragraph (d) of Rule g emphasizes that the effectiveness of any action small be carefully checked until the other vessel is finally past and clear. During restricted visibility and using one's radar, this means that after an alteration of course and I or speed, observations of the target should be taken at frequent intervals to see if the echo follows the predicted track at the predicted rate. If the echo deviates from the predicted track away from the centre of the plot, then to other vessel has taken action contributing. To safety but if on the other hand the echo deviates from the prediction he towards the centre of the plot, then the other vessel has taken action which has cancelled out or partly cancelled out our avoiding action and in the majority of cases, the wisest thing to do is to reduce the speed of own ship to a minimum at which she can be kept on her course, or to alter course and to put the other ship right astern. On ARPA the history track should be watched for alterations of course or/ and speed of targets. Finally, Rule 8 makes a mention of "close-quarters situation" and "safe passing distance". These concepts cannot be concisely formulated. It for obvious that their extent depends on many factors, such as weather conditions, state of visibility, type of vessel, manoeuvrability and observations are carried out by visual means or by radar which is far less discriminating than the naked eye in discerning changes in aspect. But even, when considering radar navigation in fog only, formulation does not come easily. For example, to pass another vessel at half a mile at three knots could be considered just as safe as passing a vessel four miles off at 15 knots. One must also take into account, when deciding what is a safe distance to pass, the direction in which vessels are shaping to pass. For instance, it could be quite reasonable and safe when overtaking a vessel to pass two miles off, whereas this could be unwise when passing a vessel on a reciprocal course with speed. In other words, it is really the relative speed and the direction of the target which should be considered when judging what is a close-quarters situation. In thick fog, however, when there is plenty of sea room, it is practical to keep the minimum radius of the close-quarters situation at about three miles in order to allow for bearing errors, unsuspected manoeuvres of the target and to keep out the range of audibility of the other ship's sound signals so that delays owing to the application of Rule 19 (e) can be avoided. Really, in order to assess the radius of the close-quarters situation, the master must rely on intuition based on experience to give him the right answer (radar simulator courses are generally useful to accelerate this experience). CPA and TCPA data should be set on 74 ARPA so that sufficient warning can be given to the O.O.W. when a close-quarters situation is approaching. Rule 10 applies to Traffic Separation Schemes and is a highly important addition to the 1972 Rules. It contains regulations adopted by IMO (see IMO-publication "Ships' Rooting Traffic Separation Schemes and Areas to be Avoided") but have become mandatory for all the published schemes. Additional paragraphs are included for small vessels and sailing ships, and exempted vessels. The more important parts of the Rule state that "a vessel using traffic-separation scheme shall, so far as practicable, keep clear of traffic-separation line or zone (b (ii) ), normally join or leave a traffic-lane the termination of a lane, but when joining or leaving from either side, shall do so at as small an angle to the general direction of traffic flow as practicable (b (iii)) and shall, so far as practicable, avoid crossing trafficlanes, but if obliged to do so, shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow (c). It goes on: "(d) Inshore traffic zones shall not normally be used by through traffic which can safely use the appropriate traffic-lane within the adjacent traffic-separation scheme. However, vessels of less than 20 meters in 'length and sailing vessels may under all circumstances use inshore traffic zones". In connection with the right-angled crossing, it is advised, if possible, to shape the new course well before the lane is reached, thus giving ships within the lane a timely indication. The Rule dealing with Traffic Separation Schemes (Rule 10) follows the Rule about Narrow Channels (Rule 9) and purposely so, as both can be grouped under Narrow Navigational Routes. There are, therefore, certain analogies between the two Rules, viz: Rule 9 (b): A vessel less than 20 m. in length or a sailing vessel shall. Not impede the passage of a vessel which can safely navigate only within a narrow channel or fairway. Rule 10 (J): A vessel less than 20 m. in length or a sailing vessel shall not impede the safe passage of a power-driven vessel following a traffic-lane. Rule 9 (c): A vessel engaged in fishing shall not impede the passage of any other vessel navigating within a narrow channel or fairway. . Rule 10 (i): A vessel engaged in fishing shall not impede the passage of any vessel following a traffic-lane. . Rule 9 (i): Any vessel shall, if the circumstances of the case admit, avoid anchoring in a narrow channel. Rule 10 (g): A vessel shall so far as practicable avoid anchoring in a traffic-separation scheme or in its areas near its termination. One of the first traffic-separation schemes was instituted in the English Channel and was followed by a marked decrease in the number of collisions. Surveillance on the conduct of vessels is carried out by radar observation from Langdon Battery, Dover (Channel Navigation Information Service), light aircraft and fast launches. Having discussed the relevant Rules of Part B (Steering and Sailing Rules), Section I of the Collision Regulation which applies to the conduct of vessels in any condition of visibility, we will skip Section II (conduct of vessels in sight of one another) and look at Section III, which is applicable to vessels in restricted visibility. This section contains only one Rule, Rule 9, which has replaced the famous Rule 16 of previous Regulations.. so well known to generations of seamen. As the Rule is so important, we will quote in full (Italics are the Author's): 75 "(0) This Rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. . . (b) Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility A power-driven vessel shall have her engines ready for immediate manoeuvre. (c:) Every vessel shall have due regard to the prevailing circumstances and conditions of restricted visibility when compiling with Rules of Section I of this Part. (d) A vessel which detects by radar alone the presence of another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration of course, so far as possible the following shall be avoided: (i) an alteration of course to port for a vessel forward of the beam .. other than for a vessel being overtaken; (ii) an alteration of course towards a vessel abeam or abaft the beam. (e) Except where it has been determined that a risk of collision does not exist,. every vessel which hears, apparently forward of her beam, fog-signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over". Paragraph (b) emphasizes the fact that the restricted visibility has to be taken in to account when determining a safe speed. It also states that.... a power-driven vessel should have her engines on "Stand-by". Paragraph (c) refers to Section I of this part, again stressing the fact allow for the additional circumstances and conditions of restricted visibility. Paragraph (d) refers to ships which have their radar in working order and makes It compulsory for these ships to use their radar for determining if a close-quarters situation is developing, for assessing the risk of eventual collision and for taking avoiding action. The paragraph also places restrictions on certain manoeuvres: (i) Do not alter course to port for vessel forward of the beam (it does not apply when one is overtaking vessel); (ii) Do not alter course towards a vessel which is abeam or abaft the beam. Note that the word "abeam" is not accurately defined within the Collision Rules and there is a choice of avoiding action (to starboard or to port) for a vessel abeam on starboard. There arc cases with a vessel forward of the beam and the bearing changing slowly clockwise in the initial stages that there is a reluctance to go to starboard and an inclination to go to port, especially when the rate of approach is fairly fast. One case is with a crossing vessel on the port bow. In such a case, with a close-quarters situation developing and the Master feeling reluctant to alter course to starboard, should reduce the speed of own ship. In fact, as will be amplified later in this Chapter, this is quite a good manoeuvre and might be one of these cases where a reduction in speed is better than an alteration of course. 76 Another difficult case is when an echo of a vessel is detected fine on the starboard bow and shaping to pass apparently. as best as can be judged from radar, close to starboard. Again, there is often a reluctance – and this applies also to the other vessel if she is using her radar - to alter course to starboard to pass ahead of the oncoming vessel, especially if the rate of approach 1S fast. In this case stopping engines is not a satisfactory manoeuvre because the other vessel might not have detected Own vessel there is quite a possibility on a dark night that the other vessel has not yet entered the fog and is not aware either of own ship or the presence of fog thus to stop engines and become immobile might place own ship in the path of the oncoming vessel and be completely at her mercy. To stop in this case could only be Justified In narrow waters or if one is hampered on both sides by other vessels. Thus if there is sea room the only answer here is to make a very bold alteration to starboard so as to put the echo of the other vessel abeam or even a little abaft the beam as quickly as possible. The advance of most ships when making this manoeuvre is usually much smaller than their corresponding head reaches when they make an emergency stop. Having made this manoeuvre the echo should be watched carefully. If the other ship keeps her course and speed, or if she alters course in a direction to support the alteration, or if she stops then all well and good. However, if the other vessel, contrary to Rule 19, alters in a direction which cancels out our own alteration it's not so good admittedly, but it's not so bad as the initial encounter - the relative motion will be much smaller than it was originally - and one can go easily "go on round" to put the echo right astern (complying with the second restriction of Rule 19 (d) and so make the relative motion even smaller. For fast ships coming from abaft the beam which present a collision hazard to own ship if they maintain their course and speed, the best action IS to alter course away from the vessel in such a way that the stern keeps pointing towards the danger. Rule 5 (Look-out) should be kept in mind (an extra look-out might be posted near the stern) and the frequency of sound signals should be increased. Note that this Rule states "shall take avoiding action in ample time" (not "may" as in previous Regulations). Paragraph (e) of Rule 19 is applicable to every vessel (not necessarily power-driven) and refers to the detection of other vessels forward of the beam of which the presence is detected either by hearing or by radar ("hears apparently ...", or "which cannot avoid a close-quarters situation's. As this paragraph includes sailing vessels the expression "stop her engines" in previous Regulations is replaced by "reduce her speed to a minimum at which she can be kept on course. The well-known proviso "a vessel, the position of which is not ascertained", which has been used so many times in court cases by Masters to defend their action for not stopping engines has been changed to the more direct wording. "Except where it has been determined that a risk of collision does not exist". Such awareness could take place in the following cases: (a) A vessel forward of the beam, nearby, disappearing in fog, but own ship having determined by visual means up to a few moments before disappearance that there exists no risk of collision between the two vessels. (b) A vessel forward of the beam, which together with own ship, is proceeding along a narrow channel or a traffic-lane. (c) Where the intentions of both vessels have been firmly established by means of V.H.F. R/T contact. Before leaving the Rules which are relevant to this book, mention 77 should be made of Rule 2. It consists of two sections, Section (a) deals with the responsibility of owner, master and crew, and the requirement of good seamanship. Section (b) states that due regard shall be had to all dangers of collision and navigation and any special circumstances, including the limitations of the vessels involved, which may make a departure from the Rules necessary to avoid immediate danger. Its application is important ill connection with Rule 19: Common sense should go hand-inhand with obedience of the Rules under special circumstances (for example, narrow channel with current, multi-target situation, large heavy vessel in traffic lane, etc.) a departure of the Rule may be necessary. Plotting has two purposes: (a) It can show us whether danger of collision exists, how close we will pass off the target (nearest approach or distance of the closest point of approach from own ship) and how much time there is left before this will take place. (b) The approximate determination of the course and the speed of the other vessel from previous observations, so that sensible avoiding action can be taken when needed. The second purpose is connected with one of the limitations of cm radar which does not show up the aspect or leading edge of an isolated' small (in relation to the horizontal beam-width) target except at very close range. Plotting does not reveal to us the shape of a target and hence not the present heading. It will inform us, however, about the motion of the target during the plotting interval. 3.1.2 Reporting and Recognition of Collision Hazards To provide the Master with information about collision hazards, about the possibility of planning avoiding action and about the taking of avoiding action, a good method to be adopted by the radar observer is to report according to a standard pattern. Such a report would consist of two main parts. 1. (a) Last bearing, drawing forward or aft (passing ahead or astern respectively); (b) Last range, decreasing or increasing; (c) Nearest approach (distance of closest point of approach from own ship) as forecast (CPA); (d) Time interval to the nearest approach (closest point of approach) from the last observation (TCPA). 2. (a) True course or relative course or aspect of target; (b) Speed of target. First consider part 1. Whenever an echo is observed on the screen, the Master, especially during reduced visibility, is naturally anxious to know whether there is an appreciable change in the bearing and if the range is increasing or decreasing. If little change in the bearing is observed, and the range is decreasing, at once the question arises how far off will the target pass if both ships maintain their course and speed and how much time is there left before this will occur. Even if there is an appreciable change in the bearing, the Master is likely to want to learn whether the target is passing ahead or astern. If it is apparent that avoiding action has to be taken, then part 2 must be completed, In that case the Master must know the approximate motion and speed of the target. This is the same as in the visual case where one can only plan avoiding action properly if the other ship's course and speed can be estimated. Alterations of course 78 Without establishing the target’s direction and speed are irresponsible actions. The aspect is defined as the relative bearing of own vessel taken from the target. A starboard or port bearing is indicated as Green or Red respectively. For example an aspect of Red 90° means that the target sport side is observed to be beam-on to own ship; a target head-on has zero aspect, stem-on 180° aspect. Strictly speaking, as we have seen, the aspect cannot be deduced from a plot, but we will assume that the most probable aspect can he deducted from the motion of the target during the plotting Interval. As seen from Fig. 13.1 the direction of movement of the target can be expressed in terms of either her aspect, or relative course or true course and it is entirely up to the Master which he prefers. Often a glance at the plot will suffice. Aspect appeals to a lot of sailors because it is so close1y related to the visual conception when they sight a ship. They automatical1y estimate the angle between her bearing and her course. It also gives us insight as to whether the target may see us on her starboard or port side or whether own ship is in the overtaking position. Such considerations may help us to form an idea about the possible reactions of the target and there are of special importance when sailing through fogbanks and the Steering and Sailing Rules 11 to 18 must be applied when the fog lifts suddenly. The report has to be enlarged if avoiding action is going to be taken. Here we must assume initially that the other ship maintains her course and speed. We cannot predict her actions with certainty. After the alteration of course or reduction In speed, the observer must watch the plot closely and re-estimate the nearest approach and the time when this will take place. If the nearest approach remains dangerous, then the best practice is generally to either reduce speed substantially or to stop own ship or to alter course to put the target right astern. The other ship in such a case, probably, has taken avoiding action at about the same time which has cancelled own ship's action. If however the avoiding action is successful and the nearest estimated approach is safe, then the radar observer can continue his report by informing the Master when the original course or speed can be resumed with safety. Two remarks must be made about the nearest approach: (a) In clear weather, when at close quarters, one can almost immediately sec what the other ship is doing and one can act accordingly, if danger of collision is involved. Radar, on the other hand, is not suited for close quarter situations in this connection. Because cm radar does not possess enough discrimination, it is very slow and sometimes unable to tell us what the other vessel is doing. The target if she has radar on board is placed in the same predicament and there is insufficient appreciation on the part of each vessel of the other's movements. If the target has no radar on board or cannot use the radar, then, in thick fog, she will be completely unaware of our position and movements. Hence in fog, when there is a great loss of information, even when both vessels are using their radar, a wide margin of safety has to be introduced. (b) There are many factors which easily cause errors in plotting and the nearest approach as obtained from the plot may differ considerably from the actual value. Therefore taking these two considerations into account, the new nearest approach selected should not be too small. Give the target a wide berth. The situation is not the same as when in clear weather. 79 It is good practice generally, in the open sea, for the Master to base his plan for taking avoiding action on a bold alteration of course and / or speed initially so that the other ship, if she is using radar, will be able to detect own ship's action as quickly as possible. After taking avoiding action careful plotting should be continued to see if the other vessel is keeping her course and speed. If she docs, then a prediction should be made from the plot when it will be safe for own vessel to resume her original course and /or speed. The following three factors should then be taken into consideration: (i) The closest distance which one considers it safe to pass the other vessel under the existing circumstances (three miles is generally accepted as a safe minimum distance for average types of merchant ships in the open sea). (ii) The time factor. In case the other vessel is using radar she should be allowed to have sufficient time to detect own ship's alteration in course and/or speed (a minimum of about twelve minutes is generally considered necessary). (iii) If own ship took action by altering course to bring the echo across from the starboard to the port side or vice versa, then, when the original course is resumed, care should be taken to avoid, if at all possible, bringing the echo back to the opposite side again (if this was done a misunderstanding of the situation might arise if both vessels suddenly came into sight of one another). The report can be computed either by plotting on a sheet of paper or directly on a screen covering the PPI, or by mechanical or electronic plotting devices, some of which will be discussed in following chapters. The motion of the echo is plotted relative to own ship, which is considered as a fixed reference point. In other words, the motion is plotted as it appears on a Relative Motion Radar Display. The centre-point of the plot represents the electronic centre of the radar screen, i.e. own ship. The heading marker, representing the fore and aft-line of own vessel, is drawn on the plot and indicates the direction of own course. All the plots shown in the following diagrams are referred to as Compass Datum Relative Plots. This means that the compass bearing scale is fixed. When course is altered, the heading marker swings round in the same way as it does on a stabilized display, and the movement of the echo is not broken up as it would be on a Head-up Relative Motion Display. The relative bearing scale is not used, though one can, if one wishes, lay-off relative bearings from the heading marker wherever this is positioned. In nearly all the diagrams it is assumed that North is "up". One may, of course, if this is preferred, always start off with the heading marker upwards, provided one turns the heading marker to the new direction when course is altered. By turning the plot bodily around one could then bring the new heading marker to the upward position again. After putting in the heading marker, the different bearings (relative to the heading marker or true) and ranges are plotted from the centre-point according to a suitable scale which should not be too small (about one inch to represent one mile is suitable). A time interval can be chosen which is related to own ship's speed, for example five minutes for a ship of twelve knots, so that the distance moved by own ship during that time is one mile. Or one can take intervals of six minutes during which the ship covers Ii. distance of one tenth of the speed. Anyhow, this can be best left to the observer as it depends on he rate of approach of the other vessel. If the rate of approach is fast, a three-minute interval between successive observations is advisable, but the plot should not be completed 80 before at least three observations of the target have been made. If the log is in operation, it should be read when observations are taken as this will indicate the distance own ship has traveled through the water. This is of special importance after reduction of speed when one is not quite sure about the average speed of own ship, and we will see later (Section 14.1) that inaccuracies in the distance covered by own ship during the plotting interval will introduce errors in the estimated course and speed of the target. Bearings and ranges of each of two echoes have been taken which are laid off from the centre. The first bearings and ranges were at 0000 hrs., the last ones at 0012 hrs. The plotted point which is laid off first (0000) of each echo is called 0 (for "Origin''), that one which is laid-off last (0012) is called A. OA represents the movement of the echo in 12 minutes as seen on the screen of a relative motion display. The nearest distance of approach is the length of the perpendicular dropped from the centre on to OA produced. The unit of time is OA, representing 12 minutes. The arrival times of the estimated closest points of approach of the two targets, whose echoes are depicted on the plot, w ill take place at 0032 hrs. and at 0046 hrs. (foot of perpendicular from centre) if all ships concerned maintain their courses and speeds. Items 1 (a). (b). (c) and (d) of the report are now known. OA is the relative motion of the echo and its length and direction are determined by the course and speed of own ship and the course and speed of the target vessel. Therefore, one may expect that if both vessels maintain their course and speed, and bearings and ranges are correct, the three ,points will lie on a straight line with the 0006 point halfway between the 0000 and 0012 points. In practice, however, one may find that the respective points are staggered even when the target follows a steady course and speed. This is so because the bearing accuracy, especially of the unstabilized display is not very high. Also, own ship may yaw and this will affect the relative motion. If the plotted points are situated nearly on a straight line, and if the distances between them are roughly proportional to the time intervals between the respective observations, one may draw a mean line through the plotted points and assume that the other vessel has kept her course and speed. We thus see that the information derived from the relative motion line is the nearest approach and the time it takes to the closest point of approach. It can also tell us if the other vessel maintained her course and speed. The relative motion plot is compact; the image of own ship is fixed and generally echoes are only plotted of targets whose ranges are decreasing. 3.2 Construct a motion triangle using M.O.T. symbols (Motion, Own, Target). The determination of the course and the speed of the target is sometimes called Completing the Plot. See Fig. 3.1. If the target has been stationary (light vessel or buoy), then at 0012 hrs. its plotted echo would have reached W (Zero Speed point) where WO is parallel to the heading marker and its length corresponds to the distance Determination of Course and Speed of Target the determination of the course and the speed of the target is sometimes called completing the plot. See Fig. 3.1. If the target has been stationary (light vessel or buoy), then at 0012 hrs. its plotted echo would have reached W (Zero Speed point) where WO is parallel to the heading marker and its 81 length corresponds to the distance travelled by own ship in 12 minutes, i.e. a distance of 12/60 X own speed. However, at the end of the plotting interval, the echo is not at W, but at A (0012 hrs.). This can only mean that WA must represent the true motion of the target during these 12 minutes. Measure WA, multiply by 60/12 and the speed of the target is obtained (in practice, one compares the length of WA to the length of WO and estimates the speed relative to own ship's speed). Measure angle OWA and the relative course of the target is known. The aspect can also be read off. In the diagram, the aspect is roughly Red Items 2 (a) and 2 (b) of the report are now established. If the WO component is plotted during the time interval, then nearest approach, course and speed of the target are obtained simultaneously. Figure 3.1 - Determination of course and speed of target An advantage of practicing the complete relative motion plot is that it enables an officer to acquire a better understanding of the relative motion display, it helps him in interpreting the true meaning of the motion and changes in motion of echoes on the display. This can be useful on those occasions when the lack of time precludes plotting or, when there are many echoes, it helps the observer to be selective in, his plotting. When plotting on a reflection plotter, the direction and length of WO can be obtained by 82 means of the parallel index, first aligning it parallel to the heading marker for diction and then swinging it through 90° for marking the distance (using china graph pencils). If the plotted points are lying on a straight or nearly straight line, but the distances between them are not proportional to the time intervals between the observations, then the other ship has altered course or altered speed or has done both. The same is true when the plotted points do not lie on a straight or nearly straight line. In all these cases it is accepted that own ship has maintained course and speed. Irregularities in the OA line tell us that manoeuvring action has been taken by the target, but it does not inform us of what type this is. This can only be ascertained by making a new plot when the new OA line is well established. One can never know the exact time when the target took manoeuvring action. Hence close observation of the echo on the screen is advised. Ship has maintained course and speed-in practice the second velocity triangle (W'O'A') need not be completed to verify this fact, because It can be seen at a glance that 0'A' is equal in direction and magnitude to OA and because own ship has maintained course and speed W'O' IS the same as WO; thus W'A' must be the same as WA vectorially. 3.2.1. Explain plotting geometry and relative motion concepts. Plotting has two purposes: (a) It can show us whether danger of collision exists, how close we will pass off the target (nearest approach or distance of the closest point of approach from own ship) and how much time there is left before this will take place. (b) The approximate determination of the course and the speed of the other vessel from previous observations, so that sensible avoiding action can be taken when needed. The second purpose is connected with one of the limitations of cm radar which does not show up the aspect or leading edge of an isolated' small (in relation to the horizontal beam-width) target except at very close range. Plotting does not reveal to us the shape of a target and hence not the present heading. It will inform us, however, about the motion of the target during the plotting interval. 3.2.1.1 Reporting and Recognition of Collision Hazards To provide the Master with information about collision hazards, about the possibility of planning avoiding action and about the taking of avoiding action, a good method to be adopted by the radar observer is to report according to a standard pattern. Such a report would consist of two main parts. 1. (a) Last bearing, drawing forward or aft (passing ahead or astern respectively); (b) Last range, decreasing or increasing; (c) Nearest approach (distance of closest point of approach from own ship) as forecast (CPA); (d) Time interval to the nearest approach (closest point of approach) from the last observation (TCPA). 2. (a) True course or relative course or aspect of target; (b) Speed of target. 83 First consider part 1. Whenever an echo is observed on the screen, the Master, especially during reduced visibility, is naturally anxious to know whether there is an appreciable change in the bearing and if the range is increasing or decreasing. If little change in the bearing is observed, and the range is decreasing, at once the question arises how far off will the target pass if both ships maintain their course and speed and how much time is there left before this will occur. Even if there is an appreciable change in the bearing, the Master is likely to want to learn whether the target is passing ahead or astern. If it is apparent that avoiding action has to be taken, and then part 2 must be completed, In that case the Master must know the approximate motion and speed of the target. This is the same as in the visual case where one can only plan avoiding action properly if the other ship's course and speed can be estimated. Alterations of course Without establishing the target’s direction and speed are irresponsible actions. The aspect is defined as the relative bearing of own vessel taken from the target. A starboard or port bearing is indicated as Green or Red respectively. For example an aspect of Red 90° means that the target sport side is observed to be beam-on to own ship; a target head-on has zero aspect, stem-on 180° aspect. Strictly speaking, as we have seen, the aspect cannot be deduced from a plot, but we will assume that the most probable aspect can he deducted from the motion of the target during the plotting Interval. As seen from Fig. 3.2 the direction of movement of the target can be expressed in terms of either her aspect, or relative course or true course and it is entirely up to the Master which he prefers. Often a glance at the plot will suffice. Aspect appeals to a lot of sailors because it is so close1y related to the visual conception when they sight a ship. They automatical1y estimate the angle between her bearing and her course. It also gives us insight as to whether the target may see us on her starboard or port side or whether own ship is in the overtaking position. Such considerations may help us to form an idea about the possible reactions of the target and there are of special importance when sailing through fogbanks and the Steering and Sailing Rules 11 to 18 must be applied when the fog lifts suddenly. The report has to be enlarged if avoiding action is going to be taken. Here we must assume initially that the other ship maintains her course and speed. We cannot predict her actions with certainty. After the alteration of course or reduction In speed, the observer must watch the plot closely and re-estimate the nearest approach and the time when this will take place. If the nearest approach remains dangerous, then the best practice is generally to either reduce speed substantially or to stop own ship or to alter course to put the target right astern. The other ship in such a case, probably, has taken avoiding action at about the same time which has cancelled own ship's action. If however the avoiding action is successful and the nearest estimated approach is safe, then the radar observer can continue his report by informing the Master when the original course or speed can be resumed with safety. Two remarks must be made about the nearest approach: (a) In clear weather, when at close quarters, one can almost immediately sec what the other ship is doing and one can act accordingly, if danger of collision is involved. Radar, on the other hand, is not suited for close quarter situations in this connection. Because cm radar does not possess enough discrimination, it is very slow and sometimes unable to tell us what the other vessel is doing. The target if she has radar on board, is placed in the same predicament and there is insufficient appreciation on the 84 part of each vessel of the other's movements. If the target has no radar on board or cannot use the radar, then, in thick fog, she will be completely unaware of our position and movements. Hence in fog, when there is a great loss of information, even when both vessels are using their radar, a wide margin of safety has to be introduced. Figure 3.2 - Target's direction, target's head and aspect (b) There are many factors which easily cause errors in plotting and the nearest approach as obtained from the plot may differ considerably from the actual value. Therefore taking these two considerations into account, the new nearest approach selected should not be too small. Give the target a wide berth. The situation is not the same as when in clear weather. It is good practice generally, in the open sea, for the Master to base his plan for taking avoiding action on a bold alteration of course and / or speed initially so that the other ship, if she is using radar, will be able to detect own ship's action as quickly as possible. After taking avoiding action careful plotting should be continued to see if the other 85 vessel is keeping her course and speed. If she docs, then a prediction should be made from the plot when it will be safe for own vessel to resume her original course and /or speed. The following three factors should then be taken into consideration: (i) The closest distance which one considers it safe to pass the other vessel under the existing circumstances (three miles is generally accepted as a safe minimum distance for average types of merchant ships in the open sea). (ii) The time factor. In case the other vessel is using radar she should be allowed to have sufficient time to detect own ship's alteration in course and/or speed (a minimum of about twelve minutes is generally considered necessary). (iii) If own ship took action by altering course to bring the echo across from the starboard to the port side or vice versa, then, when the original course is resumed, care should be taken to avoid, if at all possible, bringing the echo back to the opposite side again (if this was done a misunderstanding of the situation might arise if both vessels suddenly came into sight of one another). The report can be computed either by plotting on a sheet of paper or directly on a screen covering the PPI, or by mechanical or electronic plotting devices, some of which will be discussed further. There are two main types of plots: (a) Relative Motion Plot. (b) True Motion Plot. 3.2.1.2 Relative Motion Plot The motion of the echo is plotted relative to own ship, which is considered as a fixed reference point. In other words, the motion is plotted as it appears on a Relative Motion Radar Display. The centre-point of the plot represents the electronic centre of the radar screen, i.e. own ship. The heading marker, representing the fore and aft-line of own vessel, is drawn on the plot and indicates the direction of own course. All the plots shown in the following diagrams, are referred to as Compass Datum Relative Plots. This means that the compass bearing scale is fixed. When course is altered, the heading marker swings round in the same way as it does on a stabilized display, and the movement of the echo is not broken up as it would be on a Head-up Relative Motion Display. The relative bearing scale is not used, though one can, if one wishes, lay-off relative bearings from the heading marker wherever this is positioned. In nearly all the diagrams it is assumed that North is "up". One may, of course, if this is preferred, always start off with the heading marker upwards, provided one turns the heading marker to the new direction when course is altered. By turning the plot bodily around one could then bring the new heading marker to the upward position again. After putting in the heading marker, the different bearings (relative to the heading marker or true) and ranges are plotted from the centre-point according to a suitable scale which should not be too small (about one inch to represent one mile is suitable). A time interval can be chosen which is related to own ship's speed, for example five minutes for a ship of twelve knots, so that the distance moved by own ship during that time is one mile. 86 Or one can take intervals of six minutes during which the ship covers distance of one tenth of the speed. Anyhow, this can be best left to the observer as it depends on the rate of approach of the other vessel. If the rate of approach is fast, a three-minute interval between successive observations is advisable, but the plot should not be completed before at least three observations of the target have been made. If the log is in operation, it should be read when observations are taken as this will indicate the distance own ship has travelled through the water. This is of special importance after reduction of speed when one is not quite sure about the average speed of own ship, and we will see later that inaccuracies in the distance covered by own ship during the plotting interval will introduce errors in the estimated course and speed of the target. Figure 3.3 - Relative motion line and predicted nearest approach In figure 3.3 three bearings and ranges of each of two echoes have been taken which are laid off from the centre. The first bearings and ranges were at 0000 hrs., the last ones at 0012 hrs. The plotted point which is laid off first (0000) of each echo is called 0 (for "Origin''), that one which is laid-off last (0012) is called A. OA represents the movement of the echo in 12 minutes as seen on the screen of a relative motion display. The nearest distance of approach is the length of the perpendicular dropped from the centre on to OA produced. The unit of time is OA, representing 12 minutes. The arrival times of the estimated closest points of approach of the two targets, whose echoes are depicted on the plot, w ill take place at 0032 hrs. and at 0046 hrs. (foot of perpendicular from centre) if all ships concerned maintain their courses and speeds. Items 1 (a). (b). (c) and (d) of the report are now known. OA is the relative motion of the echo and its length and direction are determined by the course and speed of own ship and the course and speed of the target vessel. Therefore, one may expect that if both vessels maintain their course and speed, and bearings and ranges are correct, the 87 three points will lie on a straight line with the 0006 point halfway between the 0000 and 0012 points. In practice, however, one may find that the respective points are staggered even when the target follows a steady course and speed. This is so because the bearing accuracy, especially of the unstabilized display is not very high. Also, own ship may yaw and this will affect the relative motion. If the plotted points are situated nearly on a straight line, and if the distances between them are roughly proportional to the time intervals between the respective observations, one may draw a mean line through the plotted points and assume that the other vessel has kept her course and speed. We thus see that the information derived from the relative motion line is the nearest approach and the time it takes to the closest point of approach. It can also tell us if the other vessel maintained her course and speed. The relative motion plot is compact; the image of own ship is fixed and generally echoes are only plotted of targets whose ranges are decreasing. 3.2.1.3 Determination of Course and Speed of Target The determination of the course and the speed of the target is sometimes called Completing the Plot. See figure 3.4. If the target has been stationary (lightvessel or buoy), then at 0012 hrs. its plotted echo would have reached W (Zero Speed point) where WO is parallel to the heading marker and its length corresponds to the distance Figure 3.4 - Determination of course and speed of target Determination of Course and Speed of Target The determination of the course and the speed of the target is sometimes called Completing the Plot. If the target has been stationary (lightvessel or buoy), then at 0012 hrs. its plotted echo would have reached W (Zero Speed point) where WO is parallel to the heading marker and its length 88 corresponds to the distance travelled by own ship in 12 minutes, Le. a distance of 12/60 X own speed. However, at the end of the plotting interval, the echo is not at W, but at A (0012 hrs.). This can only mean that WA must represent the true motion of the target during these 12 minutes. Measure WA, multiply by 60/12 and the speed of the target is obtained (in practice, one compares the length of WA to the length of WO and estimates the speed relative to own ship's speed). Measure angle OWA and the relative course of the target is known. The aspect can also be read off. In the diagram, the aspect is roughly Red Items 2 (a) and 2 (b) of the report are now established. If the WO component is plotted during the time interval, then nearest approach, course and speed of the target are obtained simultaneously. An advantage of practicing the complete relative motion plot is that it enables an officer to acquire a better understanding of the relative motion display, it helps him in interpreting the true meaning of the motion and changes in motion of echoes on the display. This can be useful on those occasions when the lack of time precludes plotting or, when there are many echoes, it helps the observer to be selective in, his plotting. When plotting on a reflection plotter, the direction and length of WO can be obtained by means of the parallel index, first aligning it parallel to the heading marker for diction and then swinging it through 90° for marking the distance. 3.2.1.4 Target alters Course or (and) Speed If the plotted points are lying on a straight or nearly straight line, but the distances between them are not proportional to the time intervals between the observations, then the other ship has altered course or altered speed or has done both (Ship Q, figure 3.5). Figure 3.5 - Effect on relative motion line when the target alters course and/or speed 89 The same is true when the plotted points do not lie on a straight or nearly straight line (ship P and ship R, Fig. 3.5). In all these cases it is accepted that own ship has maintained course and speed. Irregularities in the OA line tell us that maneuvering action has been taken by the target, but it does not inform us of what type this is. This can only be ascertained by making a new plot when the new OA line is well established. One can never know the exact time when the target took maneuvering action. Hence close observation of the echo on the screen is advised. Ship S in fig. 3.5 has maintained course and speed-in practice the second velocity triangle (W'O'A') need not be completed to verify this fact, because It can be seen at a glance that 0'A' is equal in direction and magnitude to OA and because own ship has maintained course and speed W'O' IS the same as WO; thus W'A' must be the same as WA vectorially. 3.2.1.5 Determination of Set and Rate of Currents and Tidal Streams If it is desired to find the set and rate of a current the echoes of a stationary target, for example, a Iightvessel, should 'be plotted. The diagram, (Fig. 3.6) shows an apparent motion of the Iightvessel on the starboard bow from W to W'. This cannot be the case. As the Iightvessel cannot move towards own ship, own ship must have moved towards the Iightvessel, and the set is represented by the direction from W' to W. The drift and the rate can be deduced from the length of W'W. Also illustrated in Fig. 13.5 is the construction to determine the course and speed made good over the ground of another vessel at 00 18 hrs. Instead of using WO for one side of the triangle, W'O (parallel to the track made good) is employed. Figure 3.6 - Determination of set and rate of current It should be noted that this construction is seldom used at sea, where for collision avoidance, the observer is interested in the other ship's heading with respect to own 90 ship's head. But it is worthwhile to study the construction in order to grasp the "Ground Stabilized True Motion Display" N.B. The nearest approach which can be deduced from the direction of OA is not altered by the presence of current or a steady wind. 3.3. Interpret the motion in all display modes. 