A New Approach to GPS Phase Multipath Mitigation

D. Bétaille, J. Maenpa, H.J. Euler, P. Cross

Abstract:	There have been a number of important developments in recent years in the mitigation of code multipath in GPS receivers but very little progress has been made with the mitigation of phase multipath and it remains the singlemost important source of error in short baseline kinematic GPS and in most network Real-Time Kinematic (RTK) applications. University College London (UCL), the Laboratoire Central des Ponts et Chaussées (LCPC) and Leica Geosystems are collaborating in the development of methodologies to reduce this error source and so improve RTK GPS performance. For several years Leica Geosystems have been developing a new GPS phase multipath mitigation technique leading to the ability of a receiver to deliver additional information from a separate dedicated correlator that enables the direct correction of the multipath-biased phase output from the measurement correlator. The technique has the potential to eliminate many multipath components and is based on sampling the received signals together with their possible multipath components before and immediately after code transitions. This paper explains the basic theory behind the method and summarises the key elements of the associated patents. Two Leica System 500 receivers have been modified to incorporate the firmware necessary to use this new technique and a number of tests have been carried out to assess its capabilities both with lightweight and choke-ring antennas. These tests have been carried out at the LCPC near Nantes in France in May and June 2002. In order to create large multipath signals for some of the tests a 5 m by 2.5 m steel panel has been constructed and placed at different locations near to one of the antennas. Kinematic tests were carried out using the SESSYL kinematic positioning test bed, which consists of a carriage that can be moved in a controlled manner on an 180 m oval-shaped monorail. This enabled the roving antenna to be moved on a known trajectory in order to provide millimetre-level reference positions for the tests. Initial results from the tests are extremely encouraging and are described in detail in this paper. The size of the multipath component of double differenced phase measurements has been reduced by up to 30% when using lightweight antennas and some of the theoretical characteristics of the method have been confirmed. The method is shown to have significant potential for reducing multipath at both roving receivers and at reference receiver locations, especially in cities and on engineering sites where high levels of multipath can often occur and where the number of satellites in view is critical.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	243 - 253
Cite this article:	Bétaille, D., Maenpa, J., Euler, H.J., Cross, P., "A New Approach to GPS Phase Multipath Mitigation," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 243-253.
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