End-Of-Life Indicators For NIMA’s High Performance Cesium Frequency Standards

The National Imagery and Mapping Agency (NIMA) operates a worldwide network of GPS monitoring stations that utilize high performance Cesium Frequency Standards and geodetic quality GPS receivers. The NIMA Monitor Station Network (MSN) operates continuously, and has been in operation for more than seven years. The frequency standards are located in nonlaboratory environments and logistically challenging locations. The mean lifetime of the Cesium Beam Tube (CBT) is approximately six years; failure or end-of-life of the CBT is a significant cause in the reduction of data used to produce the NIMA GPS precise ephemeris. This paper considers methods of predicting CBT failure and end–of-life by examining a variety of data, collected over a seven-year period, by the NIMA MSN. Several operational parameters that are available from the frequency standard hardware, which are routinely collected by the NIMA GPS Monitor Station Control Center (MSNCC), are examined. The relative clock phase of the frequency standards in a zero-baseline configuration (in the stations that have redundant hardware) is also considered as a predictor, by using GPS carrier phase time transfer techniques.