3.3.1 Relative Motion Plot The motion of the echo is plotted relative to own ship, which is considered as a fixed reference point. In other words, the motion is plotted as it appears on a Relative Motion Radar Display. The centre-point of the plot represents the electronic centre of the radar screen, i.e. own ship. The heading marker, representing the fore and aft-line of own vessel, is drawn on the plot and indicates the direction of own course. All the plots shown in the following diagrams, are referred to as Compass Datum Relative Plots. This means that the compass bearing scale is fixed. When course is altered, the heading marker swings round in the same way as it does on a stabilized display, and the movement of the echo is not broken up as it would be on a Head-up Relative Motion Display. The relative bearing scale is not used, though one can, if one wishes, lay-off relative bearings from the heading marker wherever this is positioned. In nearly all the diagrams it is assumed that North is "up". One may, of course, if this is preferred, always start off with the heading marker upwards, provided one turns the heading marker to the new direction when course is altered. By turning the plot bodily around one could then bring the new heading marker to the upward position again. Figure 3.7 - Relative motion line and predicted nearest approach 91 After putting in the heading marker, the different bearings (relative to the heading marker or true) and ranges are plotted from the centre-point according to a suitable scale which should not be too small (about one inch to represent one mile is suitable). A time interval can be chosen which is related to own ship's speed, for example five minutes for a ship of twelve knots, so that the distance moved by own ship during that time is one mile. Or one can take intervals of six minutes during which the ship covers the distance of one tenth of the speed. Anyhow, this can be best left to the observer as it depends on the rate of approach of the other vessel. If the rate of approach is fast, a three-minute interval between successive observations is advisable, but the plot should not be completed before at least three observations of the target have been made. If the log is in operation, it should be read when observations are taken as this will indicate the distance own ship has travelled through the water. This is of special importance after reduction of speed when one is not quite sure about the average speed of own ship, and we will see later that inaccuracies in the distance covered by own ship during the plotting interval will introduce errors in the estimated course and speed of the target. In Fig. 3.7 three bearings and ranges of each of two echoes have been taken which are laid off from the centre. The first bearings and ranges were at 0000 hrs., the last ones at 0012 hrs. The plotted point which is laid off first (0000) of each echo is called 0 (for "Origin''), that one which is laid-off last (0012) is called A. OA represents the movement of the echo in 12 minutes as seen on the screen of a relative motion display. The nearest distance of approach is the length of the perpendicular dropped from the centre on to OA produced. The unit of time is OA, representing 12 minutes. The arrival times of the estimated closest points of approach of the two targets, whose echoes are depicted on the plot, w ill take place at 0032 hrs. and at 0046 hrs. (foot of perpendicular from centre) if all ships concerned maintain their courses and speeds. Items 1 (a). (b). (c) and (d) of the report are now known. OA is the relative motion of the echo and its length and direction are determined by the course and speed of own ship and the course and speed of the target vessel. Therefore, one may expect that if both vessels maintain their course and speed, and bearings and ranges are correct, the three points will lie on a straight line with the 0006 point halfway between the 0000 and 0012 points. In practice, however, one may find that the respective points are staggered even when the target follows a steady course and speed. This is so because the bearing accuracy, especially of the unstabilized display is not very high. Also, own ship may yaw and this will affect the relative motion. If the plotted points are situated nearly on a straight line, and if the distances between them are roughly proportional to the time intervals between the respective observations, one may draw a mean line through the plotted points and assume that the other vessel has kept her course and speed. We thus see that the information derived from the relative motion line is the nearest approach and the time it takes to the closest point of approach. It can also tell us if the other vessel maintained her course and speed. The relative motion plot is compact; the image of own ship is fixed and generally echoes are only plotted of targets whose ranges are decreasing. 92 3.3.2 True Motion A True Motion Plot is shown in Fig. 13.6. "'Own Ship' is near the bottom left-hand corner. Figure 3.8 - True motion plot Bearings and ranges are taken of the targets at 0000 hrs., 0006 hrs. (not in diagram) and 0012 hrs. The positions of own ship at these times are plotted along a line representing the course line. From these positions bearings and ranges are laid-off. A protractor can be used for laying-off the bearings, a suitable rule should be employed for the ranges; or a chart, parallel rules and dividers can be employed. Course, speed and aspect of the target can be determined directly and there is a constant check that the other vessel maintains her course and speed. As the position of own ship constantly moves along, one needs large sheet or plotting board. There are some officers who prefer the True Motion Plot to the Relative Motion Plot, because it gives them a better realization of what happens. It is advised in any case to get thoroughly acquainted with the relative motion plot and the OWA triangle at first. Once this is mastered It will not be difficult to determine the nearest approach and time from the true motion plot. This is illustrated in Fig. 3.8, where in chain lines the OWA triangle, the relative approach line and the nearest approach are shown. Note that the perpendicular, indicating the predicted nearest approach is dropped from the 0012 position of own ship, the time terminating the plotting interval; for which the triangle WOA is constructed. This type of plot is sometimes called the Complete True Motion Plot as it yields the distance and time of the estimated nearest approach besides the aspect and speed of the target. 93 Alternatively, one does not complete the True Motion Plot but uses the Table printed at the back of the book which gives the nearest approach directly from two successive ranges and bearings (provided the target maintains course and speed). It has been observed, however, that because the True Motion Plot does not provide the observer directly with the time and distance of the estimated nearest approach, selective plotting becomes more difficult and there is a tendency to try to plot too many ships at once thereby losing sight of the crucial encounters. When the True Motion Plot is carried out in the vicinity of land approaches, plotting on a large scale chart can be extremely useful. 3.4 Plan collision avoidance action using motion triangle for head up and north up. Avoiding action of own ship and its effect on the relative motion line. Suppose that own ship takes avoiding action. The questions arising are: “Will the action be effective and when is it safe to finish it?” 3.4.1 Own Ship Reduces Speed The diagram (Fig. 3.9) shows a plot carried out for two targets (one of them a lightvessel) for a 12-rninute interval First bearing and range is at 0000 hrs., last bearing and range at 0012 hrs. for each of the targets. At 1800 hrs, own ship reduces speed to one quarter of her original speed. How do we find the new relative motion assuming for the time being that the other vessel maintains her course and speed? Figure 3.9 - Effect on relative motion line when own ship changes her speed 94 A new velocity triangle (W0'A) must be constructed in which WA represents the target's present velocity. WO' represents our own ship's new velocity and 0'A represents the new relative motion. As, at this stage of plotting, the WA line (for 12 minutes) is already available on the plot, this vector is best utilized to commence the construction of the ~O'A velocity triangle. From W layoff WO' representing own, course .and distance: made good in 12 minutes; then 0'A gives the new relative motion for 12 minutes. As soon as own ship has settled down to her new speed-and this might take about ten minutes for the larger class of vessel-a new range and bearing should be taken of the other vessel’s position and transferred to the plot (the 0030 position in Fig. 3.9). A line should then be drawn through this position parallel to the 0'A line and this line from now on will be known as the Prediction line or Predicted Track. If the echo does not follow the predicted track at the predicted rate in the plot it must be taken that WA has changed and the target has carried out an alteration of course. A reduction of speed or both. and a new velocity triangle might be required. The question now is: At what time can the original speed be safely resumed, assuming the echo is following the prediction line? Once back to the original speed, the direction of the apparent motion of the target, and its rate, will be the same as before the speed reduction, provided the target maintains course and speed. The construction (assuming instantaneous new speed) is as follows: Around the centre, draw a circle of radius' representing the intended minimum distance of the nearest approach. Draw a tangent to this circle parallel to the original OA. In diagram 13.7, this tangent intersects the prediction line at the 0045 point and when the echo is at this point speed can be resumed. Keep observing the echo closely after the resumption of speed. Report if any deviation occurs from the predicted track. The time unit after 0045 is, of course, the length of the original OA (which represents 12 minutes in this example). 3.4.2 Own Ship Alters Course Suppose that own ship is going to alter course 90 degrees to starboard at 0015. The new prediction line has to be constructed assuming for the time being that the other vessel maintains her course and speed. See Fig 3.10. Again, as in the previous example, part of the original velocity triangle is used for the construction as the WA vector is already available. There is no alteration in speed of own ship and the length of the WO vector for 12 minutes will remain unchanged, although its direction will change. Therefore, circle WO round W through an angle of 90 degrees so that the new vector WO' points in the direction of the proposed course reading on the bearing scale. 0'A gives the predicted relative motion for 12 minutes after the course alteration is carried out. As soon as own ship has steadied up on her new course, a new range and bearing should be taken of the other vessel's position and transferred to the plot (the 0018 position in Fig. 3.10). A line should then be drawn through this position parallel to the 0'A line and this line represents the prediction line. If the echo does no follow the predicted track at the predicted rate on the plot, then it must be taken that the target has carried out an alteration of course, a reduction of speed or both and a new velocity triangle might have to be constructed. At what time can the original course be safely resumed? 95 After all that has been said before, the construction is obvious: Draw a circle about the centre with the radius representing the intended minimum safe nearest approach. In figure 3.10, the tangent to this circle parallel to the original OA line intersects 'he apparent motion line at the 0042 point. When the echo has followed the prediction line and has reached this point, the original course can be resumed. Keep observing the echo closely after the resumption of the original course. Maintain this until the target is abaft the beam and report till then any irregularities in the echo's track. The time unit after 0042 is, of course, the length of the original OA (representing 12 minutes in this example). Figure 3.10 - Effect on relative motion line when own ship alters course 3.4.3 Passing a given Distance off a Light vessel Fig. 3.11 shows an echo of a light vessel which is on the starboard bow. In the case of Fig. 3.11 (a), own ship is n01 affected by current, but in the case of Fig. 3.11 (b) the current sets easterly. Take Fig. 3.11 (a)first. It is the intention to alter course at 0022 (mean time of the manoeuvre) in order to pass the light vessel at two miles distance (say) on the starboard side. What alteration of course is required? Draw a circle, representing 2 miles, round the centre of the plot, then layoff a tangent to the circle from 0022. 96 At 0022 make an alteration of course equal to the difference between the original heading and the direction of the tangent towards the point marked 0022. If the current is running (Fig. 3.11 (b)) plot the light vessel as if it were a moving vessel, using the OA W triangle. Figure 3.11 - FIG. 3.4.3 Altering course to pass a given distance off a light vessel Plot the 00.22 position-altering course the same time as before-and draw the tangent. Transfer the direction of the tangent back through point A. Then circle WO round clockwise until it intersects the transferred tangent in A. The angle OWA represents the required alteration of course for 00.22. 3.4.4 Using an Unstabilized Display It can happen when plotting on a non-rotatable reflection plotter using an unstabilized Head-up display, that the observer is compelled to produce a Head-Up Datum Relative Motion Plot, which might lead to some confusion after an alteration of course by own ship. The best thing to do in this case is to start a fresh plot after the course alteration, but leaving the old plot on the plotter. Comparison with the old plot will soon reveal if the other vessel has maintained its course and speed if the other vessel has maintained her course the directions of the WAs differ by the amount of own ship's course alteration). Put the range marker on the intended minimum safe nearest approach distance and, using the Parallel Index, rotate the original OA-line through the angle of the course alteration (anti-clockwise if the alteration was to starboard). Then draw a tangent, parallel to the Index, touching the range marker. The point of intersection of this tangent line and the present echo track, represents the point where the echo should arrive before a resumption of course is contemplated-on the assumption that the other vessel maintains her course and speed. 97 3.5 Discuss situation. application of radar plotting in multi-target Assume that it is required to produce a 12-minute plot on a plotting sheet of a ship’s echo which has appeared on the radar display. Three sets of ranges and bearings are taken with an interval of six minutes. Only a Relative Motion Plot will be considered. 1. Draw a line representing the heading marker. 2. Lay-off the first position of the target, marking it "0" and indicate time. 3. Draw a line through "0", parallel to, but in opposite direction to, the heading marker. Terminate this line at "W", where WO represents the distance travelled by own ship during the plotting interval (12 minutes). Mark in the direction from W to O. 4. Plot the 6-minute and 12-minute position of the target and label the latter with “A”. Mark the time. Check that, for all practical purposes, the produced echo motion is uniform in its direction and rate. 5. Draw a line through "0" and "A", producing it well past the centre of the plot. This line represents the apparent motion of the target and its symbol is an encircled arrow. 6. Drop a perpendicular from the centre of the plot onto the OA-line produced. This yields the Closest Point of Approach (CPA). Using the OA-distance as a time unit (12 minutes) determine the time at which the target is expected to arrive at CPA (TCPA). If the range of the object is increasing it has already passed the CPA. 7. Connect W to A with a straight line. This line, from W to A, represents the true motion of the target during the plotting interval. Mark in with arrow. If the target is at anchor Wand A should coincide, provided there is no current. If there is current there will be a displacement between W and A where the line, A to W, indicates the direction and drift of the current (or tidal stream) during the plotting interval. 8. If the aspect is required, measure the angle between WA (produced) and the line connecting the two ships (line of sight), adding "Green" or "Red", when own ship is to "starboard" or to "port" of the other ship, respectively. That completes the basic plot. Suppose now that the Master decides to take avoiding action, either by an alteration of 90° to starboard or by a reduction in speed from "Full" to "Slow" (approximately 1/4 Full Speed). Execution of the manoeuvre assumed to be about 18 minutes after starting the basic plot. Alteration of Course 9. Draw in new heading marker. Turn WO, round W, through an angle of 90° . Label the new vector WO`. Reduction in Speed 9. From W, in the direction of 0, measure off WO`, where WO`= WO/4 10. Join a and A, producing the line well beyond A. The line a to A represents the relative motion for 12 minutes for the new heading or the new speed, respectively. 11. After own ship has steadied on her new heading or settled down at her new speed, take a range and bearing of the target and plot the position of the target on the plotting sheet. Mark the time. 98 12. Draw a line through this new fix parallel to the 0'A line, already plotted above. 13. Check that that the direction and rate of movement of the echo along this line is In accordance with 0'A vector. If this is not the case the target must have taken some action and it is best to start a new complete plot The “middle" part of the plot has now been completed and, on the assumption that the target has maintained its course and speed, the Master has to consider when he can order own ship to go back to the original course, or when he can put the telegraph on "Full" again 14. Around the centre of the plot, draw a circle of radius representing the minimum safe CPA distance. 15.. Draw a tangent to this circle parallel to the original OA-line. Mark the point where this tangent intersects the present relative motion track of the target. 16. When the echo has reached the above point, it will be safe to resume the original course or speed. Figure 3.12 - FIG. 3.4.4 Visual estimation of course and speed of targets 17. Having resumed the original course or speed, keep the echo under observation until the target is completely clear. If in (4) the echo motion is not uniform in direction (developing a kink) and/or in rate (unequal progress during each of the 3-minute time intervals), then one simply has to wait until the motion becomes uniform, and start the plot from then. 99 4. USE RADAR TO ENSURE SAFE NAVIGATION 4.1. Familiarization with the Radar Simulator’s Own Ship control and Characteristics 4.1.1 Introduction 4.1.1.1. Simulator Status In 1995 the Conference of Parties to the STCW Convention, having adopted amendments to the Appendix to STCW-78 Convention and an associated Code, approved the seafarers’ training and competency assessment using the simulators as standard. The Conference specified the transition period until 1 February 2002 where after all the simulators should be in compliance with new requirements. Navi-Trainer Professional 4000 (NTPro-4000) simulator manufactured by Transas Marine has Type Approval Certificate from the Sea Fleet Service of Russia’s Ministry of Transport (ROSMORFLOT) as a Vessel Manoeuvring and Vessel Control, and Bridge Team Work simulator, as well as the Certificate of Evaluation and Testing from the Marine Safety Agency (UK) as a simulator ensuring the navigators’ training and demonstration of competency in compliance with sections AII/ 1 and A-II/2 of STCW Code with 1995 amendments taken into account. 4.1.2. Simulator Capabilities of NTPro 4000 Listed below are the tasks of the navigator simulator training and competency assessment implemented in NTPro 4000 simulator. 1. Route planning and navigation in any conditions (except in ice conditions). 2. Positioning and assessment of its accuracy using any modern methods (except astronomical). 3. Determining and taking into account corrections of the principal navigational aids (log, compass and echo sounder). 4. Use of the radar and ARPA. 5. Safe watch keeping on the bridge. 6. Operating the remote control systems of the ship’s propulsion unit. 7. Manoeuvring and vessel control in any conditions. 8. Search and rescue operations. NTPro 4000 is capable of re-constructing and analysing complex navigational situations including emergencies in the actual seamanship. 4.1.3. CONNING DISPLAY STRUCTURE Necessary information on the vessel’s motion and on the process of the exercise fulfilment. The ship control facilities and navigational aids are arranged on several onscreen pages (panels). To display the required panel on the screen “press” the 100 appropriate button in the bottom part of the screen. “Pressing” of the button is understood as a single mouse click with the cursor positioned on the picture of this button. The called panel is displayed in the left bottom part of the screen. The remaining part of the display, common for all the panels, contains controls of the main engine, rudder and viewing direction, as well as some information displays (figure 4.1). Listed below are the existing pages with the indication of relevant call buttons: • Pilot Card (“Pilot Card” button); • Main Panel with the Vessel Controls (“Man. Info” button); • Autopilot (“Auto” button); • Echo Sounder (“Echo” button); • Ship’s Signals (“Signal” button); • Log and Anchors (“Log&Anchor” button); • Gyro Compass Recorder and Corrector (“Gyro” button); • Engine and Rudder Alarms (“Alarms” button); • Mooring Operation Control Panel (“Moor” button); • Navigation Aids (“Nav. Aids” button). Fig.4.1 - Conning display structure All the panels listed above can be switched to in any order with a trainee's pointing device (mouse). It is possible to adjust the illumination brightness of all the panels to suit the illumination level in the visualisation gaming area. To do this use “Light” button. “Dimmer” button allows the brightness of instrument indicators to be adjusted. These two buttons are arranged in the right hand part of panel call buttons. 4.1.4. DESCRIPTION OF THE DISPLAY'S COMMON PART Conning Display structure allows the trainee, no matter what panel, with which controls might be called, to control the ship’s main engine, rudder and thrusters, to select the viewing direction on the visualisation screens, to turn on/off the binoculars mode, to give signals and to promptly obtain necessary information on the vessel’s motion and exercise fulfilment process. The implementation of these functions is ensured by the common (for all the panels) part of the display. 4.1.4.1. Control of the Ship's Main Engine (Fig. 4.2) The engine telegraph is shown to the right hand part of the called panel. It is modified depending on the vessel type. If the ship has a single engine, the engine telegraph will have a single handle (as shown on the drawing). If the ship has two engines, the engine telegraph will have two handles. To work with one of the engines, use the mouse (tracker ball) to move the appropriate engine telegraph handle. To work with both engines (portside and starboard) simultaneously, move the mouse in the space 101 between the handles. Over the telegraph there is an engine readiness indicator (Engine Ready) which shows that it is ready for operation. For a more accurate setting of the engine telegraph handles use the buttons with arrows and digital displays arranged under the engine telegraph. The engine operation is shown by the digital displays of the propeller shaft (shafts) rate of revolutions – RPM and the propeller (propellers) pitch angle – Pitch (not for all the ships) as well as the analog Start Air gauge. Figure. 4.2 - Control of the Ship's Main Engine 4.1.4.2. Control of the Ship Steering Gear (Fig. 4.3) Arranged in the top right corner of the screen are the controls of the rudder and thrusters. Figure. 4.3 - Control of the Ship Steering Gear Over the engine telegraph there are two scales with the ship thruster control handles: for the Bow Thruster and for the Stern Thruster. The availability of one or two scales, or their absence, is determined by the number of thrusters for the given ship type. Control of the thruster is by moving the handle in the required direction (to the left or to the right) by using the mouse (drag-and-drop mode). Thruster displays show their power as the per cent of the maximum power, and the direction of the ship’s bow or thruster turn.In the top central part of the display (to the left of the thrusters panel) buttons for switching the rudder control modes are located: 102 Fig. 4.4 - thrusters panel • Auto – in this mode the autopilot is turned on (see Autopilot page), is steering the ship along that course which was set at the time of switching on; when the autopilot is operating a green light indicator is turned on to the right of the button; • FollowUp – follow up mode (helm is used); • NonFollowUp – non-follow up mode (tiller is used). The helm (in FollowUp mode) is controlled by using the scale with Rudder handle. To have the helm over to the portside or starboard, position the mouse cursor on the required scale reading in the appropriate direction and press the left mouse button. The rudder angle will not move to the set angle immediately, but with a delay required for the rudder response. Another way is to position the cursor on the handle itself and drag it to the required position by using the mouse in drag-and-drop mode. In the non-follow up mode, when “Non Follow Up” button is enabled, the ship is controlled with the tiller. The tiller will be turning the rudder to the starboard as long as “STBD” button is pressed, or to the portside while “PORT” button is pressed. When the button is released, the rudder will resume its zero position. The tiller analog gauge shows the ship’s direction and rate of turn.The tiller can be used not only in Non Follow Up mode, but also for a short time in other modes when the ship is required to be turned fast. In Follow Up mode, when the tiller button is released the rudder resumes the position set by the helm. In Auto Pilot mode the autopilot alarm is triggered off for a short time. To the left of the rudder controls there are some digital displays (from the top to the bottom): vessel’s gyro course, vessel’s magnetic course and the rate of turn. 4.1.4.3. Control of the Viewing Direction (Fig 4.5) The controls of the viewing direction on the visualisation are arranged in the top left corner of the display. They comprise: • Azimuth disk of the gyro compass card; within there is binoculars turn-on button and a graphic marker of the relative wind direction; • Buttons for moving the viewing point to the portside wing, to the centre and to the starboard wing of the navigation bridge after pressing “Advanced view” button (see below); • Buttons with arrows which can be used for changing the viewing direction on the visualisation screen “to the left”, “to the right”, “up” and “down”. The current position of the viewing sector is shown with the light green arc on the compass azimuth disk. 103 Fig 4.5 -Control of the Viewing Direction The horizontal direction of the viewing sector middle (bearing) in degrees is shown in the top window between “3“ and “4“ arrow buttons. This direction may be by the gyro or magnetic compass depending on which compass is currently turned on “Man. Info” page. The vertical direction of the viewing angle in degrees is shown in the bottom window between “6“ and “5“ arrow buttons. This direction can generally be changed by using the following procedures: • click the mouse in the required point of the azimuth disk; • drag the light green arc along the azimuth disk using the mouse in drag-anddrop mode; • click the appropriate turn direction screen button. The first two methods of changing the viewing direction are used when “Default view” button is lighted above the compass card. In order to use the third method, it is necessary to press “Advanced view” button: the compass card will be replaced with a schematic ship view with nine buttons for the selection of viewing direction (Select view point). Fig 4.6 -Select view point To change the viewing direction you can either press at once the required button on the diagram or press “Next” button for selecting, one by one, all the available viewing directions. After the final selection of the direction, press “Done” button in order to return to the compass card. A similar procedure is sued for the control of binoculars. 104 Digital displays under the gyrocompass card, on the left, show the direction (in degrees) and speed (in knots) of the current, whilst on the right the relative wind direction (in degrees) and speed (m/s) are shown. 4.1.4.4. Control of the Log and Ship Signals (Fig 4.7) To the left of the gyrocompass card there are some log displays which show the ship’s longitudinal (in the centre) and transverse velocity, on the bow and on the stern. The lighting of an arrow indicator shows the direction of the ship’s speed. In addition, there are some buttons for giving the ship’s signals on the display’s common part (to the right of the log displays). Fig 4.7 - Control of the Log and Ship Signals 4.1.4.5.Control of the Ship’s Main Engine and Rudder (Fig 4.8) The direct control of the ship’s main engine and rudder is effected from the display's common part “Man. Info” page, therefore, contains additional controls and displays for work with them. Fig 4.8 - Control of the Ship’s Main Engine and Rudder In the left hand part of the panel there are two analog gauges showing the engine’s operation: rate of revolutions (RPM) pitch angle (Pitch), whilst in the right hand part of the panel there are two rudder analog gauges: Rudder Angle, ° and the ship’s Rate Of Turn, °/min. During the vessel’s turn the pointer of ‘Rate Of Turn’ gauge may “tremble”. In this case, in order to steady the reading use ‘F’ (filter) button under it. The button has three positions, and the number of the current position is shown on the digital display to the right of the button. 105 Between the gauges, in the bottom part of the panel there are buttons of three steering gear pumps Fig 4.9: two principal pumps (Pump 1 and Pump 2) and a Standby. When the pumps are turned on, the buttons are lighted. Round light indicators over the buttons are turned on when the respective pump is under operation. In the process of the exercise the trainee can disable one of the pumps, the rudder’s rate of turn will then decrease. The ship’s main engine controls include the analog gauge of the engine start air – Start Air, p/i; ‘Emergency Run’ and ‘Emergency Stop’ which are used in case of engine faults set by the instructor during the exercise, as well as ‘Manoeuvre Program’ and “Combinator” . Fig 4.9 - steering gear pumps NAVI-SAILOR “Emergency Run” button disables the automatic engine safety system canceling Slow Down mode (drop of the engine load) when the engine operation parameters grow beyond the admissible limits. A lengthy engine operation with “Emergency Run” button depressed may cause its break down (Shut Down). Emergency Stop” button allows the engine to be stopped instantaneously by cutting off the fuel supply to the engine cylinders. “Manoeuvre Program” button is used for switching the engine to the mode most favourable during frequent manoeuvring and mooring operations. “Combinator” button is used exclusively on the vessels whose remote control systems feature this function. The trainee can obtain information on what has happened to the engine by referring to “Alarms” page. 4.1.4.6. PILOT CARD PANEL (Fig 4.10) Pilot Card information is provided on three pages: 1. Principal Vessel Specifications: Fig 4.10 - Pilot card panel 106 – Name, class, displacement, principal dimensions; – Steering particulars; – Propulsion particulars; – Information on the anchors; – Information on the thrusters if any (Thruster effect); – Draught increase in the shallow water; – Engine telegraph table. 2. Information on the Vessel's Turning Circles: Figure 4.11 – Pilot card – Turning circle diagrams – Turning Circles on the deep water at the rudder angles of 15 and 35 degrees; – Table of times for these turning circles; – Turning circle on the shallow water at the rudder angle of 35 degrees and H/T ratio = 1.2; – Collision avoidance manoeuvres (Emergency manoeuvres): Helm amidships, engine telegraph handle at full astern; Helm hard a-portside (a-starboard), engine telegraph handle at full astern; Helm hard a-portside (a-starboard), engine telegraph handle in the current position. – In the left top corner of the page there is a ComboBox for altering the starting speed of all the manoeuvres (Start speed, kn.). 3. Information on the Stopping Characteristics: – Under each track the initial and final positions of the engine telegraph handle are shown. The vertical axis of each diagram shows the distance in cables covered by the vessel. To the left of the diagram, the speed in the reference points is indicated, to the right – time in seconds. An arrow above the diagram shows the direction of deviation from the course and its magnitude in degrees; – For the passive stopping the speed in the last point is 2 knots; – For the active stopping the speed in the last point in 0 knots; 107 – For the acceleration the speed in the last point is FAH speed. 4. Information on the Magnetic Deviation. 4.1.4.7. Autopilot control panel Figure 4.12 - Autopilot Autopilot Control Panel presents an image of an actual digital ANSCHUTZ NAUTOPILOT D panel. Its control is, therefore, identical to that of an actual device. It should be noted that in order to operate Autopilot’s knob it is necessary to capture it with the pointing device and drag in the required direction checking with the gauge readouts. 4.1.4.7.1. Purpose and Construction NAUTOPILOT D permits the following modes of operation: • course control with gyro compass; • course control with magnetic compass; • course change manoeuvres at a given rate of turn. Since the output of the autopilot contains a follow – up steering control system for the implementation of steering gear control, it permits the configuration of a complete main steering control in conjunction with • a handwheel (for follow-up control mode); • a tiller (for non-follow – up control mode); • a steering mode selector switch. The following controls are provided: • a membrane keyboard for the selection of operating modes and functions; • a large control knob for setting the set course and rudder limitation, etc. The following indicators are provided: • a large 4-digit LED display for the actual course; 108 • a smaller 4 – digit LED display for the set course; • a LED strip for the indication of course and track errors; • several luminous panels for the indication of alarms and warnings. Alarms and warnings are issued as audible signals and also by flashing of the corresponding luminous panels. 4.1.4.8. PURPOSE AND PRINCIPAL OPERATION MODES 4.1.4.8. 1. NS Purpose and Capabilities Navi-Sailor series videoplotter is an electronic information and chart system used with the aim of ensuring safe navigation. The following functional capabilities are implemented in this software: • display of electronic vector and raster charts of different formats (up to 6 charts simultaneously); • data exchange with navigational sensors and external output devices enabling the vessel position coordinates to be continuously obtained and vessel controlled in accordance with changing navigational situation; • route planning and drawing up the schedule of proceeding along this route; • monitoring of approach to the dangers to navigation plotted on an electronic vector chart or on a user chart created by the navigator; • trial manoeuvre for avoiding collision with other vessels displayed on the NS screen in accordance with the information received from the ARPA or from a UAIS transponder; • solution of various kinds of navigational tasks; • other capabilities described in this Manual. 4.1.4.8.2.ABBREVIATIONS USED CMG – Course Made Good; COG – Course Over Ground; CPP – Controllable Pitch Propeller; ECDIS – Electronic Chart Display and Information System; ENC – Electronic Navigational Chart; ERBL – Electronic Range and Bearing Line; ERML – Expected Relative Motion Line; ETA – Estimated Time of Arrival; ETD – Estimated Time of Departure; ETML – Expected True Motion Line; FPP – Fixed Pitch Propeller; GMT – Greenwich Mean Time; HO – Hydrographic Office; GPS – Global Positioning System; HDG – Heading; 109 HO – Hydrographic Office; ME – Main Engine; NS – Navi-Sailor; OS – Operating System; PS – Positioning System; RAM – Random Access Memory; RML – Relative Motion Line; SENC – System Electronic Navigational Chart; SOG – Speed Over Ground; STG – Speed To Go; TML – True Motion Line; WP – Waypoint; XTE – Cross Track Error. 4.1.4.8.3.Voyage Monitoring Mode The Voyage Monitoring mode is a compulsory permanent mode which is run concurrently with other operation modes and ensures the following: • continuous vessel tracking; • automatic recording of the ownship's primary (principal) and secondary (auxiliary or reference) tracks; • recording of ARPA targets’ tracks, as well as of tracks of targets received from a UAIS transponder; • keeping of the electronic ship’s log; • obtaining information on the status of connected units; • graphically expressed summary assessment of the accuracy of vessel positioning and plotting of objects on the chart; • obtaining data on the vessel's position relative to the route; • obtaining calculated vector of current in the vessel's position and summary drift vector between COG/SOG – HDG/LOG; • display of the current electronic chart scale; • obtaining data from the auxiliary navigational sensors (depth, drift speed and direction, weather condition parameters). In addition, this mode ensures permanent monitoring of the vessel's position relative to the objects listed below. In case of a dangerous approach to one of such objects the NS gives off an appropriate alarm • safety contours; • isolated dangers (see below) with depths less than the set one; • special areas, available in the chart and/or updating database and located within a certain range from the vessel (up to 10 miles). The following vector chart objects are automatically assessed by the NS as ISOLATED DANGERS: 110 • explosives; • fish haven; • foul ground; • distinctive depth with less than the set value, submerged obstruction; • obstruction, which covers and uncovers; • oil/gas production platform; • rock; • shoal; • well; • shipwreck; • shipwreck showing any portion of hull at the level of chart datum. Besides, objects plotted on the user chart are also considered to be dangerous: • depths less than the set one • symbols of dangers to navigation 4.1.4.8.4.Navigation Mode Navigation Mode is the principal mode of the NS operation, it implies a constant display of the ownship’s position on the screen and is running concurrently with the “Voyage Monitoring Mode”. In this mode the NS provides the navigator with the following data: • ownship's position (vessel’s symbols and motion vector) and ownship’s tracks (from the primary and secondary positioning); • electronic chart with layers of automatic and manual correction, and special user information; • secondary radar information (ARPA tracked targets) in the graphic form with the relevant entries in the table; • targets acquired by the UAIS transponder; • results of the ownship trial manoeuvre (with course and/or speed) in the graphic form taking into account the vessel's dynamic characteristics and summary drift, estimated position of the own ship and targets for any moment of time (up to 24 minutes), parameters of dangerous approach to the targets; • route planning on the chart; • display of sector lights in the colour visible from the vessels’ position with the light visibility range taken into account (if the light cannot be seen the lighthouse is shown in the grey colour). It should be noted that when some of the NS functions are activated, the Navigation Mode is automatically exited from. When operating as the “Master”, the station transmits the following data via the network: • time; • ship’s position and positioning method; 111 • course and speed; • wind direction and speed; • water temperature; • drift direction and speed; • depth; • time zone; • targets; • ownship parameters; • route loaded for the Voyage Monitoring mode. 4.1.4.8.5. NS SCREEN The NS screen includes three areas (see Fig. 4.13). The left hand part is the Electronic Chart Area. The right hand part is taken up by Information Area and Menu Area. Sometimes when it is necessary to display some additional information command, e.g., route plan waypoints, ship's log entries, Help text, etc. (as required by operator), a window opens up in the left bottom part of the screen. At this time the chart is displayed in the top part of the screen. Fig. 4.13 - Screen Areas of Navi-Sailor 4.1.4.8.6 .Electronic Chart Area This area may display the following information: • electronic charts (up to 6) covering the screen; 112 • outlines and ID numbers of all the charts available in the collection (for the displayed charts the frames are shown in a bold line and numbers are in the bold type); • numbered reference grid; • additional information (such as temporary updating); • route plan with numbered waypoints; • ownship symbol – its primary position and secondary position (if any) with motion vectors: – over the ground (with two arrows) – COG; – through the water (with a single arrow) – HDG. • display of the vessel's primary and secondary tracks; • ARPA cursor and ERBL; • positions and motion vectors of targets acquired by ARPA; • positions and motion vectors of targets acquired by the UAIS transponder system; • results of the trial manoeuvre; • current vectors. The bottom right corner of the electronic chart displays an angle shaped indicator, the sides of an angle formed by a thin line and a bold line. Bold line is a graphic presentation of the maximum possible error of plotting objects on Transas chart. When a chart is displayed on the scale of the original, the linear size of the error is taken to be 2 mm. As the scale is growing, the linear dimensions of the indicator increase showing to which extent the chart information can be relied on. Thin line is a graphic presentation of a possible error of the vessel's position sensor, which is taken to be equal to 70 m. In the absence of charts suitable for loading, or with “Chart Autoload” OFF, No Data area may appear on the NS screen: a grey coloured field with no information whatsoever. Such area will be automatically covered with a chart during the screen regeneration if “Chart Autoload” is turned ON. 4.1.4.8 NS Information Area The data displayed continuously in the Information area includes the following: • indicator of the positioning methods (primary on the left and secondary on the right). The following designations are possible: GPS – GPS; DECCA – DECCA LRNC – LORAN – C GLNS – GLONASS1; GPGL – GPS and GLONASS1; ER – positioning by ARPA reference target; DR – dead reckoning; No indicator – secondary positioning is switched off. • indication that ARPA is connected and can have the following values: 113 – ARPA – ARPA is connected to the NS; – Blank space – ARPA is disconnected from the NS; • status of the workstation operating in the network (displayed in the absence of alarm messages); Figure 4.14 – ECDIS - Navigation data Transas Marine trademark (displayed in the absence of “POSITION DROPPED” indicator meaning that the vessel symbol is not shown on the chart); • current time and date: – UTC – Greenwich time and date; – LOCAL- ship’s time and date. • loaded electronic chart number; • number or name of the User Charts loaded in areas A and B; • vessel position coordinates and an offset indicator (if taken into account). The indicator is an orange coloured triangle displayed to the left of coordinates which shows that the position coordinates received from the currently used positioning system for the primary vessel position, are corrected with the offset; • vessel’s course and speed obtained from the positioning system – COG and SOG with symbols: – “p” (Positioning system) – course and speed are received from the positioning system); – “r” (echo Reference) – data is calculated from the radar target acquired by ARPA. In the Navi-Fisher and Navi-Sailor 2500 software, in the “Dead Reckoning (DR)” positioning mode, course and speed values are displayed as CMG and SMG (Course Made Good, Speed Made Good) in the NS Information Area. • gyro readings – HDG with symbols: – “m” (Manually entered) – the course is entered manually (when DR is used for the vessel positioning); – “t” (True) – true course; – “u” (Undefined) – the source which the course is obtained from is not defined. 114 In the Navi-Fisher and Navi-Sailor software, magnetic course is displayed in HDG/M line of the NS Information Area (“M” postfix in HDG line means that the course is magnetic) provided the appropriate sensor (magnetic compass) has been connected to the NS via CONFIG\Attach sensors\Compass function. Figure 4.15 – ECDIS – drifting elements Log readings – LOG with symbols: – “b” (Bottom tracking log) – value of speed over the ground; – “m” (Manually entered) – speed value is entered manually (when DR is used for vessel positioning); – “w” (Water referenced) – value of speed through the water; – “r” (Radar tracking or fixed target) – value obtained from ARPA information or relative to the ARPA acquired target; – “p” (Positioning system ground reference) – value obtained from the satellite system information; – “u” (Undefined) – the source which the speed is obtained from is not defined. Negative speed values are shown with “-” sign. There are 4 types of display in the NS Information Area: 1. Display System Mode presents general data on the sailing conditions: – set (direction the vessel is drifting in); – drift (speed the vessel is drifting at); – depth from the sounder; – calculated safety contours or their value range (e.g., from 10 to 20 m). The display of one value only – the calculated safety contour – out of the entire range (e.g., 10-) means a three-digit value of another depth contour within the range (e.g. 600 m); – calculated tidal rise in the closest reference point as of the current time; – calculated direction and speed of current in the vessel’s position as of the current time; – range and bearing to an acquired fixed object. Figure 4.16 – ECDIS – drifting elements 115 Display Route Mode presents data on the planned route: – route name; – name and number of the next (or selected) WP; – vessel's course (or the initial course value – “GC” – in sailing along the Great Circle); – cross track error with the indication of direction; – bearing to the next waypoint (or the “RL-GC” difference between the Rhumb Line and Great Circle in sailing along the Great Circle); – distance to the WP; – time to go to the next waypoint; – estimated date and time of arrival in the next waypoint; – course to steer to the next waypoint. 3. Display Pilot Mode presents data on the vessel's position relative to the current waypoint, calculated speed and time of arrival in the waypoint with the set ETA: – bearing to the current WP; – course to the next WP; number of the WP for which ETA is set in the loaded schedule; Figure 4.17 – ECDIS – monitoring elements – calculated speed to go for arrival in the WP for which ETA is set in the loaded voyage schedule. With this type of display, the following indications may appear with this parameter: STG without any marks (asterisk) corresponds to the option when the ETA is set in the voyage schedule, whilst the speeds on the route segments are calculated; STG with an asterisk means that the speed has been pre-set on one or several route segments; STG with two asterisks indicates that the speed has been pre-set in the voyage schedule for the current route segment. STG parameter value shows the set speed to go. – ETA (estimated time of arrival) in the set WP with the current speed (SOG) remaining unchanged; – time of arrival (TA) in the set WP according to the voyage schedule. 