A Complex-Ambiguity Function Approach to a GPS Receiver

J. York, J. Little, D. Munton, K. Barrientos

Abstract:	Standard GNSS signal correlators form three xedo set lags: an early, prompt, and late. This is typically adequate for general signal tracking, though in most GNSS receivers, the user has little insight, or control, over the signal processing that eventually produces observables. As a result of our investigations into multipath, we are interested in the development of real-time correlators with many, possibly hundreds, of lags. While many-lag correlators have been used with GNSS signals to accelerate signal acquisition or for o ine analysis, these systems have historically operated at very low duty cycles and in parallel with a traditional early, prompt, late correlator because the output data is not directly suitable for real-time signal tracking or precise observable generation. The algorithmic approach we have developed is referred to as the GNSS Complex Ambiguity Function (GCAF) Engine. A Fast Number Theoretic transform is used to e ciently compute hundreds or thousands of correlator lags for a GNSS signal. These can be further processed using conventional Fast Fourier Transforms to produce a complex ambiguity function. The GCAF Engine is a hardware implementation that runs in real-time and provides bit-for-bit identical results to a traditional correlator architecture. We have developed a prototype GPS C/A code receiver in which the GCAF algorithms are implemented in a Field Programmable Gate Array (FPGA) programmed with custom rmware which computes 512 points along the correlation curve for each tracked signal. As an example of the value of this approach, we describe a multipath investigation that we have conducted in which we were able to localize and then identify the source of the multipath.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2637 - 2645
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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