Signal Acquisition and Tracking of Chirp-Style GPS Jammers

R.H. Mitch, M.L. Psiaki, S.P. Powell, B.W. O'Hanlon

Abstract:	Methods of acquiring and tracking chirp-style GPS jammers have been developed. These methods will aid in the geolocation of civil GPS jammers which have become an increasing concern for the GNSS community [1,2]. These jammers, also called personal privacy devices (PPDs), are illegal to use in the United States because they broadcast in an FCC protected frequency band and are sources of intentional interference. Unfortunately, they are also useful to a subset of society. A common use of PPDs is to disable GPS tracking capabilities in a small area about a user location in order to protect the user’s privacy. There are a variety of more sinister uses, such as preventing recovery of stolen vehicles or cellphones through the use of installed GPS technology. The utility of the PPDs combined with a cost of less than one hundred dollars has caused a proliferation of these devices. This has motivated the GNSS research community to consider countermeasures, among them methods of PPD geolocation. The present work focuses on one component of this geolocation effort, that of acquiring and then tracking the PPD signal in a Kalman Filter sense in order to provide observables for use in TDOA geolocation systems, as described in Reference 3. The classic method of TDOA geolocation uses multiple stations. It takes the sampled radio-frequency (RF) data and communicates it all to one master station. The master station computes the cross correlations between the different data streams, which tend to result in simple peaks that correspond to the relative distances between the GPS jammer and the receiving stations. This is a robust technique that assumes no a priori information about the GPS jammers other than the frequency band of interest (typically GPS L1), but it also has a number of draw-backs. Firstly, this method can require enormous communication bandwidth. Suppose a system uses only two stations, in-phase and quadrature data samples at 35 MHz with 16 bits per sample and a 100 percent data use duty cycle, i.e., an uninterrupted stream of correlations. It would require a communication bandwidth of 140MB/s, which is more than can be provided by Gigabit Ethernet. This sort of system is not tractable and would require a reduction in one of the following; duty cycle, bit resolution, or sampled bandwidth. Secondly, this type of system under multiple jammers can have difficulty making unbiased estimates of TDOA and is forced to solve a data association problem [4]. Thirdly, this type of system does not automatically classify the geolocated jammer. The above draw-backs can be either mitigated or removed if a priori information about the jammer signal is assumed and the actual PPD signal is acquired and tracked, as in a frequency-locked loop. These methods are presented from a standard Kalman Filter estimation perspective, with a signal dynamics model and a measurement model. The dynamics model has been developed based on the signal characteristics of the PPDs that were investigated in Reference 1. Virtually all investigated PPDs broadcast a continuous-wave signal with linear frequency modulation, otherwise known as a “chirp” signal. These chirp style jammers can be parameterized with a polynomial type behavior for both their ramp-ups and ramp-downs in frequency. The signal amplitude and parameters of the ramp-up and ramp-down polynomials constitute the states of the discrete-time Kalman Filter. The Kalman Filter discrete-time dynamics model allows these states to vary from chirp to chirp, but it also includes some a priori information gained from measurements of previous chirps. The measurements for signal acquisition and tracking in many RF systems are the classic in-phase and quadrature accumulations. Instead, this system’s measurements will be based on FFTs. This is a useful approach for several reasons: FFTs are effectively short-time-span accumulations, they can be computed rapidly, and they provide robustness by spanning the entire system’s Nyquist range. Additional robustness is provided by stripping off FFT phase information and retaining only power/amplitude information. The Kalman Filter tracking requires analytic partial derivatives of the FFT amplitudes, these analytic derivatives are computed efficiently by using additional FFT-type calculations. The final system can acquire the PPD signals and track them in a Kalman Filter frequency-locked-loop (FLL) sense. It has been validated using real PPD data from a laboratory experiment and it is anticipated that it will be further validated on data collected from a Department of Homeland Security sponsored jamming event at White Sands Missile Range. The extension of the acquisition and tracking algorithms to handle multiple simultaneous jamming signals at one station is straightforward and it is anticipated that multi-signal results will be presented. The practical use of these methods to classify different jammers will also be discussed. References: [1] R.H. Mitch, R.C. Dougherty, M.L. Psiaki, S.P. Powell, B.W. O´Hanlon, S.P. Powell, J.A. Bhatti, T.E. Humphreys, "Signal Characteristics of Civil GPS Jammers", ION GNSS 2011 the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation, Portland OR, September 19-23, 2011, pp. 1907-1919. [2] T. Kraus, R. Bauernfeind, B. Eissfeller, “Survey of In-Car Jammers – Analysis and Modeling of the RF signals and IF samples (suitable for active signal cancellation),” ION GNSS 2011 the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation, Portland OR, September 19-23, 2011, pp. 430-435. [3] R.H. Mitch, M.L. Psiaki, B.W. O’Hanlon, S.P. Powell, J.A. Bhatti, “Civilian GPS Jammer Signal Tracking and Geolocation” ", ION GNSS 2012, Nashville TN, September 17-21, 2012, pp. 2901-2920. [4] J.A. Bhatti, T.E. Humphreys, B.M. Ledvina, “Development and Demonstration of a TDOA-Based GNSS Interference Signal Localization System”, IEEE/ION PLANS, Myrtle Beach, SC, April 21-23, 2012, pp. 455-469.
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:	2893 - 2909
Cite this article:	Mitch, R.H., Psiaki, M.L., Powell, S.P., O'Hanlon, B.W., "Signal Acquisition and Tracking of Chirp-Style GPS Jammers," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2893-2909.
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