Analysis of GPS Monitor Station Outages

B.J. Stanton and Ralph Strother

Abstract:	Prior to the Legacy Accuracy Improvement Initiative (LAII), the Air Force (AF) Operational Control Segment (OCS) had only six AF Monitor Stations, a majority of which were near the equator. Satellites in the GPS constellation were subject to periods of time where they were not in view of any of the six AF Monitor Stations. One of the goals of L-AII was to provide a 100 percent monitoring capability for the OCS by increasing the number of Monitor Stations with National Geospatial- Intelligence Agency (NGA) assets. Although there are a total of eleven NGA sites with GPS monitoring capability, there were some sites that required dedicated lines to enable real time data transfer. Between August and September 2005, L-AII added six NGA Monitor Stations. Then in September 2006, two more NGA Monitor Stations were added bringing the total to 14 (6 AF and 8 NGA). These additions provide overlapping Monitor Station coverage worldwide; now each GPS satellite can be viewed by an average of four Monitor Stations over a 24-hour period. While the subset of NGA Monitor Stations have provided redundancy in the event of the temporary loss of one or more stations or communication links, it is still possible to momentarily lose visibility of one or more GPS satellites if conditions are right. In preparation for the Air Force Operational Test & Evaluation Center (AFOTEC) - led Operational Utility Evaluation for the Architecture Evolution Plan (AEP), HQ AFSPC/A5NN tasked SI International to perform analysis of current Monitor Station coverage performance so a comparison could be made prior to, and after AEP implementation. This paper examines Monitor Station availability from both a hypothetical and an empirical perspective. The hypothetical analysis compares coverages for earlier and current families of Monitor Stations, and for various combinations of Monitor Station outages. The empirical analysis examines a 341-day period of Monitor Station availability data as derived from OCS Payload System Operator (PSO) reports. Using the Monitor Station availability data and the active GPS constellation, analysis was performed to determine actual Monitor Station coverage statistics.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	176 - 183
Cite this article:	Stanton, B.J., Strother, Ralph, "Analysis of GPS Monitor Station Outages," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 176-183.
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