Orbit Period Frequency Variations in the GPS Satellite Clocks

A history of the GPS satellite clock behavior has been accumulated as a result of the weekly precise ephemerides produced at the Naval Surface Warfare Center NSWC under sponsorship of the Defense Mapping Agency DMA. These ephemerides are produced using smoothed pseudorange data collected at a global set of ten tracking stations. The GPS satellite and station clocks are estimated simultaneously with the orbits. Time and frequency offset estimates are generated at one-hour intervals using a stochastic clock model. Studies using interferometric techniques to separate orbits and clocks have indicated that significant orbit period variations are present in the GPS satellite clocks. For PRNBNAVs, which is currently operating on a rubidium frequency standard, these variations have amplitudes as large as 50 ns during the middle of eclipse season. For the four satellites operating on cesium frequency standards, the amplitudes of these variations are less than 15 ns. It is assumed that thermal cycling is the cause of these variations. The stochastic clock model has been tuned to allow the frequency offset state to track these variations. Starting with the first GPS week in 1988, this tuning has been used in NSWCYs production processing. A brief description of the tuning experiments is given along with Allan variances computed for two satellite clocks based on the hourly estimates accumulated aver a period of thirty weeks.