Impact of GPS Modernization on Precise Carrier Phase-Based Positioning in the Presence of Multipath

L. Lau, P. Cross

Abstract:	This paper investigates the impact of modernized GPS signals on precise carrier phase-based positioning in the presence of multipath. Carrier phase-based precise positioning is widely used in attitude and orbit determination, geodesy, deformation monitoring, survey applications, Real-Time Kinematic (RTK) positioning for machine guidance, and other high-accuracy applications. However, the accuracy of precise positioning and attitude is limited by multipath error since GPS measurements are often contaminated by an indirect signal reflected from the object carrying the GPS antenna and/or nearby reflective objects. Multipath from nearby reflective objects is our major concern in this investigation. GPS modernization includes enhancements to the current L2 signal and a new L5 signal. It offers many different opportunities for the improvement of precise carrier phase GPS. 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 three-frequency data processing algorithms to mitigate multipath. Positioning accuracies using different numbers of frequencies (L1, L1 and L2, and L1, L2, and L5) 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 combinations of data in our investigation. The program is based on the well-known double-difference single-epoch least squares approach. Since multipath effects on different frequencies are different, and since more redundant measurements are available in three-frequency data, a multipath detection and rejection algorithm to improve the positioning accuracy for future three-frequency carrier phase-based precise positioning is proposed and tested. This strategy, along with the fact that more redundancy leads to better averaging within the least squares process, leads to a modernized GPS 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 additional frequency for improved ambiguity determination. As the GPS modernized signals aren't available now, a GNSS data simulator has been developed to generate three-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, we are able confidently to extrapolate our study for the L5 signal. The study concludes that three-frequency data shows about 60% and 50% improvement on positioning accuracy when compared with single and dual frequency data respectively. Exploitation of the additional redundancy provided by three-frequency data using the proposed multipath detection and rejection technique can further improve the positioning accuracy by about 40%. Therefore, we can conclude that modernized GPS will have significantly better multipath mitigation capabilities than the current system. It should be noted that the results of this study should also be of interest to users of the future European GNSS - Galileo, which is also expected to provide three high-power civil signals.
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:	2163 - 2172
Cite this article:	Lau, L., Cross, P., "Impact of GPS Modernization on Precise Carrier Phase-Based Positioning in the Presence of Multipath," 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. 2163-2172.
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