Transcript
User RAIM Integrity and Interference Mitigation Test Results with Upgraded German Galileo Test Range GATE Guenter Heinrichs, Erwin Loehnert and Elmar Wittmann IFEN GmbH Poing, Germany
GATE is the only Galileo test and development range worldwide where already today navigation is possible with realistic Galileo signals on three frequencies simultaneously in an outdoor environment. The testbed has started commercial operations in August 2008. In parallel a system upgrade has been performed in terms of upgrading GATE to the latest Galileo signal structure (ESA Galileo SIS ICD and Galileo Open Service SIS ICD) including CBOC. Hence GATE is now capable of transmitting the Galileo OS, the Galileo SoL Service (functional), the Galileo CS and a Galileo PRS dummy signal according to the latest Galileo SIS ICD version. This GATE system upgrade is being further extended now to support user integrity testing more effectively. For this reason two additional transmit stations are deployed, resulting in a number of eight stations in total. In addition, GATE will be capable to emulate simple feared events on system/satellite level after the upgrade, so that the GATE system will support GPS and GATE/Galileo dual constellation RAIM, individual user integrity test scenarios as well as test of receivers with different RAIM functionalities implemented. The paper will give an overview on the status of the work to be performed in the framework of the system upgrade of the GATE test infrastructure. It further will present first results of user RAIM integrity and interference mitigation tests obtained by test campaigns in the upgraded GATE testbed. Keywords: Galileo; Testbed; Interference; Integrity; RAIM
I.
INTRODUCTION
GATE is the only Galileo test and development range worldwide where already today - years before the full operability of the Galileo system in space - navigation is possible with realistic Galileo signals on three frequencies simultaneously in an outdoor environment. An arbitrary commercial Galileo receiver can be used there without any modifications. Thus, GATE is an important intermediate step for Galileo on its way from the laboratory to the orbit in terms of realistic (RF) signal transmission. GATE operates following the same physical principles like Galileo and GPS to allow users finding their position, velocity and time (PVT). The receiver can determine its PVT by calculating the distance to the virtual satellite, emulated by a transmitter station, and finding the intersection point. Through its infrastructure, GATE is able to radiate the original
navigation signals from Galileo satellites, to simulate natural influences like ionosphere or troposphere delays, to change characteristic parameters of signals and to adapt the signal strength as required, and thus to enable testing of standard respectively commercial receivers. The testbed is open to all users worldwide and has started commercial operations on August 1st in 2008. In Spring 2010, IFEN GmbH has been selected by the German Aerospace Center (DLR) as operating company for GATE. In parallel to the commercial service, a system upgrade has been started on October 1st, 2008, in terms of upgrading GATE to the latest Galileo signal structure (ESA Galileo SIS ICD and Galileo Open Service SIS ICD) including CBOC. The implementation of the new Galileo signal structure was finished in June 2010 and successfully validated and approved by the customer DLR in July 2010. Thus the GATE test infrastructure is now capable of transmitting the Galileo OS, the Galileo SoL Service (functional), the Galileo CS and a Galileo PRS dummy signal according to the latest Galileo SIS ICD version. Beside this signal upgrade, further goals of this system upgrade are firstly, the possible certification of GATE as accredited open-air test infrastructure to perform the necessary tests needed for the certification process to certify Galileo Safety-of-Life (SoL) equipment and secondly, to incorporate adequate means on receiver level to mitigate interferences, caused by the beams of a military radar station that impinge on the testbed. Furthermore, this GATE system upgrade is now being extended to support user integrity testing more effectively. To enable RAIM integrity processing with fault detection and exclusion (RAIM FDE), at least 6 satellite measurements are required. However, more measurements are often needed depending on the satellite geometry and to serve for higher robustness. For this reason, DLR has awarded an extension contract by mid of this year to IFEN GmbH to upgrade the GATE testbed with its current six transmit stations by two further stations, then eight in total. In addition, GATE will be capable to emulate simple feared events (FE) on system/satellite level after the upgrade through an “intentionally non-steering” of configurable satellites, either manually by the operator or according to a corresponding test
plan. In this way, single or multiple feared events of different size and duration can be configured (according to the GATE satellites selected) and thus the GATE system will support GPS and GATE/Galileo dual constellation RAIM, individual user integrity test scenarios as well as test of receivers with different RAIM functionalities implemented. After a brief summary of the testbed architecture and implementation the following sections will present the status of the activities to be performed for the system upgrade of the GATE test infrastructure. Furthermore, the first results of user RAIM integrity and interference mitigation tests that were obtained during test campaigns in the upgraded GATE testbed will be provided. II.
