Multipath Error Analysis and Calibration for GPS Differential Phase 1

Jérémie Godet, Joël Dantepal, Michel Grondin, Daniel Belot, Lise Baudenon, Claude Sourdois, Isabelle Sebbag

Abstract:	It is well known that multipath is the major remaining source of error when using differential GPS. Therefore, the accuracy of attitude determination of a vehicle using GPS carrier phase difference measurements is mainly affected by multipath. In the case of a spacecraft, the only source of multipath is that originating from the spacecraft itself. This paper presents an analysis of multipath errors using comparative measurements. First, GPS carrier differential phase is measured as a function of the line-of-sight signal direction, between antennas mounted on a mock-up satellite face. We used for this experiment a pseudolite in an anechoic chamber. Then, the phase diagrams of the antennas used are measured in the same conditions with and without the structure. Furthermore, some obstacles are placed around the antenna to intentionally create multipath. This method allows to isolate different kinds of parasite signals like single or double specular reflections, diffuse reflections, diffraction on the edges of the obstacles, and creeping signals, and to evaluate their relative power with respect to the line-of-sight signal. The results show also that multipath errors remain even without any obstacle surrounding the antennas. On the other hand, the reading of the phase diagrams leads to explain multipath errors by the corruption of the antennas phase diagrams by the structure on which the antennas are mounted. The accuracy of the anechoic chamber calibration method is evaluated using external measurements where the same structure is exposed to real GPS signals. The results are very encouraging since they show after calibration errors lowered by up to 80 percent.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	521 - 530
Cite this article:	Godet, Jérémie, Dantepal, Joël, Grondin, Michel, Belot, Daniel, Baudenon, Lise, Sourdois, Claude, Sebbag, Isabelle, "Multipath Error Analysis and Calibration for GPS Differential Phase 1," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 521-530.
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