4.1.4.8.9 Navigational symbols used in NAVI-SAILOR system IEC 61174 standard 116 Symbol description Navi-Sailor Own ship Cursor A short thick line is used for showing the direction to the ship bow. This notation is sued in order to avoid confusion in the ship orientation in case of display of a negative log speed received by the NS. Own ship track with an hourly mark (a dot stands for 10 seconds mark, a small cross stands for minute mark, a point in a circle stands for hourly mark) ERBL Cursor 117 Own ship's course over ground (COG) vector with minute marks Own ship's heading vector (HDG) with minute marks Way point (WP) with an identification number (ID) Track line and time mark “Official” chart highlighting update 4.2. Set Up and Operate the Radar Display in all Modes Relative Motion Display, Unstabilized, Head-Up 4.2.1. Introduction If there is no gyro-compass on board, or the radar display cannot be connected to a gyro repeater or magnetic transmitting compass, the presentation of the display should be "Head-Up". During a course alteration, for example to port, the heading marker remains at zero, i.e. upwards, while the picture turns round inside the tube in a clockwise direction. This will cause considerable blurring due to the retainment of the afterglow. While the alteration takes place, and for some moments afterwards, little use can be made of the picture on the screen. 118 This is a great disadvantage for blind pilotage during fog and in narrow waters, rivers and channels where frequent alterations of course take place. See Fig. 4.18. When using a conventional radar display turn the gain down during a course alteration. This will prevent blurring. Bearings are relative and when course is altered, the echo swings over the screen and the track of the echo is broken up. This makes it impossible to assess the relative motion of a ship's target during and shortly after an alteration. It also causes false "tadpole tails" when own ship is yawing. 4.2.2. Relative Motion Display, Stabilized Outer Azimuth (Bearing) Ring, Course-Up, UnstabiIized In this case the outer bearing ring is stabilized by the gyro-compass transmitter, but the picture itself is not stabilized so that blurring of echoes takes place when the ship's course is altered. The true course is read by the beading marker and true bearings can be read on the outer bearing scale, irrespective of any yawing of own ship. See Fig. 4.18. There is also an inner stationary bearing scale (not shown) which has the zero point opposite the heading marker so that relative bearings can be read. FIG. 4.18 - Relative motion display, stabilized outer azimuth ring, course-up, unstabilized 4.2.3. Relative Motion Display, Stabilized, North-Up The display is connected to a gyro compass or magnetic compass repeater system. The orientation is "North-Up" and the heading marker indicates the true course on the edge scale. Comparison with the chart is made easier. During a course alteration, the heading marker swings round to the new course indication-acting like a gyro repeater-but the picture stays aligned in the same position and remains with a North-up presentation. No blurring takes place due to a course alteration. The presentation is as clear as when proceeding on a steady course. See Fig. 4.19a. Bearings are true and there is no discontinuity in the track of a ship's echo during an alteration. Observations can be continuous. This presentation should generally be used for Navigation. For anti-collision purposes other presentations may be preferred 119 It is true that this type of presentation is a bit difficult to get used to if one has already become accustomed to a head-up presentation. However, this presentation has more desirable advantages than the head-up display. One may say that the navigator does not need to turn a chart upside down if a southerly course is laid-off in order to see whether land will show up on starboard or port of the track, but this comparison is not correct. On a chart, the ship's position progresses along, but in this presentation the position of own ship is stationary while echoes move in the opposite direction to the course and automatically the navigator relates this to the visual view presented from the bridge where starboard is on the right hand and port on the left hand side. However, clear weather practice will help and experience will soon teach the observer to get used to it. FIG. 4.19a - Unstabilized Relative Motion Display - Echo rotates after a ship changes course FIG. 4.19b - Stabilized Relative Motion Display - Heading marker rotates when ship alters course. No blurring 120 If a True Motion or Track Indicator Unit is provided, then the Relative Motion Stabilized Display must still be used for landfall navigation, as true motion is only introduced for the medium and shorter range scales where the tail of a moving echo can be observed. Figure 4.19b shows a comparison between the unstabilized and the Stabilized Relative Motion displays. 4.2.4. Course-Up Stabilized Display This display is really a Relative Motion Head-up Stabilized Display with the heading marker showing the true course: instead of the zero mark of the bearing scale. This is possible with stabilized bearing scales (4.2.2) and with electronically generated bearing scales as is done with rasterscans. Yawing will make the heading marker move and after a course alteration the heading marker has to be reset to the "Up" position. This can be done by manual means, but is often achieved automatically by means of a push button. Owing to the nature of the tube there is no blur after re-alignment for raster-scan displays. Sometimes a North- reference electronic marker is shown near the periphery of the display. Pilots like the display because the visual view can be compared with the radar picture. This type of display is now an IMO requirement for ARPAs. True Motion Course-Up Displays are not used much. 4.2.5. Relative Motion Display, Unstabilized, Head-Up, Off-Centred In some types of radar sets, especially designed for river navigation, the centre can be off-set. A greater forward range is obtained, while using the same range scale. Bearing discrimination is better in some areas of the screen than the same areas on the next scale up. Echoes near the edge of the screen possess a greater relative motion than the same echoes displayed on the next range scale up. The ordinary bearings marker scale cannot be used for observation of bearings unless a Parallel Index is provided, (an electronic bearing marker would give more accurate bearing observations). The view abaft the beam is limited. 4.2.6. Relative Motion Display, Stabilized, North-Up, Off-Centered This display can be presented when True Motion is provided. By means of the Zero Speed Switch the displacement of the electronic centre can be halted. The presentation can be useful for applying Parallel Index Techniques and for a quick estimation of the nearest approach for a ship at a range greater than that represented by the radius of the PPI at the range scale in use, because the rate of relative motion is not so clearly indicated on a longer range scale. It provides an excellent display for coastal navigation. 4.2.7. True Motion or Track Indication (Chart) Display The principle of True Motion has already been discussed. Moving targets and "own ship", if moving, are shown as moving spots on the face of the PPI, while stationary targets (no current is assumed) yield echoes which have no afterglow trail. The display is stabilized. Fewer difficulties are encountered here in the interpretation of the picturein contrast with Relative Motion Stabilized Display, North-Up-because it is similar to Chart Presentation. See Fig. 4.20. 4.2.7.1. Sea-stabilized 121 Correct heading and speed through water are fed in, but no tidal information. It is an excellent display for coastal1Uwigation, pilotage and anti·collision. Echo trails of ships' targets and trail of "own ship" give an indication of the heading of the vessels, irrespective of whether there is current or not. FIG. 4.20 - True motion north~up display 4.2.7.2.Ground-stabilized Correct heading and log speed of own ship plus direction and rate of tide are fed in. When Set and Rate Controls are provided, then observe the movement, if any, of the echo of a known land-fixed object such as a light vessel with the Rate Control on zero and the correct course and speed of own ship through the water fed into the radar display. If there is a current of any consequence the echo will now appear to move in a direction opposite to that of the set of the current so the Set Control can be adjusted accordingly; for example, if the echo appears to move S.W. then the Set Control is adjusted to read N.E. The Rate Control is then turned slowly from zero until the echo ceases to move over the display. If the radar set is the type which has a "Course Made Good Correction" Control, then the quickest method of allowing for the current is probably to construct a velocity triangle (velocity of own ship through water, velocity of the current and velocity over the ground) on the chart. This triangle should be drawn anyway, even if one was not using radar! The length of the resultant velocity vector will give an indication how the speed input should be readjusted. The direction of the resultant velocity vector will inform the operator if the "Course Made Good Correction" should be applied to starboard or to port of the steered course as shown on the gyro repeater. With the "Course Made Good Correction" control on zero, an increased (decreased) speed input would allow for a following (opposite) tidal current. This is useful when proceeding along a twisting river where the direction of the current changes continually with the direction of the river. If the current is not known initially it can-be found in a short time by observing and plotting the echo of a land-fixed object (see. Fig. 4.21) If there is no land-fixed object available, then a sea-stabilized display is generally the best presentation to use. 122 If a ground-referenced two-axes Doppler Log is available, then its output can be fed directly to the display unit. Or the Echo Reference Control can be used so that the computer will keep the display in the Ground-Stabilized mode. Note: Sometimes the expression True Motion Fixed Centre Display is used by manufacturers. It is, in fact, a relative motion display with true motion vectors (ARPAs) or a true motion display whereby the centre is reset every scan so that the echo trails show true motion, but the echoes themselves move bodily along a relative motion line A diagram showing the Sea-Stabilized True Motion Display versus the Stabilized True Motion Display is shown in Fig. 4.21. Ground- Fig.4.21 - Sea-stabilized and ground stabilized true motion display 123 4.3. The use of radar plotting in Coastal Water Navigation 4.3.1. RADAR FIXING 4.3.1.1. Radar fixing methods 4.3.1.1. 1. Range and bearing to a single object Preferably, radar fixes obtained through measuring the range and bearing to a single object should be limited to small, isolated fixed objects which can be identified with reasonable certainty. In many situations, this method may be the only reliable method which can be employed. If possible, the fix should be based upon a radar range and visual gyro bearing because radar bearings are less accurate than visual gyro bearings. A primary advantage of the method is the rapidity with which a fix can be obtained. A disadvantage is that the fix is based upon only two intersecting position lines, a bearing line and a range arc, obtained from observations of the same object. Identification mistakes can lead to disaster. 4.3.1.1. 2. Two or more bearings Generally, fixes obtained from radar bearings are less accurate than those obtained from intersecting range arcs. The accuracy of fixing by this method is greater when the center bearings of small, isolated, radar-conspicuous objects can be observed. Because of the rapidity of the method, the method affords a means for initially determining an approximate position for subsequent use in more reliable identification of objects for fixing by means of two or more ranges. 4.3.1.1. 3. Tangent bearings Fixing by tangent bearings is one of the least accurate methods. The use of tangent bearings with a range measurement can provide a fix of reasonably good accuracy. As illustrated in figure 4.22, the tangent bearing lines intersect at a range from the island observed less than the range as measured because of beam width distortion. Right tangent bearings should be decreased by an estimate of half the horizontal beam width. Left tangent bearings should be increased by the same amount. The fix is taken as that point on the range arc midway between the bearing lines. It is frequently quite difficult to correlate the left and right extremities of the island as charted with the island image on the PPI. Therefore, even with compensation for half of the beam width, the bearing lines usually will not intersect at the range arc. 124 Figure 4.22 - Fixing by tangent bearings and radar range. 4.3.1.1. 4. Two or more ranges In many situations, the more accurate radar fixes are determined from nearly simultaneous measurements of the ranges to two or more fixed objects. Preferably, at least three ranges should be used for the fix. The number of ranges which it is feasible to use in a particular situation is dependent upon the time required for identification and range measurements. In many situations, the use of more than three range arcs for the fix may introduce excessive error because of the time lag between measurements. If the most rapidly changing range is measured first, the plot will indicate less progress along the intended track than if it were measured last. Thus, less lag in the radar plot from the ship’s actual position is obtained through measuring the most rapidly changing ranges last. Similar to a visual cross-bearing fix, the accuracy of the radar fix is dependent upon the angles of cut of the intersecting position lines (range arcs). For greater accuracy, the objects selected should provide range arcs with angles of cut as close to 90° as is possible. In cases where two identifiable objects lie in opposite or nearly opposite directions, their range arcs, even though they may intersect at a small angle of cut or may not actually intersect, in combination with another range arc intersecting them at an angle approaching 90°, may provide a fix of high accuracy (see figure 4.23). The near tangency of the two range arcs indicates accurate measurements and good reliability of the fix with respect to the distance off the land to port and starboard. 125 Figure 4.23 - Radar fix. Small, isolated, radar-conspicuous fixed objects afford the most reliable and accurate means for radar fixing when they are so situated that their associated range arcs intersect at angles approaching 90°. Figure 4.24 illustrates a fix obtained by measuring the ranges to three well situated radar-conspicuous objects. The fix is based solely upon range measurements in that radar ranges are more accurate than radar bearings even when small objects are observed. Note that in this rather ideal situation, a point fix was not obtained. Because of inherent radar errors, any point fix should be treated as an accident dependent upon plotting errors, the scale of the chart, etc. While observed radar bearings were not used in establishing the fix as such, the bearings were useful in the identification of the radarconspicuous objects. Figure 24 - Fix by small, isolated radar-conspicuous objects. As the ship travels along its track, the three radar-conspicuous objects still afford good fixing capability until such time as the angles of cut of the range arcs have degraded appreciably. At such time, other radar-conspicuous objects should be selected to provide better angles of cut. Preferably, the first new object should be selected and observed before the angles of cut have degraded appreciably. Incorporating the range arc of the 126 new object with range arcs of objects which have provided reliable fixes affords more positive identification of the new object. 4.3.1.1. 5. Mixed methods While fixing by means of intersecting range arcs, the usual case is that two or more small, isolated, and conspicuous objects, which are well situated to provide good angles of cut, are not available. The navigator must exercise considerable skill in radarscope interpretation to estimate which charted features are actually displayed. If initially there are no well defined features displayed and there is considerable uncertainty as to the ship’s position, the navigator may observe the radar bearings of features tentatively identified as a step towards their more positive identification. If the cross-bearing fix does indicate that the features have been identified with some degree of accuracy, the estimate of the ship’s position obtained from the cross-bearing fix can be used as an aid in subsequent interpretation of the radar display. With better knowledge of the ship’s position, the factors affecting the distortion of the radar display can be used more intelligently in the course of more accurate interpretation of the radar display. Frequently there is at least one object available which, if correctly identified, can enable fixing by the range and bearing to a single object method. A fix so obtained can be used as an aid in radarscope interpretation for fixing by two or more intersecting range arcs. The difficulties which may be encountered in radarscope interpretation during a transit may be so great that accurate fixing by means of range arcs is not obtainable. In such circumstances, range arcs having some degree of accuracy can be used to aid in the identification of objects used with the range and bearing method. With correct identification of the object observed, the accuracy of the fix obtained by the range and bearing to a single object method usually can be improved through the use of a visual gyro bearing instead of the radar bearing. Particularly during periods of low visibility, the navigator should be alert for visual bearings of opportunity. While the best method or combination of methods for a particular situation must be left to the good judgment of the experienced navigator, factors affecting method selection include: (1) The general need for redundancy—but not to such extent that too much is attempted with too little aid or means in too little time. (2) The characteristics of the radar set. (3) Individual skills. (4) The navigational situation, including the shipping situation. (5) The difficulties associated with radarscope interpretation. (6) Angles of cut of the position lines. 4.3.1.2. Preconstruction of range arcs Small, isolated, radar-conspicuous objects permit preconstruction of range arcs on the chart to expedite radar fixing. This preconstruction is possible because the range can be measured to the same point on each object, or nearly so, as the aspect changes during the transit. With fixed radar targets of lesser conspicuous, the navigator, generally, must continually change the centers of the range arcs in accordance with his interpretation of the radarscope. 127 To expedite plotting further, the navigator may also preconstruct a series of bearing lines to the radar-conspicuous objects. The degree of preconstruction of range arcs and bearing lines is dependent upon acceptable chart clutter resulting from the arcs and lines added to the chart. Usually, preconstruction is limited to a critical part of a passage or to the approach to an anchorage. 4.3.2. AIDS TO RADAR NAVIGATION Various aids to radar navigation have been developed to aid the navigator in identifying radar targets and for increasing the strength of the echoes received from objects which otherwise are poor radar targets. 4.3.2.1. RADAR REFLECTORS Buoys and small boats, particularly those boats constructed of wood, are poor radar targets. Weak fluctuating echoes received from these targets are easily lost in the sea clutter on the radarscope. To aid in the detection of these targets, radar reflectors, of the corner reflector type, may be used. The corner reflectors may be mounted on the tops of buoys or the body of the buoy may be shaped as a corner reflector, as illustrated in figure 4.25. Each corner reflector illustrated in figure 4.25 consists of three mutually perpendicular flat metal surfaces. A radar wave on striking any of the metal surfaces or plates will be reflected back in the direction of its source, i.e., the radar antenna. Maximum energy will be reflected back to the antenna if the axis of the radar beam makes equal angles with all the metal surfaces. Frequently corner reflectors are assembled in clusters to insure receiving strong echoes at the antenna. Figure 4.25 - Corner reflectors. 4.3.2.2. RADAR BEACONS While radar reflectors are used to obtain stronger echoes from radar targets, other means are required for more positive identification of radar targets. Radar beacons are transmitters operating in the marine radar frequency band which produce distinctive indications on the radarscopes of ships within range of these beacons. There are two general classes of these beacons: racon which provides both bearing and range information to the target and ramark which provides bearing information only. However, if the remark installation is detected as an echo on the radarscope, the range will be available also. 128 Racon Racon is a radar transponder which emits a characteristic signal when triggered by a ship’s radar. The signal may be emitted on the same frequency as that of the triggering radar, in which case it is automatically superimposed on the ship’s radar display. The signal may be emitted on a separate frequency, in which case to receive the signal the ship’s radar receiver must be capable of being tuned to the beacon frequency or a special receiver must be used. In either case, the PPI will be blank except for the beacon signal. Figure 4.26 - Coded racon signal. “Frequency agile” racons are now in widespread use. They respond to both 3 and 10 centimeter radars. The racon signal appears on the PPI as a radial line originating at a point just beyond the position of the radar beacon or as a Morse code signal displayed radially from just beyond the beacon (see figures 4.26). Racons are being used as ranges or leading lines. The range is formed by two racons set up behind each other with a separation in the order of 2 to 4 nautical miles. On the PPI scope the “paint” received from the front and rear racons form the range. Some bridges are now equipped with racons which are suspended under the bridge to provide guidance for safe passage. The maximum range for racon reception is limited by line of sight. 4.3.3. Identifying a radar-inconspicuous object Situation: There is doubt that a pip on the PPI represents the echo from a buoy, a radarinconspicuous object. On the chart there is a radar-conspicuous object, a rock, in the vicinity of the buoy. The pip of the rock is identified readily on the PPI. 129 Required: Identify the pip which is in doubt. Solution: (1) Measure the bearing and distance of the buoy from the rock on the chart. (2) Determine the length of this distance on the PPI according to the range scale setting. (3) Rotate the parallel-line cursor to the bearing of the buoy from the rock (see figure 4.23). (4) With rubber-tipped dividers set to the appropriate PPI length, set one point over the pip of the rock; using the parallel lines of the cursor as a guide, set the second point in the direction of the bearing of the buoy from the rock. (5) With the dividers so set, the second point lies over the unidentified pip. Subject to the accuracy limitations of the measurements and normal prudence, the pip may be evaluated as the echo received from the buoy. Note: During low visibility a radar-conspicuous object can be used similarly to determine whether another ship is fouling an anchorage berth. Figure 4.27 - Use of parallel-line cursor to identify radar-inconspicuous object. 130 4.3.4. The use of paralel indexing 4.3.4.1. Finding course and speed made good by parallel-line cursor Situation: A ship steaming in fog detects a prominent rock by radar. Because of the unknown effects of current and other factors, the navigator is uncertain of the course and speed being made good. Required: To determine the course and speed being made good. Solution: (1) Make a timed plot of the rock on the reflection plotter. (2) Align the parallel-line cursor with the plot to determine the course being made good, which is in a direction opposite to the relative movement (see figure 4.28). (3) Measure the distance between the first and last plots and using the time interval, determine the speed of relative movement. Since the rock is stationary, the relative speed is equal to that of the ship. Note: This basic technique is useful for determining whether the ship is being set off the intended track in pilot waters. Observing a radar-conspicuous object and using the parallel-line cursor, a line is drawn through the radar-conspicuous object in a direction opposite to own ship’s course. By observing the successive positions of the radar-conspicuous object relative to this line, the navigator can determine whether the ship is being set to the left or right of the intended track. Figure 4.28 - Use of parallel-line cursor to find course and speed made good. 131 4.3.4.2. Use of parallel-line cursor for anchoring (fig. 4.29) Situation: A ship is making an approach to an anchorage on course 290°. The direction of the intended track to the anchorage is 290°. Allowing for the radius of the letting go circle, the anchor will be let go when a radar-conspicuous islet is 1.0 mile ahead of the ship on the intended track. A decision is made to use a parallel-line cursor technique to keep the ship on the intended track during the last mile of the approach to the anchorage and to determine the time for letting go. Before the latter decision was made, the navigator’s interpretation of the stabilized relative motion display revealed that, even with change in aspect, the radar image of a jetty to starboard could be used to keep the ship on the intended track. Required: Make the approach to the anchorage on the intended track and let the anchor go when the islet is 1.0 mile ahead along the intended track. Solution: (1) From the chart determine the distance at which the head of the jetty will be passed abeam when the ship is on course and on the intended track. (2) Align the parallel-line cursor with the direction of the intended track, 290° (see figure 4.25). (3) Using the parallel lines of the cursor as a guide, draw, at a distance from the center of the PPI as determined in step (2), the relative movement line for the head of the jetty in a direction opposite to the direction of the intended track. (4) Make a mark at 290° and 1.0 mile from the center of the PPI; label this mark “LG” for letting go. (5) Make another mark at 290° and 1.0 mile beyond the LG mark; label this mark “1”. (6) Subdivide the radial between the marks made in steps (4) and (5). This subdivision may be limited to 0.1 mile increments from the LG mark to the 0.5 mile graduation. (7) If the ship is on the intended track, the RML should extend from the radar image of the head of the jetty. If the ship keeps on the intended track, the image of the jetty will move along the RML. If the ship deviates from the intended track, the image of the jetty will move away from the RML. Corrective action is taken to keep the image of the jetty on the RML. (8) With the ship being kept on the intended track by keeping the image of the jetty on the RML, the graduations of the radial in the direction of the intended track provide distances to go. When the mark labeled “1” just touches the leading edge of the pip of the islet ahead, there is 1 mile to go. When the mark label “.5” just touches the leading edge of the latter pip, there is 0.5 mile to go, etc. The anchor should be let go when the mark labeled “LG” just touches the leading edge of the pip of the islet. 132 Figure 4.29 - Use of parallel-line cursor for anchoring. 4.3.4.3. Parallel Indexing (fig. 4.30) Parallel Indexing has been used for many years. It was defined by William Burger in the Radar Observers Handbook as equidistantly spaced parallel lines engraved on a transparent screen which fits on the PPI and can be rotated. This concept of using parallel lines to assist in navigation has been extensively used in Europe to assist in maintaining a specified track, altering course and anchoring. It is best suited for use with a stabilized radar. When using an unstabilized radar, it can pose some danger to an individual that is unaware of problems inherent in this type of display. With the advent of ARPA with movable EBLs (Electronic Bearing Lines) and Navigation Lines, parallel indexing on screen can be accomplished with greater accuracy. Index lines that are at exact bearings and distances off can be displayed with greater ease. A number of diagrams are included on the pages that follow to explain the use of parallel indexing techniques as well as its misuse. Cross Index Range (“C”) The distance of an object when abeam if the vessel was to pass the navigation mark. A parallel line is drawn through this mark. The perpendicular distance from the center of the display to this parallel line is the Cross Index Range (1964, Admiralty Manual of Navigation). Dead Range (“D”) The distance at which an object tracking on a parallel line would be on a new track line (ahead of or behind the beam bearing of the object). Wheel Over Point (“W”) 133 The point at which the actual maneuver is made to insure that the object being “indexed” is on the new track line taking into account the advance and transfer of the vessel. Figure 4.30 - Use of parallel-line cursor for anchoring. 4.3.5. Radar detection of ice Radar can be an invaluable aid in the detection of ice if used wisely by the radar observer having knowledge of the characteristics of radar propagation and the capabilities of his radar set. The radar observer must have good appreciation of the fact that ice capable of causing damage to a ship may not be detected even when the observer is maintaining a continuous watch of the radarscope and is using operating controls expertly. When navigating in the vicinity of ice during low visibility, a continuous watch of the radarscope is a necessity. For reasonably early warning of the presence of ice, range scale settings of about 6 or 12 miles are probably those most suitable. Such settings should provide ample time for evasive action after detection. Because any ice detected by radar may be lost subsequently in sea clutter, it may be advisable to maintain a geographical plot. The latter plot can aid in differentiating between ice aground or drifting and ship targets. If an ice contact is evaluated as an iceberg, it should be given a wide berth because of the probability of growlers in its vicinity. If ice contacts are evaluated as bergy bits or growlers, the radar observer should be alert for the presence of an iceberg. Because the smaller ice may have calved recently from an iceberg, the radar observer should maintain a particularly close watch to windward of the smaller ice. 134 4.3.5.1. Icebergs While large icebergs may be detected initially at ranges of 15 to 20 miles in a calm sea, the strengths of echoes returned from icebergs are only about 1/60 of the strengths of echoes which would be returned from a steel ship of equivalent size. Because of the shape of the iceberg, the strengths of echoes returned may have wide variation with change in aspect. Also, because of shape and aspect, the iceberg may appear on the radarscope as separate echoes. Tabular icebergs, having flat tops and nearly vertical sides which may rise as much as 100 feet above the sea surface, are comparatively good radar targets. Generally, icebergs will be detected at ranges not less than 3 miles because of irregularities in the sloping faces. 4.3.5.2. Bergy bits Bergy bits, extending at most about 15 feet above the sea surface, usually cannot be detected by radar at ranges greater than 3 miles. However, they may be detected at ranges as great as 6 miles. Because their echoes are generally weak and may be lost in sea clutter, bergy bits weighing several hundred or a few thousand tons can impose considerable hazard to a ship. 4.3.5.3. Growlers Growlers, extending at most about 6 feet above the sea surface, are extremely poor radar targets. Being smooth and round because of wave action, as well as small, growlers are recognized as the most dangerous type of ice that can be encountered. In a rough sea and with sea clutter extending beyond 1 mile, growlers large enough to cause damage to a ship may not be detected by radar. Even with expert use of receiver gain, pulse length, and anti-clutter controls, dangerous growlers in waves over 4 feet in height may not be detected. In a calm sea growlers are not likely to be detected at a range exceeding 2 miles. 135 5. USE RADAR TO AVOID COLLISIONS OR CLOSE ENCOUNTERS 5.1 Navigation and Collision Avoidance in Open Waters 5.1.1 Course Alteration Diagram Intended primarily for use in avoiding a vessel detected by radar and out of sight. Course alteration Course alteration to be done in accordance with the recommendations from diagram 5.1 Figure. 5.1: Course alteration diagram Resumption of Course After turning to starboard for a vessel on the starboard side, keep the vessel to port when resuming course. Escape Action A vessel approaching from the port side and rear sector can normally be expected to take early avoiding action. The suggested turns are recommended for use when such a 136 vessel fails to keep out of the way. As an alteration to put the bearing astern may not complement subsequent action by the other vessel, it is recommended that further turns be made to keep the vessel astern until she is well clear. 5.1.2 Speed changes 5.1.2.1 Reduction of Speed A vessel can reduce speed or stop at any time, and such action is recommended when the compass bearing of a vessel on the port bow is gradually changing in it clockwise direction (increasing). A reduction of speed should be made as an alternative to, and not in conjunction with, the suggested turn to starboard for avoiding a vessel either on the port bow or ahead. Normal speed should be resumed If it becomes apparent that a vessel on the port side has either subsequently turned to starboard in order to pass astern or stopped. 5.1.2.2 Increases of Speed It will sometimes be advantageous to increase speed if this is possible within the limitations of the requirement lo proceed at a safe speed. An increase of speed may be appropriate when the vessel to be avoided is astern or on the port quarter, or near the port beam, either initially or after taking the alter course action indicated in the diagram. 5.1.2.3 Limitations The presence of other vessels and/or lack of sea room may impose limitations on the: manoeuvres which can be made, but it should be kept in mind that small changes of course and! or speed are unlikely to be detected by radar. Caution It is essential to ensure that any action taken is having the desired effect. If not, the recommended turns can normally be applied successively for newly-developed situations with the same vessel. 5.3 Collision assessment by offset EBL The origin of the EBL can be placed anywhere with the trackball to enable measurement of range and bearing between any targets. This function is also useful for assessment of the potential risk of collision. To assess possibility of collision: 1. Press the EBL ON key to display or activate an EBL (No.1 or 2). 2. Place the cursor (+) on a target of interest (A in the illustrated example) by operating the trackball. 3. Press the EBL OFFSET key on the mode panel, and the origin of the active EBL shifts to the cursor position. Press the EBL OFFSET key again to anchor the EBL origin. 137 Figure 5.2. - Evaluating target ship’s course and CPA in relative motion mode 4. After waiting for a few minutes (at least 3 minutes), operate the EBL control until the EBL bisects the target at the new position (A’). The EBL readout shows the target ship’s course, which may be true or relative depending on the settings on the RADAR 2 menu. If relative motion is selected, it is also possible to read CPA by using a VRM as shown in figure 5.3. If the EBL passes through the sweep origin (own ship) as illustrated in figure 5.3, the target ship is on a collision course. 5. To return the EBL origin to the own ship’s position, press the EBL OFFSET key again. Figure 5.3. - Target ship on collision course Suppressing Second-trace Echoes In certain situations, echoes from very distant targets may appear as false echoes (second trace echoes) on the screen. This occurs when the return echo is received one transmission cycle later, that is, after a next radar pulse has been transmitted. To activate or deactivate the second trace echo rejector: 138 1. Press the RADAR MENU key on the plotting keypad to show the FUNCTIONS menu. 2. Press the (8) key to select menu item 8: 2ND ECHO REJ. 3. Further press the (8) key to activate (ON) or deactivate (OFF) the second trace echo rejector. 4. Press the ENTER key to conclude selection followed by the RADAR MENU key to close the FUNCTIONS menu. Adjusting Relative Brilliance Levels of Screen Data You can adjust relative brilliance levels of various marks and alphanumeric readouts displayed on the screen by following the steps shown below: 1. Press the RADAR MENU key on the plotting keypad to show the FUNCTIONS menu. 2. Press the (9) key to show the BRILLIANCE menu. 3. Select a desired menu item by pressing the corresponding numeric key. As an example, press (4) if you want to change the brilliance of echo trails. 4. Further press the same numeric key as you pressed in step 3 above to select or highlight a desired brilliance level. 5. Press the ENTER key to conclude your selection followed by the RADAR MENU key to close the FUNCTIONS menu. Set and Drift (Set and Rate) Set the direction in which a water current flows, can be manually entered on 0.1 degree steps. Drift (rate), the speed of the tide, can also be entered manually in 0.1 knot steps. Set and drift corrections are beneficial for increasing the accuracy of the vectors and target data. The correction is best made in the head up mode with true vector, watching landmasses, or other stationary targets. If they have vectors, set and drift values should be adjusted until they lose vectors. Note: Set and drift correction is available on selecting the water tracking mode only. Proceed as follows to enter set and drift (rate): 1. Press the RADAR MENU key on the plotting keyboard to show the FUNCTIONS 1 menu. 2. Press the (8) key to select menu item 8; SET, DRIFT. 3. Further press the (8) key to select OFF or MAN option. OFF: No correction against set and drift. MAN: Manual entry of set and drift data. 4. If OFF is selected, press the ENTER key. 5. If you have selected MAN in step 3 above, the highlight cursor will advance one line down requesting you to enter SET xxx.x .Enter the value of set in degrees by hitting numeric keys without omitting leading zeroes, if any, and press the ENTER key. 6. The highlight cursor will then advance to the next line DRIFT xx.x KT. Enter the value of drift in knots by hitting numeric keys without omitting leading zeroes, if any, 139 and press the ENTER key. Set and drift have the same effect on own ship and all targets. 7. Press the RADAR MENU key to close the menu. 5.4 Rules that apply to navigation and collision avoidance in open waters Radar for the Merchant Service is designed for what is known as "Surface Warning" and for anti-collision purposes its main use was during reduced visibility. With the faster ships, radar, however, is also now extensively used in clear weather as an extra aid for the look-out man. With the range-scale on 12 miles and the electronic centre offset, strong echoes of ships can be detected up to 16 miles for an average bridge height and at a time when the ship has not yet been sighted visually. Another reason for earlier detection by radar is that the white echo pip against the dark background is often more conspicuous than the appearance of a. ship against a grey sky and seas. By placing the cursor over the echo, a timely check can be kept on the bearing change. When fog-banks are expected the radar set should be at least on "Stand-by" during daytime, making it ready for immediate use; but at night the set should be left on "Transmit", as the vessel could well be streaming along near a fog bank which is giving no visual indication or is it precise. When approaching a fog-bank Rule 35 (Sound Signals must be used Visibility) must be adhered to and radar must be used to see what is inside the fog-bank. Failure to employ the radar in such a case contravenes Rule 2 and blame accordingly has been attached to ships which did not comply with this rule near a fog-bank. Upon approach of the fogbank, radar watch routine should be started, and inside the fog-bank, the observer should realize that some echoes on the screen might represent ships which are not in fog and may not exercise the same caution as his own ship. Unexplained maneuvers by other vessels as observed from the radar screen might indicate the existence of a small vessel or vessels undetected by the ship's own radar, and a close watch should be kept on the suspected area. Shipmasters have been blamed for not keeping a proper "look-out" because they were not using their radar on clear nights to detect the presence of the unlit oil-rigs with which their vessels collided. It may, therefore, be said that it is always good practice, especially for fast ships, to keep the radar working.. This also offers an opportunity to the officer of watch to maintain his plotting expertise, which is so important 18 cases when the visibility deteriorates and plotting becomes really essential. At night, when in a region where fog-banks and/or unlit obstruction can be expected, the radar must be in continuous operation. Previous Court Cases, by the way, have stressed that a shipmaster is considered to be at fault for not using radar provided for his ship and also for allowing the radar installation on his ship to remain in a defective condition for a prolonged period. At present the U.K. Government has made radar compulsory for all British ships of 500 gross tons or more, following an IMO recommendation that at least one radar must be fitted in ships of 500 g.r.t. or more (300 g.r.t. after 1st February 1995), and at least two radars must be fitted to all ships of 10,000 g.r.t. or more, each capable of operating independently of the other. Some 140 important points to be kept in mind when using radar in reduced visibility, are the following: (a) The setting of the anti-clutter control on raw radar displays. Adjust, if possible, in such a way that echoes can be traced near the spot representing own ship. Be aware of over-suppression, as this will wipe off most of the echoes of ships nearby. (b) The existence of blind and shadow sectors caused by objects on the ship itself. A slight "weaving" around the course is recommendable in such a case. (c) The selection of range scale, taking account of: (i) The speed of own ship (the faster the ship, the greater the range scale). (ii) The accuracy of bearing and range observations (shorter range scales with the echo in the outer half of the screen yields an increased accuracy). (iii) The length of the "tadpole" tails (the shorter the range scale, the longer the tails). If a True Motion Display is available, this might entail off-centering the time-base and use of the Zero Speed switch. (iv) The possibility of encountering small craft or ice growlers (easier discernable on the shorter range scales and, if possible, with long pulse selected). (v) The number of ships in the vicinity of own ship (a long-range scale can produce a confusing array of closely-packed echoes). (vi) The range at which most merchant ships are first detected (generally about 10-15 miles). In addition to the above considerations, it should be remembered that a plot on a reflection plotter mounted on a True Motion Display will become distorted if the range scale is altered during the plotting interval. This problem does not arise with Relative Motion radar. Summarizing on this problem of selecting a range scale, it is generally best to relate the range-scale used most of the time to the vessel's speed changing to shorter range scales now and again to obtain more accurate observations of bearings and ranges of any nearby objects and also to conduct a search for smaller objects. (d) The obedience of Rule 35 (Sound signals in restricted visibility) even if the screen is free from echoes on the longer range scales and one knows that the set is fully efficient. (e) The obedience of Rule 34 (0) (Maneuvering signals) only when the other vessel is in sight. 5.5 Radar and the Collision Regulations Under Part B (Steering and Sailing Rules) there are two sections which have a special bearing on this Chapter. These are Section I and Section m. The former deals with the conduct of vessels in any condition of visibility (Rules 4, 5, 6, 7. 8 and 10); the latter (Rule 19) discusses the conduct of vessels in restricted visibility. Turning our attention to Section 1 fist, it will be seen that Rule 5 specifically deals with the importance of maintaining "a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a fulJ appraisal of the situation and of the risk of collision". The word "specifically" is stressed because in previous Regulations this came under "the ordinary practice of seamen". 