Simulation of Multi-element Antenna Systems for Navigation Applications

The application of user terminals with multiple antenna inputs for use with the global satellite navigation systems like GPS and Galileo becomes more and more attraction in last years. Multiple antennas may be spread over the user platform and provide signals required for the platform attitude estimation or may be arranged in an antenna array to be used together with array processing algorithms for improving signal reception, e.g. for multipath and interference mitigation. In order to generate signals for testing of receivers with multiple antenna inputs and corresponding receiver algorithms in a laboratory environment a unique HW signal simulation tool for wavefront simulation has been developed. The signals for a number of antenna elements in a flexible user defined geometry are first generated as digital signals in baseband and then mixed up to individual RF-outputs. The paper describes the principle function of the system and addresses some calibration issues. Measurement set-ups and results of data processing with simulated signals for different applications are shown and discussed

Joint Antenna Array Attitude Tracking and Spoofing Detection Based on Phase Difference Measurements

Spoofing attacks are a serious problem for civil GNSS applications with safety content, such as airplane landing or maritime navigation in harbors. Also many strategically important infrastructures, such as electric power grids or mobile communications networks, are becoming increasingly dependent on GNSS services. Military GNSS users solve that problem by signal encryption at chip level. This reduces the threat to only allow for meaconing, i.e. retransmitting the GNSS signals from a certain location, since the exact waveform is unpredictable. Civil users cannot rely on encryption at the moment and most likely in the near future. They must be protected by additional techniques, which are able to detect and mitigate spoofing attacks. A number of receiver-autonomous solutions for the spoofing problem have been proposed in the last decade. For single antenna receivers the detection of spoofing attacks can rely on the observation of the time evolution of different signal parameters such as power and Doppler frequency shift, the PRN code delay and its rates, the correlation function shape as well as the cross-correlation of the signal components at different carrier frequencies. However, the most advanced protection against the sophisticated spoofing attacks can be provided by utilizing the spatial domain for signal processing available by using antenna arrays ([1], [2], [3], [4], [5]). A GNSS receiver with an antenna array is able to estimate the directions of arrival of the impinging waveforms and so to discriminate between the authentic and counterfeit signals. Moreover the malicious signals can be mitigated by generating a spatial zero into the array antenna reception pattern in the direction of the spoofing source(s). The use of the array-aided joint estimation of the array attitude and spoofing detection was investigated by the authors in [1], [3], [5]. A post-correlation estimation of the signal direction of arrival (DOA) was utilized as the first step of the corresponding signal processing chain. This approach however still suffers from the effects of short-term distortions in the receiver tracking loops and the resulting unavailability of the DOA estimations during the spoofing attack. Two approaches have been identified to overcome this effect. On the one hand, a more accurate direction of arrival detection and antenna calibration can be used. On the other hand, the attitude estimation can be made more robust by skipping the DOA estimation step and using instead directly the post-correlation array outputs in the underlining measurement model, similar to method 2 in [6]. The latter possibility will be exploited throughout the current paper. One of the main challenges here is to design robust and computationally effective attitude estimation when the post-correlation array outputs