Performance of Multi-path Mitigation Techniques at Low Signal to Noise Ratios

Anthony R. Pratt

Abstract:	Many multi-path mitigation schemes have been described in the literature for application to GPS ranging performance. Most of these are assumed to operate in conditions with high signal to noise ratios. These conditions may be appropriate for some operational scenarios such as surveying and aviation. However, many receivers have to operate in the lower signal to noise regimes found in personal, hand-held, mobile and asset tracking applications. QinetiQ is developing the most sensitive GPS receivers available today for use in these low signal strength environments and is continually working to improve receiver performance through better management of multi-path signals. Models describing multi-path propagation conditions in a variety of circumstances are now well established (eg Jahn et al (ref 8)) indicating that the majority of multi-path propagation with low attenuation occurs for small path length differences. It is, therefore, necessary to consider that multi-path interference vies with tracking noise as the principal cause of inaccuracy (ignoring ionospheric effects) in many GNSS receivers. To complicate matters further, the satellite signal to noise ratio may also be linked to changes in the multi-path model through changes in the statistical parameters describing the nature of the multi-path interference. Specifically, the occurrence of low signal to noise ratio may correspond with poorer multi-path conditions found inside buildings. The performance of two multi-path mitigation techniques (narrow correlators and double-delta correlators) have been analysized and simulated with respect to performance in low signal or high noise environments. As expected, both techniques suffer from a threshold effect below which performance improvements due to the mitigation technique degrade rapidly. The thresholds depend upon the parameters used – for example –the width of the narrow correlator. Furthermore, the use of the multi-path detector as part of a delay discriminator is also subject to loss of linearity in high noise conditions that may further degrade performance. The expectations of performance enhancements due to multi-path mitigation (for example in Galileo receivers) have to be balanced against the thresholds due to low signal to noise ratios.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	43 - 53
Cite this article:	Pratt, Anthony R., "Performance of Multi-path Mitigation Techniques at Low Signal to Noise Ratios," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 43-53.
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