Performance of global positioning system (GPS) on-orbit Navstar clocks

The Naval Research Laboratory (NRL) analyses the performance of the on-orbit Navstar clocks in the Global Positioning System (GPS). This work is sponsored by the GPS Joint Program Office and is done in cooperation with the GPS Master Control Station. NRL has analyzed the performance of more than 50 Navstar atomic clocks with a total lifetime of more than 150 years. The results presented in this paper are a summary ofthe performance of the 25 Navstar atomic clocks that were operational during 1995. NRL analysis of on-orbit Navstar clocks is per formed using a multi-year database that includes data from initial activation of each clock through the first quarter of 1995. The Navstar pseudorange data was observed using single and dual-frequency GPS Precise Positioning Service (PPS) receivers that correct for the effects of selective availability. Quality of clock operation is calculated using both broadcast and post-processed precise ephemerides. This presentation includes a description ofthe Navstar clocks that are currently operating in the GPS constellation. The Navstar clock performance in the time domain is characterized by frequency-stability profiles with sample times that range from. 1 to 10 days, or more. The results presented show an improvement in the frequency stability over earlier Block I Navstar clocks with the majority of the currently operating Navstar clocks having a frequency stability for a sample time of one day of 1 pp 10/sup 13/ or better.