Observed GPS Signal Continuity Interruptions

H. Stewart Cobb, David Lawrence, Jock Christie, Todd Walter, Y.C. Chao, J. David Powell, and Bradford Parkinson

Abstract:	During an autoland flight test of Stanford’s Integrity Beacon Landing System in October 1994, one approach was aborted before landing due to a temporary satellite outage. Analysis showed that both the aircraft and ground reference GPS receivers lost lock on one satellite for six seconds. In subsequent weeks, we observed simi- lar outages on most of the Block II satellites. Analysis of this data did not indicate a cause for these outages. According to the satellite operators, this is a generic spacecraft problem. Command uplinks to Block II (but not IIA) satellites occasionally cause a conflict in the spacecraft computer. A conflict causes the spacecraft to emit a non-standard PRN code during one navigation data subframe (six seconds). These conflicts occur roughly 0.3 times per satellite per day. A simple Monte Carlo analysis shows that, in the worst case, this phenomenon could reduce availability of GPS precision landing systems by a factor of ten. This type of outage is not described in the standard literature on GPS spacecraft reliability, nor is it monitored by the FAA’s Performance Analysis Network (PAN). The FAA has recently contracted to upgrade PAN to continuously monitor spacecraft signals. As a result, accurate data on spacecraft signal continuity will soon be available to researchers.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	793 - 795
Cite this article:	Cobb, H. Stewart, Lawrence, David, Christie, Jock, Walter, Todd, Chao, Y.C., Powell, J. David, Parkinson, Bradford, "Observed GPS Signal Continuity Interruptions," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 793-795.
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