GNSS Over-the-Air Testing Using Wave Field Synthesis

A. Rügamer, G. Del Galdo, J. Mahr, G. Rohmer, G. Siegert, M. Landmann

Abstract:	This paper discusses the design and requirements of an over-the-air test system using wave field synthesis and assesses its applicability to GNSS receivers testing and certification for scenarios involving the presence of jammers, spoofers and/or classified signals. Intentional interference is becoming a critical issue as an increasing number of GNSS receivers are used for security related applications. Whereas jammers are used for denial-of-service attacks, spoofers pose an even bigger threat since they can intentionally lead a receiver to estimate a fake position and/or time without recognizing it. To assess the robustness of a receiver, optimize its algorithms, or for certify its performance, a protected test range is necessary. Conventional free-field test ranges are seldom suited to test the receiver’s robustness against interference. Firstly, the operation of a powerful jammer or spoofer in an open field also affects surrounding receivers and therefore often requires a special frequency license to be legally authorized. Secondly, the test conditions are not easily reproducible which makes the direct comparison of different devices or implementations difficult (e.g. the GNSS satellite constellations, weather conditions or multipath environment might vary between for each test run). Finally, the testing, optimization, and verification of not yet certified receivers design to use classified signals (e.g. Galileo PRS) are generally not allowed in open-sky testbeds since stringent anti-tamper and limitation of radiation have to be guaranteed. "Connected tests" with an RF-GNSS emulator where the antenna is bypassed by cables that directly connect the emulator to the device under test (DUT), are regarded as the state-of-the-art. However, they suffer several limitations. Firstly, the DUT as well as jammers/spoofers may have integrated antennas that make a direct connection impossible. Secondly, cable-connected tests bypass the antenna characteristics and therefore neglect their influence on the receiver performance. This is a major limitation for mass-market receivers with integrated antennas (e.g. smartphones) as well as for very sophisticated beamforming receivers where the antenna(s) of the device are a critical factor in the performance evaluation. Especially since spatial array processing of GNSS signals with a controlled reception pattern antenna (CRPA) is one of the most promising and effective mitigation method against jammers and spoofers – regardless of the beamforming techniques used to either blank the incoming interference signals or steer a beam towards the space vehicles (SV) to be received. These array antennas can neither be tested nor certified using conventional connected tests since the effect of the array antenna itself would be excluded which is crucial for beamforming. A similar problem is present in the field of multiple input multiple output (MIMO) device testing in the communication area, e.g. for LTE mobile phones and systems. One approach used there to include the special interaction between antennas and radio environment is the application of wave field synthesis for testing wireless equipment. This is also known as over-the-air testing (OTA) and is the technology of choice for testing the full potential of MIMO radios. Fraunhofer IIS operates an “Over-the-Air Research and Testing” laboratory in an anechoic chamber. This wave field synthesis installation was constructed to emulate complex electro-magnetic wave fields in the frequency range of 0.8 to 3 (18) GHz. It enables accurate and reproducible performance assessments of wireless terminals equipped with multiple antennas by creating controlled test conditions. The system comprises up to 100 dual-polarized antennas surrounding the DUT. Each antenna radiates a signal with well-defined phase and amplitude. The coherent superposition of these individual signals constitutes wave fields that can be arbitrarily shaped to emulate MIMO fields. The current OTA system was designed to test communication terminals by emulating a virtual electromagnetic environment (e.g. for LTE Advanced). In this paper the current communication-based installation is analyzed and its suitability for GNSS testing is assessed, including scenarios involving interferences. Currently this OTA laboratory is extended and adapted for GNSS use. A commercial GNSS constellation simulator provides the necessary parameters for the spatial mapping of the simulated satellites to the OTA waveform synthesizer module whereby the delay between the satellites and the DUT is applied to the individual signals of each satellite in the GNSS signal emulator. The connection between the GNSS signal emulator and the OTA waveform synthesizer is realized in digital baseband. The correct signal power constellations are adjusted in the OTA waveform synthesizer depending on the chosen propagation conditions. The baseline is to support the multi-constellation emulation of either 12 GNSS signals (with up to 80 MHz bandwidth) in one frequency band or 6 GNSS signals in two frequency bands. 32 OTA antennas in an anechoic chamber are used to emulate a realistic wave field for the DUT which can be a GNSS receiver with antenna an integrated antenna or with an array used for beamforming. In this environment the spoofer and jammer can be either emulated by a point source (single antenna) or by applying wave field syntheses including the directional propagation channel between jammer/spoofer and DUT. In this way mobile and time invariant scenarios can be modeled. The scientific and technological challenges are hardware and resource efficient spatial temporal emulation of the wave field for the GNSS satellites as well as for the jammer/spoofer. Depending on the grade of the DUT an appropriate OTA test solution will be discussed in this contribution. To conclude, in contrast to open field test ranges, the OTA approach can be used to exactly replay scenarios. This enables the direct and fair comparison of different algorithms as well as the identification of optimal receiver settings. Moreover receivers with array antennas as well as integrated antennas can be tested while fully accounting for the antenna influences. Thanks to the anechoic chamber in which the wave field synthesis works, jammer and spoofers can be operated without any constraints. Similarly, the testing of classified signals, like Galileo PRS, with non-certified receivers is feasible since the anechoic chamber inherently provides an excellent isolation to and from the outside world.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1931 - 1943
Cite this article:	Rügamer, A., Del Galdo, G., Mahr, J., Rohmer, G., Siegert, G., Landmann, M., "GNSS Over-the-Air Testing Using Wave Field Synthesis," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1931-1943.
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