141 The inclusion of "as well as by all available means" refers obviously to a radar watch (the use of guard rings on the radar display will be helpful in this connection), but it also incorporates a V.H.F. R/T watch and the words "full appraisal" may be taken to include proper radar plotting procedures and active V.H.F. Radio-Telephony communications. Although this section deals with clear weather conditions and conditions of restricted visibility, the master is given some latitude in making use of radar and R/T information by the addition "appropriate in the prevailing circumstances and conditions". Rule 6 introduces a new concept, namely Safe Speed. When, about 45 years ago radar was introduced on board ships, one of the greatest difficulties with which Mariners were confronted, was the term "Moderate Speed". What. in fact, was a moderate speed using radar? A concise answer was not possible. It could be argued that a moderate speed, using radar, could in some cases mean "Full speed with engines on Stand-by", but in other cases could mean a slower speed than a Mariner without radar might consider "moderate". From the legal and philosophical point of view these arguments are quite correct but in the literary sense they are unsatisfactory. A Safe Speed as defined in the 1972 Rules is not based only on the state of visibility (as in the 1960 Rules) but "Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions". Besides the state of visibility (i) the following factors should be taken into account in determining a safe speed: (ii) "the traffic density, including concentrations of fishing-vessels or any other vessels"; (iii) "the manoeuvrability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions"; The manoeuvrability depends on the stern power of the vessel, the number and type of screws, the provision of a bow-thruster. the size of the ship and her loaded condition while the prevailing conditions are mainly governed by the wind and wave directions, wind force and wave height, and current and tidal conditions. (iv) "at night the presence of background light such as from shore lights or from back scatter of her own lights"; (v) "the state of wind, sea and current, and the proximity of navigational hazards"; (vi) "the draught in relation to the available depth of water. These factors, which detennine a safe speed in general, are applicable to all ships. Vessels which use their radar need, in addition, take the following conditions into account:(i) "the characteristics, efficiency and limitations of the radar equipment"; The age and reliability of the equipment, the number of radars and displays, interswitching facilities, types of display presentations, plotting devices and facilities for automatic plotting etc., are all factors to consider. (ii) "any constraints imposed by the radar range scale in use"; A constraint may be imposed on a particular range scale owing to strong radar or electrical interference, or for a very fast vessel the use of the l2--mile range scale (the 24-mile range scale is. too small for effective plotting on a reflection plotter) for echo observation, might compel her to reduce speed. (iii) "the effect on radar detection of the sea state, weather and other sources of interference"; Excessive "noise" due to wave, sea, ram-drops, snow crystals, other ships' 142 radar pulses or electrical interference may swamp the signal and essential information .may be lost. . . (iv) "the possibility that small vessels, Ice and other floating objects may not be detected by radar at an adequate range". This "possibility" can be a result of atmospheric conditions such as sub-refraction or it might be caused by a small reflection coefficient of the object. (v) "the number, location and movement of vessels detected by radar". (vi) "the more exact assessment of the visibility that may be possible when radar is used to determine the range of vessels or other objects 10 the vicinity". In addition, we may say that the number of men for keeping radar watch and a plot, and their efficiency could influence the master’s opinion about what is, or what is not a safe speed. . Rule 7 deals with the "Risk of Collision". The Rule stresses again the use of "all the available means appropriate to the prevailing circumstances and conditions to determine if the risk of collision exists". This includes the 'listening to V.H.F. R/T messages of other ships and shore radar stations, but no guidance is give about actual active participation. There is a very important last sentence in the first paragraph: "If there IS any doubt such risk shall be deemed to exist". This might remove the possible element of indecision in a radar encounter. Rule 7 (b) stresses the importance of making proper use of radar equipment, including early warning of collision risk on the long-range scales. It furthermore emphasizes the practice of radar plotting or "equivalent systematic observation of detected objects" (recording in writing and tabulation, automatic plotting aids). Rule 7 (e) states: "''Assumptions shall not be made on the basis of scanty information, especially scanty radar information". The omission of a plot, an incomplete plot or a plot based on an insufficient number of observations, in short the determination of the position of another vessel without finding her movement, might be termed as "scanty". ARPA provides a solution here. The last paragraph of Rule 7 states how risk of collision can be obtained from compass bearings and gives a warning that an appreciable change in bearing does not always indicate a safe passing (large vessel, Or a tow, or a ship at close range; see also Fig. 14.4). Bearings should be recorded as compass bearings and not as relative bearings as is so easily done on an unstabilized display. It is not possible to compare relative bearings when own ship is subject to yaw or makes alterations of course, and often the Master has been led to believe, that. By making a small alteration of course, the situation improved because the relative bearing changed and he did not realize that the change in the relative bearing was mainly due to own ship's alteration of course. If he had converted the relative bearings to compass bearings, he would have noticed that danger of collision after the alteration had become greater instead of less. Rule 8 is headed "Action to avoid Collision". Paragraph (a) states that, if the circumstances of the case admit, any action shall "be positive, made in ample time and with due regard to the observance of good seamanship". The word "positive" in this connection means "effective" and bears no relationship to the conventional adaptations "'positive and negative actions", mentioned in certain papers about collision-avoidance (more about these later). 143 Paragraph (b) is an extension of paragraph (a), stating that "Any alteration of course and! or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and (or speed should be avoided". The Rule requires substantial action in order to make clear one's intention to all vessels in the neighborhood ("another vessel" is not necessarily the vessel for which avoiding action was taken) both in clear weather as well as in fog. This requirement should be kept in mind when an agreement is reached about collision-avoiding tactics between two vessels via V.H.F. R/T and also when using the 'Trial Manoeuvre' facility on ARPA. The remainder of the Rule (paragraphs (c), (d) and (e» emphasizes that an alteration of course, provided there is sufficient sea room, may be the most effective action to avoid a close quarters situation on condition that it is made in good time, is substantial and does not result in another close-quarters situation. It stresses the safe passing distance and warns that effectiveness of the action shall be carefully checked until the other vessel is finally past and clear. If necessary, or to allow more time to assess the situation, a vessel shall slacken her speed or take out way off by stopping Or reversing her means of propulsion. In short, what this Rule is saying is that if avoiding action for another vessel is going to be taken such action should be bold both in clear weather and in conditions of restricted visibility so that the intention of the vessel taking the action becomes readily apparent to other vessels in the vicinity. Seen in this light, an alteration of course is generally more effective than an alteration in speed. There are some contributory factors for substantial action to be taken where radar navigation in fog is concerned. The first factor is that for a collision encounter between two vessels meeting ends on or crossing – and each forward of the other's beam - an alteration of course or speed by one of the vessels shows up far less pronounced in the relative track (direction or rate) on the other vessel's display or plot when a relative presentation is used than if a true motion presentation were used. This is understandable when one remembers that the relative motion line is produced as the result of two vectors, of which only one is changed in this case. The reverse is also true. If our own ship, for example makes an alteration of course of 30 degrees, another crossing vessel, with approximately the same speed, forward of the beam, involving risk of collision, will observe a change in her relative motion line of about IS degrees. To make, therefore, a course alteration - and this holds also for alterations in speed - readily apparent and on the assumption that other vessels in the vicinity use a relative motion display presentation, a substantial alteration is required by own ship. The second reason for making substantial alterations is thal errors in plotting and a wrong estimation of the direction of the relative motion line can easily take place especially when the display is unstabilized. The observer may, for example, conclude that the other vessel is on a collision course or will be passing on her port side while, in fact, the other ship, if she maintains her course and speed will be passing on her starboard side. If, in this case, own ship makes a small alteration to starboard, then, instead of improving the situation, the nearest approach between the two vessels will become even smaller. If later on, own ship makes a second alteration to starboard, and perhaps even a third one, then this may lead to collision. This type of action whereby one ship makes a succession of small alterations of course has become known as The Cumulative Turn and the majority of collisions in fog have been caused by this type of action. 144 In many of these collision cases, while one ship carried out the cumulative turn, the other vessel maintained her course and speed, simply because she had not detected the effect of the turn on her display. Although, for her, the bearing opened out it did not open out sufficiently (Rule 7 (d) (ii)) and in the final stages of the encounter it became steady. Hence the warning in Rule 8 (b) against a succession of small alterations of course and/ or speed. Rule 8 (c) states that if there is sufficient sea room, alterations of course alone may be the most effective action to avoid a close-quarters situation. The sea room, however, could be restricted by navigational dangers or a fair amount of traffic with ships crossing from different directions. In such cases substantial alterations of course may not be possible and may make the situation even more dangerous when ships which, before the alteration, were not on collision courses, may after the alteration, involve a risk of collision. If it becomes necessary to avoid collision, a substantial reduction in speed must be made (Rule g (e)). Paragraph (d) of Rule g emphasizes that the effectiveness of any action small be carefully checked until the other vessel is finally past and clear. During restricted visibility and using one's radar, this means that after an alteration of course and I or speed, observations of the target should be taken at frequent intervals to see if the echo follows the predicted track at the predicted rate. If the echo deviates from the predicted track away from the centre of the plot, then to other vessel has taken action contributing. To safety but if on the other hand the echo deviates from the prediction he towards the centre of the plot, then the other vessel has taken action which has cancelled out or partly cancelled out our avoiding action and in the majority of cases, the wisest thing to do is to reduce the speed of own ship to a minimum at which she can be kept on her course, or to alter course and to put the other ship right astern. On ARPA the history track should be watched for alterations of course or/ and speed of targets. Finally, Rule 8 makes a mention of "close-quarters situation" and "safe passing distance". These concepts cannot be concisely formulated. It for obvious that their extent depends on many factors, such as weather conditions, state of visibility, type of vessel, maneuverability and observations are carried out by visual means or by radar which is far less discriminating than the naked eye in discerning changes in aspect. But even, when considering radar navigation in fog only, formulation does not come easily. For example, to pass another vessel at half a mile at three knots could be considered just as safe as passing a vessel four miles off at 15 knots. One must also take into account, when deciding what is a safe distance to pass, the direction in which vessels are shaping to pass. For instance, it could be quite reasonable and safe when overtaking a vessel to pass two miles off, whereas this could be unwise when passing a vessel on a reciprocal course with speed. In other words, it is really the relative speed and the direction of the target which should be considered when judging what is a close-quarters situation. In thick fog, however, when there is plenty of sea room, it is practical to keep the minimum radius of the close-quarters situation at about three miles in order to allow for bearing errors, unsuspected manoeuvres of the target and to keep out the range of audibility of the other ship's sound signals so that delays owing to the application of Rule 19 (e) can be avoided. Really, in order to assess the radius of the close-quarters situation, the master must rely on intuition based on experience to give him the right answer (radar simulator courses are generally useful to accelerate this experience). CPA and TCPA data should be set on 145 ARPA so that sufficient warning can be given to the O.O.W. when a close-quarters situation is approaching. Rule 10 applies to Traffic Separation Schemes and is a highly important addition to the 1972 Rules. It contains regulations adopted by IMO (see IMO-publication "Ships' Rooting Traffic Separation Schemes and Areas to be Avoided") but have become mandatory for all the published schemes. Additional paragraphs are included for small vessels and sailing ships, and exempted vessels. The more important parts of the Rule state that "a vessel using traffic-separation scheme shall, so far as practicable, keep clear of traffic-separation line or zone (b (ii) ), normally join or leave a traffic-lane the termination of a lane, but when joining or leaving from either side, shall do so at as small an angle to the general direction of traffic flow as practicable (b (iii)) and shall, so far as practicable, avoid crossing trafficlanes, but if obliged to do so, shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow (c). It goes on: "(d) Inshore traffic zones shall not normally be used by through traffic which can safely use the appropriate traffic-lane within the adjacent traffic-separation scheme. However, vessels of less than 20 meters in 'length and sailing vessels may under all circumstances use inshore traffic zones". In connection with the right-angled crossing, it is advised, if possible, to shape the new course well before the lane is reached, thus giving ships within the lane a timely indication. 146 6. OPERATIONAL USE OF ARPA SYSTEM 6.1.Introduction to ARPA 6.1.1 Introduction The availability of low cost microprocessors and the development of advanced computer technology during the 1970s and 1980s have made it possible to apply computer techniques to improve commercial marine radar systems. Radar manufactures used this technology to create the Automatic Radar Plotting Aids (ARPA). ARPAs are computer assisted radar data processing systems which generate predictive vectors and other ship movement information. The International Maritime Organization (IMO) has set out certain standards amending the International Convention of Safety of Life at Sea requirements regarding the carrying of suitable automated radar plotting aids (ARPA). The primary function of ARPAs can be summarized in the statement found under the IMO Performance Standards. It states a requirement of ARPAs....“in order to improve the standard of collision avoidance at sea: Reduce the workload of observers by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target”. As we can see from this statement the principal advantages of ARPA are a reduction in the workload of bridge personnel and fuller and quicker information on selected targets. A typical ARPA gives a presentation of the current situation and uses computer technology to predict future situations. An ARPA assesses the risk of collision, and enables operator to see proposed maneuvers by own ship. While many different models of ARPAs are available on the market, the following functions are usually provided: 1. True or relative motion radar presentation. 2. Automatic acquisition of targets plus manual acquisition. 3. Digital read-out of acquired targets which provides course, speed, range, bearing, closest point of approach (CPA, and time to CPA (TCPA). 4. The ability to display collision assessment information directly on the PPI, using vectors (true or relative) or a graphical Predicted Area of Danger (PAD) display. 5. The ability to perform trial maneuvers, including course changes, speed changes, and combined course/speed changes. 6. Automatic ground stabilization for navigation purposes. ARPA processes radar information much more rapidly than conventional radar but is still subject to the same limitations. ARPA data is only as accurate as the data that comes from inputs such as the gyro and speed log. 147 6.1.2 Stand-alone and integral ARPA’s Over the past 10 years, the most significant changes to the ARPA systems has been in their design. The majority of ARPAs manufactured today integrate the ARPA features with the radar display. The initial development and design of ARPAs were Stand-alone units. That is they were designed to be an addition to the conventional radar unit. All of the ARPA functions were installed on board as a separate unit but needed to interfaced with existing equipment to get the basic radar data. The primary benefits were cost and time savings. This of course was not the most ideal situation and eventually it was the integral ARPA that gradually replaced the stand-alone unit. The modern integral ARPA combines the conventional radar data with the computer data processing systems into one unit. The main operational advantage is that both the radar and ARPA data are readily comparable. The following paragraphs describe the features and operating instructions of the Furuno Heavy-Duty High Performance Raster Scan Radar and ARPA Model FR/FAR-28x5 series. Only selected portions of the Furuno operating instructions are presented in this manual. For the complete operating instructions you should contact a Furuno dealer or representative. The purpose of this section is to provide a sample of the technical instructions that should be available to the officer. As a radar observer you should thoroughly read and understand the operating instructions for the radar units that you will be using. Operating instructing will of course differ not only between different radar manufactures’ but also with different models for the same manufacturer. As with all equipment, the operator should be completely familiar with the safety instructions prior to turning on the radar. There are a number of dangers, warnings and cautions that should be followed by those operating these radars. Failure to follow the appropriate safety instructions could result in serious injury or death. 6.1.3 Features The FR-2805 and FAR-2805 series of Radar and ARPAs are designed to fully meet the exacting rules of the International Maritime Organization (IMO) for installations on all classes of vessels. The display unit employs a 28 inch diagonal multicolored CRT. It provides an effective radar picture of 360 mm diameter leaving sufficient space for on screen alpha-numeric data. Target detection is enhanced by the sophisticated signal processing technique such as multi-level quantization (MLQ), echo stretch, echo average, and a built-in radar interference rejector. Audible and visual guard zone alarms are provided as standard. Other ship’s movement is assessed by trails of target echoes or by electronic plotting. The FAR-2805 series ARPA further provides target assessment by historical plots, vectors and target data table. On screen data readouts include CPA, TCPA, range, bearing, speed/course on up to 3 targets at a time. The ARPA functions include automatic acquisition of up to 20 targets, or manual acquisition of 40 targets. In addition, the ARPA features display of a traffic lane, buoys, dangerous points, and other important reference points, 6.1.4 General Features - Daylight-bright high-resolution display 148 - 28 inch diagonal CRT presents radar picture of 360 mm effective diameter with alphanumeric data area around it - User friendly operation by combination of tactile backlit touchpads, a trackball and rotary controls - Audio-visual alert for targets in guard zone - Echo trail to assess targets’ speed and course by simulated afterglow - Electronic plotting of up to 10 targets in different symbols (This function is disabled when ARPA is activated) - Electronic parallel index lines - Interswitch (optional) built in radar or ARPA display unit - Enhanced visual target detection by Echo Average, Echo Stretch, Interference Rejector, and multi-level quantization - Stylish display - Choice of 10, 25 or 50 KW output for X-band; 30 KW output for S-band, either in the transceiver aloft (gearbox) or RF down (transceiver in bridge) - Exclusive FURUNO MIC low noise receiver 6.1.5 ARPA Features - Acquires up to 20 targets automatically - Movement of tracked targets shown by true or relative vectors (Vector length 1 to 99 min. selected in 1 min steps) - Setting of nav lines, buoy marks and other symbols to enhance navigation safety - On-screen digital readouts of range, bearing, course, speed, CPA, TCPA, BCR (Bow Crossing Range) and BCT (Bow Crossing Time) of two targets out of all tracked targets. - Audible and visual alarms against threatening targets coming into operator-selected CPA/TCPA limits, lost targets, two guard rings, visual alarm against system failure and target full situation - Electronic plotting of up to 10 targets in different symbols (This function is disabled when ARPA is activated) - Electronic parallel index lines - Interswitching (optional) built in radar or ARPA display unit - Enhanced visual target detection by Echo Average, Echo Stretch, Interference Rejector, and multi-level quantization - Stylish display - Choice of 10,25 or 50 kW output for X-band; 30kw output for S-band, either in the transceiver aloft (gearbox) or RF down (transceiver in bridge) - Exclusive FURUNO MIC low noise receiver 6.1.6 display controls - mode panel HM OFF-Temporarily erases the heading marker. 149 ECHO TRAILS - Shows trails of target echoes in the form of simulated afterglow. MODE - Selects presentation modes: Head-up, Head-up/TB, North-up, Course-up, and True Motion. GUARD ALARM - Used for setting the guard alarm. EBL OFFSET - Activates and deactivates off-centering of the sweep origin. BKGR COLOR - Selects the background color. INDEX LINES - Alternately shows and erases parallel index lines. X2 ZOOM - enlarges a user selected portion of picture twice as large as normal. (R-type only) CU, TM RESET - Resets the heading line to 000 in course-up mode; moves own ship position 50% radius in stern direction in the true motion mode. INT REJECT - Reduces mutual radar interference RANGE RINGS - Adjusts the brightness of range rings. VECTOR TRUE/REL - Selects true or relative vector. VECTOR TIME - Sets vector length in time. RADAR MENU - Opens and closes RADAR menus. E-PLOT, AUTO PLOT MENU - Opens and closes E-plot and AUTO PLT menus. NAV MENU- Opens and closes NAV menu. KEYS 0-9- Select plot symbols. Also used for entering numeric data. CANCEL- Terminates plotting of a specified target or all tracked targets. ENTER - Used to save settings on menu screen. TARGET DATA- Displays the acquired target data. TARGET BASED DATA - Own ship’s speed is measured relative to a fixed target. AUTO PLOT - Activates and deactivates the Auto Plotter. TRIAL- Initiates a trial maneuver. LOST TARGET - Silences the lost target audible alarm and erases the lost target symbol. HISTORY - Shows and erases past positions of tracked targets. MARK - Enter/erase mark. CHART ALIGN -Used to align chart data. VIDEO PLOT - Turns the video plotter on/off. 6.1.7 Turning on power The POWER switch is located at the lower right corner of the display. Push it to switch on the radar set. To turn off the radar, push it again; the switch will extend. The screen shows the bearing scale and digital timer approximately 15 seconds after power-on. The timer counts down three minutes of warm-up time. During this period the magnetron, or the transmitter tube, is warmed for transmission. When the timer has reached 0:00, the legend STBY appears indicating that the radar is now ready to transmit pulses. In warm-up and standby condition, you will see the message BRG SIG MISSING. This is normal because a bearing signal is not yet generated when the antenna is not rotating. ON TIME and TX TIME values shown at the bottom of the screen are the time counts in hours and tenths of hour when the radar has been powered on and transmitted. 6.1.8 Transmitter on When the STANDBY status is displayed on the screen, press the Transmit switch labeled ST-BY/TX on the control panel of the display unit. The radar is initially set to previously used range and pulse width. Other settings such as brilliance levels, VRMs, ELBs and menu option selections are also set to previous settings. The Transmit switch toggles the radar between STANDBY and TRANSMIT status. The antenna stops in STANDBY status and rotates in TRANSMIT status. Notes: 150 1. If the antenna does not rotate in TRANSMIT status, check whether the antenna switch in the tuning compartment is in the OFF position. 2. The magnetron ages with time resulting in a reduction of output power. It is highly recommended that the radar be set to STANDBY status when not used for an extended period of time. 6.1.9 CRT brilliance Operate the BRILL control on the control panel of the display unit to adjust the entire screen brightness. Note that the optimum point of adjustment varies with ambient light conditions, especially between daytime and nighttime. Note: The CRT brilliance should be adjusted before adjusting relative brilliance levels on the BRILLIANCE menu to be explained later. 6.1.10 Tuning the receiver 6.1.10.1 Auto tune The radar receiver is tuned automatically each time the power is turned on, thus there is no front panel control for tuning purpose. The tuning indicator and the label AUTO TUNE at the top right corner of the display unit show the tuning circuit is working. If the label AUTO TUNE is not displayed, check that the TUNE selector in tuning compartment is the AUTO position. 6.1.10.2 Manual tune If you are not satisfied with the current auto tune setting, follow these steps to fine-tune the receiver: 1. Push the tune control so that it pops up. 2. Set the TUNE selector in the tuning compartment to MAN for manual tuning. 3. While observing the picture on the 48 mile scale, slowly adjust TUNE control and find the best tuning point. 4. So the TUNE selector to AUTO and wait for about 10 seconds or four scanner rotations. 5. Make sure that the radar has been set to the best tuning point. This condition is where the tuning indicator lights to about 80% of its total length. 6. Push the TUNE control into the retracted position. 6.1.10.3 Video Lockup Recovery Video lockup, or picture freeze, can occur unexpectedly on digital rasterscan radars. This is mainly caused by heavy spike noise in the power line and can be noticed by carefully watching the nearly invisible sweep line. If you suspect that the picture is not updated every scan of the antenna or no key entry is accepted notwithstanding the apparently normal picture, do Quick Start to restore normal operation: 1. Turn off the power switch and turn it on again within five seconds. 2. Push the ST-BY switch in the tuning compartment. 151 3. Push the Transmit switch labeled ST-BY/TX for Transmit status. 6.1.10.4 Degaussing the CRT screen Each time the radar is turned on, the degaussing circuit automatically demagnetizes the CRT screen to eliminate color contamination caused by earth’s magnetism or magnetized ship structure. The screen is also degaussed automatically when own ship has made a significant course change. While being degaussed, the screen may be disturbed momentarily with vertical lines. If you wish to degauss by manual operation at an arbitrary time, open and press the Degauss switch in the tuning compartment. 6.1.10.5 Initializing the gyro readout Provided that your radar is interfaced with a gyrocompass, ship’s heading is displayed at the top of the screen. Upon turning on the radar, align the onscreen GYRO readout with the gyrocompass reading by the procedure shown below. Once you have set the initial heading correctly, resetting is not usually required. However, if the GYRO readout goes wrong for some reason, repeat the procedure to correct it. 1. Open the tuning compartment and press the HOLD button. The Gyro LED lights. 2. Press the UP or DOWN button to duplicate the gyrocompass reading at the on screen GYRO readout. Each press of these buttons changes the readout by 0.1-degree steps. To change the readout quickly, hold the UP or DOWN button for over two seconds. 3. Press the HOLD switch when the on screen GYRO readout has matched the gyrocompass reading. The Gyro LED goes out. Note: The HOLD button is used to disengage the built-in gyro interface from the gyrocompass input in the event that you have difficulty in fine-adjusting the GYRO readout due to ship’s yawing, for example. When initializing the GYRO readout at a berth (where the gyrocompass reading is usually stable), you may omit steps 1 and 3 above. 6.1.11 Presentation modes This radar has the following presentation modes: 6.1.11.1 Relative Motion (RM) Head-up: Unstabilized Head-up TB: Head-up with compass-stabilized bearing scale (True Bearing) Course-up: Compass-stabilized relative to ship’s intended course North-up: Compass-stabilized with reference to north) 6.1.11.2 True Motion (TM) North-up: Ground or sea stabilized with compass and speed inputs 6.1.12 Selecting presentation mode Press the MODE key on the mode panel. Each time the MODE key is pressed, the presentation mode and mode indication at the upper-left corner of the screen change cyclically. Loss of Gyro Signal: When the gyro signal is lost, the presentation mode 152 automatically becomes head-up and the GYRO readout at the screen top shows asterisks (***.*). The message SET HDG appears at the upper of the screen. This warning stays on when the gyro signal is restored, to warn the operator that the readout may be unreadable. Press the MODE key to select another presentation mode (the asterisks are erased at this point). Then, align the GYRO readout with the gyrocompass reading and press the CANCEL key to erase the message SET HDG. 6.1.12.1 Head-up Mode A display without azimuth stabilization in which the line connecting the center with the top of the display indicates own ship’s heading. The target pips are painted at their measured distances and in their directions relative to own ship’s heading. A short line on the bearing scale is the north marker indicating compass north. A failure of the gyro input will cause the north marker to disappear and the GYRO readout to show asterisks (***.*) and the message SET HDG appears on the screen. Figure 6.1 - Head-up Mode 6.1.12.2 Course-up Mode An azimuth stabilized display in which a line connecting the center with the top of the display indicates own ship’s intended course (namely, own ship’s previous heading just before this mode has been selected). Target pips are painted at their measured distances and in their directions relative to the intended course which is maintained at the 0 position while the heading marker moves in accordance with ship’s yawing and course changes. This mode is useful to avoid smearing of picture during course change. After a course change, press the (CU, TM RESET) key to reset the picture orientation if you wish to continue using the course up mode. 153 Figure 6.2 - Course-up Mode 6.1.11.3 Head-up TB (True Bearing) Mode Radar echoes are shown in the same way as in the head-up mode. The difference from normal head-up presentation lies in the orientation of the bearing scale. The bearing scale is compass stabilized, that is, it rotates in accordance with the compass signal, enabling you to know own ship’s heading at a glance. This mode is available only when the radar in interfaced with a gyrocompass. 154 Figure 6.3. - Head-up TB (True Bearing) Mode 6.1.12.4 North-up Mode In the north-up mode, target pips are painted at their measured distances and in their true (compass) directions from own ship, north being maintained UP of the screen. The heading marker changes its direction according to the ship’s heading. If the gyrocompass fails, the presentation mode changes to head-up and the north marker disappears. Also, the GYRO readout shows asterisks (***.*) and the message SET HDG appears on the screen. Figure 6.4 - North-up Mode 6.2. Acquisition of Targets 6.2.1 AUTOMATIC ACQUISITION 6.2.1.1 Target acquisition Targets can be acquired manually by the operator or automatically using operator definable auto-acquisition zones. When a target enters an auto-acquisition zone, an alarm is raised and the target is automatically “acquired”. Auto acquisition zones are available in all presentation and motion modes. Targets cannot be acquired within 0.25 nm of own ship. 6.2.1.2 Target tracking limitations 155 • When the maximum numbers of targets are being tracked, the TRACKS FULL alarm is raised and another target cannot be acquired until one or more targets are cancelled; • If the radar is switched to standby, all targets will be cancelled automatically; • Already acquired targets are dead reckoned (DR) when within 0.25 nm of own ship. The integrity of ARPA tracking is a function of many variables which include clutter conditions, signal-to-noise ratio and sensor errors (log, compass, main input etc.). The design of the tracker minimises the effects of these errors but the operator must be aware that such errors will produce discrepancies in derived tracked target information such as true speed, course, bearing, CPA and TCPA. The possibility of target swoop is minimised by the use of damped plot predictions in the tracker. The ARPA tracker employs advanced rain and sea clutter rejection techniques independent of the display settings. A fully established tracked target will not be affected by large levels of sea or rain clutter, however attempting to acquire a target at close range in severe clutter conditions, may cause the occasional appearance of the lost target symbol and its associated alarm. When changing from one speed mode to another, and particularly between a water speed and a ground speed mode, the vectors take some time to resettle. Three minutes should be allowed to obtain full accuracy when switching between speed modes. 6.2.1.3 Compass errors If targets are being tracked, a compass error will cause affected target tote data to change from green to red. The affected data being TBRG, CPA, TCPA, COG (or CSE), SOG (or STW), BCR and BCT. After 1 minute all tracked targets will be cancelled; auto acquisition zones, mapping facilities, the constant radius turn and plots will be switched off and it will not be possible to use these facilities, or select a stabilised mode, until a valid compass heading is available. The system will reset to the H-Up presentation mode. 6.2.1.4 Target alarm symbols If an alarm is raised against a target currently in the video circle, an alarm symbol is displayed. This symbol flashes until the alarm is acknowledged. The alarm symbol then remains displayed as long as the alarm condition exists. Even if the target is not currently displayed in the video circle, an alarm will still be raised. An unacknowledged alarm always has a higher priority than an acknowledged alarm. The following alarm symbols, listed in order of priority, are used: If the radar hasn't been able to obtain successfully the position of a target, which is being used as an echo reference, during the last three radar scans, a LOST REF alarm is raised. If an acquired target infringes the bow crossing limits, a BOW CROSSING alarm is raised. If an acquired target infringes the CPA and TCPA limits, a CPA/TCPA alarm is raised. When a target enters an auto-acquisition zone, an AZ ENTRY alarm is raised. If the radar hasn't been able to obtain successfully a target's position during the last six radar scans, a LOST TARGET alarm is raised. 156 6.2.2 Guard Zones Two annular guard zones are available which are always displayed relative to own ship's head. Guard zones are active on all ranges in all motion and presentation modes. When a target enters a guard zone an alarm is raised. If 60 infringements have been detected, a ZONES FULL alarm is raised. 6.2.2.1 Accessing the Guard Zones Menu 1. Position the screen cursor over the “AZ” soft key. 2. Left click to reveal the Guard Zones menu. A left click on the “EXIT GUARD ZONES” soft key will close the GUARD ZONES menu. Figure 6.5 – Selection menu for Guard Zones 6.2.2.2. Turning Guard Zones On/Off Note: Guard zones retain their definitions when turned off. 1. Position the screen cursor over a “ZONE” line in the menu. 2. Left click to toggle the selected zone ON and OFF. 6.2.2.3. Defining a Guard Zone Note: A guard zone is not active while it is being defined. 1. Position the cursor over an “EDIT” soft key. 2. Left click to select edit mode for the associated guard zone. 3. Move the cursor to the centre of the video circle from where the zone can be edited. The other zone will temporarily be displayed at its last setting for reference purposes (provided the range in use is suitable), but will not detect any infringements unless it is currently ON. The selected zone is displayed in a different colour and the associated ZONE ON/OFF line in the menu shows EDIT. 157 4. Edit the zone as described in Annular Zone Editing later in this chapter. 5. Select another soft key (EDIT or EXIT GUARD ZONES) to store the new zone and automatically switch it on. 6.2.2.3. Annular Zone editing. Changing the Start/Stop Bearing 1. Position the cursor over the start or stop bearing as required. 2. Press and hold down the left key. 3. Drag the start or stop bearing to its new position. 4. Release the key. 6.2.2.2.3.1. Changing the Range of a Zone 1. Place the cursor over the inner arc of the annulus. 2. Press and hold down the left key. 3. Drag the entire zone to its new position. 4. Release the key. 6.2.2.2.3.2. Altering the Depth of the Zone 1. Place the cursor over the outer arc of the annulus. 2. Press and hold down the left key. 3. Drag the outer arc to its new position. 4. Release the key. The ARPA can acquire up to 40 targets (20 automatically and 20 manually or all 40 manually). If AUTO ACQ is selected after more than 20 targets have been manually acquired, only the remaining capacity of targets can be automatically acquired. For example, when 30 targets have been acquired manually, then the ARPA is switched to AUTO ACQ. Only 10 targets can be acquired automatically. A target just acquired automatically is marked with a broken square and a vector appears about one minute after acquisition indicating the target’s motion trend. Three minutes after acquisition, the initial tracking stage is finished and the target becomes ready for stable tracking. At this point, the broken square mark changes to a solid circle. (Targets automatically acquired are distinguished from those acquired manually, displayed by bold symbol). Up to 40 targets can be track simultaneously. These targets can be manually acquired using the PLOT key (renamed MANUAL ACQ for ARPA), or with one or both of the guard zones set, targets will be automatically acquired as they enter the zones. Once the total number of targets being tracked reaches 39, the TRACKS FULL alarm is raised to indicate that only one more target can be acquired. Each guard zone can have up to ten acquired targets in it any time. Violation symbols (red inverted triangle) are displayed and the GZ1 or GZ2 ALARM is raised whenever a new target enters the guard zone. When a target leaves a guard zone, it becomes a manually acquired target, and thus allows another target to be automatically acquired in the guard zone. When the total number of targets being tracked in a zone exceeds ten, new guard zone violations are shown as lines in the guard zone, and will not be tracked. The tracking sequence is the same for all targets: 158 6.2.3. Tracking sequence 1. On the first scan of the target, a broken white square is displayed over the target; this symbol is called the initial tracking symbol. For a manual target a solid green square is also displayed; this symbol is called the tote target symbol and means that information for this target is now shown in the menu box. Initially only range and bearing information is displayed. 2. After 16 successful scans of the target, the initial tracking symbol is removed and the vector (green line) for the target is displayed. The vector has a green dot at the target end of the line. At this stage, the target is said to be established and the vector shows the approximate (trend) course and speed of the target. The information shown in the menu box will now be completed for that target (that is the target with the tote target symbol). After approximately three minutes, full operational accuracies will be achieved on target vector and information. 3. If an alarm condition arises for a tracked target, the target symbol is replaced by a red alarm symbol which flashes until the ALARM ACK key is pressed. 6.2.4. Autodrop As the name implies, tracked targets will be automatically deleted from the tracked target list when certain conditions obtain. 6.2.4.1. Autodrop rules 1. No video information (lost target) is found for 15 scans (except for an echo reference target). 2. The radar has been set to STANDBY. 6.2.4.2. Target information When a target is being tracked, full target information is displayed in the menu box, as shown below: The full target information is as follows: • Target Name* and Identification Number; • CPA – Closest Point of Approach; • TCPA – Time to Closest Point of Approach; • RANGE – target Range from own ship; • BRG – target Bearing from own ship; • T CO – target Course (see below); • SPEED – target speed. * if previously allocated via Autotrack/ARPA menu. Some of the target information depends on the speed mode selected via the Speed menu. Using Manual Speed or Serial Logs means that the picture is Sea Stabilised, whereas Echo Ref, Nav G, and some Serial Logs provide Ground Stabilisation. With Sea Stabilisation, target information is also sea stabilised i.e., the course is the water track and the speed is the speed through the water. When operating with Ground Stabilisation, 159 the target course and speed are Course Made Good (CMG) and Speed Made Good (SMG) respectively. 6.2.5 - Target tote (ARPA Only) An alternative display, which shows limited target information for up to five selected targets, can be displayed by a short press of the DATA SELECT key with the cursor positioned over a clear area of the display. The limited 5-Target information is as follows: ID – target Identification Number; TCPA – Time to Closest Point of Approach; CPA – Closest Point of Approach. Initially, the Tote display is blank. To select targets for display, move the cursor over selected targets and press the Data/Select key. The selected target identification numbers will be displayed against the target and its information will be shown in the Tote. To delete a target from the tote, position the cursor over it and press Data/Select. This action does not cancel the target from the tracked target list. A long key press of the Data/Select removes the target table from the screen, and the operator can obtain data on any one selected target. To show table of the targets again, place the cursor in a clear area and short press Data/Select. 6.2.5.1. Past positions The Past Positions of all targets can be displayed by selection via the ARPA Menu. Past Positions are displayed as a series of dots on the 0.75 to 24 n.mile range scales inclusive: four dots at 3-minute time intervals. The term Past Positions is used in accordance with IMO definitions. In some earlier ARPA radars, the term History Dots was used for this function. 6.2.6. Enabling and disabling auto acquisition 1. Press the E, AUTO PLOT key if the ARPA is not yet activated. Note that the label ARPA appears in the box at the upper right on the screen. 2. Press the E, AUTO PLOT MENU key to show the ARPA 1 menu. 3. Press the (1) key to select menu item 1 AUTO ACQ. 4. Further press the (1) key to select (or highlight) ON (enable auto acquisition) or OFF (disable auto acquisition) as appropriate. 5. Press the ENTER key to conclude your selection followed by the E, AUTO PLOT MENU key to close the AUTO PLOT 1 menu. Note that the label AUTO MAN is displayed in the box at the upper right on the screen when auto acquisition is enabled; MAN when auto acquisition is disabled. Note: When the ARPA has acquired 20 targets automatically, the message AUTO TARGET FULL is displayed in the box at the right hand side of the screen. 160 6.2.7. Setting auto acquisition areas Instead of limits lines, auto acquisition areas are provided in the system. There are two setting methods: 3, 6 Nautical Miles: Two predefined auto acquisition areas; one between 3.0 and 3.5 nautical miles and the other between 5.5 and 6.0 nautical miles. SET: Two sector shaped or full circle auto acquisition areas set by using the trackball. To activate two predefined auto acquisition areas (3 & 6 NM): 1. Press the E, AUTO PLOT MENU key to show the ARPA 1 menu. 2. Press the (2) key to select menu item 2 AUTO ACQ AREA. 3. Further press the (2) key to select (or highlight) menu option 3, 6 nautical miles. 4. Press the ENTER key to confirm your selection followed by the E, AUTO PLOT MENU key to close the ARPA 1 menu. To set auto acquisition areas with trackball: 1. Press the E, AUTO PLOT MENU key to show the ARPA 1 menu. 2. Press the (2) key to select menu item 2 AUTO ACQ AREA. 3. Further press the (2) key to select (or highlight) SET option. 4. Press the ENTER key to conclude your selection. At this point the AUTO ACQ SETTING menu is displayed at the screen bottom. 5. Press the (2) key to select menu item 2 1/2 and press the ENTER key. 6. Place the cursor at the outer counter clockwise corner of the area and press the ENTER key. 7. Place the cursor at the clockwise edge of the area and press the ENTER key. Note: If you wish to create an auto acquisition area having a 360 degree coverage around own ship, set point B in almost the same direction (approx. +/-3) as point A and press the ENTER key. 8. Repeat steps 5 and 7 above if you want to set another auto acquisition area with the trackball. 9. Press the (1) key followed by the E, AUTO PLOT MENU key to close the ARPA 1 menu. An auto acquisition area like the example shown above appears on the display. Note that each auto acquisition area has a fixed radial extension width of 0.5 nautical miles. Note that the auto acquisition areas are preserved in an internal memory of the ARPA even when auto acquisition is disabled or the ARPA is turned off. 6.2.8. Terminating tracking of targets When the ARPA has acquired 20 targets automatically, the message AUTO TARGET FULL is displayed in the box at right hand side of the screen and no more auto acquisition occurs unless targets are lost. You may find this message before you set an auto acquisition area. Should this happen, cancel tracking of less important targets or perform manual acquisition. 161 6.2.8.1. Individual targets Place the cursor (+) on a target to cancel tracking by operating the trackball. Press the CANCEL key. 6.2.8.2. All targets Press and hold the CANCEL key down more than 3 seconds. In the automatic acquisition mode, acquisition begins again. 6.2.8.3. Discrimination between landmass and true targets A target is recognized as a landmass and thus not acquired if it is 800 meters or more in range or bearing direction. 6.2.9. Manual acquisition In auto acquisition mode (AUTO ACQ ON), up to 20 targets can be manually acquired in addition to 20 auto acquired targets. When auto acquisition is disabled (AUTO ACQ OFF), up to 40 targets can be manually acquired and automatically tracked. To manually acquire a target: 1. Place the cursor (+) on a target of interest by operating the trackball. 2. Press the ACQ key on the control panel. The selected plot symbol is marked at the cursor position. Note that the plot symbol is drawn by broken lines during the initial tracking stage. A vector appears in about one minute after acquisition indicating the target’s motion trend. If the target is consistently detected for three minutes, the plot symbol changes to a solid mark. If acquisition fails, the target plot symbol blinks and disappears shortly. Notes: 1. For successful acquisition, the target to be acquired should be within 0.1 to 32 nautical miles from own ship and not obscured by sea or rain clutter. 2. When you have acquired 40 targets manually, the message MAN TARGET FULL is displayed at the screen bottom. Cancel tracking of non threatening targets if you wish to acquire additional targets manually. 