Solutions to the Cross-Correlation and Oscillator Stability Problems for Indoor C/A Code GPS

P.G. Mattos

Abstract:	Traditional automotive GPS systems see a signal of -120 to -140 dBm,(53 to 33dB C/No), stronger than the specification due to good antennas, on good ground-planes, with over-spec satellites. Indoor GPS signals may be degraded by obstruction and poor antennas to as low as -155dBm, (-185dBW).(18dB C/No) The receiver can be upgraded to find these signals by increasing the integration time, with data from an assistance source rather than from the satellites. However the sensitivity improvement is dependent on successful integration and detection of the correlation peak. The laboratory sensitivity results of coherent integration are excellent... but this approach is unsuitable because of the narrowing of the bandwidth, and because any strong GPS signal (or interfering carrier), is mis-detected, mis-identified as the desired satellite, due to cross correlation..The narrow bandwidth problems are fixed in our solution by having a fully flexible integration time divisible between coherent and incoherent on the fly to match the signal environment. Thus the bandwidth is not excessively narrowed, but the maximum benefit is achieved compatible with the signal strength and the quality of the oscillator. The cross correlation problem is much more difficult. There are logical methodologies available to minimise the problem... for example look for all satellites at normal sensitivity, then switch to more sensitive mode, looking for the others but omitting the frequency of the previously found satellites. While it is a good first step, this approach falls down in a dynamic environment, with changing obstructions.... as the strong satellite can appear later. Additionally, cross correlation occurs not only on-frequency, but also at multiples of 1KHz away from the strong satellite. Thus a more advanced approach is required. Our approach is to use the high-sensitivity acquisition engine on each satellite not once but multiple times, with significant time separation. A sophisticated algorithm is then applied to the multiple sets of correlation results in three dimensions... time, frequency and amplitude, to identify which of the many possible correlation peaks is the valid one. It has been demonstrated that using this method, a cross-correlation (or interfering carrier) that exists in all sets of data, and is many dB stronger than the wanted correlation, will still be eliminated, allowing the correct correlation peak to be detected. The algorithm has been demonstrated on real indoor GPS signals, and will be incorporated in silicon products during 2004.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	654 - 659
Cite this article:	Mattos, P.G., "Solutions to the Cross-Correlation and Oscillator Stability Problems for Indoor C/A Code GPS," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 654-659.
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