GPS Satellite Clock Event on SVN Impact on Augmented Navigation

Andrew Hansen, Todd Walter, Per Enge, David Lawrence

Abstract:	Recent analysis of GPS data collected by the FAA’s National Satellite Test Bed (NSTB) indicates that a clock event occurred 10 March 1998 on SVN 27, a block 11A satellite carrying a cesium standard. The NST13 reference network maintains receivers from three different manufactures. All receivers viewing SVN 27 observed this clock event which induced coherent L1 carrier and C/A code range rates of 141 m/s and range accelerations of at least |2.5| m/s^2 over a period of up to 6 seconds. .Note that Selective Availability- has a velocity specification Not To Exceed (NTE) 2 m/s. The NSTB receivers (all dual-frequency) also tracked L2 carrier phase fluctuations that were exactly the same as the L1 anomaly. Further, the Doppler measurements on both the L1 and L2 signals followed the clock event, Consequently, the pseudorange error seen by a dynamic platform may not be autonomously detectable without external information from a static reference station. An augmentation system such as WAAS or LAAS with appropriate monitoring algorithms is capable of detecting the erroneous signals. By taking the appropriate precautions, the aircraft can be warned using the augmentation system’s integrity channel. More importantly, the faulty signals must be isolated before allowing them to leak into the correction estimates, ephemeris or ionosphere models for WAAS or pseudorange corrections for LAAS. We present a monitoring algorithm implemented at Stanford for use in GPS augmentation systems which detects and isolates this type of anomaly to both protect integrity and minimize the impact on continuity of service.
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:	1665 - 1673
Cite this article:	Hansen, Andrew, Walter, Todd, Enge, Per, Lawrence, David, "GPS Satellite Clock Event on SVN Impact on Augmented Navigation," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1665-1673.
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