Method for Mitigating Multipath Environment Impact on GPS Relative Navigation Solution Accuracy

David Igli, Michael Osborne

Abstract:	An analysis technique for determining multipath environment for omni antennas on complex structures has been employed using a Geometric Theory of Diffraction (GTD) code. The phase shift of the reflected signals is computed using a group delay technique. The direct vs. reflected signal ratio and the phase shift are then combined in an early/late gate error equation to solve for the multipath errors at the desired incoming angles. In addition to multipath effects on the tracking loop model, the analysis also provides signal distortion information to allow the analyst to estimate the amount of Doppler Measurement (Pseudorange Rate) error which can be expected in the RF environment. This technique was applied to determine an estimate of the multipath and blockage pattern for a GPS antenna mounted on a hypothetical set of spacecraft with different structural configuration. The subsequent antenna patterns corrupted by multipath and blockage were then incorporated in an error budget analysis for estimating the performance of a relative (differential) navigation solution generated by co- orbiting GPS Receivers. The performance estimates were compared to other error budget formulations typically used for a spacecraft environment. Instead of using a noise process to simulate the multipath delay, the computed multipath delay and power ratio relative to the direct simulation were used to establish a mapping of multipath response values for the performance analysis. A sensitivity to the multipath and blockage environment is demonstrated for differing space vehicle structures as well as the effectiveness of various remedies to maintain navigation solution accuracy in a noisy RF environment and underdetermined GPS Relative Navigation solution. Performance improvement methods included higher fidelity dynamic models in the navigation filter and applying integrated Doppler techniques to carrier phase measurements to overcome the multipath induced errors in pseudorange measurements.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	227 - 241
Cite this article:	Igli, David, Osborne, Michael, "Method for Mitigating Multipath Environment Impact on GPS Relative Navigation Solution Accuracy," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 227-241.
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