Frequency Domain Techniques for Fast GPS Acquisition and Interference Detection/Rejection

Benjamin Peterson, Richard Hartnett, Ralf Fiedler, and Adam Nebrich

Abstract:	In earlier papers we presented methods for using Fast Fourier Transform (FFT) techniques to process digitized GPS and GLONASS signals. We showed in the laboratory that these methods could potentially offer superior performance over conventional GPS template cross correlation methods in the urban canyon environment since satellite “glimpses” of as little as a few milliseconds provide meaningful information. Processing in the frequency domain also allows one to handle the narrow band interference problem in a very natural way. In this paper we present data collected among the largest buildings in Hartford, CT. Consistent with our findings in the lab, our field analysis suggests that the FFT method is able to extract the time of arrival (TOAs) of the satellites given only brief glimpse of the signals. The end result is a receiver that can track more satellites than conventional receivers when operating in urban canyons allowing for more successful navigation. In addition we present a method based on dynamic programming which makes use of local maxima in the cross correlation function to find the optimal trajectory, thereby further increasing fix availability. In Hudson MA near Boston, we examined potential causes for GPS user unavailability around the Channel-66 transmitting antenna. Using the Plessey GPS Builder II receiver, we never lost our navigation capability even while operating between 200 to 4000 yards from the transmitter and in heavily wooded areas. We did find that the fundamental TV signal transmitted by the antenna drives the GPS antenna pre-amplifier non-linear which creates additional harmonics which could interfere with the GPS signal. Finally we present a method of implementing the optimum matched filter for colored noise and interference. We show the filter improves performance even when noise and interference is limited to white noise and other satellites, and we illustrate notching/filtering noise as wide as 1.44 Mhz centered in the GPS band.
Published in:	Proceedings of the 1996 National Technical Meeting of The Institute of Navigation January 22 - 24, 1996 Loews Santa Monica Hotel Santa Monica, CA
Pages:	339 - 348
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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