Frequency-Domain GPS Baseband Processor Design And Initial Test Results

Chun Yang

Abstract:	A frequency-domain approach to GPS baseband processor design is presented in this paper. The use of the fast Fourier transform (FFT) is considered as a tool for domain conversions in digital realization. By circular correlation, FFT operates on blocks of incoming and replica code samples and provides simultaneous search over the entire block of code phases. The proposed approach further manipulates the signal spectrum for narrowband interference suppression and digital downconversion to baseband with the residual Doppler removed. Although it is possible to emulate conventional GPS receivers by creating a software version of NCOs and tracking loops, the frequency-domain approach described in this paper presents a different measurement mechanism. It performs time and frequency tracking in the frequency domain with a feedback introduced from the acquired Doppler and code phase. This feedback is actually used more for computation reduction than for linearization as required in conventional GPS receivers. In a sense, the proposed GPS baseband processor design blurs the line between acquisition and tracking and measures the signal time and frequency parameters directly from each and every 1ms data snapshot in a quasi open loop manner. Benefits of such an implementation include fast acquisition, high dynamics, and flexible programmability, among others. This paper also addresses key issues for practical implementation. This includes code Doppler and clock error compensation, alignment to ms boundary, data bit sync, signal parameter extraction, and GPS observable generation among others. Initial test results of a particular frequency-domain implementation of an L1 C/A-code GPS baseband processor are presented based on recorded GPS signals.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2859 - 2870
Cite this article:	Yang, Chun, "Frequency-Domain GPS Baseband Processor Design And Initial Test Results," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2859-2870.
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