AF/NGA GPS Monitor Station High-Performance Cesium Frequency Standard Stability 2007/2008: From NGA Kalman Filter Clock Estimates

摘要

The National Geospatial-Intelligence Agency (NGA) and the United States Air Force (USAF/AF) operate a worldwide network of GPS monitoring stations. The NGA and USAF monitor stations utilize high-performance cesium frequency standards (CFS) and geodetic quality GPS receivers. Products derived from the NGA Monitor Station Network (MSN) include the postprocessed GPS satellite ephemerides, and GPS broadcast ephemeris and clock parameters. The NGA monitor stations are unmanned and operate in non-laboratory environments. As such, the NGA stations must be monitored frequently to ensure data integrity. The Monitor Station Network Control Center in St. Louis monitors all NGA stations on a 24/7 basis. The USAF operates their stations in a similar manner, with the USAF GPS Operational Control Segment (OCS) at Schriever AFB incorporating data from ten NGA monitor stations in their Kalman filter process. This paper summarizes USAF and NGA MSN clock stability for 2007 and 2008, using the Kalman filter clock estimate data computed daily at NGA St. Louis. The clock stability of six USAF and ten NGA monitor stations is presented. Also presented are site-specific observations, along with an overview of station configuration and planned upgrades. Results show that the USAF/NGA station clock estimates generally meet or exceed specifications for high-performance cesiums, providing accurate, stable, and reliable clocks in support of the GPS program.