GATE INFRASTRUCTURE AND TEST AREA
A. GATE System Architecture The GATE system is partitioned into four segments Transmit Segment (GATS), Mission Segment (GAMS), Control Segment (GCS), and the Support Segment. Fig. 1 presents an overview of the GATE system architecture. The ground-based transmitters, which are part of the GATE Transmit Segment (GATS), emit signals on all Galileo frequencies. They are flexible in signal generation and adaptive to changes in signal structure. As GATE is a real-time system, it is necessary to feed the navigation message in real-time to the transmitters. They are also equipped with stable atomic clocks. The GATE Mission Segment (GAMS) monitors the navigation signals by using two GATE Monitoring Stations (GMS), performs the time synchronisation of all system clocks and generates navigation messages and steering commands to be sent to the eight transmitters. The tasks denoted above are mainly performed by the two GAMS core elements, the GATE Processing Facility (GPF) and the GATE Monitor Receivers (GMRx), both developed by IFEN GmbH.
control the entire GATE system, to host and operate the control centre to provide accurate and stable GATE system time aligned with the International Atomic Time (TAI), and to archive GATE mission data. The ground-based transmitters, which are part of the GATE Transmit Segment (GATS), emit all frequencies foreseen for Galileo. Therefore they have to be flexible in signal generation and adaptive to changes in signal structure. As GATE is a realtime system it is necessary to feed the navigation message in real-time to the transmitters. They are also equipped with stable atomic clocks. Fig. 2 shows the locations of the six transmit stations of the primary GATE constellation, as well as the transmitter rack and the corresponding transmit antenna. The GATE Control Segment (GCS) includes all the functionality and facilities that are required for the mission control and operation. The main tasks it has to perform are to monitor and control the entire GATE system, to host and operate the control centre, which serves as operational node of GATE including e.g. the mission planning, to host and provide the GATE system time and to archive the GATE mission data. The main tasks of the GATE Support Segment (GSS) finally comprise the appropriate user individual preparation, i.e. simulation and planning of the actual user experiments with dedicated software tools of the GATE Mission Support Facility (GMSF), as well as the provision of the GATE User Terminals equipped with a combined Galileo/GPS receiver. B. GATE Test Area Berchtesgaden / Germany The GATE test area is located in the region of Berchtesgaden in the very south-eastern part of Germany/ Bavaria. The service area is depicted in the maps shown in Fig. 2. The two monitoring stations are located at the GATE office which is situated quite in the centre of the GATE test area. As can be seen on Fig. 3, Berchtesgaden is surrounded by high mountains that are rising up to over 2000 meters. The installation of the GATE transmitters on well exposed positions allows for emission of the GATE signals with average elevation angles between 10 to 15 degrees from a user’s point of view, when located within the GATE test area.
