Mitigation of CW PPDs via Signal Tracking Suppression

T. Kraus, B. Eissfeller

Abstract:	Personal Privacy Devices (PPDs) are small devices, powered from a battery or the cigarette lighter of a car, transmitting a high power signal within the GNSS frequency band. This kind of active transmission of signals in the GNSS band is called “intentional interference”. Given that the navigation signal is weak, when received by a GNSS receiver on earth, it can be easily interfered by jammers. The jammer prevents receivers in its vicinity from determining a position and degrades the position accuracy over a wide area. In the last year several papers were presented, which provided the detailed signal characterization of these jammers. All of them are transmitting continuous wave signals. Either they have a single-tone CW or a linear chirp signal structure. Since the last years several DSP mitigation algorithms have been developed to regain the GNSS performance reduced by interferences. Major ones are notch filtering, zeroing or clipping with FFT/IFFT, STFT, FrFT, KLT, and wavelet transformation in combination with blanking against pulsed interferences. Each of these methods do belong to the group of destructive processes, because part of the GNSS signal energy are removed by the mitigation process, too. In this paper a novel time-domain DSP signal processing method will be introduced called Interference Tracking Suppression (ITS), which suppresses just the interferer without affecting GNSS signals. The idea of ITS is to reconstruct a replica of the CW interference signal through an appropriate tracking loop. The undesired CW signal can be suppressed by subtracting the replica from the IF-sample. In case of a perfect match of the replica (theoretical consideration) the CW interference will disappear and keeps the GNSS signals untouched. That’s why this approach belongs to the category of non-destructive methods. For tracking of a CW signal, the CW power must be higher than the noise floor of the receiver (IF-samples). The demonstration of the ITS algorithm will be given in three steps: simulation, lab environment, and outdoor tests. First of all, a simulation with NI LabView shows the capability of the ITS principle. Real GNSS signals will be recorded and a single-tone CW signal with constant amplitude will be added in the simulation. The scientific navigation software receiver SX-NSR from IFEN GmbH will be utilized to compare the GNSS performance of the original GNSS signal, the jammed GNSS signal, and the GNSS signal after the mitigation process in the post-processing mode. As indicator for the performance the C/N0-ratio, the code-error- and phase-error measurement will be used. The second demonstration will be done in the lab environment. Therefore, real GNSS signals will be recorded together with a low-budget CW jammer, which will be added via a direct cable connection over an HF combiner between the GNSS antenna and the different front-ends. In our lab we have an 8-bit front-end from Fraunhofer IIS and two different software defined radios from National Instruments (NI) at the moment. The low-cost solution from NI is a USRP and the high-end the VSA (PXIe-5665) both with a quantization of 16 bits. It’s planned to take further 8-bit front-ends into account like the TeleOrbit GTEC and the next generation of the IFEN NavPort. Compared to the simulations, the ITS algorithm has do deal with phase noise, frequency drift and amplitude variations additionally. The final demonstration will be made as an outdoor test. The jammer signal will be transmitted close to the GNSS antenna, which is connected directly to the front-ends. Our institute has got a special license for such kind of outdoor jamming tests. Compared to the lab environment the ITS algorithms has do handle with larger amplitude variations and multipath effects of the jammer signal now. Especially, the multipath effect of the interferences causes higher stress on the interference tracking loop for the jammer replica creation. Each mentioned performance evaluation will be further compared with state-of-the-art mitigation algorithms like notch-filter and FFT/IFFT via excision of anomalous spectral lines. Theoretically, the maximum dynamic of the signal energy is 48.2 dB and 96.2 dB in an 8 bit and 16 bit system, respectively. First tests in our lab have shown that with an 8-bit quantization of the IF-stream an interference suppression of 30 dB can be achieved with the ITS method in the pre-correlation stage. The interference replica had constant amplitude within one IF sample package. With an amplitude-variant replica model a higher suppression rate can be expected. The peak power of the low-budget jammer signal was 40 dB higher than the noise-floor level of the receiver measured in the IF-stream. The front-ends show still a perfect linear behavior in presents of CW signal, which has a 45 dB higher signal peak power than the noise floor. In present of high power jammer signals the high frequency path of the front-ends is the most critical element in the whole receiver chain. That’s why this paper provides a detailed characterization of the front-ends used for the ITS demonstration. It provides the reader the parameter of maximum signal dynamic, 1-dB compression point and third-order intercept point, which are valid in presents of in-band CW interference signals. Finally, a detailed discussion of the CW PPDs signal needs to be provided. We’ve seen in recorded IF-Streams, that the amplitude of the CW signal is more than doubled about every two milliseconds and it holds this gain boost for approximately 2.5 microseconds every time. This signal anomaly will be discussed and first results of counter measures for the improved interference replica construction will be presented.
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:	3290 - 3294
Cite this article:	Kraus, T., Eissfeller, B., "Mitigation of CW PPDs via Signal Tracking Suppression," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3290-3294.
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