Inside GNSS Media & Research

JUL-AUG 2018

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34 Inside GNSS J U L Y / A U G U S T 2 0 1 8 ing standards to demonstrate they are clearly defined, useful, and practical; and second, to assess performance of a variety of receivers against real- world threats detected by the STRIKE3 monitoring network. Using real-world threats detected at the monitoring sites enables interested stakeholders (e.g., certification bodies, application developers, receiver manufacturers, etc.) to better assess the risk to GNSS performance during operations and to develop appropriate countermeasures. e remainder of this article pres- ents some illustrative examples for multi-GNSS mass-market and profes- sional grade receiver testing against a single interference type that is very commonly detected at STRIKE3 moni- toring sites, namely a triangular chirp swept frequency signal as depicted in Figure 1 . e test platform used is shown in Figure 2 . e clean GNSS signal is generated from a multi-constellation, multi-frequency Spectracom GSG-6 hardware simulator, whereas the threat signature is generated using a Key- sight Vector Signal Generator (VSG) N5172B through the replay of raw I/Q (In-phase/Quad-phase) sample data. Raw I/Q data captured in the field for a real-world event is used as input to the VSG which then re-creates the detected threat by continuously replaying the data in a loop. Both the GNSS signal simulator and the VSG are controlled via so- ware in order to automate the testing process. e automation script is used to control these devices remotely and to limit human intervention. e script also provides synchronization between the two instruments in order to ensure repeatability of the tests and the reli- ability of the results. e clean GNSS signal and the interference signal are combined using an RF combiner, and the interference- contaminated GNSS signal is fed to the Receiver Under Test (RUT), which produces its own output metrics. For the validation of baseline performance under nominal signal conditions, the VSG does not generate any interference signal. In this case, the input signal to the RUT is only the clean GNSS signal produced by the GNSS constellation simulator. A laptop is used to record and ana- lyze the performance of the receiver against the different threat signals. e analysis is performed using a MAT- LAB-based script that processes the NMEA output messages from the RUT. For each receiver category — name- ly mass-market and professional grade — three different test methodologies are performed: • Baseline – a clean GNSS signal in the absence of interference is fed to the RUT to validate its performance under nominal conditions. e total duration of this test is 60 minutes. • Time To Re-compute Position (TTRP) solution – this test is used to measure the time taken for the RUT to recover aer a strong inter- ference event. In this test, the inter- ference is switched on 14 minutes aer the simulated scenario starts and it is applied for 90 seconds. e interference power is fixed to a value such that the receiver immediately loses its position solu- tion. In this test case scenario the interference power corresponds to a Jamming-to-Signal (J/S) ratio of ~90 dB. e time taken between switching off the interference source and the first position fix is recorded as the TTRP. e profile of this test methodology, whose total duration is 30 minutes, is illustrated in Figure 3 . • Sensitivity – this test scenario is conducted by varying the power of the interfering signal. e interfer- ence is turned on 10 minutes aer the simulation starts and it follows a two-peak ramp power profile. e initial interference power is such that J/S is ~5 dB, and then the inter- ference power is increased by 5 dB every 45 seconds until reach- ing a J/S of 65 dB. Aer the first peak has been reached, the interfer- ence power is decreased in a reverse FIGURE 1 Normalized power spectrum (left plot) and the correspond- ing spectrogram (right plot) of a triangular chirp swept frequency signal 12 9 6 3 0 –3 0 –4 –8 4 8 [MHz] [db] 6 4 2 0 –2 –4 –6 –8 40 20 0 6 0 80 [MHz] [MHz] FIGURE 2 Receiver test platform Vector Signal Generator Automation Script Clean GNSS Signal RF Combiner Receiver Under Test Interference Signal Interference Contaminated GNSS Signal I/Q Samples for interference signals based on recorded threat signatures Laptop with analysis software GNSS SOLUTIONS

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