consist of the superposition of the authentic and counterfeit signals. This problem, for example, is not adequately handled in [6] and [7]. In the aforementioned approaches, the estimation of the actual direction of arrival in terms of (antenna local) azimuth and elevation was done explicitly before the attitude was estimated. The approach presented in the paper will avoid this (computationally expensive) step, by introducing an adequate measurement model. This model connects the measured relative phases between the antennas elements (spatial signature) to the ones expected from the almanac. This interconnection involves the receiver attitude, which is the state to be estimated. In a second step, the model fit (i.e. residuals of least square fit) is used to detect anomalies. Further processing is done by comparing the spatial signature for different satellites. Contrary to using the cyclic nature of PRN codes to detect the direction in the pre-correlation domain as described in [2], the spatial signature in the post-correlation domain is used. If one dominant direction is present, the likelihood of spoofing or meaconing is considered high. If detected, a second processing stage is triggered, capable of spatially filtering out the spoofers signature (post-correlation nulling). Finally a second run of the aforementioned procedure is done to estimate the antennas attitude using a spatially filtered signal. Theoretical results as well as hardware simulations ([8]) show, that if a GPS/CA or Galileo receiver already tracks a certain PRN, the likelihood of success is very low for an unsynchronized spoofer. In this context (un)synchronized is related to the PRNs current frequency shift (caused by the Doppler Effect), as well as code delay. The code delay error should not be larger than one chip in general. The tolerable frequency mismatch however, highly depends on the receivers implementation (i.e. FLL and PLL parameters and stages), but should not be bigger than a few multiples of 50 Hz. A synchronized spoofer or meaconing signal which is turned on when the receiver already tracks the corresponding PRN will be considered in the context of the paper. The described methods will be evaluated using software simulations. Scenarios without spoofing or meaconing are used to demonstrate the attitude estimation. Scenarios with repeaters will be used to demonstrate the two-stage approach with spatial filtering. [1] M. Meurer, A. Konovaltsev, M. Cuntz, and C. Hättich, “Robust Joint Multi-Antenna Spoofing Detection and Attitude Estimation using Direction Assisted Multiple Hypotheses RAIM,” in Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), September 2012, Nashville, TN, USA., 2012. [2] S. Daneshmand, A. Jafarnia-Jahromi, A. Broumandon, and G. Lachapelle, “A low-complexity GPS anti-spoofing method using a multi-antenna array,” in Proc. ION GNSS 2012, 2012, pp. 1233–1243. [3] A. Konovaltsev, M. Cuntz, C. Haettich, and M. Meurer, “Autonomous Spoofing Detection and Mitigation in a GNSS Receiver with an Adaptive Antenna Array,” in Proc. ION GNSS+ 2013, 2013, p. 12. [4] M. Appel, A. Konovaltsev, and M. Meurer, “Robust Spoofing Detection and Mitigation based on Direction of Arrival Estimation,” in Proc. ION GNSS+ 2015, 2015, pp. 3335–3344. [5] M. Meurer, A. Konovaltsev, M. Appel, M. Cuntz, E. M. Meurer, A. Konovaltsev, M. Appel, and M. C. De, “Direction-of-Arrival Assisted Sequential Spoofing Detection and Mitigation,” in ION ITM 2016, 2016. [6] M. Markel, E. Sutton, and H. Zmuda, “An antenna array-based approach to attitude determination in a jammed environment,” in Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), 2001, pp. 2914–2926. [7] S. Daneshmand, N. Sokhandan, and G. Lachapelle, “Precise GNSS Attitude Determination Based on Antenna Array Processing,” in Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation, ION GNSS+ 2014, Tampa, Florida, September 8-12, 2014, 2014. [8] M. Appel, A. Hornbostel, and C. Haettich, “Impact of meaconing and spoofing on galileo receiver performance,” 7th ESA Workshop on Satellite Navigation Technologies NAVITEC, 2014

