Use of the Correct Satellite Repeat Period to Characterize and Reduce Site-Specific Multipath Errors

Penina Axelrad, Kristine Larson and Brandon Jones

Abstract:	Multipath caused by reflectors in the vicinity of GPS ground reference stations is a dominant error source in differential GPS and high rate precise positioning applications. Techniques for calibration and correction of these errors often rely on the repeatability of the GPS satellite geometry. While it is generally assumed that the repeat occurs at a time shift equal to the difference between the solar day and a sidereal day (23 hours, 56 minutes, 4 sec), the actual repeat shift is slightly different for each satellite, depending on its true orbital period. The average for the constellation is approximately 244 - 246 s (rather than 236 s) shy of 24 hours. The correct shift can be determined based on the broadcast ephemeris parameters or by comparing local geometry from day to day. This paper provides an overview of multipath effects on code, SNR, and carrier phase measurements including expressions to estimate distance to the reflector for simple geometries. We then present several methods for computing the correct time shift for each satellite and apply these shifts to compare code and SNR observations from day to day. Results from three ground stations show a reduction in the code multipath when the observables are differenced from day to day. This has the potential to provide significant benefit for differential GPS, WAAS, JPALS, and common view time transfer.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2638 - 2648
Cite this article:	Axelrad, Penina, Larson, Kristine, Jones, Brandon, "Use of the Correct Satellite Repeat Period to Characterize and Reduce Site-Specific Multipath Errors," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2638-2648.
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