GPS III URA and URRA Information for Optimal User Performance

R. DiEsposti, J. DiLellio, D. Galvin, C. Kelley, J-M. Shih

Abstract:	The GPS III will provide enhanced accuracy and integrity relative to the current GPS. Timely update of broadcast ephemeris and clock parameters will reduce Age-of-Data (AOD) and ranging error associated with Space Vehicle (SV) orbital and clock state prediction. Up-to-date, reliable statistical accuracy information in the form of broadcast User Range Accuracy (URA) and User Range Rate Accuracy (URRA) would enhance the availability of accuracy and integrity for certain operations. The current GPS AOD varies significantly from SV to SV due to staggered upload operations. The Signal in Space (SIS) User Range Error (URE) can vary significantly over time and between SVs due to differences or changes in AOD, uncertainty in SV dynamics, SV clock behavior, geometry/observability, etc. The current URA Index binning structure definition for civilian signals (defined in ICD-GPS-200C) has insufficient resolution to adequately represent the current and projected decrease in SIS errors. Also, the current URA information is not updated frequently enough for applications that require integrity. New, proposed broadcast URA and URRA (or URAdot) information on the modernized signals would provide greater resolution and more closely correspond to the observed SIS accuracy, which would allow user equipment navigation filters to reliably implement weighted Line-of-Sight (LOS) measurement error processing to optimize PVT solutions. Frequent and timely updates would allow this information to be used for applications that require integrity, such as for calculation of protection levels and enhanced Receiver Autonomous Integrity Monitoring (RAIM) performance. Results are presented which show that global performance is significantly enhanced when user equipment implement the above processing approach (e.g. weighted least squares) where the SIS error weighting is given by the reliable, up-to-date broadcast URA information, having sufficient resolution. Global performance statistics are given for the case of the current SV staggered upload operational methodology, for a three-hour AOD for all SV, and for the limiting case of zero AOD for all SV.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	504 - 512
Cite this article:	DiEsposti, R., DiLellio, J., Galvin, D., Kelley, C., Shih, J-M., "GPS III URA and URRA Information for Optimal User Performance," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 504-512.
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