A Multi Antenna Receiver for Galileo SoL Applications

One of the main features of the Galileo Satellite Navigation System is integrity. To ensure a reliable and robust navigation for Safety of Life applications, like CAT III aircraft landings, new receiver technologies are indispensable. Therefore, the German Aerospace Centre originated the development of a complete safety-of-life Galileo receiver to demonstrate the capabilities of new digital beam-forming and signal-processing algorithms for the detection and mitigation of interference. To take full advantage of those algorithms a carefully designed analogue signal processing is needed. The development addresses several challenging questions in the field of antenna design, frontend development and digital signal processing. The paper will give an insight in the activity and will present latest results

Comparison of SAGE and classical multi-antenna algorithms for multipath mitigation in real-world environment

The performance of the Space Alternating Generalized Expectation Maximisation (SAGE) algorithm for multipath mitigation is assessed in this paper. Numerical simulations have already proven the potential of SAGE in navigation context, but practical aspects of the implementation of such a technique in a GNSS receiver are the topic for further investigation. In this paper, we will present the first results of SAGE implementation in a real world environmen

Developing a Spoofer Error Envelope for Tracking GNSS Signals

Global navigation satellite systems (GNSSs) are the most significant service for global positioning and timing. The high relevance and wide spread of these systems contrast with the risk for interference or even manipulations of GNSS signals. One specific threat is GNSS spoofing. A spoofer counterfeits satellite signals to mislead the receiver to an erring position/time estimation. The technological progress enabling affordable and easy-to-use spoofer hardware further increases the relevance of this threat. To maintain the integrity of the position/time information, it is mandatory to be able to assess the errors induced by spoofing. The paper at hand derives a bound of the code tracking bias in relevant spoofing scenarios extending the well-known Multipath Error Envelope. These new bounds can be used as a tool to estimate the position/time error, especially but not exclusively for receivers that are collateral damage of a spoofing attack

Enabling RTK Positioning Under Jamming: Mitigation of Carrier-Phase Distortions Induced by Blind Spatial Filtering

ew GNSS applications demand resilience against radio interference and high position accuracy. Separately, these demands can be fulfilled by multi-antenna systems using spatial filtering and carrier-phase positioning algorithms like real-time kinematic (RTK), respectively. However, combining these approaches encounters a severe issue: The spatial filtering induces a phase offset into the measured carrier phase leading to a loss of position accuracy. This paper presents a new approach to compensate for the phase offset in a blind manner, (i.e., without knowing the antenna array radiation pattern or the direction of arrival of the signals). The proposed approach is experimentally validated in two jamming scenarios. One includes a jammer with increasing power and the other includes a moving jammer. The results demonstrate that the approach successfully compensates for the phase offset and, hence, allows for the combined use of RTK positioning and spatial filtering even under jamming

Investigation of Potentially Critical Interference Environments for GPS/Galileo Mass Market Receivers

Positioning and timing services provided by the U.S. Global Positioning System (GPS) have already given rise for various applications almost in all fields of our everyday life. With the advent of European Galileo significant performance improvements for civil users are expected due to the doubled number of satellites and the availability of multiple open signals. The L1/E1 frequency band is expected to be the most probable candidate for mass market GPS/Galileo receivers. It is currently a common belief that the L1/E1 band is relatively quiet in terms of radio frequency interference. However, evidence or counterevidence of this belief is still an open issue. The opportunity to prove this belief has occurred during an interference measurement campaign carried out by the German Aerospace Center (DLR) within the framework of the GJU project GIRASOLE [1]. For this purpose a dedicated measurement van built and further adapted to DLR’s needs by Joanneum Research in Austria was provided by ESA. It was equipped with one Rohde Schwarz FSH6 and one Agilent E4443A spectrum analyzers, several directional and one hemispherical antenna and one dedicated measurement unit by Joanneum Research. The measurements have been carried out in exemplary large cities in Germany (Munich, Augsburg, Hamburg) and were primarily focused on investigating the radio interference situations in the new Galileo frequency bands corresponding to the safety of life services (E5/E1). The L1 band among others has been systematically scanned for the presence of radio interferers. As an interesting result of this campaign, it turned out that in several urban area scenarios high power pulsed interferers were partially situated in L1. The interference signals observed have bandwidths of several MHz and were detected at different frequency offsets from L1 centre frequency. Roughly speaking, these interferers are invisible for non-professional GPS L1 receivers with low RF-Front -End bandwidths but may cause problems to future GPS/Galileo receivers designed to operate with broader bandwidth signals. For example the interference signals were observed to partly overlap with the frequency range of 4 MHz around 1575.42 MHz which contains most of energy of future Galileo E1B and E1C BOC(1,1) Open Service signals. This paper aims at assessing the sensitivity of GPS and Galileo receivers with respect to pulsed interferers observed near L1 band. Two different approaches will be applied for the assessment using both software and hardware simulations. Following the first approach, we use GPS/Galileo acquisition and tracking MATLAB software for determining the corresponding levels of maximum tolerable interference under interference conditions similar to those observed within the GIRASOLE measurement campaign. When performing simulations we assume that the receivers use RF-Front-End configurations (e.g. 2 or 4 bit ADC, 4 MHz bandwidth etc.) which are typical of mass-market GPS/Galileo receivers. Moreover, since mass-market GPS/Galileo receivers are considered, standard signal processing algorithms are applied for acquisition and tracking without utilizing dedicated high sophisticated methods for radio interference mitigation. To enable realistic simulations of pulsed interference signals a parametric interference signal model is developed based on the results of the measurement campaign using characterization both in time and frequency domain. The parametric model easily allows for studying the sensitivity of GPS/Galileo signal processing to different parameters of the interfering signals like pulse repetition rate, power and spectrum separation from the L1 centre frequency. Following the second assessment approach, hardware simulations are performed making use of the combination of the modified Spirent GSS 7790 GNSS Simulator and the Agilent E8267D Signal Generator. The digital signal samples containing the detected L1 interferers are used to create RF L1 interference signals which act as external interference source in the Spirent GNSS Simulator. The output of the simulator, which is a mixture of radio interference signals and signals of GPS satellites, is fed into a hardware GPS receiver in order to determine the effect of the interference signals onto the acquisition and tracking performance and to identify critical interference levels. The results of the hardware and software simulations are compared. The work tends to provide valuable insights in and results about the degree of interference threat related to pulsed interference signals partially overlapping with L1 GPS/Galileo. The assessment of maximum tolerable levels of this interference for mass-market receivers without dedicated mitigation algorithms will be provided. Extensive software and hardware simulations are carried out to corroborate the gained results