6.2.10. Criteria for selecting targets for tracking The FURUNO ARPA video processor detects targets in midst of noise and discriminates radar echoes on the basis of their size. Target whose echo measurements are greater than those of the largest ship in range or tangential extent are usually land and are displayed only as normal radar video. All smaller ship sized echoes which are less than this dimension are further analyzed and regarded as ships and displayed as small circles superimposed over the video echo. When a target is first displayed, it is shown as having zero true speed but develops a course vector as more information is collected. In accordance with the International Maritime Organization Automatic Radar Plotting Aid requirements, an indication of the motion trend should be available in 1 minute and full vector accuracy in 3 minutes of plotting. The FURUNO ARPAs comply with these requirements. 162 6.2.10.1. Acquisition and tracking A target which is hit by 5 consecutive radar pulses is detected as a radar echo. Manual acquisition is done by designing a detected echo with the trackball. Automatic acquisition is done in the acquisition areas when a target is detected 5-7 times continuously depending upon the congestion. Tracking is achieved when the target is clearly distinguishable on the display for 5 out of 10 consecutive scans whether acquired automatically or manually. Targets not detected in 5 consecutive scans become “lost targets”. 6.2.10.2. Quantization The entire picture is converted to a digital from called “Quantized Video”. A sweep range is divided into small segments and each range elements is “1” if there is radar echo return above a threshold level, or “0” if there is no return. The digital radar signal is then analyzed by a ship sized echo discriminator. As the antenna scans, if there are 5 consecutive radar pulses with l’s indicating an echo presence at the exact same range, a target “start” is initiated. Since receiver noise is random, it is not three bang correlated, and it is filtered out and not classified as an echo. 6.3. Tracking Capabilities and Limitations 6.3.1. Minimum Range The minimum range is defined by the shortest distance at which, using a scale of 1.5 or 0.75 nm, a target having an echoing area of 10 square meters is still shown separate from the point representing the antenna position. It is mainly dependent on the pulse length, antenna height, and signal processing such as main bang suppression and digital quantization. It is good practice to use a shorter range scale as far as it gives favorable definition or clarity of picture. The IMO Resolution A. 477 (XII) and IEC 936 require the minimum range to be less than 50m. All FURUNO radars satisfy this requirement. 6.3.2. Maximum Range The maximum detecting range of the radar, Rmax, varies considerably depending on several factors such as the height of the antenna above the waterline, the height of the target above the sea, the size, shape and material of the target, and the atmospheric conditions. Under normal atmospheric conditions, the maximum range is equal to the radar horizon or a little shorter. The radar horizon is longer than the optical one about 6% because of the diffraction property of the radar signal. It should be noted that the detection range is reduced by precipitation (which absorbs the radar signal). 6.3.3. X-BAND and S-BAND In fair weather, the above equation does not give a significant difference between X and S band radars. However, in heavy precipitation condition, an S band radar would have better detection than X band. 6.3.3. Radar Resolution There are two important factors in radar resolution: bearing resolution and range resolution. 163 6.3.4. Bearing Resolution Bearing resolution is the ability of the radar to display as separate pips the echoes received from two targets which are at the same range and close together. It is proportional to the antenna length and reciprocally proportional to the wavelength. The length of the antenna radiator should be chosen for a bearing resolution better than 2.5 (IMO Resolution). This condition is normally satisfied with a radiator of 1.2 meters (4 feet) or longer in the X band. The S band radar requires a radiator of about 12 feet (3.6 meters) or longer. 6.3.5. Range Resolution Range resolution is the ability to display as separate pips the echoes received from two targets which are on the same bearing and close to each other. This is determined by pulselength only. Practically, a 0.08 microsecond pulse offers the discrimination better than 25 meters as do so with all Furuno radars. Test targets for determining the range and bearing resolution are radar reflectors having an echo area of 10 square meters. 6.3.6. Bearing Accuracy One of the most important features of the radar is how accurately the bearing of a target can be measured. The accuracy of bearing measurement basically depends on the narrowness of the radar beam. However, the bearing is usually taken relative to the ship’s heading, and thus, proper adjustment of the heading marker at installation is an important factor in ensuring bearing accuracy. To minimize error when measuring the bearing of a target, put the target echo at the extreme position on the screen by selecting a suitable range. 6.3.7. Range Measurement Measurement of the range to a target is also a very important function of the radar. Generally, there are two means of measuring range: the fixed range rings and the variable range marker (VRM). The fixed range rings appear on the screen with a predetermined interval and provide a rough estimate of the range to a target. The variable range marker’s diameter is increased or decreased so that the marker touches the inner edge of the target, allowing the operator to obtain more accurate range measurements. 6.3.8. FALSE ECHOES Occasionally echo signals appear on the screen at positions where there is no target or disappear even if there are targets. They are, however, recognized if you understand the reason why they are displayed. Typical false echoes are shown below. 6.3.8.1. Multiple echoes Multiple echoes occur when a transmitted pulse returns from a solid object like a large ship, bridge, or breakwater. A second, a third or more echoes may be observed on the display at double, triple or other multiples of the actual range of the target. Multiple reflection echoes can be reduced and often removed by decreasing the gain (sensitivity) or properly adjusting the A/C SEA control. 164 Fig 6.6 - Multiple echoes 6.3.8.2. Side lobe echoes Every time the radar pulse is transmitted, some radiation escapes on each side of the beam, called “sidelobes”. If a target exists where it can be detected by the side lobe as well as the main lobe, the side echoes may be represented on both sides of the true echo at the same range. Side lobes show usually only on short ranges and from strong targets. They can be reduced through careful reduction of the gain or proper adjustment of the A/C SEA control. 6.3.8.3. Indirect Echoes Indirect echoes are caused by the reflection of the outgoing pulse against part of the superstructure (funnel or foremast) or against a building cliff river bank bridge or ship etc. nearby. The echo pulse returns the same way and due to reflection, it paints its echo in the wrong direction on the screen. What really happens is that we see the target Via a mirror. See fig 6.7. The most offending parts are the cross-trees of the foremast. Their mirror effect is worse than that of the funnel because the pulse reflected by the cross-trees contains more concentrated energy than that reflected by the funnel. Containers on deck can also be the cause of strong indirect echoes, An excellent remedy is to cover the cross-trees with angle bars or a piece of corrugated metal, so that the reflected energy returns directly back? The scanner when the receiver is paralyzed or is scattered m all directions, thereby losing all its energy. Radar absorbent material (RAM) is also available for this purpose. In some radars transmissions switched off when the radar beam passes through a ship's blind sector. Owing to its curvature, the funnel may distort the .echo. Ann indirect echo, from a ship approaching from ahead, reflected via the funnel, will show up on the screen as an overtaking vessel. An indirect echo reflected via the cross-trees from a receding cliff or building appears on the screen as a target ahead of own ship, moving away and possessing twice her speed. 165 Large ships passing close by in confined waters may give rise to part of the picture being re-produced in reverse on the screen.. These strong indirect echoes move very fast in a circular path over the display. A ship dead astern sometimes gives rise to four echoes on the screen. Three of them nearly in one line and perpendicular to the course, show up forward and one shows up abaft the beam. The three indirect echoes forward' of the beam are caused by reflection via the samson posts on the fore ship and the foremast, the fourth one is the true echo. When there are many echoes on the screen, indirect echoes are likely to escape notice. It is when the screen is reasonably clear teat one starts wondering about some echo, which one knows, from experience or visual observations should not be there. Indirect echoes can appear from targets at quite large ranges provided the reflecting surface has an excellent aspect. They can be wiped off the display by reduction of gain or clutter. Characteristics of indirect echoes can be summed up as follows: (a) They usually appear in blind or shadow sectors and areas because the obstructions producing these regions of reduced response often themselves act as "mirrors". See Fig. 6.7. (b) When caused by the ship's obstructions, they will appear on the same relative bearing (foremast or funnel) although the bearing of the target may change. This is also the case when the ship is stationary and the indirect echo is caused by shore object (Fig. 6.7 (e). If the ship is moving, in the latter case indirect echoes will appear only for a very short time. (e) When caused by an obstruction on own ship the true echo and its image, the false echo, appear at about the same rampage. This is because the range discrimination is not sufficient to measure the difference in distance between the two routes of the pulse (Fig. 6.7 (a) and (b). (d) When caused by an obstruction not on own ship the range of the false echo will be the range of the obstruction from own ship plus the range of the true echo from the obstruction. See Fig. 6.7(e). (e) Movements of false echoes are abnormal if they are compared with the movements on the screen of the true echoes. (j) There is a distortion in shape (via the funnel) and in presentation on the screen as only the best reflecting surfaces of a large target can produce a false echo (for example, a false echo of a river bank could be presented on the screen as a single spot). (g) When caused by obstructions ("mirrors on own ship an alteration of course will make the false echo disappear although another false echo may appear on the same relative bearing. 166 Fig 6.7 - Indirect echoes 6.3.8.3. Virtual image A relatively large target close to your ship may be represented at two positions on the screen. One of them is the true echo directly reflected by the target and the other is a false echo which is caused by the mirror effect of a large object on or close to your ship. If your ship comes close to a large metal bridge, for example, such a false echo may temporarily be seen on the screen. Shadow sectors Funnels, stacks, masts, or derricks in the path of the antenna block the radar beam. If the angle subtended at the scanner is more than a few degrees, a non-detecting sector may be produced. Within this sector targets cannot be detected. 167 7. OPERATE AN ARPA SYSTEM 7.1. ARPA Displays 7.1.1. Introduction From the time radar was first introduced to the present day the radar picture has been presented on the screen of a cathode ray tube. Although the cathode ray tube has retained its function over the years, the way in which the picture is presented has changed considerably. From about the mid-1980s the first raster-scan displays appeared. The radial-scan PPI was replaced by a raster-scan PPI generated on a television type of display. The integral ARPA and conventional radar units with a raster-scan display will gradually replace the radial-scan radar sets. The development of commercial marine radar entered a new phase in the 1980s when raster-scan displays that were compliant with the IMO Performance Standards were introduced. The radar picture of a raster-scan synthetic display is produced on a television screen and is made up of a large number of horizontal lines which form a pattern known as a raster. This type of display is much more complex than the radialscan synthetic display and requires a large amount of memory. there are a number of advantages for the operator of a raster-scan display and concurrently there are some deficiencies too. The most obvious advantage of a raster-scan display is the brightness of the picture. This allows the observer to view the screen in almost all conditions of ambient light. Out of all the benefits offered by a raster-scan radar it is this ability which has assured its success. Another difference between the radial-scan and raster-scan displays is that the latter has a rectangular screen. The screen size is specified by the length of the diagonal and the width and height of the screen with an approximate ratio of 4:3. 7.1.2. Features The FR-2805 and FAR-2805 series of Radar and ARPAs are designed to fully meet the exacting rules of the International Maritime Organization (IMO) for installations on all classes of vessels. The display unit employs a 28 inch diagonal multicolored CRT. It provides an effective radar picture of 360 mm diameter leaving sufficient space for on screen alpha-numeric data. Target detection is enhanced by the sophisticated signal processing technique such as multi-level quantization (MLQ), echo stretch, echo average, and a built-in radar interference rejector. Audible and visual guard zone alarms are provided as standard. Other ship’s movement is assessed by trails of target echoes or by electronic plotting. The FAR-2805 series ARPA further provides target assessment by historical plots, vectors and target data table. On screen data readouts include CPA, TCPA, range, bearing, speed/course on up to 3 targets at a time. The ARPA functions include automatic acquisition of up to 20 targets, or manual acquisition of 40 targets. In addition, the ARPA features display of a traffic lane, buoys, dangerous points, and other important reference points. 168 Figure 7.1 – ARPA radar 7.1.3. General features · Daylight-bright high-resolution display · 28 inch diagonal CRT presents radar picture of 360 mm effective diameter with alphanumeric data area around it · User friendly operation by combination of tactile backlit touch pads, a trackball and rotary controls · Audio-visual alert for targets in guard zone · Echo trail to assess targets’ speed and course by simulated afterglow · Electronic plotting of up to 10 targets in different symbols (This function is disabled when ARPA is activated) · Electronic parallel index lines · Interswitch (optional) built in radar or ARPA display unit · Enhanced visual target detection by Echo Average, Echo Stretch, Interference Rejector, and multi-level quantization · Stylish display · Choice of 10, 25 or 50 KW output for X-band; 30 KW output for S-band, either in the transceiver aloft (gearbox) or RF down (transceiver in bridge) · Exclusive FURUNO MIC low noise receiver 7.1.4 ARPA features · Acquires up to 20 targets automatically · Movement of tracked targets shown by true or relative vectors (Vector length 1 to 99 min. selected in 1 min steps) · Setting of nav lines, buoy marks and other symbols to enhance navigation safety · On-screen digital readouts of range, bearing, course, speed, CPA, TCPA, BCR (Bow Crossing Range) and BCT (Bow Crossing Time) of two targets out of all tracked targets. 169 · Audible and visual alarms against threatening targets coming into operator-selected CPA/TCPA limits, lost targets, two guard rings, visual alarm against system failure and target full situation · Electronic plotting of up to 10 targets in different symbols (This function is disabled when ARPA is activated) · Electronic parallel index lines · Interswitching (optional) built in radar or ARPA display unit · Enhanced visual target detection by Echo Average, Echo Stretch, Interference Rejector, and multi-level quantization · Stylish display · Choice of 10,25 or 50 kW output for X-band; 30kw output for S-band, either in the transceiver aloft (gearbox) or RF down (transceiver in bridge) · Exclusive FURUNO MIC low noise receiver 7.1.5. Degaussing the CRT screen Each time the radar is turned on, the degaussing circuit automatically demagnetizes the CRT screen to eliminate color contamination caused by earth’s magnetism or magnetized ship structure. The screen is also degaussed automatically when own ship has made a significant course change. While being degaussed, the screen may be disturbed momentarily with vertical lines. If you wish to degauss by manual operation at an arbitrary time, open and press the Degauss switch in the tuning compartment. 7.1.6. Initializing the gyro readout Provided that your radar is interfaced with a gyrocompass, ship’s heading is displayed at the top of the screen. Upon turning on the radar, align the onscreen GYRO readout with the gyrocompass reading by the procedure shown below. Once you have set the initial heading correctly, resetting is not usually required. However, if the GYRO readout goes wrong for some reason, repeat the procedure to correct it. 1. Open the tuning compartment and press the HOLD button. The Gyro LED lights. 2. Press the UP or DOWN button to duplicate the gyrocompass reading at the on screen GYRO readout. Each press of these buttons changes the readout by 0.1-degree steps. To change the readout quickly, hold the UP or DOWN button for over two seconds. 3. Press the HOLD switch when the on screen GYRO readout has matched the gyrocompass reading. The Gyro LED goes out. Note: The HOLD button is used to disengage the built-in gyro interface from the gyrocompass input in the event that you have difficulty in fine-adjusting the GYRO readout due to ship’s yawing, for example. When initializing the GYRO readout at a berth (where the gyrocompass reading is usually stable), you may omit steps 1 and 3 above. 7.1.7. PRESENTATION MODES This radar has the following presentation modes: 170 7.1.7.1. Relative Motion (RM) Head-up: Unstabilized Head-up TB: Head-up with compass-stabilized bearing scale (True Bearing) Course-up: Compass-stabilized relative to ship’s intended course North-up: Compass-stabilized with reference to north) 7.1.7.2. True Motion (TM) North-up: Ground or sea stabilized with compass and speed inputs 7.1.8. Selecting presentation mode Press the MODE key on the mode panel. Each time the MODE key is pressed, the presentation mode and mode indication at the upper-left corner of the screen change cyclically. Loss of Gyro Signal: When the gyro signal is lost, the presentation mode automatically becomes head-up and the GYRO readout at the screen top shows asterisks(***.*). The message SET HDG appears at the upper of the screen. This warning stays on when the gyro signal is restored, to warn the operator that the readout may be unreadable. Press the MODE key to select another presentation mode (the asterisks are erased at this point). Then, align the GYRO readout with the gyrocompass reading and press the CANCEL key to erase the message SET HDG. 7.1.8.1. Head-up Mode (Figure 7.2) A display without azimuth stabilization in which the line connecting the center with the top of the display indicates own ship’s heading. The target pips are painted at their measured distances and in their directions relative to own ship’s heading. A short line on the bearing scale is the north marker indicating compass north. A failure of the gyro input will cause the north marker to disappear and the GYRO readout to show asterisks (***.*) and the message SET HDG appears on the screen. Figure 7.2 - Head-up Mode 171 7.1.8.2. Course-up Mode (Figure 7.3) An azimuth stabilized display in which a line connecting the center with the top of the display indicates own ship’s intended course (namely, own ship’s previous heading just before this mode has been selected). Target pips are painted at their measured distances and in their directions relative to the intended course which is maintained at the 0 position while the heading marker moves in accordance with ship’s yawing and course changes. This mode is useful to avoid smearing of picture during course change. After a course change, press the (CU, TM RESET) key to reset the picture orientation if you wish to continue using the course up mode. Figure 7.3.- Course-up Mode 7.1.8.3. Head-up TB (True Bearing) Mode (Figure 7.4) Radar echoes are shown in the same way as in the head-up mode. The difference from normal head-up presentation lies in the orientation of the bearing scale. The bearing scale is compass stabilized, that is, it rotates in accordance with the compass signal, enabling you to know own ship’s heading at a glance. This mode is available only when the radar in interfaced with a gyrocompass. Figure 7.4 - Head-up TB (True Bearing) Mode 172 7.1.8.4. North-up Mode Figure 7.6) In the north-up mode, target pips are painted at their measured distances and in their true (compass) directions from own ship, north being maintained UP of the screen. The heading marker changes its direction according to the ship’s heading. If the gyrocompass fails, the presentation mode changes to head-up and the north marker disappears. Also, the GYRO readout shows asterisks (***.*) and the message SET HDG appears on the screen. 7.1.8.5. True Motion Mode (Figure 7.5) Own ship and other moving objects move in accordance with their true courses and speeds. All fixed targets, such as landmasses, appear as stationary echoes. When own ship reaches a point corresponding to 75% of the radius of the display, the own ship is automatically reset to a point of 50% radius opposite to the extension of the heading marker passing through the display center. Resetting can be made at any moment before the ship reaches the limit by pressing the (CU, TM RESET) key. Automatic resetting is preceded by a beep sound. If the gyrocompass fails, the presentation mode is changed to the head-up mode and the north marker disappears. The GYRO readout at the top of the screen shows asterisks (***.*) and the message SET HDG appears on the screen. Figure 7.5 - True Motion Mode 173 Figure 7.6 - North-up Mode 7.1.8. True or relative vector True vectors can be displayed relative to own ship’s heading (Relative) or with reference to the north (True). Press the VECTOR TRUE/REL key to select the proper indication. This feature is available in all presentation modes (gyrocompass must be working correctly). The current vector mode is indicated at the upper right corner of the screen. 7.1.9. Vector time Vector time (or the length of vectors) can be set to 30 sec, 1, 2, 3, 6, 12, 15 or 30 minutes and the selected vector time is indicated at the upper right corner of the screen. Press the VECTOR TIME key until the desired vector time is reached. The vector tip shows an estimated position of the target after the selected vector time elapses. It can be valuable to extend the vector length to evaluate the risk of collision with any target. 7.1.10. Target data The radar calculates motion trends (range, bearing, course, speed, CPA, and TCPA) of all plotted targets. In head up and head up true bearing modes, target bearing, course and speed shown in the upper right target data field become true (suffix “T”) or relative (suffix “R”) to own ship in accordance with true/relative vector setting. In north up, course up, and true motion modes, the target data field always displays true bearing, true course and speed over the ground or through the water. 7.1.11. Reading the target data Press the corresponding plot symbol key, and the following target data is displayed. RNG/BRG: (Range/Bearing): Range and bearing from own ship to last plotted target with suffix “T” or “R” plot symbol. CSE/SPD: (Course/Speed): Course and speed are displayed for the last plotted target with suffix “T” or “R” plot symbol. CPA/TCPA: CPA is a closest range the target will approach to own ship. 174 TCPA is the time to CPA. Both are automatically calculated. TCPA is counted up to 99.9 minutes and beyond this., it is indicated as TCPA >*99.9 MIN. BCR/BCT: BCR (Bow Cross Range) is the range at which target will cross own ship’s bow. BCT (Bow Cross Time) is the estimated time at which target will cross own. 7.1.12. Terminating target plotting With E-plot you can plot up to 10 targets. You may wish to terminate plotting of less important targets to newly plot other threatening targets. By Symbol: To terminate plotting of a certain target, press the corresponding plot symbol key. Then press the CANCEL key. With Trackball: Place the cursor (+) on a target which you do not want to be tracked any longer by operating the trackball and press the CANCEL key. All Targets: To terminate plotting of all targets at once, press and hold the CANCEL key until all plot symbols and marks disappear in about 3 seconds. 7.1.13. Entering own ship’s speed EPA requires an own ship speed input and compass signal. The speed can be entered from a speed log (automatic) or through the plotting keypad 7.1.14. Automatic speed input 1. Press the RADAR MENU key on the plotting keypad to show the functions menu, 2. Press the (6) key to select menu item 6 SHIP’S SPEED. 3. Press the (6) key to select (or Highlight) LOG option. 4. Press the ENTER key to confirm your selection followed by the RADAR MENU key to close the FUNCTIONS menu. The ship’s speed readout at the screen top shows own ship’s speed fed from the speed log preceded by the label “LOG”. Notes: 1. IMO Resolution A.823(19) for ARPA recommends that a speed log to be interfaced with an ARPA should be capable of providing through-the-water speed data. 2. Be sure not to select LOG when a speed log is not connected. If the log signal is not provided, the ship’s speed readout at the screen top will be blank. 7.1.15. Manual speed input If the radar is not interfaced with a speed log, or the speed log does not feed correct speed enter the ship’s speed as follows: 1. Press the RADAR MENU key on plotting keypad to show the FUNCTIONS menu. 2. Press the (6) key to select menu 6 SHIP’S SPEED. 3. Press the (6) key to select menu 6 SHIP’s SPEED. 4. Press the ENTER key to confirm selection. At this point, “MAN+XX.KT” appears at the bottom of the FUNCTIONS menu. 175 5. Enter the ship speed by hitting corresponding numeric keys followed by the ENTER without omitting leading zeros, if any. As an example, if the ship speed is 8 knots, punch (0) (8) (ENTER). 6. Press the RADAR MENU key to close FUNCTIONS menu. The ship speed displayed at the screen top shows own ship speed entered by the label “MAN”. 7.1.16. TARGET TRAILS (ECHO TRAILS) Echo trails are simulated afterglow of target echoes that represent their movements relative to own ship or true movements with respect to true north in a single tone or gradual shading depending on the settings on the RADAR 1 menu. 7.1.16.1. True or relative trails You may display echo trails in true or relative motion. Relative trails show relative movements between targets and own ship. True motion trails require a gyrocompass signal and own ship speed input to cancel out own ship’s movement and present true target movements in accordance with their over the ground speeds and courses. Refer to the automatic and manual speed input procedures for entering own ship’s speed information. Note: When true trail is selected on the RM mode, the legend TRUE TRAIL appears in red. No true relative selection on TM, it is only TRUE TRAIL on TM mode. To select true or relative echo trail presentation: 1. Press the RADAR MENU key on the plotting keypad to show the FUNCTIONS menu. 2. Press the (0) key to show the SYSTEM SETTING 1 menu. 3. Press the (2) key to show the RADAR 1 menu. 4. Press the (6) key to select menu item 6 TRAIL REF. 5. Press the (6) key to select (or highlight) REL (Relative) or TRUE option. 6. Press the ENTER key to confirm your selection followed by the RADAR MENU key to close the menu. 7.1.16.2. Trail gradation Echo trails may be shown in monotone or gradual shading. Gradual shading paints the trails getting thinner with time just like the afterglow on an analog PPI radar. Selection of monochrome or gradual shading requires almost the same operation as for true or relative trails setup procedure described above except that you should: · Press the (7) key to select menu item 7 TRAIL GRAD (graduation) in step 4, and · Press the (7) key to select (or highlight) GGL (single tone) or MULT (multiple shading) option in step 5. 7.1.16.3. Displaying and erasing echo trails Press the ECHO TRAILS key to activate or deactivate the echo trails feature. Each press of the ECHO TRAILS key within 5 seconds cyclically changes echo trail length (time) to 30 seconds, 1, 3, 6, 15, and 30 minutes, continuous echo trailing and OFF. The current echo trail setting is displayed at the lower right corner of the screen. 176 Suppose that “3 MIN” has just been selected. If the ECHO TRAILS key is hit more than 5 seconds later, echo trails are removed from the display (memory) still alive with echo trail timer count going on). Next hitting of the key calls out the echo trails on the screen. To proceed to longer plot intervals, successively push the ECHO TRAILS key with a hit and release action. The larger the echo trail length, the larger the larger the echo trail plot interval. Note: Holding the ECHO TRAILS key depressed for about 3 seconds will cause a loss of echo trail data so far stored in an in memory. 7.1.16.4. Resetting echo trails To reset (or clear) the echo trail memory, hold the ECHO TRAILS key depressed for about 3 seconds. Echo trails are cleared and the trailing process restarts from time count zero at current echo trail plot interval. When memory assigned to echo trailing becomes the echo trail timer at the lower right corner of the screen freezes and the oldest trails are erased to show the latest trails. 7.2. Target Information Target Data - The target box defaults to showing data for a single target. Acquired Target Data - The following data is shown: The Auto Plotter calculates motion trends (range, bearing, course, speed, CPA and TCPA) of all plotted targets In head up and head up true bearing modes, target bearing, course and speed shown in the upper right target data field become true (suffix “T”) or relative (suffix “R”) to own ship in accordance with the true/relative vector setting. In north up, course up and true motion modes, the target data field always displays true bearing, true course and speed over the ground. Place the cursor on the desired target and press the TARGET DATA key on the plotting keypad. Data on the selected target is displayed at the upper right corner of the screen. A typical target data display is shown in figure 7.7. Figure 7.7 - Target Data 177 Figure 7.8 – Target data - “TARGET” Target identification number/name. - “RANGE” Range of target from own ship. - “T BRG” Bearing of target from own ship. - “CPA” Closest point of approach to own ship. - “TCPA” Time to closest point of approach. - “CSE”/ ”COG” Target's Course through the water (CSE) or Course Over the Ground (COG). - “STW”/ ”SOG” Target's Speed Through the Water (STW) or Speed Over the Ground (SOG). - “BCR” Bow crossing range. - “BCT” Bow crossing time Figure 7.9 - True and relative vectors The target, for which data is shown, can be selected by left clicking on an acquired target in the video circle. The selected target [2] is identified in the video circle by a small ” ” symbol centred on the plot origin. - RNG/BRG: Range and bearing from own ship to the selected target with suffix “T” (True) or “R” (Relative). - CSE/SPD: Course and speed are displayed for the selected target with suffix “T” or “R”. CSE/SPD: CPA (Closest Point of Approach) is the closest range a target will approach to own ship. TCPA is the time to CPA. Both CPA and TCPA are automatically calculated. When a target ship has passed clear of own ship, CPA is prefixed with an asterisk such as, CPA * 1.5NM. TCPA is counted to 99.9 min and beyond this, it reads TCPA.*99.9MIN. 178 BCR/BCT: Bow crossing range is a range of a target which will pass dead ahead of own ship at a calculated distance. BCT is the time when BCR occurs. 7.2.1.True or relative vector Target vectors can be displayed relative to own ship’s heading (relative) or with reference to the north (true). Press the VECTOR TRUE/REL key to select true or relative vectors. This feature is available in all presentation modes (gyrocompass must be working correctly). The current vector mode is indicated at the upper right corner of the screen. 7.2.2.True vector In the true motion mode, all fixed targets such as land, navigational marks and ships at anchor remain stationary on the radar screen with vector length zero. But in the presence of wind and/or current, true vectors appear on fixed targets representing the reciprocal of set and drift affecting own ship unless set and drift values are properly entered (see figure7.10). Figure 7.10 - True vectors in head-up mode 7.2.3 Relative vector Relative vectors on targets which are not moving over the ground such as land, navigational marks and ships at anchor will represent the reciprocal of own ship’s ground track. A target of which vector extension passes through own ship is on the collision course. (See figure 7.2.5. - dotted lines are for explanation only). Figure 7.11 - Relative vectors in head-up mode 179 7.2.4 Vector time Vector time (or length of vectors) can be set to 30 seconds, 1, 2, 3, 6, 12, 15 or 30 minutes and the selected vector time is indicated at the upper right corner of the screen. Press the VECTOR TIME key to select desired vector time. The vector tip shows an estimated position of the target after the selected vector time elapses. It can be valuable to extend the vector length to evaluate the risk of collision with any target. 7.2.5 Past positions The ARPA displays equally time spaced dots marking the past positions of any targets being tracked. A new dot is added every minute (or at preset time intervals) until the present number is reached. If a target changes it speed, the spacing will be uneven. If it changes the course, its plotted course will not be a straight line. 7.2.5.1 Displaying and erasing past positions To display past positions, press the HISTORY key to display past positions of targets being tracked. The label HISTORY appears at the upper right corner of the screen. To erase past positions, press the HISTORY key again. 7.2.5.2 Selecting the number of dots and past position intervals 1. Press the E, AUTO PLOT MENU key on the plotting keyboard to show the ARPA 1 menu. 2. Press the (7) key to select menu item 7 HISTORY POINTS. 3. Further press the (7) key to select a desired number of past positions (5, 10, 20, 30, 100, 150 or 200). The IMO-type has the selection of only 5 or 10. 4. Press the ENTER key to confirm your selection. 5. Press the (8) key to select menu item 8 HISTORY INTERVL. 6. Further press the (8) key to select a desired past position plot interval (30 seconds, 1, 2, 3 or 6 minutes). 7. Press the ENTER key to conclude your selection. 8. Press the E, AUTO PLOT MENU key to close the menu. 7.2.6. Trial Maneuver Trial simulates the effect on all tracked targets against own ship’s manoeuvre without interrupting the updating of target information. There are two types of trial manoeuvres: STATIC and DYNAMIC. 7.2.6.1 Dynamic trial manoeuvre A dynamic trial manoeuvre displays predicted positions of the tracked targets and own ship. You enter own ship’s intended speed and course with a certain “delay time”. Assuming that all tracked targets maintain their present speeds and courses, the targets’ and own ship’s future movements are simulated in one second increments indicating their predicted positions in one minute intervals. 180 The delay time represents the time lag from the present time to the time when own ship will actually start to change her speed and/or course. You should therefore take into consideration own ship’s maneuvering characteristics such as rudder delay, turning delay and acceleration delay. This is particularly important on large vessels. How much the delay is set the situation starts immediately and ends in a minute. Note that once a dynamic trial manoeuvre is initiated, you cannot alter own ship’s trial speed, course or delay time until the trial manoeuvre is terminated. 7.2.6.2 Static trial manoeuvre A static trial manoeuvre displays only the final situation of the simulation. If you enter the same trial speed, course and delay time under the same situation as in the aforementioned example of dynamic trial manoeuvre, the screen will instantly show position OS7 for own ship, position A7 for target A and position B7 for target B, omitting the intermediate positions. Thus, the static trial manoeuvre will be convenient when you wish to know the manoeuvre result immediately. Note: For accurate simulation of ship movements in a trial manoeuvre, own ship’s characteristics such as acceleration and turning performance should be properly set in initial settings at the time of installation. To perform a trial manoeuvre: 1. Press the E, AUTO PLOT MENU key on the plotting keypad followed by the (0) key to show the ARPA 2 menu. 2. Press the (2) key to select 2 TRIAL MANEUVER. 3. Further press the (2) key to select (or highlight) STATIC or DYNAMIC trial manoeuvre option as appropriate. 4. Press the ENTER key to conclude your selection followed by the E, AUTO PLOT MENU key to close the ARPA 2 menu. 5. Press the VECTOR TRUE/REL key to select true or relative vector. 6. Press the TRIAL key. The TRIAL DATA SETTING menu appears at the screen bottom associated with the current own ship’s speed and course readouts. Note: The second line reads (STATIC MODE) in the event of a static trial manoeuvre. 7. Enter own ship’s intended speed, course and delay time in the following manner: Speed: Set with the VRM control. Course: Set with the EBL control. Delay time: Enter in minutes by hitting numeral keys. This is the time after which own ship takes a new situation, not the time the simulation begins. Change the delay time according to own ship loading condition, etc. 8. Press the TRIAL key again to start a trial manoeuvre. Trial manoeuvre takes place in three minutes with the letter “T” displayed at the bottom of the screen. If any tracked target is predicted to be on a collision course with own ship (that is, the target ship comes within preset CPA/TCPA limits), the target plot symbol changes to a triangle and flashes. 181 If this happens, change own ship’s trial speed, course or delay time to obtain a safe manoeuvre. The trial manoeuvre is automatically terminated and the normal radar picture is restored three minutes later. 7.3 Interpretation Errors These can be: (i) Misinterpretation of Display Presentation and Vector Mode. The combination of different display and vector (plus eventual history tracks) are so many that mistakes are easily made in interpretation. Sometimes spring-loaded switches are provided for certain vector modes and this can be helpful. A brief guide is now provided:First the display presentation:(a) Relative Motion North-up Stabilized. (b) Relative Motion Course-up Stabilized. (c) True Motion North-up Stabilized. (d) True Motion Course-up Stabilized. Combined with the vector and "Past (History) Track" mode. the following Table is obtained. In the True Motion vector mode, using a Relative Motion display, a vector will be attached to the point representing own ship although the point remains stationary on the radar screen. Note also that in some cases the past track does not coincide with the afterglow (for example TM past track on a RM display). (ii) Misinterpretation of the Trial Manoeuvre (Simulation). Here, also. the type of display presentation has to be appreciated. With static simulation, showing the predicted situation immediately after the manoeuvre, it seems best to use a :Relative Motion Display with Relative Motion vectors of moderate length. With dynamic simulation, showing the predicted developing situation up to thirty minutes after the manoeuvre has been carried out, it will be better to have a True Motion Display, for good understanding, plus Relative Motion vectors (if possible). Although "Simulation" will give guidance for a predicted safe manoeuvre, the observer should keep the "Rule of the Road" in mind, especially Rule 19, during poor visibility. The former prediction, which is based merely upon the other vessel keeping her course and speed, may clash with the latter requirement. (ill) Misinterpretation of the Input Speed (Velocity). In open sea the input speed to ARPA is generally manual sea speed or one-axis "water-locked" speed. In calm water which is often the case during fog conditions - one can be reasonably certain from the true motion vector what the target's aspect will be. Near the coast or in estuaries, it is often advisable to use the "Auto-Track" or "&ho-Reference" facility if these are available. The true motion vectors will then show the ground velocity giving a good idea where the ships are going to (this arrangement, under restricted visibility conditions does not clash with Rule 19). This facility can be used with a True Motion or a Relative Motion Display. Whatever the speed input, one must make certain what the type is - sea or ground speed, one-axis; sea or ground speed dual axes (sea or ground velocity) - to appreciate the meaning of and to understand the interpretation of the true motion vectors. Also, during rough weather, one should realise that some vessels will have wind drift (leeway) superimposed on their directed motion and their real aspect may 182 differ from the one shown on the display or read out digitally. Error in the speed or velocity input does not affect the accuracy of range, bearing and RM past track. (iv) Misinterpretation of Display Symbols. One has to be careful with older ARPA sets where different manufacturers used different symbols (circles. triangles, squares, diamonds etc.) for the same message. For example, depending on the ARPA make, a square symbol may indicate "Acquired", or "Stationary Target" or "Passing within the set CPA distance". Errors present in displayed data on ARPA screen or alphanumeric read-out will affect decision making. The observer therefore should have a knowledge of the level of accuracy that can be expected and the errors which will affect it. It is not a simple matter to specify the accuracy because it depends on , among other things, the geometry of the plotting triangle. For this reason, the IMO Performance Standards specify accuracy terms of what must be achieved by the ARPA. Errors of interpretation are not within the system, but are those likely to be made by the operator through misunderstanding, inexperience or casual observation. Errors generated in ARPA itself. These are: (i) Smoothing Errors. Especially, owing to rolling and pitching errors (a combined effect of scanner movement and gyro-compass errors) slight changes in vector quantities and digital read-outs are continuously taking place for all targets in rough weather. It should, however, be remembered that a target's velocity vector, even under ideal conditions, is always subject to slight changes, depending on type of steering facilities employed, weather and ship's parameters. When own ship or the target ship change their velocity vectors, smoothing will oppose the change and true velocity information of targets (vector and digital read-out) becomes unreliable. Some ARPAs stop tracking during these periods. The reason for this is that in most ARPAs, calculations are based on the relative motion velocity vector. In one particular ARPA, however, position and velocity of tracked targets are stored in true motion format, so that true motion vectors of targets do not need to be re-established after a change in relative motion. A very good check on the smoothing process is the study of the past history track. At very short range, due to rapid bearing changes, the tracker may lose the target. (ii) Computer Calculation Errors. These are nearly always due to course and speed input errors. (a) Effect on True Motion Velocity. With certain exceptions – the operator's true motion vectors do not possess the same reliability as relative motion vectors. (b) Effect on Predicted Relative Motion Vectors. As in (a) errors are introduced when course and speed input is not correct. (c) Effect on ppc. The effect of speed input error is illustrated in Fig. 7.3.1. (a) and (b). It is reassuring to see that, with the ships on collision courses, the PPC, although displaced, will remain on the heading marker. On the assumption that the heading marker is correctly aligned, course input errors do not affect the ppe positions with respect to the heading marker. However, picture and heading marker will be disorientated inside the tube; a correction has to be applied to obtain the true course to avoid a PAD. 183 (iii) Vector Jumping (a) This may occur when targets are close to each other and their two echoes are in the same tracking window. The two vectors may interchange and so will the digital information (target information Swoop) or sometimes they combine or, when in manual acquisition mode, one target may lose all its information while the other target may yield data for the first time, but they are the wrong data. Target swoop should be overcome by "rate-aiding", the forecast of the target(s) predicted position ahead of the echo during the next scan (so that the proper vector can be drawn if the position is later confirmed), and by making the tracking window as small as possible after the initial acquisition. (b) It can also take place that while in automatic acquisition mode false echoes are received due to side-lobe effect or indirect reflection via superstructures on own ship. The remedy is to switch over to the manual acquisition mode or to put into action a minimum tracking andJ or acquisition range. (iv) Spurious information owing to acquisitioning of rain and sea clutter echoes and to tracking information of land-hased objects. This can happen while using the automatic acquisition mode. Not only does the observer get far too much unwanted information, it will also make the radar picture confusing to look at. Lastly it may saturate the tracking capacity of the computer and some of the targets may be dropped or ignored even though they are important to the observer. In these cases one should go back to the manual acquisition mode or apply acquisition restriction for a minimum desired range and use the Area Rejection Boundaries or Zones. Strong clutter echoes can give rise to acquisition and vectors so obtained may combine with real vectors attached to echoes of a target ship resulting in a rotating vector when the target ship is near. 184 Figure 7.12 (a) and (b) - Effect of speed errors on PPC 7.3.1 Errors with vector system In case of vector systems, the most common misunderstanding arise because of the observer, either form lack of observation due to stress of the moment or through lack of knowledge, confuses relative vectors with true vectors. Typical blunders are: measuring the CPA as the target’s distance at which the true vector passes the origin; mistaking the direction of the relative vector for the target`s true heading. A further source of error sometimes occurs where the to see the dynamic development of the situation (which in itself is an useful ploy) to assist in determining collision avoidance strategy, but deduces the closest point of approach as the point where the vectors cross. This is of course only the case in case of a collision. Attempts to find passing distance by trial and error using this technique also frequently mislead the observer and are not necessary when the CPA is so easily available from other sources 185 Some manufacturers fit spring-loaded switches or hold down buttons to ensure that equipment always reverts to either true or relative vector mode in an attempt to reduce the chance of misinterpretation of data in this way. Other common errors include the confusion of real an trial values of CPA and omitting to set the correct trial speed where analog controls are provided. Where the displayed vectors and history of a different type are simultaneously presented, the difference between the two may be mistaken for a maneuver by the target. During the second and third minute of tracking, the vectors will be stabilizing ad care must be taken not to be misled into assuming that this is an alteration by the target or that it is yawing 7.3.2 Errors with PPC and PAD systems In the case of PPC and PAD systems, the commonest mistake arise when attempting to Interpolate or extrapolate data from the display. Typical errors arise because of failure to appreciate the following: The line joining the target to the collision point is not a time-related vector and does not indicate speed. The collision point does not give an indication about miss distance Changes in collision point position does not necessarily indicate a change in the target’s true course or speed. The area of danger does not change symmetrically with a change in the selected miss distance . The collision point is not necessarily at the center of the danger area It is always important to realize that the areas of danger generated on screen apply only to own ship and the target, and that they do not always give warning of mutual threat between two targets. If two areas of danger overlap, it is reasonable to suppose that the two targets will also pass each other within the stated miss distance, but separated danger areas do not imply safe passing between the targets. Two targets may eventually have a close passing although their danger areas, as applied to own ship, appear to be separated. 