Figure 1. GATE infrastructure overview
The GATE Control Segment (GCS) includes all the functionality and facilities that are required for the mission control and operation. The main tasks are to monitor and
Figure 2. GATE service area Berchtesgaden/Germany (core test area marked red, transmitters and monitoring station labelled
Figure 5. The two new autonomous GATE transmit stations GTS7 & 8
Figure 3. View into the GATE test area (from GTS Gruenstein)
In addition to the six transmit stations of the primary GATE constellation depicted in Fig. 4 the system is currently being extended by two further stations. First of all a thorough analysis was performed in order to identify various locations that are particularly suited for the establishment of the additional GTS with regard to their visibility in wide parts of the test area. More than ten different potential installation sites have been investigated, finally resulting in two favourite candidates that allow for wide-area signal propagation due to their exposed topographic situation including an acceptable accessibility for installation and maintenance activities. Nevertheless, these both locations do not feature any available infrastructure that could be used for the installation of the GTS, e.g. in terms of platform mounting, power supply, communication links etc. Thus, the concept of the only existing autonomous GATE station so far, i.e. the GTS Gruenstein, was applied for the new stations: Two dedicated containers equipped with solar power plants and GSM/WLAN communication infrastructures were installed (see Fig. 5). For each station the preparation of a solid foundation was required. As both sites are only accessible via hiking trails the transportation of all construction materials and equipment was done by helicopters.
The two new GTS locations, i.e. the mountains "Rauhenkopf" (GTS 7) in the northern part and "Brettgabel" (GTS 8) in the South-western part of the test area are indicated with purple labels in the map (Fig. 2). III.
SIGNAL STRUCTURE UPGRADE OF THE GATE TEST INFRASTRUCTURE
In the framework of the GATE signal conformity and certification phase (GATE SKZ, prime contractor IFEN GmbH) the GATE test infrastructure was upgraded in terms of the inclusion of the updated Galileo Open Service (OS) signal structure (CBOC), as published by the GSA end of April 2008 in the latest version of the Public Galileo OS SIS ICD, on one hand and to achieve the certification of GATE as officially accredited open-air test infrastructure to perform the necessary tests needed for the certification process to certify Galileo Safety-of-Life (SoL) equipment, on the other. Before the upgrade the Galileo signals emitted in GATE were according to the ESA SIS-ICD Version 9. After completion of the signal upgrade end of 2009, the GATE test infrastructure is now capable of transmitting the new CBOC signal structure on the Galileo E1 frequency band and a broader bandwidth of 92.07 MHz on the Galileo E5 frequency band in accordance with the ESA Galileo SignalIn-Space Interface Control Document (SIS ICD) Version 13 as well as the European GNSS Supervisory Authority (GSA) Public Galileo Open Service ICD (Issue 1.0).
Figure 4. GATE transmitter rack and transmitter locations overview: Primary constellation including GTS 1 to 6
In addition, GATE is capable of transmitting the Galileo SoL Service on functional basis, i.e. providing integrity alert messages (however, no real Galileo integrity computation is performed) and the Galileo Commercial Service CS on functional basis, i.e. providing the Galileo dummy C/NAV message (C/NAV-0) without encryption. Besides the prime contractor IFEN GmbH, the following companies are involved on subcontractor level in the GATESKZ activities: EADS Astrium GmbH, VCS AG, NavCert GmbH and Work GmbH. IV.
INTERFERENCE MITIGATION
During the GATE phase C/D system performance tests in 2008 some interference effects on the Galileo E5b and E6 frequency bands have been detected that caused a noticeable performance degradation. After extensive spectrum evaluation and analysis, it was found that the interference effects are caused by military aerial surveillance radar, operating in the Galileo E5b and E6 frequency bands A. Interference Measurements in the GATE Area In the framework of the system AIV (Assembly, Integration and Verification) activities extensive system tests have been performed. For the evaluation of the system performance within the GATE core area a dedicated measuring vehicle equipped with the GATE user receiver and terminal was used. In order