Transcript
DATA LINK FUNCTIONS AND ATTRIBUTES OF AN UNMANED AERIAL VEHICLE (UAV) SYSTEM USING BOTH GROUND STATION AND SMALL SATELLITE Hamid R. Saeedipour1, Md. Azlin Md. Said2, and P. Sathyanarayana3 School of Aerospace Engineering, University of Science Malaysia (USM), 14300 Nibong Tebal, Pulau Pinang, Malaysia Phone: +6-0129780016, Fax: +604-5941026, Email:
[email protected]
ABSTRACT The interaction between the small satellite, the ground station, the data link and the rest of the UAV system is complex and multifaceted. This paper reviews the data link functions and characteristic of Unmanned Aerial Vehicle (UAV) to identify the primary performance, complexity and drivers of both small satellite and ground station for data link purposes. The options available to a UAV system designer are discussed for achieving required system performance within the constraints of various level of capability for the data-link subsystem, the ground station and the small satellite. The critical requirement in data link design is anti-jam (AJ) capability and it may lead to data-rate restriction. The paper provides the necessary discussions in world-wide availability of frequency allocation/assignment, resistance to unintentional interference, low probability of intercept, security, resistance to deception, anti-ARM and anti-jam capability. 1. INTRODUCTION One of the objectives of this paper is to assist the reader to understand how to structure the system and technology efforts related with a UAV data-link using both ground station and small satellite. The data link needs to be balance and integrate with other features of the UAV system including sensor design and on-board data-link design. On the air battlefield, the UAV system may face a variety of threats in electronic warfare (EW) including direction-finding used to target artillery, anti-radiation munitions (ARMs), interception and exploitation, deception, and both unintentional and intentional jamming of the data link. [1], [2], [4] 2. DATA-LINK FUNCTION The functions of a UAV data link are: a) an up-link that allows the ground station and the small satellite to control the UAV and its payload, b) a down-link that provides two 1 2
3
Corresponding Author, Lecturer, PhD, MSc, MBS, BEng, IT Lecturer, Associate Professor, PhD, MSc, BEng
Lecturer, PhD, MSc, BEng
channels that transmit UAV status (or telemetry) and transmit sensor data to the ground and/or the small satellite, and c) measuring the range and azimuth to the UAV from the ground antenna and/or the small satellite. [3],[5] 3. DATA-LINK SYSTEM The data link located at UAV includes an air data terminal (ADT) and antennas. The ADT includes the radio frequency (RF) receiver and transmitter. Modems are required to interface the receiver and transmitter to the rest of the system. The processors are required to compress the data before transmitting in a way that it will fit within the bandwidth limitation of the down-link. The satellite data terminal (SDT) and the ground data terminal (GDT) usually consist of several antennas, an RF receiver and transmitter, modems, and processors to reconstruct the sensor data if it has been compressed before transmission. The GDT may be packaged in several pieces, often including an antenna vehicle that can be place some distance from the UAV ground control station (GCS), a local data link from the fixed or remote antenna (on the ground, in air or space) to the GCS, and some processors and interfaces within the GCS. Telemetry receiver and transmitter have been used in UAV datalink that operated under highly controlled condition on a particular test range. The data link transmits digital data using digital modulation of the carrier. Preprocessing of sensor data on board is also digital for error-detection, tolerance to intermittent interference through redundant transmission, encryption, and authentication codes.[5],[6],[7] 4. DATA-LINK TRADEOFF Operating range, data rate, AJ margin, and cost are strongly interacting factors in datalink design. The effect of range on the tradeoff can be considered a step function – one set of consideration applies to links that operate within line-of-sight range from the ground station and the small satellite and a different set of consideration applies to links that must operate beyond that range. Data rate and AJ margin are continuous variables that are inversely related for any given range and cost. Generally, increasing any of the other three parameters will increase the cost of the data link.[6] Operating range is driven directly by mission requirements and may be the easiest parameter to fix. For line-of-sight ranges, ground and space antennas’ gain can be substituted for processing gain at reasonable cost (up to 30 or 40 dB) to allow higher data rates for the same AJ margin. This allows a 4-way tradeoff of data rate, processing gain, AJ margin, and ground and space antenna size and cost (including active antenna processing), with cost as a parameter of the tradeoff. For beyond-line-of-sight ranges, antennas’ gain is not available in the tradeoff unless a large airborne relay vehicle is provided. Such a relay vehicle would be expensive in cost and manpower for operations and support. Using either low frequencies for direct propagation or a small relay vehicle (or both), the data-link tradeoff is limited to 3 factors: data rate, processing gain, and AJ margin. Even for moderate AJ margin, it is likely that the available transmission bandwidth will be fully utilized, so that the tradeoff becomes a direct trade of data rate for AJ margin. [3], [7]