7.3.3 Resumption of course Where a “chained” bearing cursor is available and the cursor chain division are an indication of time, care must be exercised in the measurement of time to resume course. The marker correctly indicates the time own ship will cross ahead or astern on the target’s track, but the time at which the required miss distance occurs cannot be determined. 186 7.3.4 Errors in interpretation a) Target A is faster and target B is slower than own ship, despite appearances. Note: vectors will show this b) The solid line shows the track of the PPC from P. The apparent track of the echo is shown by the broken line The broken line shows the PAD for 2 miles. The solid line show the PAD for 1 mile 187 Targets A and B will collide, although this is not apparent from the display Target A and B will not collide although they may pass within miss distance 7.3.6 The misleading effect of afterglow Because the vector mode is not necessarily the same as the radar presentation which has been selected, vectors and afterglow trails may not mach. When true vectors are selected on a relative-motion presentation, the vectors and the afterglow will not correlate. When relative vectors are selected while using a true motion presentation, the true after glow will not mach the relative vector. 7.3.7 Accuracy of the presented data Over-reliance on, and the failure to appreciate inaccuracies in, presented data which has been derived from imperfect inputs should be avoided at all costs. It must always be borne in mind a vector/PAD/alphanumeric read-out is not absolutely accurate, just because it has been produced by a computer, no matter how many microprocessors it may boast. An indication by the ARPA that a target will pass on cable clear from own ship should not be regarded as an indication for standing-on into such a situation 188 7.4 System Operational Tests Identify the characteristic to be measured or the defect type of concern. 7.4.1 Bearing Accuracy One of the most important features of the radar is how accurately the bearing of a target can be measured. The accuracy of bearing measurement basically depends on the narrowness of the radar beam. However, the bearing is usually taken relative to the ship’s heading, and thus, proper adjustment of the heading marker at installation is an important factor in ensuring bearing accuracy. To minimize error when measuring the bearing of a target, put the target echo at the extreme position on the screen by selecting a suitable range. 7.4.2 Range Measurement Measurement of the range to a target is also a very important function of the radar. Generally, there are two means of measuring range: the fixed range rings and the variable range marker (VRM). The fixed range rings appear on the screen with a predetermined interval and provide a rough estimate of the range to a target. The variable range marker’s diameter is increased or decreased so that the marker touches the inner edge of the target, allowing the operator to obtain more accurate range measurements. Occasionally echo signals appear on the screen at positions where there is no target or disappear even if there are targets. They are, however, recognized if you understand the reason why they are displayed. Typical false echoes are shown below. 7.4.3 Select the measuring instrument. The measuring instrument is usually either a physical piece of measuring equipment such as a micrometer, magnetic compass, gyrocompass; or alternatively, a visual check. Whenever a visual check is used, it is necessary to state whether normal eyesight is to be used or a visual aid such as a magnifying glass. In the example, normal eyesight is sufficient. On some occasions, it may also be necessary to state the distance the observer should be from the item being checked. In general, the closer the observer, the more detail will be seen. In the example, a clear visual indication is given of acceptable and unacceptable, so the observer needs to be in a position where the decision can be made. When completing a visual check, the type of lighting may also need to be specified. Certain colours and types of light can make defects more apparent. 7.4.5 Describe the test method. The test method is the actual procedure used for taking the measurement. When measuring time, the start and finish points of the test need to be specified. When taking any measurement, the degree of accuracy also needs to be stated. For instance, it is important to know whether time will be measured in hours, minutes, or seconds. 7.4.6 State the decision criteria. The decision criteria represents the conclusion of the test. Does the problem exist? Is the item correct? Whenever a visual check is used, a clear definition of acceptable versus 189 unacceptable is essential. Physical examples or photographs of acceptable and unacceptable, together with written support, are the best definitions. 7.4.7 Document the operational definition. It is important that the operational definition is documented and standardized. Definitions should be included in training materials and job procedure sheets. The results of steps 1 through 4 should be included in one document. The operational definition and the appropriate standards should be kept at the work station. 7.4.8 Test the operational definition. It is essential to test the operational definition before implementation. Input from those that are actually going to complete the tests is particularly important. The operational definition should make the task clear and easy to perform. The best way to test an operational definition is to ask different people to complete the test on several items by following the operational definition. Watch how they perform the test. Are they completing the test as expected? Are the results consistent? Are the results correct? 7.4.8.1.Operational definition: What is it? An operational definition, when applied to data collection, is a clear, concise detailed definition of a measure. The need for operational definitions is fundamental when collecting all types of data. It is particularly important when a decision is being made about whether something is correct or incorrect, or when a visual check is being made where there is room for confusion. For example, data collected will be erroneous if those completing the checks have different views of what constitutes a fault at the end of a glass panel production line. Defective glass panels may be passed and good glass panels may be rejected. Similarly, when invoices are being checked for errors, the data collection will be meaningless if the definition of an error has not been specified. When collecting data, it is essential that everyone in the system has the same understanding and collects data in the same way. Operational definitions should therefore be made before the collection of data begins. 7.4.8.2. Operational definition: When is it used? Any time data is being collected, it is necessary to define how to collect the data. Data that is not defined will usually be inconsistent and will give an erroneous result. It is easy to assume that those collecting the data understand what and how to complete the task. However, people have different opinions and views, and these will affect the data collection. The only way to ensure consistent data collection is by means of a detailed operational definition that eliminates ambiguity 7.4.8.3. Operational definition: When is it used? Any time data is being collected, it is necessary to define how to collect the data. Data that is not defined will usually be inconsistent and will give an erroneous result. It is easy to assume that those collecting the data understand what and how to complete the task. However, people have different opinions and views, and these will affect the data collection. The only way to ensure consistent data collection is by means of a detailed operational definition that eliminates ambiguity. 190 7.4.9. Calibration The range to a target can be measured most accurately on the PPI(Plan position indicator) when the leading edge of its pip just touches a fixed range ring. The accuracy of this measurement is dependent upon the maximum range of the scale in use. Representative maximum error in the calibration of the fixed range rings is 75 yards or 11/2 percent of the maximum range of the range scale in use, whichever is greater. With the indicator set on the 6-mile range scale, the error in the range of a pip just touching a range ring may be about 180 yards or about 0.1 nautical mile because of calibration error alone when the range calibration is within acceptable limits. On some PPI’s, range can only be estimated by reference to the fixed range rings. When the pip lies between the range rings, the estimate is usually in error by 2 to 3 percent of the maximum range of the range scale setting plus any error in the calibration of the range rings. Radar indicators usually have a variable range marker (VRM) or adjustable range ring which is the normal means for range measurements. With the VRM calibrated with respect to the fixed range rings within a tolerance of 1 percent of the maximum range of the scale in use, ranges as measured by the VRM may be in error by as much as 21/2 percent of the maximum range of the scale in use. With the indicator set on the 8-mile range scale, the error in a range as measured by the VRM may be in error by as much as 0.2 nautical mile. 7.4.9.1. Pip and VRM Alignment The accuracy of measuring ranges with the VRM is dependent upon the ability of the radar observer to align the VRM with the leading edge of the pip on the PPI. On the longer range scales it is more difficult to align the VRM with the pip because small changes in the reading of the VRM range counter do not result in appreciable changes in the position of the VRM on the PPI. 7.4.9.2. Range Scale The higher range scale settings result in reduced accuracy of fixed range ring and VRM measurements because of greater calibration errors and the greater difficulty of pip and VRM alignment associated with the higher settings. 7.4.9.3. PPI Curvature Because of the curvature of the PPI, particularly in the area near its periphery, range measurements of pips near the edge are of lesser accuracy than the measurements nearer the center of the PPI. 7.4.9.4. Radarscope Interpretation Relatively large range errors can result from incorrect interpretation of a landmass image on the PPI. The difficulty of radarscope interpretation can be reduced through more extensive use of height contours on charts. For reliable interpretation it is essential that the radar operating controls be adjusted properly. If the receiver gain is too low, features at or near the shoreline, which would reflect echoes at a higher gain setting, will not appear as part of the landmass image. If the receiver gain is too high, the landmass image on the PPI will “bloom”. With blooming the shoreline will appear closer than it actually is. A fine focus adjustment is necessary to obtain a sharp landmass image on the PPI. Because of the various factors introducing errors in radar 191 range measurements, one should not expect the accuracy of navigational radar to be better than + or - 50 yards under the best conditions. 7.4.9.5. Range Calibration The range calibration of the indicator should be checked at least once each watch, before any event requiring high accuracy, and more often if there is any reason to doubt the accuracy of the calibration. A calibration check made within a few minutes after a radar set has been turned on should be checked again 30 minutes later, or after the set has warmed up thoroughly. The calibration check is simply the comparison of VRM and fixed range ring ranges at various range scale settings. In this check the assumptions are that the calibration of the fixed range rings is more accurate than that of the VRM, and that the calibration of the fixed range rings is relatively stable. One indication of the accuracy of the range ring calibration is the linearity of the sweep or time base. Since range rings are produced by brightening the electron beam at regular intervals during the radial sweep of this beam, equal spacing of the range rings is indicative of the linearity of the time base. Representative maximum errors in calibrated fixed range rings are 75 yards or 1.5 percent of the maximum range of the range scale in use, whichever is greater. Thus, on a 6-mile range scale setting the error in the range of a pip just touching a range ring may be about 180 yards or about 0.1 nautical mile. Since fixed range rings are the most accurate means generally available for determining range when the leading edge of the target pip is at the range ring, it follows that ranging by radar is less accurate than many may assume. One should not expect the accuracy of navigational radar to be better than plus or minus 50 yards under the best conditions. Each range calibration check is made by setting the VRM to the leading edge of a fixed range ring and comparing the VRM range counter reading with the range represented by the fixed range ring. The VRM reading should not differ from the fixed range ring value by more than 1 percent of the maximum range of the scale in use. For example, with the radar indicator set on the 40-mile range scale and the VRM set at the 20-mile range ring, the VRM range counter reading should be between 19.6 and 20.4 miles. 7.5. Errors in displayed data 7.5.1. Target Swoop When two targets are close to each other, it is possible for association of past and present echoes to be confused so that the processor is loaded with erroneous data. The result is that the historical data on one target may be transferred to another target and the indicated relative (true) track of that ship will be composed of part of the track of two different target motions. Target swap can occur with any type of tracker but it is least likely in those which use a diminishing gate size as confidence in the track in the track increases and those which adopt rate aiding. It is most likely to occur when two targets are close together for comparatively long time and one target echo is much closer than the other. 192 7.5.2. Track errors The motion of a target is rarely completely steady and even steady motion will return position s which are randomly scattered about the actual track, due to basic radar limitations. Quantification errors in range and bearing which are introduced by translation of the basic radar information to the processor database further exacerbate the effects of these systems errors. The only way in which the tracker can deal with these is to use some form of smoothing over a period of time by applying more or less complicated filtering techniques. The aim of the filtering is to give the best possible indication of the steady track and at the same time detect real changes quickly. . a) At position 6 tracker transfers at position 6 b) At position 6, and later, the profusion of echo confounds tracker accuracy. The tracker may easily pick up random clutter instead of the target ship. 193 c) Target travel close together for a period, then separate The tracker may not follow the diverging target ship. d) Two tracked targets pass close to each other, so both are in the tracking gate at the same time. Note in this case longer rate aiding may be an advantage. Given the need to satisfy these two conflicting requirements it is inevitable that the tracker will be limited in its ability to predict precisely the relative an true motion of a tracked target at any instant and thus the track errors will appear. The effect of these errors should of course fall within the limits set out, but a prudent observer will use suitable clear weather opportunities to gain some evaluation of the practical performance of the tracker which is producing the ARPA data. Such performance can be usefully judged against two important criteria. These are the stability of the track shown on the display for a vessel which is observed to be standing-on and the rapidity with which the track responds to a target which appears to manoeuvre. It has to be 194 recognized that alteration of course are easy to be observed visually whereas seed changes are not. However, in clear weather the former are more frequent than the latter, except in very confined waters. Accuracy is most difficult to achieve with targets whose track movement is slow. In the case of relative storage this will affect a target whose course and speed are close to those of the observing vessel. The length of the relative track will be small and thus the system errors are a much more significant proportion of track length than would be the case with a target having a rapid relative motion. Thus the inherent accuracy of CPA data will be low. Conversely, in case of true track storage a near stationary will suffer the same low accuracy in the prediction of its true motion In general the tracker is likely to offer the best indication of both the relative motion and the true motion when both the target and the observing vessel maintain their course and speed for a smoothing period. In the changing situation the track errors will depend on the nature of the changes and the form of track storage adopted Where only the targets manoeuvres there will be a finite response time in which the displayed vector will seek to follow the change and to stabilize on the new track. Under these circumstances, irrespective of whether the tracker smoothes relative or true tracks there should in theory be no difference in the tracker performance when measured in terms of the accuracy with which it provides output of both relative and true vectors. In both cases the vectors may be hieratic when the processor reverts to smoothing overshot periods. Where only the observing vessel manoeuvres, the method of storing is irrelevant because the relative track of all targets will be curves for the duration of the manoeuvre. If the smoothing is applied to relative tracks, the observer will be tasked with trying to produce a straight line from a curve and will hence obtain a mean track. Errors in the relative track will result in the relative vectors of all targets may be hieratic on the short term. True data motion derived from this will also be in error, just where a manual plotter construct an OAW triangle on the basis if an apparent motion which is not consistent. The effect may be exacerbated by the fact that during the manoeuvre the path traced by the mass of the vessel an thence the aerial may differ significantly from that indicated by the gyro compass and log. Systems witch smooth true tracks should derive a more accurate indication of the target’s true track during the own ship’s manoeuvre. If the target is tracked down to very close ranges, the relative motion will give rise to a very rapid changes of bearing and it may be impossible for the tracker to follow the target; thus the target lost indication may appear, not because the echo is weak but because the gate cannot open fast enough or open up sufficiently to find it. It is also worth remembering that the use of vectors as a indication of the target’s heading is based on the assumption that the target is moving through the water in the direction in which is heading. Leeway is the prime example where this is not correct. Unless one may see the target it is impossible to make an estimate of the leeway. In poor visibility and high winds the observer must be alert to the possibility and use of the displayed data with additional caution. In summary, it must be remembered that whenever the steady state conditions are disrupted there will be a period in which the data will be particularly liable to the track errors described above, in the same way as is the case when a target is first acquired. In the event the steady rate is acquired, accuracy and stability will improve, first over the 195 short smoothing period and later over the long period. Any track data extracted during periods of non-steady-state conditions may be viewed with suspicion. 7.5.3 The effect of vectors and incorrect speed From the theory of radar plotting it is evident that it is possible to deduce the relative motion of a target without using a knowledge of the true motion of the observing vessel) other than to produce stabilised bearings). Deduction of the true motion of the target requires knowledge of the true motion of the observing vessel to allow resolution of the OAW triangle and the accuracy of the course and speed data used. Extrapolation of this reasoning suggests that the accuracy of relative vectors and the associated CPA data are independent of the accuracy of the course and speed input, whereas the accuracy of true vectors and the associated data are dependent on the accuracy input course and speed. In the case of systems which smooth relative tracks, this is invariably correct; in the case of systems which store true tracks, it is correct subject to the qualification that the input error are constant. In the case of a fluctuating error input, the two storage approaches will produce different results. For this reason the effect of steady input errors on the relative and true vectors is discussed in this chapter. Whatever approach the tracker uses, it is essential for the observer to ensure that the correct course and speed inputs are fed in when setting up and that the regular and frequent checks are made to ensure that the values remain correct. Failure t do this will in general result in erroneous display of true data which may seriously mislead the navigator when choosing a suitable avoiding manouvre strategy. 7.5.3.1. Relative vectors The relative vectors associated with the CPA/TCPA data should be unaffected if he observer allows a fixed erroneous input of course and speed data to be applied. In the case of relative storage the information is not used in the calculation. In the case of true storage it is used twice and one could say that this illustrates a classic case of „two wrong makes it right” A fixed gyro compass error present at the point which the bearings are digitized would result in the picture being slewed but would not affect the CPA/TCPA data. Figure 7.13 – Effect of incorrect speed input 196 In the figure 7.13 the effect of incorrect speed input on a true-vector presentation. Vector B is correct. Speed input “too high” shows vector A, crossing slow ship. Speed vector input” too low” shows vector C, a fast target on near parallel crossing. 7.5.3.3 True vectors The true vectors will be displayed incorrectly if the observer allows an erroneous input of course and speed data to be applied, irrespective of the storage format. This may give the observer seriously misleading impression of the other vessel’s heading and speed and may prompt an unsafe manoeuver, Such a situation is depicted above, where a target which is moving at a similar speed to the observing vessel, is showing a red light and which will pass clear down the starboard side. Incorrect speed input could make this appears to be a slower vessel in a board crossing situation or a faster vessel ship passing green to green. The effect of an incorrect heading input will have a similar capacity to produce misleading results, as illustrated below. 7.5.4 The effect of fluctuating input error The most probable source of a fluctuating input error is the log. There are a number of circumstances in which this might arise, An example in this case of some doppler logs which tend to give erratic outputs in bad weather when the traducer has to operate through an aerated layer. The fluctuating effect will affect the display of true vectors whatever the form of storage but the behaviour of the relative vectors will depend on the mode of storage used. Where the relative tracks are smoothed the fluctuating error will have no effect on the relative vectors and the associated CPA/TCPA data. Figure 7.14 – Effect of fluctuating input error In this figure 7.14 is shown the effect of incorrect compass input on a true-vector presentation. Vector B is correct. Incorrect compass input to the left at ά shows a target broad crossing to port and faster than own ship. Incorrect compass input to their right shows a target fine crossing to starboard and slower than own ship Where true tracks are smoothed, the relative vector will tend to change erratically in sympathy with the input fluctuations, since the relative vector is derived from smoothed true track and the instantaneous input course and speed data. 197 The difference in effect can be considered by an example shown below. Consider the case where the observing vessel has been on a steady course and speed for a full smoothing period and the correct speed and course data has been consistently fed in. Both methods of smoothing will have settled to produce the correct relative and true vectors. Suppose that the log develops and instantaneous fault and reads half the correct speed. The relative track system will show no change in the relative vector(because the relative track is smoothed) but the true vector will immediately go to the erroneous values because it is derived from smoothed relative track and the instantaneous course and speed input. The true track system will show no immediate change in the true vector, because it is insulated by the smoothing, whereas the vector will go immediately to an erroneous value since is derived from a smooth true track and the instantaneous course and speed input. If no further changes take place in the error and the vectors are observed over a smoothing period , the true vectors will change over a full smoothing period, the true vector will gradually change to the erroneous value while the relative vector will gradually come back to the correct value as the previously smoothed track is progressively discarded. Thus in this type of storage, if the speed input fluctuates the relative vector will also fluctuate. This disadvantage has to set against the advantage gained in being able to maintain a stable relative vector when the observing vector is manoeuvring. An erratic course input would have a similarly disruptive effect. However, such a condition would also affect the digitization of bearings. This would tend to cause targets to jump and it would be fairly obvious to even a casual observer that something was wrong. 7.5.5. Comparison of relative and true vectors Given that one or other of the vectors can be affected by input errors in a way which may be dependent on the tracker philosophy, it is important to stress the need for the observer`s continuously to compare one data source with a another to ensure that, in all cases indications given from relative vectors and true vectors sensibly agree. Figure 7.15 - Comparison of relative and true vectors (correct speed input) 198 In figure 7.15 the course and speed impute are correct for a smooth period of time Figure 7.16 - Comparison of relative and true vectors (incorrect speed input) In figure 7.16 the speed input suddenly drops to half the correct value due to log error Figure 7.17 - Comparison of relative and true vectors (smoothing period) In figure 7.17 transient conditions part away through smoothing period 199 Figure 7.18 - Comparison of relative and true vectors (full smoothing period) In figure 7.18 vectors settle after one full smoothing period of steady state conditions 7.5.6. The effect of incorrect data input on the PPC 7.5.6.1. Errors in speed input If incorrect speed is input to a collision situation the collision point will still appear on the heading marker, but at an incorrect range, and will move down the heading marker at an incorrect speed. In the case where there is, in fact, a miss distance, the collision point will appear in the wrong position, which may rise to a misjudgment of danger or urgency of the situation. The figures below show how the collision point may be displaced due to speed error in the two cases where the target is crossing ahead and crossing astern Figure 7.19 - effect of incorrect data input on the PPC 200 In figure 7.19 the effect on the PPC of the speed error target A passing astern, correct speed used. B passing astern, correct speed used.A1, B1, for greater speed, and A2, B2 if lower speed. 7.6. The risk of over reliance on radars and ARPA Accidents have occurred where the primary cause has been over-reliance on a single electronic navigational aid. Watch-keepers must always ensure that positional information is regularly cross-checked using other equipment, as well as visual aids to navigation. In other cases accidents have occurred where the watch-keeper was not fully conversant with the operation of equipment or its limitations. Collisions have been frequently caused by failure to make proper use of radar and radar plotting aids in both restricted visibility and clear weather. Common errors have been deciding to alter course on the basis of insufficient information and maintaining too high a speed, particularly when a close-quarters situation is developing. Information provided by radar and radar plotting aids in clear weather conditions can assist the watch-keeper in maintaining a proper lookout in areas of high traffic density. It is most important to remember that navigation in restricted visibility can be more demanding and great care is needed even with all the information available from the radar and radar plotting aids. Where continuous radar watch keeping and plotting cannot be maintained even greater caution must be exercised. A "safe speed" should at all times reflect the prevailing circumstances. 7.6.1. Electronic radar plotting aids Radars must be equipped with plotting aids, the type of which depends upon the size of ship as follows; a) Electronic Plotting Aid (EPA) EPA equipment enables electronic plotting of at least 10 targets, but without automatic tracking (Ships between 300 and 500 Gross Tonnage (GT)). b) Automatic Tracking Aid (ATA) ATA equipment enables manual acquisition and automatic tracking and display of at least 10 targets (Ships over 500 GT). On ships of 3000 GT and over the second radar must also be equipped with an ATA, the two ATAs must be functionally independent of each other. c) Automatic Radar Plotting Aid (ARPA) ARPA equipment provides for manual or automatic acquisition of targets and the automatic tracking and display of all relevant target information for at least 20 targets for anti-collision decision making. It also enables trial manoeuvre to be executed (Ships of 10000 GT and over). The second radar must incorporate ATA if not ARPA. Manual plotting equipment is no longer acceptable except for existing vessels still complying with SOLAS V/74. Watch-keepers must be fully conversant with the operation and limitations of these plotting facilities and should practice using them in clear-weather conditions to improve their skills. 201 In addition to the advice given above and the instructions contained in the Operating Manual(s), users of radar plotting aids should ensure that: (i) performance of the radar is monitored and optimised, (ii) test programmes provided are used to check the validity of the plotting data, and (iii) speed and heading inputs to the ARPA/ATA are satisfactory. Correct speed input, where provided by manual setting of the appropriate ARPA/ATA controls or by an external input, is vital for correct processing of ARPA/ATA data. Serious errors in output data can arise if heading and/or speed inputs to the ARPA/ATA are incorrect. 7.6.2. Plotting To estimate risk of collision with another vessel the closest point of approach (CPA) must be established. Choice of appropriate avoiding action is facilitated by the knowledge of the other vessel's track using the manual or automatic plotting methods. The accuracy of the plot, however obtained, depends upon accurate measurement of own ship's track during the plotting interval. It is important to note that an inaccurate compass heading or speed input will reduce the accuracy of true vectors when using ARPA or ATA. This is particularly important with targets on near-reciprocal courses where a slight error in own-ship's data may lead to a dangerous interpretation of the target vessel's true track. The apparent precision of digital read-outs should be treated with caution. If two radars are fitted (mandatory for ships of 3000 GT and over) it is good practice, especially in restricted visibility or in congested waters, for one to be designated for anti-collision work, while the other is used to assist navigation. If only one of the radars is fitted with ARPA then this should be the one used for anti-collision work and the other for navigation. 7.6.3. Interpretation It is essential for the operator to be aware of the radar's current performance which is best ascertained by the Performance Monitor. The echo return from a distant known target should also be checked. Be aware of the possibility that small vessels, ice floes or other floating objects such as containers may not be detected. Echoes may be obscured by sea- or rain-clutter. Correct setting of clutter controls will help but may not completely remove this possibility. When plotting larger targets on a medium range scale, the display should be periodically switched to a shorter range, and the clutter controls adjusted, to search for less distinct targets. The observer must be aware of the arcs of blind and shadow sectors on the display caused by masts and other on-board obstructions. These sectors must be plotted on a diagram placed near the radar display. This diagram must be updated following any changes which affect the sectors. 7.6.4. Choice of range scale Although the choice of range scales for observation and plotting is dependent upon several factors such as traffic density, speed of own ship and the frequency of observation, it is not generally advisable to commence plotting on a short range scale. Advance warning of the approach of other vessels, changes in traffic density, or proximity of the coastline, should be obtained by occasional use of longer range scales. This applies particularly when approaching areas where high traffic density is likely, 202 when information obtained from the use of longer range scales may be an important factor in determining a safe speed. 7.6.5. Appreciation A single observation of the range and bearing of an echo will give no indication of the track of a vessel in relation to own ship. To estimate this, a succession of observations must be made over a known time interval. The longer the period of observation, the more accurate the result will be. This also applies to ARPA/ATA which requires adequate time to produce accurate information suitable for assessing CPA/TCPA and determining appropriate manoeuvres. Estimation of the target's true track is only valid up to the time of the last observation and the situation must be kept constantly under review. The other vessel, which may not be keeping a radar watch or plotting, may subsequently alter its course and/or speed. This will take time to become apparent to the observer. Electronic plotting will not detect any alteration of a target's course or speed immediately and therefore should also be monitored constantly. The compass bearing, either visual or radar, should be used to assess risk of collision. The relative bearing of a target should not be used when own ship's course and/or speed alters, as risk of collision may still exist even where the relative bearing is changing. Mariners should also be aware that at close range, risk of collision may exist even with a changing compass bearing. Radar displays may be equipped to display AIS target data. Such information may be used to assist the observer in assessing the situation and taking correct action to avoid a close-quarters situation. Watch-keepers should be aware that not all vessels transmit AIS data. In addition it is possible that not all the AIS data displayed will be accurate, particularly data which is inputted manually on the target vessel. 7.6.6. Clear weather practice Radar should be used to complement visual observations in clear weather to assist assessment of whether risk of collision exists or is likely to develop. Radar provides accurate determination of range enabling appropriate action to be taken in sufficient time to avoid collision, taking into account the manoeuvring capabilities of own ship. It is important that watch-keepers should regularly practice using radar and the electronic plotting system in clear weather. This allows radar observations and the resulting electronic vectors to be checked visually. It will show up any misinterpretation of the radar display or misleading appraisal of the situation, which could be dangerous in restricted visibility. By keeping themselves familiar with the process of systematic radar observations, and comparing the relationship between radar and electronically plotted information and the actual situation, watch keepers will be able to deal rapidly and competently with the problems which may confront them in restricted visibility. 7.6.7. Operation Radar if fitted should be operating at all times. When weather conditions indicate that visibility may deteriorate, and at night when small craft or unlit obstructions such as ice are likely to be encountered, both radars if fitted should be operating, with one dedicated to anti-collision work. This is particularly important when there is a likelihood of occasional fog banks, so that vessels can be detected before entering the 203 fog. Radars are designed for continuous operation and frequently switching them on and off could damage components. 7.6.8. Parallel Index technique Investigations into cases where vessels have run aground have often shown that, when radar was being used as an aid to navigation, inadequate monitoring of the ship's position was a contributory factor. Parallel Index techniques provide the means of continuously monitoring a vessel's position in relation to a pre-determined passage plan, and would in some cases have helped to avoid these groundings. Parallel indexing should be practiced in clear weather during straightforward passages, so that watch-keepers remain thoroughly familiar with the technique and confident in its use in more demanding situations (in confined waters, restricted visibility or at night). The principles of parallel index plotting can be applied, using electronic index lines. A number of index lines may be pre-set and called up when required on all modes of display: electronic index lines remain at the set cross index range (CIR) enabling the operator to change range without corrupting the range of the index line. Care should be exercised when activating preset parallel index lines that the correct line(s) for the passage are being displayed. a) Parallel indexing on a relative motion display On a relative motion compass-stabilised radar display, the echo of a fixed object will move across the display in a direction and at a speed which is the exact reciprocal of own ship's ground track: parallel indexing uses this principle of relative motion. Reference is first made to the chart and the planned ground track. The index line is drawn parallel to the planned ground track at a perpendicular distance (cross index range or offset) equal to the planned passing distance off an appropriate fixed target. Observation of the fixed object's echo movement along the index line will indicate whether the ship is maintaining the planned track: any displacement of the echo from the index line will immediately indicate that own ship is not maintaining the desired ground track, enabling corrective action to be taken. b) Parallel indexing on a true motion display The use of a true motion radar presentation for parallel indexing requires an ability to ground-stabilized the display reliably. Parallel index lines are fixed relative to the trace origin (i.e. to own ship), and consequently move across the display at the same rate and in the same direction as own ship. Being drawn parallel to the planned charted track and offset at the required passing distance off the selected fixed mark, the echo of the mark will move along the index line as long as the ship remains on track. Any displacement of the fixed mark's echo from the index line will indicate that the ship is off track, enabling corrective action to be taken. c) Integration with ECDIS Where the radar display is integrated with an Electronic Chart Display and Information System (ECDIS) the practice of parallel indexing continues to enable the navigator to monitor the ship's position relative to the planned track and additionally provides a means of continuously monitoring the positional integrity of the ECDIS system. 204 d) Precautions Some older radars may still have reflection plotters. It is important to remember that parallel index lines drawn on reflection plotters apply to one range scale only. In addition to all other precautions necessary for the safe use of radar information, particular care must therefore be taken when changing range scales. The use of parallel indexing does not remove the requirement for position fixing at regular intervals using all appropriate methods available including visual bearings, since parallel indexing only indicates if the ship is on or off track and not its progress along the track. When using radar for position fixing and monitoring, check: (i) the identity of fixed objects, (ii) the radar's overall performance, (iii) the gyro error and accuracy of the heading marker alignment, (iv) that parallel index lines are correctly positioned on a suitable display, and (v) the accuracy of the variable range marker, bearing cursor and fixed range rings. 7.6.9 Chart Radar Some radars are provided with electronic chart overlays. These charts may have a limited amount of data and are not the equivalent to an Electronic Navigational Chart (ENC) used in the ECDIS or paper charts. They should not therefore be used as the primary basis for navigation. 7.6.10 Regular operational checks Frequent checks of the radar performance must be made to ensure that the quality of the display has not deteriorated. The performance of the radar should be checked using the Performance Monitor before sailing and at least every four hours whilst a radar watch is being maintained. Misalignment of the heading marker, even if only slightly, can lead to dangerously misleading interpretation of potential collision situations, particularly in restricted visibility when targets are approaching from ahead or fine on own ship's bow. It is therefore important that checks of the heading marker should be made periodically to ensure that correct alignment is maintained. If misalignment exists it should be corrected at the earliest opportunity. The following procedures are recommended: a) Check that the heading marker is aligned with the true compass heading of the ship. b) Ensure that the heading marker line on the display is aligned with the fore-and-aft line of the ship. This is done by selecting a conspicuous but small object with a small and distinct echo which is clearly identifiable and lies as near as possible at the edge of the range scale in use. Measure simultaneously the relative visual bearing of this object and the relative bearing on the display. Any misalignment must be removed in accordance with the instructions in the equipment manual. To avoid introducing serious bearing errors, adjustment of the heading marker should not be carried out: (i) when alongside a berth by using the berth's alignment. 205 (ii) using bearings of targets which are close to the vessel, not distinct or have not been identified with certainty both by radar and visually. 7.6.11 Stabilization modes It is important to select the optimum stabilization mode for the radar display. To assess risk of collision the relative motion of a target gives the clearest indication of CPA and may be monitored by observing either the direction of the target's relative trail, or the CPA predicted by the relative vector. By default, relative motion will display relative target trails and true motion will display true target trails. Where true target trails is selected, a sea stabilized display will indicate all targets' motion through the water. A ground stabilized display will indicate all targets' motion over the ground. In coastal, estuarial and river waters where a significant set and drift may be experienced, a sea stabilized display will produce significant target trails from all fixed (stationary) objects possibly producing an unacceptably high level of clutter and masking. In such circumstances a ground stabilized display may reduce its effect and enable the observer to detect clearly the trails of moving targets, thus enhancing the observer's situational awareness. It should be noted that the observed and predicted relative motion of a target is unaffected by the choice of sea or ground stabilization, allowing the same assessment of CPA and risk of collision. If switching between sea and ground stabilization, the observer should be aware of the time required for the radar equipment to reprocess the stabilization input data. 7.6.12 Speed Input It should be noted that in determining a target's aspect by radar; the calculation of its true track is dependent on the choice and accuracy of the own ship's course and speed input. A ground-stabilized target plot may accurately calculate the ground track of the target, but its heading may be significantly different from its track when experiencing set, drift or leeway. Similarly, a sea stabilized target plot may be inaccurate when own ship and the target, are experiencing different rates of set, drift or leeway. 7.6.13 Gyro failure In cases of gyro failure when the radar's heading data is provided from a Transmitting Magnetic Heading Device (TMHD), watch-keepers should determine and apply the magnetic compass errors. The true vector function of automatic plotting and tracking equipment should be operated with caution when the heading input is derived from a Transmitting Magnetic Compass (TMC). ARPA prediction is reliant on steady state tracking, where course and speed remain steady: In a seaway a transmitting magnetic compass may not produce a sufficiently steady heading resulting in unreliable vectors. 