to provide an independent and very accurate position reference a GPS RTK solution was applied using high-precision differential corrections. The accuracy of the RTK positions was typically in the range of a few centimetres in static scenarios and several decimetres within dynamic tests. In this way it is made sure that on the one hand the steering of the signal transmitters in the Extended Base Mode and Virtual Satellite Mode is adjusted properly. On the other hand the precise RTK position serves as a reliable reference for the performance evaluation with respect to the absolute GATE accuracy achieved. The GATE user receivers as well as a dualfrequency GPS RTK receiver for the position reference were installed in the test vehicle. Various tests with duration of at least eight hours have been performed for the three different GATE modes in order to verify both, system stability and performance during the maximum daily operating time specified. The results obtained from these tests gave proof of the compliance of the GATE system with the performance specifications, at least for the E1 band. The GATE specification for the horizontal real-time navigation requests a positioning accuracy with GATE signals better than 10 m (2 sigma) with an adequate HDOP (Horizontal Dilution of Precision) value. The tests showed that this accuracy can also be achieved with the E5a and E5b signals, however, often some degradations of the positioning performance were observed for these frequencies as presented in [1]. After various analyze of the E5 test results there was clear evidence to suggest that those degradations may be caused by signal interference within the E5 band. Thus, intensive investigations were made with respect to potential
GATE/Galileo interference sources in the area of Berchtesgaden and surroundings. In the scope of these analyses several measurement campaigns were performed in order to detect potential interference sources as well as to evaluate the impact of interference sources already identified. Measurements have been performed by IFEN GmbH and by the Bundesnetzagentur BNetzA (German Federal Network Agency), which is responsible for securing efficient and interference-free use of frequencies in Germany. B. Interference Measurement and Spectrum Evaluation Results in GATE Analysis of the spectrum analyzer measurements for the Galileo frequencies showed that interference signals are present just outside of the ITU-reserved E5 band and right within the E6 band. The interference signals exhibited quite similar characteristics on two frequencies of interest: E5 band: • center frequency: 1217 MHz • “always on” E6 band: • center frequency: 1290 MHz • “sporadic presence” Common: • measured bandwidth: > 5 MHz • pulse spectral power: ca. –75 dBm • complex pulse sequence and shaping • average PDC: ~ 5% The 1217 MHz signal is constantly present, while the 1290 MHz signal is only present at certain times of the day. After clear reception for a couple of hours, the signal ceases for another couple of hours, and then comes back. There are indications the signal is switched to other frequencies meanwhile. No obvious schedule for this characteristic could be established. Snapshots of the interference signals observed at the spectrum analyzer are depicted in Fig. 6 below (E5 left and E6 right).
Figure 6. Interference signals observed at GATE monitoring station
The interference signals received at the GMS have then been analyzed in the time domain. Fig. 7 shows the E5 recurring pulse sequence. The pulse sequence is rather complex with pulses of variable duration from 50 to 200 microseconds. Each of the pulses is itself shaped in a nonrepetitious form. An enlarged representation of a single 200
microsecond pulse is presented in Fig. 8. The pulse gaps are 1.6 ms and 3.5 ms respectively. A 24.7 ms long sequence of 8 short pulses and 3 long pulses is repeated continuously. Fig. 8 also illustrates the signal measurements in the receiver front-end.
Figure 7. Interference pulse sequence on E5
The signal displayed green was measured before the AGC (Automatic Gain Control) amplifier and the one shown in pink after AGC. The latter gives clear evidence of the AGC going into compression during the pulse. Essentially what happens is pulse clipping in the analogue domain. This is a desirable effect because the total power entering the next stage, the A/D converter (ADC), is limited. An elongation of the pulse due to discharging of capacitors in the RF system after it has been driven into saturation (recovery time) is not evident in Fig. 8.