As a general rule, higher AJ margins will require higher frequencies. Higher frequencies will increase hardware costs. Operating frequency is involved in the above tradeoff via its effect on: a) Availability of antenna gain, b) Line-of-sight versus beyond-line-of-sight propagation characteristics, and c) Limits on transmission bandwidth and thus on processing gain. Data rate is an important factor in the data-link tradeoff that is most controllable by the system designer and user. On-board processing due to recent advances in electronics can significantly reduce the volume of data that must be transmitted for given information content. Appropriate design of control loops and system software can accommodate time delays due to data-rate reduction and allow successful mission performance at lower data rates. Finally, choices of how to use the UAV system that are made with an awareness of data-rate restrictions, may allow similar operational results without requiring procedures that exceed those restrictions.[3], [8], [9] 5. DATA-LINK ATTRIBUTES The data-link attributes are ranging from those that are easy (no complexity) and low cost to achieve to those that are extremely difficult (most complexity) and high cost. Easy attributes include; Resistance to unintentional interference, Protection from ARMs, Remote distribution of sensor data (without AJ) from ground or space, Geometrical AJ (antenna gain only) at line-of-sight ranges, and finally, High data rate down link without processing gain. Moderately Difficult attributes include; AJ capable up-link, Resistance to exploitation and deception, Moderate AJ margin on the down-link for 1-2 Mbps and long range, Low-probability-of-intercept up-link, and finally, Navigation data at line-of-sight ranges. Very Difficult attributes include; High AJ margin on down-link for 10-12 Mbps and line-of-sight ranges, or somewhat lower AJ margin on down-link for 1-2 Mbps and beyond-line-of-sight ranges. Extremely Difficult attributes include; High AJ margin on down-link for 10-12 Mbps and beyondline-of-sight ranges.[3], [8], [9] Except for the last category, there is no question all of these attributes can be provided in a data link that can be used with a UAV. Note that the ranking by difficulty here represents growth in complexity and cost. Technical risk is probably not more than moderate for any of these attributes, but schedule and cost risk could be high for more difficult combinations of attributes. However, there is some uncertainty between the “easy” and “moderately difficult” categories, depending on how many of the attributes listed are combined in a single system and on some basic choices in the UAV system design. However, there is no doubt that the attributes listed under “very difficult” and “extremely difficult” belong to data links that are at least expensive, if not risky. Based on this presumption, a “low-cost, jam-resistant” data link probably should fall in the “moderately difficult” category. If so, it should not be expected to have data rates above 1-2 Mbps unless it is limited to line-of-sight ranges. [3], [6] The discrete change in characteristics that occurs at the transition from line-of-sight to beyond-line-of-sight ranges of the UAV suggests that a common data link that attempts to cover both of these range requirements probably will result a more expensive design than either of two different links designed for the different range conditions. This distinction remains unclear for the most capable data links, since they
have already been driven to the most expensive configurations in order to meet data-rate and AJ requirements. [3], [9], [10] 6. UAV NAVIGATION AND TARGET LOCATION If data-link azimuth and range measurements are used to determine UAV position, the location and orientation of the antenna on the ground and the small satellite must be precisely known as a starting point for the calculation. These problems, as well as the need for a line-of-sight data-link path and a large ground antenna for high angular resolution, are eliminated if the air vehicle has an independent means of determining its own location. Global Positioning System (GPS) receivers have become so inexpensive and small that it seems clear that they should be considered a standard navigation system for UAVs. [1], [2], [4] The GPS uses simultaneous measurements of the range