7.6.14 Warnings and alarms Audible operational warnings and alarms may be used to indicate that a target has closed on a pre-set range, enters a user-selected guard zone or violates a preset CPA or TCPA limit. 206 When the ARPA is in automatic acquisition mode, these alarms should be used with caution, especially in the vicinity of small radar-inconspicuous targets. Users should familiarize themselves with the effects of error sources on the automatic tracking of targets by reference to the ARPA Operating Manual. Such alarms do not relieve the user from the duty to maintain a proper lookout by all available means. Practical Examples Many casualties to ships have resulted from serious disregard for the basic principals of good seamanship and prudent navigation in bad visibility. Sensible use of radar and other aids to navigation greatly assists the conduct of ships in fog, but these aids have not reduced the need to comply fully with the Collision Regulations: to proceed at a safe speed, pay especial attention to good watch keeping, and navigate with proper caution. The following brief outline of two casualties shows how lack of sensible caution, combined with over-reliance on radar leads to accidents. A medium-sized cargo ship left port intending to proceed to sea, in fog so dense that the forecastle could not be seen from the bridge, a distance of 100 meters. To reach the sea it was necessary to navigate a river though a Channel with depths at low water of about 1.8 meters; the vessel’s draught was 8 meters and she sailed on a falling tide. The channel is in places narrow and several bends have to be negotiated. The tide runs at up to 4 knots, falls at a rate of as much as 0.5 meters in 10 minutes, and in places sets across the channel. Great care is therefore necessary at all times, and to attempt the passage on a falling tide in dense fog was very foolhardy, even with the aid of radar. Not surprisingly the ship stranded. A large container ship was in transit through the Dover Strait Traffic Separation Scheme, and despite very thick fog she was steaming at about 18 knots. The bridge was manned by the Master, Officer of the Watch and a lookout. Both radar’s (one of which was an ARPA) were being used, but although they were found to be in good working order, when inspected after the casualty it is apparent that not all possible echoes were being displayed, perhaps due to the masking effect of clutter: there was a force 5 breeze and a considerable sea running. When radar clutter is experienced even a careful search by both automatic and manual clutter controls may not reveal the presence of small craft, and this fact should have been recognized by those on watch. Nevertheless, and despite a close quarter encounter with a fishing vessel in which the ship had to take last minute avoiding action to avert collision, she continued at 18 knots and, later, collided with a trawler which was not seen on either radar. The trawler was stopped and hauling her nets at the time; she was severely damaged though she was able to make port. As well as demonstrating the folly of high speed in fog, this accident emphasizes the need for fisherman while working, to maintain prudent navigation and watch keeping. In all cases those responsible for the ship’s navigation sacrificed seamen for expediency. They failed to recognise the limitations of aids to navigation; or to follow the requirements of the Collision Regulations and the advice of Marine Notices. It is worth stressing that the ships involved were all well equipped vessels in the charge of men with sound qualifications; it was not skill or experience that was lacking, but the proper seaman like approach to the situation. Whatever the pressure on Masters to make a quick passage or to meet the wishes of owners, operators, charters or port operators, it does not justify ships and those on board them being put unnecessarily at risk. Proper standards must be maintained, and will take 207 appropriate action which may lead to the loss of their certificates, against officers who in future jeopardize their ships, or the lives and property of others. It has long been established, and the ISM Code now expressly provide, that it is the duty of the Company to take all responsible steps to secure that the ship is operated in a safe manner. The Company must have established and implemented an effective safety management system which includes procedures to ensure safe operation of ships, as well as reporting accidents and non-conformities. In the well-known case of THE LADY GWENDOLEN, the Court of Appeal said that “excessive speed in fog is a grave breach of duty, and ship owners should use their influence to prevent it”. Because of their failure to do so, it was held in that case that the owners could not limit their liability. 7.7. Obtaining information from ARPA Displays 7.7.1. Target Information The full target information is as follows: • Target Name* and Identification Number; • CPA – Closest Point of Approach; • TCPA – Time to Closest Point of Approach; • RANGE – target Range from own ship; • BRG – target Bearing from own ship; • T CO – target Course (see below); • SPEED – target speed. * if previously allocated via Autotrack/ARPA menu. Some of the target information depends on the speed mode selected via the Speed menu. Using Manual Speed or Serial Logs means that the picture is Sea Stabilised, whereas Echo Ref, Nav G, and some Serial Logs provide Ground Stabilisation. With Sea Stabilisation, target information is also sea stabilised i.e., the course is the water track and the speed is the speed through the water. When operating with Ground Stabilisation, the target course and speed are Course Made Good (CMG) and Speed Made Good (SMG) respectively. 7.7.2. Target Tote (ARPA Only) An alternative display, which shows limited target information for up to five selected targets, can be displayed by a short press of the DATA SELECT key with the cursor positioned over a clear area of the display. This type of presentation is shown below: The limited 5-Target information is as follows: ID – target Identification Number; TCPA – Time to Closest Point of Approach; CPA – Closest Point of Approach. 208 Initially, the Tote display is blank. To select targets for display, move the cursor over selected targets and press the Data/Select key. The selected target identification numbers will be displayed against the target and its information will be shown in the Tote. To delete a target from the tote, position the cursor over it and press Data/Select. This action does not cancel the target from the tracked target list. A long key press of the Data/Select removes the target table from the screen, and the operator can obtain data on any one selected target. To show table of the targets again, place the cursor in a clear area and short press Data/Select. 7.7.3 Past Positions The Past Positions of all targets can be displayed by selection via the ARPA Menu. Past Positions are displayed as a series of dots on the 0.75 to 24 n.mile range scales inclusive: four dots at 3-minute time intervals. The term Past Positions is used in accordance with IMO definitions. In some earlier ARPA radars, the term History Dots was used for this function. 7.7.4 Standby Mode The radar always appears in standby mode. Fig. 7.7.4 Standby Mode 209 Selection of TX A (S) or TX A (X) type of transiver: Fig. 7.4.5. Selection of TX A (S) or TX A (X) In standby mode, a number of functions are available which allow the display to be set up for operation. The following functions can be accessed from standby mode: • Brilliance (DEFAULT BRILL, BRILLIANCE +, BRILLIANCE -, to select the display palette: DAY and 3 NIGHT); • Cursor data; • Range selection; • User data; • Heading; • Speed; • Presentation & Motion modes; ARPA Limits and Settings; • Alarms; • System (exit from BridgeMaster E radar). 7.7.5 Transmit mode After adjusting of the required functions in standby mode, press TRANSMIT soft key. 7.7.6 Using the Radar Controls 7.7.6.1 Control Panels There are two types of control panel in current use for controlling the radar, a Simple Control Panel and an optional Dedicated Control Panel. 7.7.6.2 Simple Control Panel The simple control panel is made up of a number of modules which are usually mounted immediately under the display monitor. A simple pointing device (joystick or tracker-ball), with two associated keys (left and right), is used to control the radar and its display. The joystick/tracker-ball controls the position of the on-screen cursor which is displayed as a small white arrow when positioned outside the radar circle, see The On-screen Cursor later in this chapter. Selections are made by positioning the on-screen cursor over an object or caption and clicking (press and release) with the “left” key. The left key is duplicated on the left 210 hand side of the control panel, to enable two handed operation. The “right” key is used on some items to provide additional functionality when available. Note: Throughout this manual, instructions to “left click” or “right click” relate to a press-and-release of either of the left keys or the right key. Similarly, references to the “cursor control” relate to the joystick or tracker ball depending on which is fitted. 7.7.6.3 Optional Dedicated Control Panel A Dedicated Control Panel, which contains a number of additional push buttons and rotary controls, can be fitted as an optional extra. The Simple Control Panel is always fitted. The Dedicated Control Panel provides individual tactile controls for specific functions. These functions would normally be accessed and adjusted using the cursor control and associated left/right keys of the Simple Control Panel. The controls available are as follows: Push Buttons: “RANGE UP”, “RANGE DOWN”; “TM/RM”, “TRUE/REL VECTORS”; “CENTRE”, “ACK ALARM”. Rotary Controls: “GAIN”, “RAIN” (Clutter), “SEA” (Clutter); “EBL 1”, “VRM 1”, “PANEL” (Brightness). 7.7.7 The On-screen cursor When the on-screen cursor is outside the video circle it is displayed as a small white arrow, referred to as the screen cursor. As the cursor passes into the video circle it changes and is displayed as a small white cross, referred to as the video cursor. Fig. 7.7.7.a Screen cursor Fig. 7.7.7.b Video cursor 211 7.7.7.1 Screen cursor As the screen cursor moves over a caption or item which can be accessed, its box is highlighted (drawn in white), and two small boxes (representing the left and right keys) appear next to the arrowhead cursor. One or both of these boxes is filled in white to indicate which key(s) are active and available for selection, see example left. Fig. 7.7.7.1 Screen cursor Note: For reasons of clarity and to avoid conflicting information, the screen cursor is shown without its associated left/right key boxes in the diagrams throughout the rest of the manual. If a caption box is not highlighted as the cursor passes over it, it indicates that the caption or item inside the box cannot be accessed in the current mode. Drop down menu options are highlighted in yellow as the cursor passes over them. If a particular option is not available it will not be highlighted. Options, which can never be selected because of the current radar configuration, are NOT shown. If an adjustable parameter is selected, the cursor will disappear and the parameter is displayed in yellow (as a number or control bar). If an adjustment is not made within 10 seconds, the parameter will be automatically deselected, and the cursor will reappear. 7.7.7.2 Video cursor Whenever the video cursor is displayed, a dialog box giving readout of the cursor's position within the video circle, replaces the usual function soft keys shown in the bottom right hand corner of the display. By default this box gives cursor range and bearing (from own ship) and cursor lat/long. Note: Soft keys are small, boxed areas of the screen, usually containing a single caption, which respond in much the same way as the dedicated function keys of a computer keyboard. In TRANSMIT mode, the range and bearing of the cursor are relative to own ship's position. In STANDBY mode, the range and bearing are relative to the centre of the video circle. Note: If, when in TRANSMIT, own ship's position is lost, or there is a compass error, the lat/long readings are replaced by dashes. 7.7.8 Drop down menus Where there are a number of fixed selections for a particular parameter, for example RANGE in the top left hand corner of the display, a left click will reveal a drop down menu of the alternatives available. 212 Fig. 7.7.8 Drop down menus A drop down menu is usually displayed in the vicinity of the screen cursor when the selection is made. Once a menu is displayed, the cursor is restricted to the area within the menu and selections are made with a left click. A right click will close the menu without taking further action (i.e. Cancel). 7.7.9 Selecting a Mode of Operation From the STANDBY display, there are two mode selections available. 7.7.9.1 Transmit The normal operational mode. The antenna is rotating and the transceiver transmits and receives radar pulses enabling a radar picture to be displayed. 7.7.9.2 Initialisation The system initialisation mode. This is used to set up the system parameters during installation. The soft keys for selecting these modes of operation are located in the bottom left hand corner of the display. 7.7.9.3 To Select a Mode 1. Use the cursor control to position the screen cursor over the soft key for the mode required. (Usually TRANSMIT). 2. Left click to select. 7.7.10 Basic Operation 7.7.10.1 Introduction When TRANSMIT is selected from Standby, the system is switched to transmit. Slave radars can only be set to transmit if the associated Master is already transmitting. 213 Transmit Display Most of the captions and soft keys associated with the TRANSMIT display are available for selection, and are highlighted individually as the screen cursor moves over them. Most of the basic radar functions are covered in this chapter. Other, more specific functions are covered in individual chapters. 7.7.10.2. User Specified Data The user data area of the display is located at the bottom right hand side, above the help area and function soft keys. The area is used to show information relating to own ship and is available in both Standby and Transmit modes. The following types of data can be displayed: • Own Ship's Position; • Wind and Depth. 7.7.10.3 Selecting the Data Type 1. Position the screen cursor over the top line of text in the User Data box. 2. Left click to select the type of data required. Each click will cycle the display to the next type. Alternatively a right click will reveal a drop down menu containing a list of data types, left click on the type required, or right click to close the menu without further action. 7.7.10.4 Data Displays When specific data is unavailable, the associated readout is replaced with dashes. 7.7.10.5 Own Ship's Position The source can be any one of the following: (DGPS), (GPS). If there is a Position alarm, the lat/lon is displayed in RED. 214 7.7.10.6. Wind and Depth Either TRUE or REL (Relative) wind speed is displayed depending on the data received from the sensor. The wind bearing is displayed relative to own ship's heading. Fig. 7.7.10.6. Wind and Depth 7.7.10.7. Range Scales & Range Rings The radar range scale can be selected from a list of preset values. A set of fixed range rings, displayed as a number of equally spaced concentric circles (normally six), can also be switched ON or OFF. Range scale selection can be made in both Standby and Transmit modes. Range rings cannot be selected or displayed in Standby. Fig. 7.7.10.7.a Range Scales The current range scale and range ring selections are given in the top left hand corner of the display. The ranges are displayed in nm. Fig. 7.7.10.7.b Range Scales 7.7.10.8. Target Data The target box defaults to showing data for a single target. 215 7.7.10.9. Acquired Target Data The following data is shown: Fig7.7.10.9. Acquired Target Data - “TARGET” Target identification number/name. - “RANGE” Range of target from own ship. - “T BRG” Bearing of target from own ship. - “CPA” Closest point of approach to own ship. - “TCPA” Time to closest point of approach. - “CSE”/ ”COG” Target's Course through the water (CSE) or Course Over the Ground (COG). - “STW”/ ”SOG” Target's Speed Through the Water (STW) or Speed Over the - Ground (SOG). - “BCR” Bow crossing range. - “BCT” Bow crossing time. The target, for which data is shown, can be selected by left clicking on an acquired target in the video circle. The selected target [2] is identified in the video circle by a small ” ” symbol centred on the plot origin. 7.7.11 Trial Manoeuvres A trial manoeuvre can be carried out to see the effect of a proposed manoeuvre of own ship. 1. Position the screen cursor over the “TRIAL” soft key. 2. Left click to reveal the TRIAL MANOEUVRE menu shown in the example left. Fig. 7.7.11.a Trial Manoeuvres Note: Own ship's course and speed are used as the default settings in the Trial Manoeuvre menu. 216 Fig. 7.7.11 Trial Manoeuvres 7.7.11.1 Running a Trial Manoeuvre Final Course of Own Ship Enter the proposed course of own ship to be followed after the manoeuvre. 1. Left click on the course line (CSE in the example) to activate. 2. Move the cursor control left or right to set the course required. 3. Left click to accept. Speed of Manoeuvre If you intend to change speed, enter the proposed speed of own ship to be maintained during and after the manoeuvre. 1. Left click on the speed line (STW in the example) to activate. 2. Move the cursor control left or right to set the required speed. 3. Left click to accept. Manoeuvre Delay Enter the proposed time delay between switching the trial manoeuvre ON and actually starting the manoeuvre. 1. Left click on the DELAY line to activate. 2. Move the cursor control left or right to set the required delay. 3. Left click to accept. Vector Type Select TRUE or REL (Relative) vector type as follows: 1. Position the screen cursor over the vector type selection field in the Trial Manoeuvre menu. 2. Consecutive left clicks will toggle the type between TRUE (T) and RELATIVE (R) VECTORS. Vector Time Enter the proposed vector time. 217 1. Position the screen cursor over the vector time field in the Trial Manoeuvre menu. 2. Left click to access. 3. Move the cursor control left or right to change the time. 4. Left click to accept. Note: Entering a longer vector time will allow you to see further into the trial manoeuvre. The above procedures (for Vector Type and Vector Time) will overwrite any selections made earlier (see Vector Node) and will remain in force until changed. If required, reset the vector time after the trial manoeuvre is completed. Manoeuvre Switch-ON Left click on the RUNNING OFF line to switch the manoeuvre ON. The “manoeuvre delay” entered earlier will start to count down. The manoeuvre vectors are displayed until the time for the manoeuvre expires, or the manoeuvre is switched-OFF. (When the manoeuvre is running, a left click on the RUNNING ON line will switch-OFF the manoeuvre.) The trial manoeuvre vectors are displayed when the trial manoeuvre is running. If true (T) vectors are selected, the trial vector is shown for own ship only, as shown in the example below. This shows own ship's true course during the manoeuvre. If relative (R) vectors are selected, the trial vectors are applied to every, acquired target, with own ship's vector suppressed, and show the course of the targets relative to own ship. Note: “Vector type” (T or R vectors) and “vector time” may be changed at any time before or during the manoeuvre. While the trial manoeuvre is running and the Trial Manoeuvre Menu is displayed, the word “TRIAL” will appear at the bottom of the video circle. Once the delay has elapsed, the word “TRIAL” is removed from the display and the manoeuvre is turned OFF. 7.8. Navigate the vessel Saffely using ARPA Under STCW, the OOW may not act as sole lookout except in daylight and only then in suitable conditions. He is to ensure that the look-out is posted on the bridge or forward if required. The watch shall call the lookout in restricted visibility, restricted waters, or other wise in accordance with the master standing orders. It is usual practice for managed container ships to have the lookout on the bridge at all times when in coastal waters. The look-out will not leave the bridge during the watch. Arrangements will be made for calling the watch where the lookout does not need to leave the bridge. If a suitable telephone system is not available alternative and suitable calling methods will be made. The lookout is required to report all lights, land, shipping or other to the OOW and a professional working relationship is to be encouraged. No unnecessary conversation should be held with the look-out man to distract from his duties. Other personnel who are not on duty are not allowed on the bridge except by permission of the Master. The OOW shall ensure that the lookouts are adequately sheltered and clothed for the prevailing weather conditions; the efficiency of the look-outs and the reports obtained from them depends on the relative comfort of their post. 218 Where the lookout is required to take the helm, the OOW shall assume (or another person shall be assigned to) his duties until he is released from helm duties. 7.8.1 General Watch-keeping All officers of the watch shall sign and follow the Master's Bridge Standing orders. 7.8.1.1 Lookout The Officer of the Watch (OOW) should keep his watch on the bridge: in no circumstances should he leave the bridge until properly relieved. It is of particular importance that he ensures that an efficient lookout is maintained at all times. In a vessel with a separate chart room the Officer of the Watch may visit it when essential, for a short period in order to carry out his navigational duties, but he should first satisfy himself that it is safe to do so and that a good lookout is being kept. The Officer of the Watch should not hand over the watch if he has any reason to believe that the relieving officer is suffering from disability (including illness, drink, drugs or fatigue) which would preclude him from carrying out his duties effectively. If in doubt, he should consult the master. All officers shall read and sign the master night orders before taking over a 'night watch'. The relieving Officer of the Watch should ensure that members of his watch are fully capable of performing their duties and in particular that they are adjusted to night vision. He should not take over the watch until his vision is fully adjusted to the prevailing light conditions and he has personally satisfied himself concerning the items of the watch change over checklist. Only after the relieving Officer is satisfied may he take over. In case he is in any doubt, the Master should be informed at once. Handing over the watch should be postponed when the ship is, or is about to be, engaged in a collision avoidance manoeuvre or a navigational alteration of course. A fundamental responsibility of the Officer of the Watch is to ensure the maintenance of a continuous and alert watch; this is one of the most important considerations in the avoidance of collisions, groundings and other casualties. This includes: - an alert all-round visual and aural (sound) lookout to allow a full grasp of the current situation, including the presence of ships and landmarks in the vicinity; - close observation of the movements and bearing of approaching vessels; - identification of ship and shore lights; - observation of the radar, AIS and echo sounder displays; - observation of ECDIS and GPS (Highway or XTE) displays; - observation of changes in the weather, especially the visibility. - maintain and efficient watch on VHF Channels 16 and 13, and relevant coastal traffic channels, together with the GMDSS watch. The Officer of the Watch should make regular checks to ensure that: - the helmsman or the automatic pilot is steering the correct course; 219 - the standard magnetic compass error is established at least once a watch and also if possible after any major alteration of course; - the standard magnetic and gyro compasses are compared frequently and gyro repeaters synchronised; - the automatic pilot is tested in the manual position at least once a watch; - the navigation and signal lights and other navigational equipment are functioning properly. 7.8.2. Manoeuvring The Officer of the Watch should bear in mind that the engines are at his disposal for assistance in manoeuvring. He should not hesitate to use them in case of need, although timely notice of an alteration of engine movements should be given whenever possible. The Officer of the Watch shall not hesitate in taking whatever action may be required to avoid immediate danger, including alteration of course and / or speed and if necessary operating the engines astern. He shall regulate speed immediately as required to achieve a safe speed in poor visibility, informing the Master that he has done so. The Officer of the Watch should be fully aware of the manoeuvring capabilities of his ship, including the time and distance taken to achieve emergency and routine stops in both "open sea" and manoeuvring conditions. The Officer of the Watch should bear in mind the need to station the helmsman and change over the steering to manual control in good time to allow any potentially hazardous situation to be dealt with in a safe manner. 7.8.2.1 Colreg Rulles Review Rule 7 deals with the "Risk of Collision". The Rule stresses again the use of "all the available means appropriate to the prevailing circumstances and conditions to determine if the risk of collision exists". This includes the 'listening to V.H.F. R/T messages of other ships and shore radar stations, but no guidance is give about actual active participation. . There is a very important last sentence in the first paragraph: "If there IS any doubt such risk shall be deemed to exist". This might remove the possible element of indecision in a radar encounter. Rule 7 (b) stresses the importance of making proper use of radar equipment, including early warning of collision risk on the long-range scales. It furthermore emphasizes the practice of radar plotting or "equivalent systematic observation of detected objects" (recording in writing and tabulation, automatic plotting aids). Rule 7 (e) states: "''Assumptions shall not be made on the basis of scanty . This problem does not arise with Relative Motion radar. information, especially scanty radar information". The omission of a plot, an incomplete plot or a plot based on an insufficient number of observations, in short the determination of the position of another vessel without finding her movement, might be termed as "scanty". ARPA provides a solution here. The last paragraph of Rule 7 states how risk of collision can be obtained from compass bearings and gives a warning that an appreciable change in bearing does not always indicate a safe passing (large vessel, Or a tow, or a ship at close range; Bearings should 220 be recorded as compass bearings and not as relative bearings as is so easily done on an unstabilized display. It is not possible to compare relative bearings when own ship is subject to yaw or makes alterations of course, and often the Master has been led to believe, that. by making a small alteration of course, the situation improved because the relative bearing changed and he did not realize that the change in the relative bearing was mainly due to own ship's alteration of course. If he had converted the relative bearings to compass bearings, he would have noticed that danger of collision after the alteration had become greater instead of less. Rule 8 is headed "Action to avoid Collision". Paragraph (a) states that, if the circumstances of the case admit, any action shall "be positive, made in ample time and with due regard to the observance of good seamanship". The word "positive" in this connection means "effective" and bears no relationship to the conventional adaptations "'positive and negative actions", mentioned in certain papers about collision-avoidance (more about these later). Paragraph (b) is an extension of paragraph (a), stating that "Any alteration of course and! or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and (or speed should be avoided". The Rule requires substantial action in order to make clear one's intention to all vessels in the neighbourhood ("another vessel" is not necessarily the vessel for which avoiding action was taken) both in clear weather as well as in fog. This requirement should be kept in mind when an agreement is reached about collision-avoiding tactics between two vessels via V.H.F. R!T and also when using the 'Trial Manoeuvre' facility on ARPA. The remainder of the Rule (paragraphs (c), (d) and (e» emphasizes that an alteration of course, provided there is sufficient sea room, may be the most effective action to avoid a close quarters situation on condition that it is made in good time, is substantial and does not result in another close-quarters situation. It stresses the safe passing distance and warns that effectiveness of the action shall be carefully checked until the other vessel is finally past and clear. If necessary, or to allow more time to assess the situation, a vessel shall slacken her speed or take all way off by stopping Or reversing her means of propulsion. In short, what this Rule is saying is that if avoiding action for another vessel is going to be taken such action should be bold both in clear weather Rule 10 applies to Traffic Separation Schemes and is a highly important addition to the 1972 Rules. It contains regulations adopted by IMO (see IMO-publication "Ships' Rooting Traffic Separation Schemes and Areas to be Avoided") but have become mandatory for all the published schemes. Additional paragraphs are included for small vessels and sailing ships, and exempted vessels. The more important parts of the Rule state that "a vessel using traffic-separation scheme shall, so far as practicable, keep clear of traffic-separation line or zone (b (ii) ), normally join or leave a traffic-lane the termination of a lane, but when joining or leaving from either side, shall do so at as small an angle to the general direction of traffic flow aspracticable (b (iii)) and shall, so far as practicable, avoid crossing trafficlanes, but if obliged to do so, shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow (c). It goes on: "(d) Inshore traffic zones shall not normally be used by through traffic which can safely use the appropriate traffic-lane within the adjacent traffic-separation scheme. However, vessels of less than 20 meters in 'length and sailing vessels may under all circumstances use inshore traffic 221 zones". In connection with the right-angled crossing, it is advised, if possible, to shape the new course well before the lane is reached, thus giving ships within the lane a timely indication. 7.8.3 Selecting a Mode of Operation for radar From the STANDBY display, there are two mode selections available. 7.8.3.1 Transmit The normal operational mode. The antenna is rotating and the transceiver transmits and receives radar pulses enabling a radar picture to be displayed. 7.8.3.2 Initialisation The system initialisation mode. This is used to set up the system parameters during installation. The soft keys for selecting these modes of operation are located in the bottom left hand corner of the display. 7.8.3.3 To s elect a Mode 1. Use the cursor control to position the screen cursor over the soft key for the mode required. (Usually TRANSMIT). 2. Left click to select. 7.8.4 Basic Operation 7.8.4.1 Introduction When TRANSMIT is selected from Standby, the system is switched to transmit. Slave radars can only be set to transmit if the associated Master is already transmitting Most of the captions and soft keys associated with the TRANSMIT display are available for selection, and are highlighted individually as the screen cursor moves over them. Most of the basic radar functions are covered in this chapter. Other, more specific functions are covered in individual chapters. 7.8.4.2 User Specified Data The user data area of the display is located at the bottom right hand side, above the help area and function soft keys. The area is used to show information relating to own ship and is available in both Standby and Transmit modes. The following types of data can be displayed: • Own Ship's Position; • Wind and Depth. 7.8.4.3 Selecting the Data Type 222 1. Position the screen cursor over the top line of text in the User Data box. 2. Left click to select the type of data required. Each click will cycle the display to the next type. Alternatively a right click will reveal a drop down menu containing a list of data types, left click on the type required, or right click to close the menu without further action. 7.8.4.4 Data Displays When specific data is unavailable, the associated readout is replaced with dashes. 7.8.4.5 Own Ship's Position The source can be any one of the following: (DGPS), (GPS). If there is a Position alarm, the lat/lon is displayed in RED. 7.8.4.6. Wind and Depth Either TRUE or REL (Relative) wind speed is displayed depending on the data received from the sensor. The wind bearing is displayed relative to own ship's heading. 7.8.5 Using the Radar Controls 7.8.5.1 Control Panels There are two types of control panel in current use for controlling the radar, a Simple Control Panel and an optional Dedicated Control Panel. 7.8.5.2 Simple Control Panel The simple control panel is made up of a number of modules which are usually mounted immediately under the display monitor. A simple pointing device (joystick or trackerball), with two associated keys (left and right), is used to control the radar and its display. The joystick/tracker-ball controls the position of the on-screen cursor which is displayed as a small white arrow when positioned outside the radar circle, see The Onscreen Cursor later in this chapter. Selections are made by positioning the on-screen cursor over an object or caption and clicking (press and release) with the “left” key. The left key is duplicated on the left hand side of the control panel, to enable two handed operation. The “right” key is used on some items to provide additional functionality when available. Note: Throughout this manual, instructions to “left click” or “right click” relate to a press-and-release of either of the left keys or the right key. Similarly, references to the “cursor control” relate to the joystick or tracker ball depending on which is fitted. 7.8.5.3 Optional Dedicated Control Panel A Dedicated Control Panel, which contains a number of additional push buttons and rotary controls, can be fitted as an optional extra. The Simple Control Panel is always fitted. The Dedicated Control Panel provides individual tactile controls for specific functions. These functions would normally be accessed and adjusted using the cursor control and associated left/right keys of the Simple Control Panel. The controls available are as follows: 223 Push Buttons: “RANGE UP”, “RANGE DOWN”; “TM/RM”, “TRUE/REL VECTORS”; “CENTRE”, “ACK ALARM”. Rotary Controls: “GAIN”, “RAIN” (Clutter), “SEA” (Clutter); “EBL 1”, “VRM 1”, “PANEL” (Brightness). 7.8.5.4 The On-screen cursor When the on-screen cursor is outside the video circle it is displayed as a small white arrow, referred to as the screen cursor. As the cursor passes into the video circle it changes and is displayed as a small white cross, referred to as the video cursor. Fig. 7.8.5.4.a Screen cursor .Fig.7.8.5.4. b Video cursor 7.8.5.5 Screen cursor As the screen cursor moves over a caption or item which can be accessed, its box is highlighted (drawn in white), and two small boxes (representing the left and right keys) appear next to the arrowhead cursor. One or both of these boxes is filled in white to indicate which key(s) are active and available for selection, see example left. 7.8.5.5.a Screen cursor Note: For reasons of clarity and to avoid conflicting information, the screen cursor is shown without its associated left/right key boxes in the diagrams throughout the rest of the manual. If a caption box is not highlighted as the cursor passes over it, it indicates that the caption or item inside the box cannot be accessed in the current mode. 224 Drop down menu options are highlighted in yellow as the cursor passes over them. If a particular option is not available it will not be highlighted. Options, which can never be selected because of the current radar configuration, are NOT shown. If an adjustable parameter is selected, the cursor will disappear and the parameter is displayed in yellow (as a number or control bar). If an adjustment is not made within 10 seconds, the parameter will be automatically deselected, and the cursor will reappear. 7.8.5.6 Video cursor Whenever the video cursor is displayed, a dialog box giving a readout of the cursor's position within the video circle, replaces the usual function soft keys shown in the bottom right hand corner of the display. By default this box gives cursor range and bearing (from own ship) and cursor lat/long. Note: Soft keys are small, boxed areas of the screen, usually containing a single caption, which respond in much the same way as the dedicated function keys of a computer keyboard. In TRANSMIT mode, the range and bearing of the cursor are relative to own ship's position. In STANDBY mode, the range and bearing are relative to the centre of the video circle. Note: If, when in TRANSMIT, own ship's position is lost, or there is a compass error, the lat/long readings are replaced by dashes. 7.8.5.7 Drop Down Menus Where there are a number of fixed selections for a particular parameter, for example RANGE in the top left hand corner of the display, a left click will reveal a drop down menu of the alternatives available. Fig. 7.8.5.7 Drop Down Menus A drop down menu is usually displayed in the vicinity of the screen cursor when the selection is made. Once a menu is displayed, the cursor is restricted to the area within the menu and selections are made with a left click. A right click will close the menu without taking further action (i.e. Cancel). 225 7.8.6 Selecting a Mode of Operation From the STANDBY display, there are two mode selections available. 7.8.6.1 Transmit The normal operational mode. The antenna is rotating and the transceiver transmits and receives radar pulses enabling a radar picture to be displayed. 7.8.6.2 Initialisation The system initialisation mode. This is used to set up the system parameters during installation. The soft keys for selecting these modes of operation are located in the bottom left hand corner of the display. 7.8.6.3 To Select a Mode 1. Use the cursor control to position the screen cursor over the soft key for the mode required. (Usually TRANSMIT). 2. Left click to select. 7.8.7 Basic Operation 7.8.7.1 Introduction When TRANSMIT is selected from Standby, the system is switched to transmit. Slave radars can only be set to transmit if the associated Master is already transmitting. Fig. 7.8.7 Transmit Display Most of the captions and soft keys associated with the TRANSMIT display are available for selection, and are highlighted individually as the screen cursor moves over them. Most of the basic radar functions are covered in this chapter. Other, more specific functions are covered in individual chapters. 7.8.7.2 User Specified Data 226 The user data area of the display is located at the bottom right hand side, above the help area and function soft keys. The area is used to show information relating to own ship and is available in both Standby and Transmit modes. The following types of data can be displayed: • Own Ship's Position; • Wind and Depth. 7.8.7.3 Selecting the Data Type 1. Position the screen cursor over the top line of text in the User Data box. 2. Left click to select the type of data required. Each click will cycle the display to the next type. Alternatively a right click will reveal a drop down menu containing a list of data types, left click on the type required, or right click to close the menu without further action. 7.8.7.4 Data Displays When specific data is unavailable, the associated readout is replaced with dashes. 7.8.7.5 Own Ship's Position The source can be any one of the following: (DGPS), (GPS). If there is a Position alarm, the lat/lon is displayed in RED. 7.8.7.6 Wind and Depth Either TRUE or REL (Relative) wind speed is displayed depending on the data received from the sensor. The wind bearing is displayed relative to own ship's heading. Fig.7.8.7.6 Wind and Depth 7.8.7.7 Range Scales The radar range scale can be selected from a list of preset values. A set of fixed range rings, displayed as a number of equally spaced concentric circles (normally six), can also be switched ON or OFF. Range scale selection can be made in both Standby and Transmit modes. Range rings cannot be selected or displayed in Standby. The current range scale and range ring selections are given in the top left hand corner of the display. The ranges are displayed in nm. 227 Fig. 7.8.7.7 Range Scales 7.8.8 Target Data The target box defaults to showing data for a single target. 7.8.8.1 Acquired Target Data The following data is shown: Fig. 7.8.8.1 Acquired Target Data - “TARGET” Target identification number/name. - “RANGE” Range of target from own ship. - “T BRG” Bearing of target from own ship. - “CPA” Closest point of approach to own ship. - “TCPA” Time to closest point of approach. - “CSE”/ ”COG” Target's Course through the water (CSE) or Course Over the - Ground (COG). - “STW”/ ”SOG” Target's Speed Through the Water (STW) or Speed Over the - Ground (SOG). - “BCR” Bow crossing range. - “BCT” Bow crossing time. The target, for which data is shown, can be selected by left clicking on an acquired target in the video circle. The selected target [2] is identified in the video circle by a small ” ” symbol centred on the plot origin. 228 7.8.9 Trial Manoeuvres A trial manoeuvre can be carried out to see the effect of a proposed manoeuvre of own ship. 1. Position the screen cursor over the “TRIAL” soft key. 2. Left click to reveal the TRIAL MANOEUVRE menu shown in the example left. Fig. 7.8.9.a Trial Manoeuvres Note: Own ship's course and speed are used as the default settings in the Trial Manoeuvre menu. Fig. 7.8.9.b Trial Manoeuvres 7.8.9.1 Running a Trial Manoeuvre Final Course of Own Ship Enter the proposed course of own ship to be followed after the manoeuvre. 1. Left click on the course line (CSE in the example) to activate. 2. Move the cursor control left or right to set the course required. 3. Left click to accept. Speed of Manoeuvre If you intend to change speed, enter the proposed speed of own ship to be maintained during and after the manoeuvre. 1. Left click on the speed line (STW in the example) to activate. 2. Move the cursor control left or right to set the required speed. 3. Left click to accept. 229 Manoeuvre Delay Enter the proposed time delay between switching the trial manoeuvre ON and actually starting the manoeuvre. 1. Left click on the DELAY line to activate. 2. Move the cursor control left or right to set the required delay. 3. Left click to accept. Vector Type Select TRUE or REL (Relative) vector type as follows: 1. Position the screen cursor over the vector type selection field in the Trial Manoeuvre menu. 2. Consecutive left clicks will toggle the type between TRUE (T) and RELATIVE (R) VECTORS. Vector Time Enter the proposed vector time. 1. Position the screen cursor over the vector time field in the Trial Manoeuvre menu. 2. Left click to access. 3. Move the cursor control left or right to change the time. 4. Left click to accept. Note: Entering a longer vector time will allow you to see further into the trial manoeuvre. The above procedures (for Vector Type and Vector Time) will overwrite any selections made earlier (see Vector Node) and will remain in force until changed. If required, reset the vector time after the trial manoeuvre is completed. Manoeuvre Switch-ON Left click on the RUNNING OFF line to switch the manoeuvre ON. The “manoeuvre delay” entered earlier will start to count down. The manoeuvre vectors are displayed until the time for the manoeuvre expires, or the manoeuvre is switched-OFF. (When the manoeuvre is running, a left click on the RUNNING ON line will switch-OFF the manoeuvre.) The trial manoeuvre vectors are displayed when the trial manoeuvre is running. If true (T) vectors are selected, the trial vector is shown for own ship only, as shown in the example below. This shows own ship's true course during the manoeuvre. If relative (R) vectors are selected, the trial vectors are applied to every, acquired target, with own ship's vector suppressed, and show the course of the targets relative to own ship. Note: “Vector type” (T or R vectors) and “vector time” may be changed at any time before or during the manoeuvre. While the trial manoeuvre is running and the Trial Manoeuvre Menu is displayed, the word “TRIAL” will appear at the bottom of the video circle. Once the delay has elapsed, the word “TRIAL” is removed from the display and the manoeuvre is turned OFF. 230 7.9. Navigation in Confined and Congested Waters and Collision Avoidance 7.9.1. Introduction Safe navigation is the most fundamental attribute of good seamanship. An increasingly sophisticated range of navigational aids can today complement the basic skills of navigating officers, which have accumulated over the centuries. But sophistication brings its own dangers and a need for precautionary measures against undue reliance on technology. Experience shows that properly formulated bridge procedures and the development of bridge teamwork are critical to maintaining a safe navigational watch. If more than one person is involved in navigating it is essential to agree the passage plan and to communicate the way the voyage objectives are to be achieved consistently and without ambiguity. The process starts with company instructions to the ship, as encompassed by a safety management system supported by master's standing orders and reinforced by discussion and bridge orders. Existing local pilotage legislation should also be ascertained to enable the master to be guided accordingly. The master has the ultimate responsibility for the safety of the ship. Delegation of authority to the officer of the watch (OOW) should be undertaken in accordance with agreed procedures and reflect the ability and experience of the watchkeeper. Similarly, when a pilot boards the master may delegate the conduct of the ship to the pilot, bearing in mind that pilotage legislation varies from country to country and from region to region. Pilotage can range from optional voluntary pilotage that is advisory in nature, to compulsory pilotage where the responsibility for the conduct of the navigation of the ship is placed upon the pilot. The master cannot abrogate responsibility for the safety of the ship and he remains in command at all times. If the master delegates the conduct of the ship to the pilot, it will be because he is satisfied that the pilot has specialist knowledge, shiphandling skills and communications links with the port. In doing so the master must be satisfied that the pilot's intentions are safe and reasonable. The OOW supports the pilot by monitoring the progress of the ship and checking that the pilot's instructions are correctly carried out. Where problems occur which may adversely affect the safety of the ship, the master must be advised immediately. The process of delegation can be the cause of misunderstanding and so it is recommended that a clear and positive statement of intention be made whenever handing over and receiving conduct of the ship. When navigating with the master on the bridge it is considered good practice, when it is ascertained that it is safe to do so, to encourage the OOW to carry out the navigation, with the master maintaining a monitoring role. The watch system provides a continuity of rested watchkeepers, but the watch changeover can give rise to errors. Consequently routines and procedures to monitor the ship's position and to avoid the possibility of mistakes must be built into the organisation of the navigational watch. The risks associated with navigation demand positive reporting at all times, self verification, verification at handover and regular checks of instrumentation and bridge procedures. The course that the ship is following and compass errors must be displayed and checked, together with the traffic situation, at regular intervals and at every course change and watch handover. 