Robust Spoofing Detection and Mitigation based on Direction of Arrival Estimation.

The threat of spoofing attacks is a serious problem for civil GNSS applications with safety content, such as airplane landing or ship navigation in a harbor. Also many strategically important infrastructures, such as electric power grids or mobile communications networks, are becoming increasingly dependent on GNSS services. In contrast to military GNSS users which solve the problem to a large extent by utilizing encrypted signals, civil GNSS receivers have to live today and most probably in the near and mid future with unencrypted signals of open GNSS services. Therefore, such receivers have to be protected by additional receiver-sided techniques, which are able to detect and mitigate spoofing attacks. Adequate solutions for the GNSS spoofing problem are the subject of intensive research. A number of receiver-autonomous spoofing detection techniques have been proposed. In order to detect the presence of a spoofing attack these techniques rely on the observation of the signal power, the Doppler frequency offset, the PRN code delay and its change rate, the correlation function shape as well as the cross-correlation of the signal components at different carrier frequencies. The advanced protection against even the most sophisticated spoofing attacks can be provided by the use of multiple antennas. This comes from the fact that the differential carrier phases of a signal, observed at different antennas, depend on the direction of arrival of the signal. Using this, a receiver with an antenna array is able to estimate the directions of arrival of the GNSS signals and detect the spoofing attack, if a large part of the signals come from a single direction. Moreover the malicious signals can be mitigated by generating a spatial zero in the array antenna reception pattern in the direction of the spoofing source. The use of the multi-antenna based approach for spoofing detection and mitigation was investigated by the authors in [1][2]. A technique for joint spoofing detection and antenna attitude estimation by using estimated signal directions of arrival was developed. This technique was implemented in an experimental receiver [3] with an adaptive antenna array where the direction-of-arrival (DOA) is estimated in each tracking channel at the post-correlation stage. On the one hand DOA information is used to constrain the digital beamforming process. On the other hand the proposed technique uses this information also for detection of spoofing attacks. The detection is based on testing the observed DOAs of the satellite signals against the predicted DOAs. The latter are obtained in the local east-north-up (ENU) coordinates while solving the PVT problem and using the computed satellite positions and user position solution. Because the attitude of the antenna array in the local ENU coordinates is not necessarily known, the spoofing detection is therefore treated as a joint detection (i.e. of spoofing attack) and estimation (i.e. of attitude) problem. It was practically demonstrated that the observed DOAs can be used to identify the direction to the spoofing source and produce a spatial null in the array reception pattern for mitigating this type of radio frequency interference. However the technique developed in [1][2] still suffers from the effects of short-term inaccuracies in the direction of arrival estimation occurring during the spoofing attack. On the one hand the algorithm itself can be improved by advanced techniques. On the other hand this problem can be effectively solved by using sequential estimation approach for the array attitude combined with an adequate user motion model. The results of practical tests reported in [2] also indicate that the DOA estimation performance under spoofing attack should be improved in order to maintain reliable spoofing detection and pointing of spatial null toward the spoofing source. This is especially important at the initial stage of a spoofing attack where the counterfeit signals just appear with relative weak power levels and therefore are difficult to detect. The paper presents a new sequential approach for solving the problem of DOA-based attitude