to three satellites (whose positions precisely know) to determine the position of a receiver on the surface of the earth. If the range four satellites are known, the altitude of the receiver also can be determined. Accuracies of 5 to 15 m are available in the restricted military version of the system, while accuracies of 100 m are available from the civilian version. The ground stations can be 100 km from the GPS receiver that takes advantage of their signal. Using so-called “differential GPS” approach, the addition of ground stations and small satellites allows accuracies of the order of 1 to 5 m even for the civilian version of GPS. [10], [11], [12] 7. CONCLUSIONS From a technology standpoint, the highest leverage with regard to data links appears to be in the area of improved on-board processing to reduce data-rate requirements and better understanding of operator task performance to allow design of procedures that make best use of available data rates. It is critical to understand the applicable limitations and options of UAV system and to select system designs, mission profiles, and operator procedures that allow mission performance within affordable data-link constraints of the UAV. Major areas in which interface issues are likely to arise with regard to UAV data links include mechanical, electrical, data rate restriction, control loop delays, interoperability, interchangeability, and commonality. A UAV system is sometimes designed to make use of high data rates available with little or no AJ margin in a low-cost data link. If an attempt is made to upgrade the data link at a later time to provide high AJ capability, the choices may be limited to a) Major redesigns to the UAV system, including major changes to training and mission profiles, or b) A very expensive data link using large airborne relays with tracking, high-gain antennas, or c) AJ margins that is not adequate for the EW environment. To avoid this, it is necessary to take the attributes of the objective data link into account in the original design. This requires determining what AJ margin eventually will be required and determining what implications this will have on data rate and cost. The system (and the procedure in which it will be used) must be designed in such way that adequate mission performance is reasonably divided between the various UAV
subsystems to produce an affordable objective system that meets all essential requirements. While GPS is resistant to jamming or deception; it is not protected. If, as appears to occurring, the military becomes highly dependent on GPS in areas ranging from navigation to weapon guidance, then GPS will become an attractive target for enemy electronic warfare. The GPS signals from the satellites are transmitted in a direct spread-spectrum mode which makes them resistant to interference jamming, and spoofing. Differential GPS could use jam resistant signal formats, although most present civilian systems do not do so. 8. REFERENCES [1] H.R. Saeedipour, & P. Sathyanarayana, Total Error in Navigation and Target Location of an Unmanned Aerial Vehicle (UAV) Using Differential Global Positioning System (GPS), The 2nd International Conference on Mechatronics 2005 (ICOM’05), Kuala Lumpur, Malaysia, (May 2005), Paper accepted. [2] S.S. Abdul Rahman & H.R. Saeedipour, The Mission Planning and Control Station (MPCS) of Unmanned Aerial Vehicles (UAV), Proceedings of National Postgraduate Colloquium (NAPCOL), University of Science Malaysia (USM), Penang, Malaysia (Dec. 2004) [3] P.G. Fahlstrom, & T.J. Gleason, Introduction to UAV systems, UAV Systems Inc., Columbia, Midland, USA, (Jan. 2001) [4] H.R. Saeedipour, B. Basuno, E. Rachman, & R. Razali, On The Design of a Multipurpose Highsubsonic Single-engined Jet-RPV; a concise approach, Regional Conference On Aeronautical Science, Technology & Industry (RC-ASTI), Institute of Technology Bandung (ITB), Bandung, Indonesia, (18-19 May 2004) [5] http://www.fas.org/irp/program/collect/gcscon.jpg [6] Janes UAV 1999, Boeing MPCS, Janes Publishing Ltd., UK, 1999 [7] www.eese.bee.qut.edu.au/QUAV/Postgraduate/Research/Mission%20Planning/mainpage.php [8] E.N. Johnson & S. Mishra, Flight Simulation for the Development of an Experimental UAV, Georgia Institute of Technology, Atlanta, USA, (2002) [9] J.S. Dittrich, Design and Integration of an Unmanned Aerial Vehicle Navigation System, Georgia Institute of Technology, Atlanta, USA, (2002) [10] T. Netter, & N. Franceschini, A Robotic Aircraft that Follows Terrain Using a Neuromorphic Eye, Marseille, France, (Oct. 2002). [11] Q. Li, M. DeRosa, & D. Rus, Distributed Algorithms for Guiding Navigation across a Sensor Net, Department of Computer Science, Dartmouth College, USA, 2003 [12] P. Sathyanarayana & H.R. Saeedipour, On the Radar Design of Unmanned Aerial Vehicle (UAV) using Synthetic Aperture Radar (SAR), The 2nd International Conference on Mechatronics 2005 (ICOM’05), Kuala Lumpur, Malaysia (May 2005), Paper accepted.
