A Weak Signal Tracking Technique for a Stand-Alone Software GPS Receiver

David M. Lin and James B.Y. Tsui

Abstract:	This paper uses simulated data sampled at 5 MHz and assumes that the initial phase of the C/A code, the initial phase of the navigation code and the Doppler frequency with an accuracy of ± 1Hz have been acquired. One-second of data is used for C/A code and carrier phase tracking. A special bi-phase decoding approach is used to find the navigation code. One second coherent integration results obtained from early and late gates are used to calculate the pseudo range. At the end of every second, the initial phase of C/A code reference is adjusted by the results of the prompt, early and late gates. Better than 1 Hz resolution can be obtained by an amplitude comparison or a phase comparison method. Theoretically, the receiver sensitivity is limited by the sensitivity of the navigation code detection, which is a C/N0 =24 dB[3]. By using the simulated data, this method can track a signal of C/N0 =24 dB for 12 seconds without a navigation code error. For signal of C/N0 <24 dB , the C/A code and frequency tracking can still be maintained even with a few navigation data errors. The receiver can still function because the navigation data error can be corrected from several repeated sub-frames. The details of the tracking method, navigation code detection method, and pseudo range calculation will be discussed. The performance of the pseudo range measurement, the frequency measurement, and the navigation code detection with respect to various noise levels will be reported by using simulated data. Real data with artificially added noise will be used to demonstrate the performance.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2534 - 2538
Cite this article:	Lin, David M., Tsui, James B.Y., "A Weak Signal Tracking Technique for a Stand-Alone Software GPS Receiver," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2534-2538.
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