Performance of Ship Relative GPS (SRGPS) Reference Station Diversity Reception Alternatives

Jim Sennott, Dave Senffner and Keith Najmulski

Abstract:	In the ship relative GPS system, multiple reference antennas and diversity processing can be utilized for improving signal- in-space accuracy and integrity in the presence of jamming, blockages and multipath. This paper compares conventional post-loop diversity processing with post-correlator diversity processing. With post-loop diversity (PLD), roll-pitch-yaw compensated loop outputs from separate receiver channels and antennas are cross-checked for consistency prior to forming a combined DGPS correction for a given satellite. In interference and jamming this approach offers no advantage over single antenna operation. Moreover, transient multipath or blockage at one or more antennas will damage the individual tracking loops, thereby impacting system continuity. By contrast, with post-correlator diversity (PCD), correlator data derived from multiple antennas is cross-checked for reception anomalies prior to loop updates, and qualified correlator data is optimally combined in an extended Kalman filter. Compared with PLD, this form of diversity reception offers potential for significant threshold improvement in jamming, as well as detection and isolation of anomalies. In the paper, SRGPS system dynamics, which fundamentally limit tracking performance, are first established, followed by definitions of attenuation, multipath, and interference reception events to be evaluated. The signal simulation approach is then discussed. System simulations are then carried out for both diversity reception approaches, as a function of the number of antennas available, with discussion of RMS phase error and hold time statistics. In the simulations, PCD is found to out-perform PLD in environments with single and multiple attenuations. Under strong specular multipath conditions, with and without direct path blockage, PCD was able to detect and reject corrupted data at the correlator level, thereby mitigating loop contaminations experienced with PLD. In jamming and interference, the PCD was found to extend tracking thresholds by as much as 6 dB. With data aiding, thresholds approaching 10 dB-Hz were realized with the 4-antenna PCD configuration. These improvements are significant to safety critical DGPS applications, both military and civilian.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	1317 - 1324
Cite this article:	Sennott, Jim, Senffner, Dave, Najmulski, Keith, "Performance of Ship Relative GPS (SRGPS) Reference Station Diversity Reception Alternatives," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 1317-1324.
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