Tracking of Weak GPS Signal with BACIX

Chun Yang and Shaowei Han

Abstract:	In our ION-GNSS’06 paper [Yang and Han, 2006] the technique of block accumulating coherent integration over extended interval (BACIX) was introduced for acquisition of weak GPS signals. Although possible, the direct application of the BACIX technique for tracking is not cost-effective in terms of computation and performance. In this paper we present the BACIX algorithm adapted for tracking of weak GPS signals. In the tracking mode, the code and carrier loops are closed upon the information handed over from the acquisition mode. This may include a satellite ID number (SVN) and the coarse estimates of code delay, carrier frequency, and data bit as well as the extended interval over which the weak signal was accumulated and detected. It is over this extended interval that the tracking loop will perform coherent integration so as to boost the signal strength while averaging out noise. To perform such a coherent integration over an extended interval in the tracking mode, the BACIX is modified as described in this paper. In a sense, a refined search is conducted in the tracking mode over a much reduced uncertainty zone to deal with changes in code delay and carrier frequency when the integration is long and particularly for unknown data bits. This is achieved by maintaining and pruning partial sums of coherently integrated short-length correlations (e.g., 1 ms). Each sum corresponds to a successive sign reversal for data bit sync, which is pre- and post-padded with zeros for coherent integration with FFT/DFT. The sum that corresponds to the most likely sequence of data bits will produce the largest value from which code and carrier errors are estimated for tracking loop closure. Recorded GPS data are processed wherein extra noise is injected to emulate weak GPS signals of different levels. Processing results are presented together with analyses to show the operation and performance of the proposed approach for weak GPS signal tracking.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2797 - 2807
Cite this article:	Yang, Chun, Han, Shaowei, "Tracking of Weak GPS Signal with BACIX," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2797-2807.
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