The Effects of Phase Ripple on GPS Receivers: Sources and Solutions

A.K. Tetewsky, J.W. Youngberg, J.A. Soltz

Abstract:	We have identified a mechanism that causes certain GPS receivers to lose lock at times when the carrier to noise-power density (CNo) ratio and vehicle dynamics should support continuous tracking. This loss of lock is caused by the receiver manager acting upon erroneously low estimates of the carrier to noise-power density output by the receiver's CNo estimator. We have found that a popular CNo estimator is sensitive to small deviations in carrier phase that can be imparted by vehicle rotation or by flexure of an offset antenna. We also note a surprising result that GPS simulators can produce unexpected phase deviations when a high dynamics trajectory file is converted within the simulator. The challenge for developers and test engineers is to diagnose the correct cause when a receiver "fails" a performance test. Did the unit under test actually fail or did the test "fail?" The paper introduces the problem and provides supporting definitions. It reviews a particular CNo estimator and identifies its sensitivity to phase deviations. It then reviews the process of commanding and generating simulated RF, methods of detecting unintentional phase deviations, and techniques for preventing it. It describes an experiment that demonstrates the effect. The paper recommends a means for testers to avoid the problem when testing receivers that use that CNo estimator.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	221 - 232
Cite this article:	Tetewsky, A.K., Youngberg, J.W., Soltz, J.A., "The Effects of Phase Ripple on GPS Receivers: Sources and Solutions," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 221-232.
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