Capture, Analysis and Mitigation of Multipath in a High Sensitivity GPS Receiver

J. Bickerstaff, R. Frayling-Cork, T. Haddrell

Abstract:	With the widespread use of high sensitivity receivers the presence of multipath has taken on a new significance. GPS receivers optimized for indoor signal levels are easily able to detect and track multipath signals that have suffered multiple bounces on their way to the antenna. Additionally, some of the more traditional methods of mitigating multipath are no longer sufficient or the architecture for high sensitivity devices may not support them. The paper provides an outline of some of these contrasts between how multipath influences highsensitivity GPS and traditional GPS receivers, since, of course, the physics of multipath has not itself changed. How, then, can the same device that operates adequately in a shopping mall or office building, where multipath may be the only signal, be capable of also operating competently in urban canyons and areas where the multipath may be of higher signal strength than encountered indoors? The performance of the end user equipment is often the subject of debate and competitive comparison, leading to a quest for a better understanding of the signal environment itself. This paper reports on a project enacted at GloNav to capture visualize, and analyse the signal environment in order to aid continuing development of mitigation techniques for inclusion in the company’s high sensitivity GPS designs. The basis for the research is the GPS 4000 GSP, containing 14 multiple correlator tracking channels, which for this experiment were set at ¼ chip spacing. Special software directly observed and stored the values during normal tracking, to look 3 ahead and behind the current signal tracking point. When the receiver correctly tracks a direct signal, then we can see any delayed correlation peak from far-multipath, or a modified correlation function shape corrupted by near-multipath. If the receiver tracks a reflected signal, then we would see the direct signal appear ahead of the tracking time. The paper describes the trials equipment and presents results of a representative set of these trials in graphical form, enabling the reader to visualize the presence of complex multipath signals and the interactions between direct and bounced components. The paper presents the authors’ analysis of the various scenarios that are found in the results. A range of potential strategies for mitigating the effects of multipath arriving in the receiver are examined - one asset fortunately available in many high sensitivity receivers is the large number of correlators available. Real trials, with and without mitigating software have been conducted, and the results are compared. We conclude by suggesting further work and trials to be made to advance the science, and to provide validation for chosen mitigation techniques under all possible signal circumstances.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	1696 - 1705
Cite this article:	Bickerstaff, J., Frayling-Cork, R., Haddrell, T., "Capture, Analysis and Mitigation of Multipath in a High Sensitivity GPS Receiver," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 1696-1705.
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