Method for Calculating Multipath Environment and Impact on GPS Receiver Solution Accuracy

Wendy Lippincott, Thomas Milligan and David Igli

Abstract:	An analysis technique for determining multipath environment for omui antennas on complex structures has been developed. With this technique, the ratio of the direct and reflected signals for specific incoming angles is computed using a Geometric Theory of Diffraction (GTD) code. Next, 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 Range 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 helical antenna mounted on a spacecraft. As expected, the response of the helical antenna on a spacecraft structure was degraded by signal scattering, diffraction and blockage. Multipath levels were also computed for reflected signals from proximate surfaces. The subsequent antenna pattern corrupted by multipath and blockage was then incorporated in an error budget analysis for estimating the performance of a navigation solution generated by a GPS Receiver. 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 as well as the effectiveness of various remedies to maintain navigation solution accuracy in a noisy RF environment and underdetermined GPS Navigation solution. Performance improvement methods included higher’fidelity dynamic models in the navigation filter, a more stable reference frequency source, and compensation of the pseudorange measurement based on the pattern mapping generated by the GTD analyses.
Published in:	Proceedings of the 1996 National Technical Meeting of The Institute of Navigation January 22 - 24, 1996 Loews Santa Monica Hotel Santa Monica, CA
Pages:	707 - 722
Cite this article:	Lippincott, Wendy, Milligan, Thomas, Igli, David, "Method for Calculating Multipath Environment and Impact on GPS Receiver Solution Accuracy," Proceedings of the 1996 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1996, pp. 707-722.
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