231 Under STCW, the OOW may not act as sole lookout except in daylight and only then in suitable conditions. He is to ensure that the look-out is posted on the bridge or forward if required. The watch shall call the lookout in restricted visibility, restricted waters, or other wise in accordance with the master standing orders. It is usual practice for managed container ships to have the lookout on the bridge at all times when in coastal waters. The look-out will not leave the bridge during the watch. Arrangements will be made for calling the watch where the lookout does not need to leave the bridge. If a suitable telephone system is not available alternative and suitable calling methods will be made. The lookout is required to report all lights, land, shipping or other to the OOW and a professional working relationship is to be encouraged. No unnecessary conversation should be held with the look-out man to distract from his duties. Other personnel who are not on duty are not allowed on the bridge except by permission of the Master. The OOW shall ensure that the lookouts are adequately sheltered and clothed for the prevailing weather conditions; the efficiency of the look-outs and the reports obtained from them depends on the relative comfort of their post. Where the lookout is required to take the helm, the OOW shall assume (or another person shall be assigned to) his duties until he is released from helm duties. 7.9.2 General Watch-keeping All officers of the watch shall sign and follow the Master's Bridge Standing orders. 7.9.2.1 Lookout The Officer of the Watch (OOW) should keep his watch on the bridge: in no circumstances should he leave the bridge until properly relieved. It is of particular importance that he ensures that an efficient lookout is maintained at all times. In a vessel with a separate chart room the Officer of the Watch may visit it when essential, for a short period in order to carry out his navigational duties, but he should first satisfy himself that it is safe to do so and that a good lookout is being kept. The Officer of the Watch should not hand over the watch if he has any reason to believe that the relieving officer is suffering from disability (including illness, drink, drugs or fatigue) which would preclude him from carrying out his duties effectively. If in doubt, he should consult the master. All officers shall read and sign the master night orders before taking over a 'night watch'. The relieving Officer of the Watch should ensure that members of his watch are fully capable of performing their duties and in particular that they are adjusted to night vision. He should not take over the watch until his vision is fully adjusted to the prevailing light conditions and he has personally satisfied himself concerning the items of the watch change over checklist. Only after the relieving Officer is satisfied may he take over. In case he is in any doubt, the Master should be informed at once. 232 Handing over the watch should be postponed when the ship is, or is about to be, engaged in a collision avoidance manoeuvre or a navigational alteration of course. A fundamental responsibility of the Officer of the Watch is to ensure the maintenance of a continuous and alert watch; this is one of the most important considerations in the avoidance of collisions, groundings and other casualties. This includes: - an alert all-round visual and aural (sound) lookout to allow a full grasp of the current situation, including the presence of ships and landmarks in the vicinity; - close observation of the movements and bearing of approaching vessels; - identification of ship and shore lights; - observation of the radar, AIS and echo sounder displays; - observation of ECDIS and GPS (Highway or XTE) displays; - observation of changes in the weather, especially the visibility. - maintain and efficient watch on VHF Channels 16 and 13, and relevant coastal traffic channels, together with the GMDSS watch. The Officer of the Watch should make regular checks to ensure that: - the helmsman or the automatic pilot is steering the correct course; - the standard magnetic compass error is established at least once a watch and also if possible after any major alteration of course; - the standard magnetic and gyro compasses are compared frequently and gyro repeaters synchronised; - the automatic pilot is tested in the manual position at least once a watch; - the navigation and signal lights and other navigational equipment are functioning properly. 7.9.2.2 Manoeuvring The Officer of the Watch should bear in mind that the engines are at his disposal for assistance in manoeuvring. He should not hesitate to use them in case of need, although timely notice of an alteration of engine movements should be given whenever possible. The Officer of the Watch shall not hesitate in taking whatever action may be required to avoid immediate danger, including alteration of course and / or speed and if necessary operating the engines astern. He shall regulate speed immediately as required to achieve a safe speed in poor visibility, informing the Master that he has done so. The Officer of the Watch should be fully aware of the manoeuvring capabilities of his ship, including the time and distance taken to achieve emergency and routine stops in both "open sea" and manoeuvring conditions. The Officer of the Watch should bear in mind the need to station the helmsman and change over the steering to manual control in good time to allow any potentially hazardous situation to be dealt with in a safe manner. 7.9.2.3 Calling the Master The Officer of the Watch should notify the master immediately under any of the following circumstances: 233 - if visibility deteriorates to the level laid down in the master’s standing instructions; - if the movements of other vessels are causing concern; - if difficulty is experienced in maintaining course due to heavy traffic or to meteorological or sea conditions; - on failure to sight land or a navigation mark or to obtain soundings by the expected time; - if either land or a navigation mark is sighted unexpectedly or if an unexpected reduction of sounded depth occurs; - on the breakdown of the engines, steering gear or any essential navigational equipment; - if in any doubt about the possibility of weather damage; - in any other situation about which he is in doubt. - as directed by the Master in his night or standing orders. Notwithstanding the requirement to notify the master immediately in the above circumstances, the officer of watch should not hesitate to take immediate action to ensure the safety of the ship whenever circumstances so require. 7.9.2.4 Navigation in restricted waters Navigation in restricted or pilotage waters (with or without a pilot on board) calls for a special vigilance and first-class bridge procedures. When a long passage within narrow waters is envisaged, the watch on the bridge should be arranged so as to avoid fatigue. Whenever vessels are in narrow or restricted waters the Master should be on the bridge throughout, especially when Junior Officers are on watch and in particular when course alterations are required. Although auto pilot may be in use, a wheel man should be standing by to check the course along with the officers of the watch. All available navigational aids should be used as and when appropriate. The ships progress should be monitored continuously using transits, parallel indexes, clearing bearings and clearing distances as appropriate. Where fitted ECDIS and/or radar mapping must be used to display tracks, no-go areas and identify points of interest. Positions should be plotted at appropriate intervals to record the track of the vessel. 7.9.3 Navigation in restricted visibility When restricted visibility is encountered or expected, the first responsibility of the Officer of the Watch is to comply with the 1972 International Regulations for Preventing Collisions at Sea (COLREGS) and the Master's standing orders which must contain guidance on what he considers to be restricted visibility for the vessel. Visibility of less than the vessel's stopping distance must always be considered as "restricted visibility". The managers would usually consider visibility of 3 miles or less to be considered as 'restricted' In conditions of restricted visibility the OOW will consider: safe speed; posting lookout(s); 234 engaging hand steering; making appropriate sound signals on the whistle; exhibiting navigation lights; operating the radar; informing the master and engine room; Stand by' bridge watch composition All these actions should be taken in good time before the visibility deteriorates. Masters must implement appropriate bridge organisation for restricted visibility utilising the navigational aids and manpower on board in ways that suit the prevailing circumstances including: the expected duration of the restricted visibility; the number of personnel available; the complexity of the navigation; other weather factors; traffic; 7.9.3.1 Navigation in heavy weather Under no circumstances should the vessel be forced to proceed in rough weather at speeds which could cause severe damage to the vessel's structure and engines and endanger the lives of the crew. Engine rpm must be reduced to the extent that the vessel makes headway without causing shuddering and excessive vibration. Masters naturally are more aware of heavy weather on deck as this is visible when seas are breaking over the bows; however, even seas that are not shipped aboard have a powerful effect on the vessel's hull. Although this is part of good seamanship, Masters have in the past not complied with this basic rule. The safety of life and of the vessel is of the utmost priority and must not be compromised in any way. Failure to comply will result in disciplinary action being taken. In the event of heavy weather, appropriate announcements should be made and access to the weather decks restricted and closely monitored. Prior to the onset of heavy weather, the head of each department should inspect their respective area of responsibility to identify heavy objects which could cause damage if they move; all such objects should be securely chocked and lashed. Anchor lashings should be checked and tightened as necessary. 7.9.5 Colreg rules that apply to navigation in confined and congested waters and collision avoidance Radar for the Merchant Service is designed for what is known as "Surface Warning" and for anti-collision purposes its main use was during reduced visibility. With the faster ships, radar, however, is also now extensively used in clear weather as an extra aid for the look-out man. With the range-scale on 12 miles and the electronic centre off- 235 set, strong echoes of ships can be detected up to 16 miles for an average bridge height and at a time when the ship has not yet been sighted visually. Another reason for earlier detection by radar is that the white echo pip against the dark background is often more conspicuous than the appearance of a. ship against a grey sky and seas. By placing the cursor over the echo, a timely check can be kept on the bearing change. When fog-banks are expected the radar set should be at least on "Stand-by" during daytime, making it ready for immediate use; but at night the set should be left on "Transmit", as the vessel could well be steaming along near a fog bank which is giving no visual indication or its presence. When approaching a fog-bank Rule 35 (Sound Signals in Restricted Visibility) must be adhered to and radar must be used to see what is inside the fog-bank. Failure to employ the radar in such a case contravenes Rule 2 and blame accordingly has been attached to ships which did not comply with this rule near a fog-bank. Upon approach of the fogbank, radar watch routine should be started, and inside the fog-bank, the observer should realize that some echoes on the screen might represent ships which are not in fog and may not exercise the same caution as his own ship. Unexplained manoeuvres by other vessels as observed from the radar screen might indicate the existence of a small vessel or vessels undetected by the ship's own radar, and a close watch should be kept on the suspected area. Shipmasters have been blamed for not keeping a proper "look-out" because they were not using their radar on clear nights to detect the presence of the unlit oil-rigs with which their vessels collided. It may, therefore, be said that it is always good practice, especially for fast ships, to keep the radar working.. This also offers an opportunity to the officer of watch to maintain his plotting expertise, wh1ch IS so 1mportant 18 cases when the visibility deteriorates and plotting becomes really essential. At night, when in a region where fogbanks and/or unlit obst~ctio1LS can be expected, the radar must be in continuous operation. Previous Court Cases, by the way, have stressed that a shipmaster is considered to be at fault for not using radar provided for his ship and also for allowing the radar installation on his ship to remain in a defective condition for a prolonged period. At present the U.K. Government has made radar compulsory for all British ships of 500 gross tons or more, following an IMO recommendation that at least one radar must be fitted in ships of 500 g.r.t. or more (300 g.r.1. after 1st February 1995), and at least two radars must be fitted to all ships of 10,000 g.r.t. or more, each capable of operating independently of the other. Some important points to be kept in mind when using radar in reduced visibility, are the following: (a) The setting of the anti-clutter control on raw radar displays. Adjust, if possible, in such a way that echoes can be traced near the spot representing own ship. Be aware of over-suppression, as this will wipe off most of the echoes of ships nearby. (b) The existence of blind and shadow sectors caused by objects on the ship itself. A slight "weaving" around the course is recommendable in such a case. (c) The selection of range scale, taking account of: (i) The speed of own ship (the faster the ship, the greater the range scale). 236 (ii) The accuracy of bearing and range observations (shorter range scales with the echo in the outer half of the screen yields an increased accuracy). (iii) The length of the "tadpole" tails (the shorter the range scale, the longer the tails). If a True Motion Display is available, this might entail off-centring the time-base and use of the ZeroSpeed switch. (iv) The possibility of encountering small craft or ice growlers (easier discernable on the shorter range scales and, if possible, with long pulse selected). (v) The number of ships in the vicinity of own ship (a long-range scale can produce a confusing array of closely-packed echoes). (vi) The range at which most merchant ships are first detected (generally about 10-15 miles). In addition to the above considerations, it should be remembered that a plot on a reflection plotter mounted on a True Motion Display will become distorted if the range scale is altered during the plotting interval Summarizing on this problem of selecting a range scale, it is generally best to relate the range-scale used most of the time to the vessel's speed changing to shorter range scales now and again to obtain more accurate observations of bearings and ranges of any nearby objects and also to conduct a search for smaller objects. (d) The obeyance of Rule 35 (Sound signals in restricted visibility) even if the screen is free from echoes on the longer range scales and one knows that the set is fully efficient. (e) The obeyance of Rule 34 (0) (Manoeuvring signals) only when the other vessel is in sight. Under Part B (Steering and Sailing Rules) there are two sections which have a special bearing on this Chapter. These are Section I and Section m. The former deals with the conduct of vessels in any condition of visibility (Rules 4, 5, 6, 7. 8 and 10); the latter (Rule 19) discusses the conduct of vessels in restricted visibility. Turning our attention to Section 1 first, it will be seen that Rule 5 specifically deals with the importance of maintaining "a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a fulJ appraisal of the situation and of the risk of collision". The word "specifically" is stressed because in previous Regulations this came under "the ordinary practice of seamen". The inclusion of "as well as by all available means" refers obviously to a radar watch (the use of guard rings on the radar display will be helpful in this connection), but it also incorporates a V.H.F. R/T watch and !he words "full appraisal" may be taken to include proper radar plotting procedures and active V.H.F. Radio-Telephony communications. Although this section deals with clear weather conditions and conditions of restricted visibility, the master is given some latitude in making use of radar and R/T information by the addition "appropriate in the prevailing circumstances and conditions". Rule 6 introduces a new concept, namely Safe Speed. When, about 45 years ago radar was introduced on board ships, one of the greatest difficulties with which Mariners were confronted, was the term "Moderate Speed". What. in fact, was a moderate speed using radar? A concise answer was not possible. It could be argued that a moderate speed, using radar, was not possible. It could be argued that a moderate speed, using radar, could in some cases mean "Full speed with engines on Stand-by", but in other cases 237 could mean a slower speed than a Mariner without radar might consider "moderate". From the legal and philosophical point of view these arguments are quite correct but in the literary sense they are unsatisfactory. A Safe Speed as defined in the 1972 Rules is not based only on the state of visibility (as in the 1960 Rules) but "Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions". Besides the state of visibility (i) the following factors should be taken into account in determining a safe speed: (ii) "the traffic density, including concentrations of fishing-vessels or any other vessels"; (iii) "the manoeuvrability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions"; The manoeuvrability depends on the stern power of the vessel, the number and type of screws, the provision of a bow-thruster. the size of the ship and her loaded condition while the prevailing conditions are mainly governed by the wind and wave directions, wind force and wave height, and current and tidal conditions. (iv) "at night the presence of background light such as from shore lights or from back scatter of her own lights"; (v) "the state of wind, sea and current, and the proximity of navigational hazards"; (vi) "the draught in relation to the available depth of water. These factors, which determine a safe speed in general, are applicable to all ships. Vessels which use their radar need, in addition, take the following conditions into account: (i) "the characteristics, efficiency and limitations of the radar equipment"; The age and reliability of the equipment, the number of radars and displays, inters witching facilities, types of display presentations, plotting devices and facilities for automatic plotting etc., are all factors to consider. (ii) "any constraints imposed by ~he radar range scale in use"; A constraint may be imposed on a particular range scale owing to strong radar or electrical interference, or for a very fast vessel the use of ~he l'2--mile range scale (the 24-mile range scale is. too small for effective plotting on a reflection plotter) for echo observation, might compel her to reduce speed. (iii) "the effect on radar detection of the sea state, weather and other sources of interference"; Excessive "noise" due to wave, sea, ram-drops, snow crystals, other ships' radar pulses or electrical interference may swamp the signal and essential information .may be lost. (iv) "the possibility that small vessels, Ice and other floating objects may not be detected by radar at an adequate range". This "possibility" can be a result of atmospheric conditions such as sub-refraction or it might be caused by a small reflection coefficient of the object. (v) "the number, location and movement of vessels detected by radar". (vi) "the more exact assessment of the visibility that may possible when radar is used to determine the range of vessels or other objects 10 the vicinity". . 238 In addition, we may say that the number of men for keeping radar watch and a plot, and their efficiency could influence the master s opinion about what is, or what is not a safe speed. . Rule 7 deals with the "Risk of Collision". The Rule stresses again the use of "all the available means appropriate to the prevailing circumstances and conditions to determine if the risk of collision exists". This includes the 'listening to V.H.F. R/T messages of other ships and shore radar stations, but no guidance is give about actual active participation. . There is a very important last sentence in the first paragraph: "If there IS any doubt such risk shall be deemed to exist". This might remove the possible element of indecision in a radar encounter. Rule 7 (b) stresses the importance of making proper use of radar equipment, including early warning of collision risk on the long-range scales. It furthermore emphasizes the practice of radar plotting or "equivalent systematic observation of detected objects" (recording in writing and tabulation, automatic plotting aids). Rule 7 (e) states: "''Assumptions shall not be made on the basis of scanty . This problem does not arise with Relative Motion radar. information, especially scanty radar information". The omission of a plot, an incomplete plot or a plot based on an insufficient number of observations, in short the determination of the position of another vessel without finding her movement, might be termed as "scanty". ARPA provides a solution here. The last paragraph of Rule 7 states how risk of collision can be obtained from compass bearings and gives a warning that an appreciable change in bearing does not always indicate a safe passing (large vessel, Or a tow, or a ship at close range; Bearings should be recorded as compass bearings and not as relative bearings as is so easily done on an unstabilized display. It is not possible to compare relative bearings when own ship is subject to yaw or makes alterations of course, and often the Master has been led to believe, that. by making a small alteration of course, the situation improved because the relative bearing changed and he did not realize that the change in the relative bearing was mainly due to own ship's alteration of course. If he had converted the relative bearings to compass bearings, he would have noticed that danger of collision after the alteration had become greater instead of less. Rule 8 is headed "Action to avoid Collision". Paragraph (a) states that, if the circumstances of the case admit, any action shall "be positive, made in ample time and with due regard to the observance of good seamanship". The word "positive" in this connection means "effective" and bears no relationship to the conventional adaptations "'positive and negative actions", mentioned in certain papers about collision-avoidance (more about these later). Paragraph (b) is an extension of paragraph (a), stating that "Any alteration of course and! or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and (or speed should be avoided". The Rule requires substantial action in order to make clear one's intention to all vessels in the neighbourhood ("another vessel" is not necessarily the vessel for which avoiding action was taken) both in clear weather as well as in fog. This requirement should be kept in mind when an agreement is reached about collision-avoiding tactics between two vessels via V.H.F. R!T and also when using the 'Trial Manoeuvre' facility on ARPA. 239 The remainder of the Rule (paragraphs (c), (d) and (e» emphasizes that an alteration of course, provided there is sufficient sea room, may be the most effective action to avoid a close quarters situation on condition that it is made in good time, is substantial and does not result in another close-quarters situation. It stresses the safe passing distance and warns that effectiveness of the action shall be carefully checked until the other vessel is finally past and clear. If necessary, or to allow more time to assess the situation, a vessel shall slacken her speed or take all way off by stopping Or reversing her means of propulsion. In short, what this Rule is saying is that if avoiding action for another vessel is going to be taken such action should be bold both in clear weather Rule 10 applies to Traffic Separation Schemes and is a highly important addition to the 1972 Rules. It contains regulations adopted by IMO (see IMO-publication "Ships' Rooting Traffic Separation Schemes and Areas to be Avoided") but have become mandatory for all the published schemes. Additional paragraphs are included for small vessels and sailing ships, and exempted vessels. The more important parts of the Rule state that "a vessel using traffic-separation scheme shall, so far as practicable, keep clear of traffic-separation line or zone (b (ii) ), normally join or leave a traffic-lane the termination of a lane, but when joining or leaving from either side, shall do so at as small an angle to the general direction of traffic flow as practicable (b (iii)) and shall, so far as practicable, avoid crossing trafficlanes, but if obliged to do so, shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow (c). It goes on: "(d) Inshore traffic zones shall not normally be used by through traffic which can safely use the appropriate traffic-lane within the adjacent traffic-separation scheme. However, vessels of less than 20 meters in 'length and sailing vessels may under all circumstances use inshore traffic zones". In connection with the right-angled crossing, it is advised, if possible, to shape the new course well before the lane is reached, thus giving ships within the lane a timely indication. The Rule dealing with Traffic Separation Schemes (Rule 10) follows the Rule about Narrow Channels (Rule 9) and purposely so, as both can be grouped under Narrow Navigational Routes. There are, therefore, certain analogies between the two Rules, Rule 9 (b): A vessel less than 20 m. in length or a sailing vessel shall. Not impede the passage of a vessel which can safely navigate only within a narrow channel or fairway. Rule 10 (J): A vessel less than 20 m. in length or a sailing vessel shall not impede the safe passage of a power-driven vessel following a traffic-lane. Rule 9 (c): A vessel engaged in fishing shall not impede the passage of any other vessel navigating within a narrow channel or fairway. Rule 10 (i): A vessel engaged in fishing shall not impede the passage of any vessel following a traffic-lane. . Rule 9 (i): Any vessel shall, if the circumstances of the case admit, avoid anchoring in a narrow channel. Rule 10 (g): A vessel shall so far as practicable avoid anchoring in a traffic-separation scheme or in its areas near its termination. One of the first traffic-separation schemes was instituted in the English Channel and was followed by a marked decrease in the number of collisions. Surveillance on the conduct of vessels is carried out by radar 240 observation from Langdon Battery, Dover (Channel Navigation Information Service), light aircraft and fast launches. Having discussed the relevant Rules of Part B (Steering and Sailing Rules), Section I of the Collision Regulation which applies to the conduct of vessels in any condition of visibility, we will skip Section II (conduct of vessels in sight of one another) and look at Section III, which is applicable to vessels in restricted visibility. This section contains only one Rule, Rule 9, which has replaced the famous Rule 16 of previous Regulations.. so well known to generations of seamen. As the Rule is so important, we will quote in full (Italics are the Author's): "(0) This Rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. . . (b) Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility A power-driven vessel shall have her engines ready for immediate manoeuvre. (c:) Every vessel shall have due regard to the prevailing circumstances and conditions of restricted visibility when compiling with Rules of Section I of this Part. (d) A vessel which detects by radar alone the presence of another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration of course, so far as possible the following shall be avoided: (i) an alteration of course to port for a vessel forward of the beam .. other than for a vessel being overtaken; (ii) an alteration of course towards a vessel abeam or abaft the beam. (e) Except where it has been determined that a risk of collision does not exist,. every vessel which hears, apparently forward of her beam, fog-signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over". Paragraph (b) emphasizes the fact that the restricted visibility has to be taken in to account when determining a safe speed. It also states that.... a power-driven vessel should have her engines on "Stand-by". Paragraph (c) refers to Section I of this part, again stressing the fact allow for the additional circumstances and conditions of restricted visibility. Paragraph (d) refers to ships which have their radar in working order and makes It compulsory for these ships to use their radar for determining if a close-quarters situation is developing, for assessing the risk of eventual collision and for taking avoiding action. The paragraph also places restrictions on certain manoeuvres: (i) Do not alter course to port for vessel forward of the beam (it does not apply when one is overtaking vessel); (ii) Do not alter course towards a vessel which is abeam or abaft the beam. Note that the word "abeam" is not accurately defined within the Collision Rules and there is a choice of avoiding action (to starboard or to port) for a vessel abeam on starboard. 241 There arc cases with a vessel forward of the beam and the bearing changing slowly clockwise in the initial stages that there is a reluctance to go to starboard and an inclination to go to port, especially when the rate of approach is fairly fast. One case is with a crossing vessel on the port bow. In such a case, with a close-quarters situation developing and the Master feeling reluctant to alter course to starboard, should reduce the speed of own ship. In fact, as will be amplified later in this Chapter, this is quite a good manoeuvre and might be one of these cases where a reduction in speed is better than an alteration of course. Another difficult case is when an echo of a vessel is detected fine on the starboard bow and shaping to pass apparently. as best as can be judged from radar, close to starboard. Again, there is often a reluctance – and this applies also to the other vessel if she is using her radar - to alter course to starboard to pass ahead of the oncoming vessel, especially if the rate of approach 1S fast. In this case stopping engines is not a satisfactory manoeuvre because the other vessel might not have detected Own vessel there is quite a possibility on a dark night that the other vessel has not yet entered the fog and is not aware either of own ship or the presence of fog thus to stop engines and become immobile might place own ship in the path of the oncoming vessel and be completely at her mercy. To stop in this case could only be Justified In narrow waters or if one is hampered on both sides by other vessels. Thus if there is sea room the only answer here is to make a very bold alteration to starboard so as to put the echo of the other vessel abeam or even a little abaft the beam as quickly as possible. The advance of most ships when making this manoeuvre is usually much smaller than their corresponding head reaches when they make an emergency stop. Having made this manoeuvre the echo should be watched carefully. If the other ship keeps her course and speed, or if she alters course in a direction to support the alteration, or if she stops then all well and good. However, if the other vessel, contrary to Rule 19, alters in a direction which cancels out our own alteration it's not so good admittedly, but it's not so bad as the initial encounter - the relative motion will be much smaller than it was originally and one can go easily "go on round" to put the echo right astern (complying with the second restriction of Rule 19 (d) and so make the relative motion even smaller. For fast ships coming from abaft the beam which present a collision hazard to own ship if they maintain their course and speed, the best action IS to alter course away from the vessel in such a way that the stern keeps pointing towards the danger. Rule 5 (Look-out) should be kept in mind (an extra look-out might be posted near the stern) and the frequency of sound signals should be increased. Note that this Rule states "shall take avoiding action in ample time" (not "may" as in previous Regulations). Paragraph (e) of Rule 19 is applicable to every vessel (not necessarily power-driven) and refers to the detection of other vessels forward of the beam of which the presence is detected either by hearing or by radar ("hears apparently ...", or "which cannot avoid a close-quarters situation's. As this paragraph includes sailing vessels the expression "stop her engines" in previous Regulations is replaced by "reduce her speed to a minimum at which she can be kept on course. The well-known proviso "a vessel, the position of which is not ascertained", which has been used so many times in court cases by Masters to defend their action for not stopping engines has been changed to the more direct wording. "Except where it has been determined that a risk of collision does not exist". Such awareness could take place in the following cases: 242 (a) A vessel forward of the beam, nearby, disappearing in fog, but own ship having determined by visual means up to a few moments before disappearance that there exists no risk of collision between the two vessels. (b) A vessel forward of the beam, which together with own ship, is proceeding along a narrow channel or a traffic-lane. (c) Where the intentions of both vessels have been firmly established by means of V.H.F. R/T contact. Before leaving the Rules which are relevant to this book, mention should be made of Rule 2. It consists of two sections, Section (a) deals with the responsibility of owner, master and crew, and the requirement of good seamanship. Section (b) states that due regard shall be had to all dangers of collision and navigation and any special circumstances, including the limitations of the vessels involved, which may make a departure from the Rules necessary to avoid immediate danger. Its application is important ill connection with Rule 19: Common sense should go hand-in-hand with obedience of the Rules under special circumstances (for example, narrow channel with current, multi-target situation, large heavy vessel in traffic lane, etc.) a departure of the Rule may be necessary. Plotting has two purposes: (a) It can show us whether danger of collision exists, how close we will pass off the target (nearest approach or distance of the closest point of approach from own ship) and how much time there is left before this will take place. (b) The approximate determination of the course and the speed of the other vessel from previous observations, so that sensible avoiding action can be taken when needed. The second purpose is connected with one of the limitations of cm radar which does not show up the aspect or leading edge of an isolated' small (in relation to the horizontal beam-width) target except at very close range. Plotting does not reveal to us the shape of a target and hence not the present heading. It will inform us, however, about the motion of the target during the plotting interval. 7.9.6 Reporting and recognition of collision hazards To provide the Master with information about collision hazards, about the possibility of planning avoiding action and about the taking of avoiding action, a good method to be adopted by the radar observer is to report according to a standard pattern. Such a report would consist of two main parts. 1. (a) Last bearing, drawing forward or aft (passing ahead or astern respectively); (b) Last range, decreasing or increasing; (c) Nearest approach (distance of closest point of approach from own ship) as forecast (CPA); (d) Time interval to the nearest approach (closest point of approach) from the last observation (TCPA). 2. (a) True course or relative course or aspect of target; (b) Speed of target. First consider part 1. Whenever an echo is observed on the screen, the Master, especially during reduced visibility, is naturally anxious to know whether there is an 243 appreciable change in the bearing and if the range is increasing or decreasing. If little change in the bearing is observed, and the range is decreasing, at once the question arises how far off will the target pass if both ships maintain their course and speed and how much time is there left before this will occur. Even if there is an appreciable change in the bearing, the Master is likely to want to learn whether the target is passing ahead or astern. If it is apparent that avoiding action has to be taken, then part 2 must be completed, In that case the Master must know the approximate motion and speed of the target. This is the same as in the visual case where one can only plan avoiding action properly if the other ship's course and speed can be estimated. Alterations of course Without establishing the target’s direction and speed are irresponsible actions. The aspect is defined as the relative bearing of own vessel taken from the target. A starboard or port bearing is indicated as Green or Red respectively. For example an aspect of Red 90° means that the target sport side is observed to be beam-on to own ship; a target head-on has zero aspect, stem-on 180° aspect. Strictly speaking, as we have seen, the aspect cannot be deduced from a plot, but we will assume that the most probable aspect can he deducted from the motion of the target during the plotting Interval. The direction of movement of the target can be expressed in terms of either her aspect, or relative course or true course and it is entirely up to the Master which he prefers. Often a glance at the plot will suffice. Aspect appeals to a lot of sailors because it is so close1y related to the visual conception when they sight a ship. They automatical1y estimate the angle between her bearing and her course. It also gives us insight as to whether the target may see us on her starboard or port side or whether own ship is in the overtaking position. Such considerations may help us to form an idea about the possible reactions of the target and there are of special importance when sailing through fogbanks and the Steering and Sailing Rules 11 to 18 must be applied when the fog lifts suddenly. The report has to be enlarged if avoiding action is going to be taken. Here we must assume initially that the other ship maintains her course and speed. We cannot predict her actions with certainty. After the alteration of course or reduction In speed, the observer must watch the plot closely and re-estimate the nearest approach and the time when this will take place. If the nearest approach remains dangerous, then the best practice is generally to either reduce speed substantially or to stop own ship or to alter course to put the target right astern. The other ship in such a case, probably, has taken avoiding action at about the same time which has cancelled own ship's action. If however the avoiding action is successful and the nearest estimated approach is safe, then the radar observer can continue his report by informing the Master when the original course or speed can be resumed with safety. Two remarks must be made about the nearest approach:. (a) In clear weather, when at close quarters, one can almost immediately sec what the other ship is doing and one can act accordingly, if danger of collision is involved. Radar, on the other hand, is not suited for close quarter situations in this connection. Because cm radar does not possess enough discrimination, it is very slow and sometimes unable to tell us what the other vessel is doing. The target if she has radar on board, is placed in the same predicament and there is insufficient appreciation on the part of each vessel of the other's movements. If the target has no radar on board or cannot use the radar, then, in thick fog, she will be completely unaware of our position 244 and movements. Hence in fog, when there is a great loss of information, even when both vessels are using their radar, a wide margin of safety has to be introduced. (b) There are many factors which easily cause errors in plotting and the nearest approach as obtained from the plot may differ considerably from the actual value. Therefore taking these two considerations into account, the new nearest approach selected should not be too small. Give the target a wide berth. The situation is not the same as when in clear weather. It is good practice generally, in the open sea, for the Master to base his plan for taking avoiding action on a bold alteration of course and / or speed initially so that the other ship, if she is using radar, will be able to detect own ship's action as quickly as possible. After taking avoiding action careful plotting should be continued to see if the other vessel is keeping her course and speed. If she docs, then a prediction should be made from the plot when it will be safe for own vessel to resume her original course and /or speed. The following three factors should then be taken into consideration: (i) The closest distance which one considers it safe to pass the other vessel under the existing circumstances (three miles is generally accepted as a safe minimum distance for average types of merchant ships in the open sea). (ii) The time factor. In case the other vessel is using radar she should be allowed to have sufficient time to detect own ship's alteration in course and/or speed (a minimum of about twelve minutes is generally considered necessary). (iii) If own ship took action by altering course to bring the echo across from the starboard to the port side or vice versa, then, when the original course is resumed, care should be taken to avoid, if at all possible, bringing the echo back to the opposite side again (if this was done a misunderstanding of the situation might arise if both vessels suddenly came into sight of one another). The report can be computed either by plotting on a sheet of paper or directly on a screen covering the PPI, or by mechanical or electronic plotting devices. The motion of the echo is plotted relative to own ship, which is considered as a fixed reference point. In other words, the motion is plotted as it appears on a Relative Motion Radar Display. The centre-point of the plot represents the electronic centre of the radar screen, Le. own ship. The heading marker, representing the fore and aft-line of own vessel, is drawn on the plot and indicates the direction of own course. All the plots shown in the following diagrams, are referred to as Compass Datum Relative Plots. This means that the compass bearing scale is fixed. When course is altered, the heading marker swings round in the same way as it does on a stabilized display, and the movement of the echo is not broken up as it would be on a Head-up Relative Motion Display. The relative bearing scale is not used, though one can, if one wishes, lay-off relative bearings from the heading marker wherever this is positioned. In nearly all the diagrams it is assumed that North is "up". One may, of course, if this is preferred, always start off with the heading marker upwards, provided one turns the heading marker to the new direction when course is altered. By turning the plot bodily around one could then bring the new heading marker to the upward position again. After putting in the heading marker, the different bearings (relative to the heading marker or true) and ranges are plotted from the centre-point according to a suitable scale which should not be too small (about one inch to represent one mile is suitable). A time 245 interval can be chosen which is related to own ship's speed, for example five minutes for a ship of twelve knots, so that the distance moved by own ship during that time is one mile. Or one can take intervals of six minutes during which the ship covers Ii. distance of one tenth of the speed. Anyhow, this can be best left to the observer as it depends on he rate of approach of the other vessel. If the rate of approach is fast, a three-minute interval between successive observations is advisable, but the plot should not be completed before at least three observations of the target have been made. If the log is in operation, it should be read when observations are taken as this will indicate the distance own ship has travelled through the water. This is of special importance after reduction of speed when one is not quite sure about the average speed of own ship, and we will see later that inaccuracies in the distance covered by own ship during the plotting interval will introduce errors in the estimated course and speed of the target. Bearings and ranges of each of two echoes have been taken which are laid off from the centre. The first bearings and ranges were at 0000 hrs., the last ones at 0012 hrs. The plotted point which is laid off first (0000) of each echo is called 0 (for "Origin''), that one which is laid-off last (0012) is called A. OA represents the movement of the echo in 12 minutes as seen on the screen of a relative motion display. The nearest distance of approach is the length of the perpendicular dropped from the centre on to OA produced. The unit of time is OA, representing 12 minutes. The arrival times of the estimated closest points of approach of the two targets, whose echoes are depicted on the plot, w ill take place at 0032 hrs. and at 0046 hrs. (foot of perpendicular from centre) if all ships concerned maintain their courses and speeds. Items 1 (a). (b). (c) and (d) of the report are now known. OA is the relative motion of the echo and its length and direction are determined by the course and speed of own ship and the course and speed of the target vessel. Therefore, one may expect that if both vessels maintain their course and speed, and bearings and ranges are correct, the three ,points will lie on a straight line with the 0006 point halfway between the 0000 and 0012 points. In practice, however, one may find that the respective points are staggered even when the target follows a steady course and speed. This is so because the bearing accuracy, especially of the unstabilized display is not very high. Also, own ship may yaw and this will affect the relative motion. If the plotted points are situated nearly on a straight line, and if the distances between them are roughly proportional to the time intervals between the respective observations, one may draw a mean line through the plotted points and assume that the other vessel has kept her course and speed. We thus see that the information derived from the relative motion line is the nearest approach and the time it takes to the closest point of approach. It can also tell us if the other vessel maintained her course and speed. The relative motion plot is compact; the image of own ship is fixed and generally echoes are only plotted of targets whose ranges are decreasing. 246