Analysis of GPS Monitor Station Exclusions with Architecture Evolution Plan

B.J. Stanton, Ralph Strother, Walter P. Petrofski

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 when 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 11 NGA sites with GPS monitoring capability, some require dedicated lines to enable real time data transfer. Between August and September 2005, L-AII added six NGA Monitor Stations. In both September 2006 and March 2008, two NGA Monitor Stations were added to bring the overall total to 16 (6 AF and 10 NGA). These additions provide overlapping Monitor Station coverage worldwide so each GPS satellite can now be viewed by an average of almost five 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 under certain circumstances. In preparation for the Air Force Operational Test & Evaluation Center (AFOTEC) - led Operational Utility Evaluation for the Architecture Evolution Plan (AEP), HQ AFSPC/A5NN (now HQ AFSPC/A5P) tasked SI International to perform analysis of current Monitor Station coverage performance so a comparison could be made before and after AEP implementation. This paper examines Monitor Station availability from both a hypothetical and an empirical perspective. The hypothetical analysis compares coverages for previous and current families of Monitor Stations and for various combinations of Monitor Station exclusions. It specifically shows the improvements provided as more NGA Monitor Stations were added. The empirical analysis examines two datasets of Monitor Station availability data as derived from OCS Payload System Operator (PSO) “Missing K-Point” reports: a 341- day period before Architecture Evolution Plan (AEP) installation, and a 291-day period after AEP installation. Using the Monitor Station availability data and the active GPS constellation, analysis determined actual Monitor Station coverage statistics to compare differences before and after AEP installation.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	2199 - 2208
Cite this article:	Stanton, B.J., Strother, Ralph, Petrofski, Walter P., "Analysis of GPS Monitor Station Exclusions with Architecture Evolution Plan," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2199-2208.
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