Performance Limits of Multi-path Mitigation for Short Delay

A. Pratt

Abstract:	It is well recognised that multi-path errors are at least as significant as ionospheric errors in determining the accuracy of open service GNSS receivers. This includes GPS, Galileo and GLONASS systems. Historically, considerable reliance has been placed on the envelope of multipath sensitivity bounded by single difference carrier phase (between direct and delayed path signals). The main drivers for the area of the multi-path envelope are the receiver bandwidth and the early/late replica spacing in narrow or double difference correlator structures (1). Most phenomenological studies of multi-path characteristics demonstrate the dominance of short delay multi-path both in the number of paths and the strength of the reflection. Most of the high amplitude multi-path echoes have path length difference less than 20-50m. The attenuation of these echoes are typically in the 5-20dB range. Narrow and double difference correlators perform to limit the maximum multi-path error. One manufacturer recently announced a new mitigation method (called the Vision correlator) in which a least means square (LMS) algorithm is used to estimate the strength and delay of several delayed replicas of the received satellite’s code signal (3)l. In outside conditions, the direct path would be modelled as the largest signal and subsequent delayed and attenuated signals would all be classed as due to multi-path. With current limitations of processor power, only two such paths can be computed at the present time. This paper develops two ideas in order to establish the performance limitations for all multi-path mitigation schemes. The ideas have been published previously (3, 4) but the concept of an (ultimate) performance bound has not. The bound has several components of which the critical one is system bandwidth (partly due to the receiver but mainly to the transmitted signal). A second equally critical component is the transmitted signal modulation waveform. This is not normally within the remit of the receiver designer to change. However, during this period of GNSS system design, the options for optimisation through modulation waveform design may be open. The third principle effect is due to thermal noise or rather the signal to noise ratio upon reception. The concept of a bound on multi-path mitigation shows how the SNR affects the ability to distinguish between different path length. It is clear from the published material on the Vision correlator that such effect are real. This paper shows how the determine the extent which multi-path mitigation can operate as the signal to noise ration degrades.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	1723 - 1732
Cite this article:	Pratt, A., "Performance Limits of Multi-path Mitigation for Short Delay," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 1723-1732.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In