estimation. The approach is described in details and its performance is analyzed with the help of computer simulations. The performance improvement for spoofing detection with respect to the former snapshot approach from [1] and the approach using a simple Kalman filter [2] are discussed. Since accurate DOA information is of large importance for the spoofing detection with the proposed approach, some effort is spent in the frame of this study on improving the direction of arrival estimation. The effect of mutual coupling of the array elements is accounted for by using two-stage DOA estimation and applying the corresponding corrections, especially for signals arriving at low elevation angles. It is shown that the corrections can be derived either from anechoic antenna measurements or through the array calibrations using live satellite signals. Both types of corrections are analyzed and compared with respect to their performance. Further, the subspace orthogonal projection method is applied to improve the DOA estimation in the situations where both authentic and counterfeit signals with the same spreading code are simultaneously observed at correlator outputs of a tracking channel. If both signals have significantly different powers, the DOA estimation with commonly used techniques such as MUSIC or ESPRIT often fails to resolve the weaker signal [2]. Therefore at the second estimation run the orthogonal projection is applied in order to minimize the effect of the stronger signal and allow for resolving the direction of arrival of the weaker signal. In order to obtain representative practical results the performance of the spoofing detection and mitigation with the sequential estimation approach is assessed by post-processing raw signal data collected during field trials in scenarios with a GPS repeater. The DLR’s experimental array receiver platform GALANT [3] with a 2-by-2 rectangular array is used for collecting the raw data. [1] M. Meurer, A. Konovaltsev, M. Cuntz, and C. Hättich, “Robust Joint Multi-Antenna Spoofing Detection and Attitude Estimation using Direction Assisted Multiple Hypotheses RAIM,” in Proc. ION GNSS 2012, 2012. [2] A. Konovaltsev, M. Cuntz, C. Haettich, and M. Meurer, “Autonomous Spoofing Detection and Mitigation in a GNSS Receiver with an Adaptive Antenna Array,” in Proc. ION GNSS+ 2013, 2013, p. 12. [3] M. Cuntz, A. Konovaltsev, M. Heckler, A. Hornbostel, L. Kurz, G. Kappen, and T. Noll, “Lessons Learnt: The Development of a Robust Multi-Antenna GNSS Receiver,” in ION GNSS 2010, 2010

Robust Joint Multi-Antenna Spoofing Detection and Attitude Estimation using Direction Assisted Multiple Hypotheses RAIM

The paper presents an approach for detection of spoofing/meaconing signals using the direction-of-arrival (DOA) measurements available in a multi-antenna navigation receiver. The detection is based on comparison and statistically testing of the measured DOAs against the expected DOAs. The expected DOAs are computed in the receiver using the almanac and ephemeris information while performing the estimation of the user position. The attitude of the antenna array is assumed to be unknown and therefore has to be estimated as well. Consequently, the detection of spoofing/meaconing signals using this approach is treated as a joint detection/estimation problem. The solution to this problem is described in this paper. In addition, the performance of the proposed approach is analyzed through simulations in exemplary artificial scenarios and by processing real DOA measurement data collected during measurement campaigns

Autonomous Spoofing Detection and Mitigation with a Miniaturized Adaptive Antenna Array

The performance of a spoofing detection and mitigation technique that makes use of the directions of arrival (DOAs) information about the navigation signals has been assessed. The directions of arrival have been estimated by utilizing a miniaturized antenna array developed for the reception of Galileo navigation signals of the public regulated service in E1 and E6 frequency bands. The performance assessment has been performed by using realistic post-correlation data which were collected in field with a multi-antenna receiver tracking GPS L1 signals