Investigations into Multipath Effects on GNSS Multiple-Frequency Single Epoch High Precision Positioning

Lawrence Lau

Abstract:	This paper investigates the impact of modernized GPS signals, European GNSS (named Galileo) multiple signals, and the combined Galileo and GPS multiple signals on precise carrier phase-based positioning in the presence of multipath. The accuracy of precise positioning and attitude is limited by multipath error since GNSS measurements are often contaminated by an indirect signal reflected from the object carrying the GNSS antenna and/or nearby reflective objects. Multipath from nearby reflective objects is the major concern in this investigation. GPS modernization includes enhancements to the current L2 signal and a new L5 signal, and Galileo will provide L1, E5a, E5b, and E6 signals. It offers many different opportunities for the improvement of precise carrier phase GNSS. The most significant of these will arise from the use of all available data from all frequencies. This is the motivation to investigate the potential of multiple-frequency data processing algorithm to mitigate multipath. Positioning accuracies using different GNSS and combined GNSS data in the presence of multipath are investigated. In order to do this a GNSS data processing program has been developed to treat the different GNSS data in this investigation. The program is based on the well-known double-difference single-epoch least squares approach. Since multipath effects on different frequencies are different, improvement on the positioning accuracy for future multiple-frequency carrier phase-based precise positioning is tested. It is due to the fact that more redundancy leads to better averaging within the least squares process resulting in multiple-frequency GNSS having the potential for greater multipath mitigation than is the case with current system. This point has so far received very little attention in the literature, which has tended to concentrate on the use of the multiple-frequency for improved ambiguity determination. As the GPS modernized signals and Galileo signals aren’t available now, a GNSS data simulator has been developed to generate multiple-frequency data for this investigation. The simulated multipath is validated with real measurements collected in an experiment with known reflector geometry. It is shown that the simulator is capable of generating GPS phase data that has multipath characteristics that agree well with those of real data. With these good agreements, the author is able confidently to extrapolate this study for the GPS L5 signal and Galileo signals. The study concludes that using Galileo + GPS multiple-frequency data shows about 75% improvement on positioning accuracy when compared with using the present GPS reliable single-frequency data. Therefore, the author can conclude that the coming modernized and new GNSS will have significantly better multipath mitigation capabilities than the current system.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1169 - 1180
Cite this article:	Lau, Lawrence, "Investigations into Multipath Effects on GNSS Multiple-Frequency Single Epoch High Precision Positioning," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1169-1180.
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