Figure 8. E5 interference “long” pulse
In addition to the characteristics described above, the received interference signal exhibited a superimposed 10 second oscillation. The 10 second interval is assumed to stem from a rotating transmission antenna, which is typical for a radar installation. The overall signal characteristics let us assume that the interference is emitted by a military long range air surveillance radar. The results of the corresponding interference measurements performed by the BNetzA largely confirmed the observations described above. According to the BNetzA statement the interference effects were clearly caused by radar transmission, presumably from military airspace surveillance. An inquiry at the German air-traffic control (DFS) confirmed this assumption. Based on the measurements and the corresponding further investigations, an Austrian military radar installation located at a mountain
near Salzburg could be identified as the most probable interference source which is described in more detail in [1]. Hence one important subject that had to be tackled in the framework of the subsequent GATE phase SKZ activities was the mitigation of the E5 interference effects on the GATE monitoring and user receivers. To this end a digital pulse-blanker was implemented at the receiver baseband. It is located in the digital signal conditioning and performs mainly the following tasks: •
Zero out signal if a maximum threshold is detected
•
Estimate the noise floor before / after pulse blanking
•
Estimate the observed duty cycle and average pulse duration of the interferer based on the threshold detection
With the implementation of this pulse-blanking mechanism the interference effects observed from the radar station are eliminated successfully. This can be clearly seen from the exemplary comparison of two GATE E5a measurements depicted in Fig. 9: At one receiver the signal was tracked without pulse-blanker activated while the other one was operated with active pulse-blanking. It is obvious that there are periodic (10 seconds) degradations of the C/N0 that can lead to a degradation of the other raw data output. witch is remedied with the new receiver implementation.
Figure 9. C/N0 comparison of E5a observations with pulse blanking disabled (top) and enabled (bottom)
V.
FIRST INTEGRITY / RAIM EXPERIMENTS WITHIN GATE
The goal of future or modernized GNSS (Global Navigation Satellite System) is not only to provide higher accuracy as comparable systems today but also to satisfy the safety critical needs of certain applications areas like in the aeronautics, maritime or rail domain by providing a corresponding Safety-of-Life (SoL) service as it is planned for Galileo. Safety critical applications thus require a “trustable” position, denoted in GNSS as integrity. According to [2], a navigation system shall further deliver an alarm when the error in the computed user position exceeds an allowable threshold (alarm limit). This warning has to be issued to the user within a given period of time (time to alarm) and with a given probability (integrity risk). Integrity is obtained by implementing appropriate mechanisms/
algorithms at several levels. The integrity evaluation is typically performed either: •
At system level: the system provides integrity when it can detect errors and warn the users in a timely manner. In case of Galileo the Global Integrity Concept (GIC) etc.
•
At user level: through receiver autonomous integrity monitoring, RAIM etc.
satellites) with the Septentrio user receiver performed in the GATE testbed close to the GATE office. Because GATE is specified only for a horizontal position accuracy of better than 10 meters (2 sigma) and not for the vertical due to its terrestrial nature, the integrity results presented are focused on the horizontal position and corresponding horizontal protection limits (HPL) and alert limits (HERL).
The new approach for Galileo to provide integrity assumes that the integrity task is performed mainly on user level by applying RAIM algorithms within the user receiver. For RAIM integrity processing at user level a certain number of available satellites are required. To enable RAIM FD (Fault detection in RAIM), at least 5 measurements / satellites are required, and to enable RAIM FDE (Fault detection in RAIM with the ability to exclude faulty data), at least 6 measurements are required. For a dual constellation scenario (GPS + GATE/Galileo/IOV), one additional satellite is required. In order to test meaningful integrity scenarios in GATE, two further considerations have to be taken into account: •
•
GATE initiates from time to time a so called PRN change in order to maintain good horizontal DOP performance which means that only the maximum number of satellites minus one can be used during this time.
Figure 10. Position with RAIM parameters (HERL=16.42m) and planimetric plot
Fig. 11 depicts the receiver channels status with the GATE/Galileo satellites tracked on the SoL frequencies E1 and E5b for this experiment.
Due to the topography and shadowing effects by buildings and vegetation within the GATE test area, there are not always the maximum number of satellites (GATE transmit stations, GTS) visible.
