90th Annual Precise Time and Time Interual (PTTI) Meeting GPS BLOCK IIR RUBIDIUM ATOMIC FREQUENCY STANDARD LIFE TEST

The rubidium clocks in the GPS Block ZIR spacecrafl have no prmious flight experience, and will be the only atomic clocks used on the Block IIR satellites. The US. Naval Research Laboratory (NRL),) in cooperation with the Global Positioning System Joint Program OfJice (GPS-JPO), the GPS control Segment, Lockheed Martin, I n, and EGbG, is conducting a three year IiJe-test of two flight qualfied EG&G Rubidium Atomic Frequency Standards (RAFS) selectedfrom the operational spacecraft inventoy. The two RAFS, serial numbers 28 and 30, were selected as representative of the flight conjiguration buildfor Block IIR by ITT and provided by the JPO to NRL for this test. The test is intended to build confidence in these units as operational spacecra3 clocks and establish a database of findamental performance characteristics. The two units are being operated in independent thermal vacuum chambers with high resolution monitoring of the clock's frequency compared to the NRL hydrogen maser references. The units ' internal monitors that would normally be sent as telemehy and environmental test conditions are monitored and recorded with high resolution. Outputfiom one of the two units has been used as input to the NRL Time Keeping System Simulator ( T W. The TKSS was originalb built to evaluate the implementation and algorithms used in the Block IIR on-board Time Keeping System (TKS), which determines the satellite's clock signal to the transmitter. Data @om the TKSS have been used to evaluate TKS on-orbit performance and serve as an analystr reference. RAFS data fiom the beginning of the test on 31 March 1997 to August 1998 and representatfie TKSS data will be presented. Frequency stability results of the RAFS have routinely shown Allan deviation performance in parts in 10-"' ' at one day, much better than initially expected In addition to the data from the two test units, on-orbit data from the Block IIR clock is being added to the database for life data analysis as sutellites are put into service