Furthermore, more measurements are often needed depending on the satellite geometry and to serve for higher robustness. Thus, to perform robust RAIM FDE integrity tests within GATE at least 8 satellites should be available. Note that this was the reason to extend the testbed by two additional GTS from six to eight transmit stations. Note further, that the GATE system provides no RAIM capable GATE user receiver GURx, however, the GURx is principally capable to detect - large enough feared Events (FE) induced - pseudorange errors and to exclude the corrupted signal (again if large enough) for positioning by using implemented simple signal quality check algorithms. In the following functional integrity / RAIM test results are presented performed with an upgraded GATE testbed of eight stations and a RAIM capable Septentrio user receiver. As GATE is neither designed nor capable to validate the real Galileo satellite navigation system with actual integrity performance parameters (horizontal/vertical alert limits, integrity risk, time to alarm etc.), the integrity test scenarios performed in GATE have to be regarded on a functional basis / qualitative level only. Fig. 10 shows the dual frequency E1/E5b user position, the position accuracy, the RAIM parameters and the planimetric plot (true offset of receiver position solution to the reference position) of a static integrity test (experiment I: normal operation with eight
Figure 11. Receiver channels status, showing E1/E5b PVT solution
Fig. 12 shows the corresponding results of the horizontal protection limits (HPL). As it can be seen from the figure the resulting HPLs are around 7 meters, indicating “Normal Operation”, detailed to 100% for “APV-I Operation and 100% for “CAT-I Operation” according to the limits set.
Figure 12. Experiment I: Horizontal protection and alert limits, showing “HPL/HERL normal operation”
G
In the experiment II (see Fig. 13), one single satellite out of the eight satellites was removed from the position solution (PVT). The HERL increased from 16.42 m to a value of 40.60m resulting to HERL “system unavailable”.
Figure 15. Experiment III: HPL Plot, indicating “Hazardously Misleading Information” HERL of 35.15m, leading to “HERL system unavailable”
Because an RAIM algorithm is implemented in the Septentrio test receiver, the receiver is capable to detect and to exclude (FDE) the feared event corrupted signal PRN 25 and finally it rejects this signal from the internal PVT solution (see Fig. 16).
Figure 13. Experiment II: HERL of 40.60m, leading to “HERL system unavailable”
Finally, in the “integrity” experiment III, a feared event (FE) was introduced in this way that the steering algorithm of the GATE processing facility was de-activated for a single satellite (PRN 25 in the test), which causes the virtual satellite to drift slowly away (see Fig. 14).
Figure 16. Receiver channels status, showing E1/E5b PVT solution and the rejected (FE corrupted) PRN 25 signal
CONCLUSION The Galileo Test Range GATE has reached its full operational capability (FOC) on August 1, 2008. GATE provides the opportunity for receiver, application and service developers to perform realistic field-tests of hardware and software for Galileo at an early stage, i.e. several years before the full operability of Galileo. The realism of the signal environment includes also the presence of typical pulsed interference. Successful remedial measures have been taken to mitigate the effects of these interference signals on the GATE monitoring and user receivers. As it was further presented in the paper, the GATE testbed with its eight transmit stations is also capable to perform functional integrity / RAIM test scenarios. Further up-to-date information about GATE can be found on the official homepage www.gate-testbed.com ACKNOWLEDGMENT
Figure 14. Exp. III: Position with RAIM parameters (HERL=35.15m) and planimetric plot, indicating user postion drift due to induced PRN 25 FE
This position drift results after some time into a Hazardously Misleading Information for several epochs (see HPL plot in Fig. 15 with corresponding 22 alarm epochs). Furthermore, due to the increased HERL value of 35.15m, an “HERL system unavailable” is raised (see Fig. 15).
GATE was developed on behalf of the DLR (German Aerospace Center, Bonn-Oberkassel) under contract number FKZ 50 NA 0604 and GATE SKZ is performed on behalf of the DLR under contract number FKZ 50 NA 0802, both funded by the BMWi (German Federal Ministry of Economics and Technology). REFERENCES [1]
[2]
G. Heinrichs, E. Loehnert, E. Wittmann, "Interference Effects on Galileo E5b and E6 Frequencies observed in the Galileo Test and Development Environment GATE", September 2008, ION GNSS 2008, Savannah/Georgia, USA C. Pecchioni, A. Zin, L. Scaciga, S. Di Raimondo, F. Luongo, O. Crest, “Galileo Test User Receiver Development for Safety of Life Applications: Integrity Processing Overview”, 2009 - May 6, 2009. Naples, Italy
