The rubidium clocks in the GPS Block IIR spacecraft have no previous flight experience, and will be the only atomic clocks used on the Block IIR satellites. The U.X.Naval Research Laboratory (NRL), in cooperation with the Global Positioning System Joint Program Office (GPS-JPO), the GPS Control Segment, Lockheed Martin, ITT, and EG&G, is conducting a three year life-test of two flight qualified EG&G Rubidium Atomic Frequency Standards (RAFS) selected from the operational spacecraft inventory. The two RAFS, serial numbers 28 and 30, were selected as representative of the flight configuration build for 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 spacecraft clocks and establish a database of fundamental 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 telemetry and environmental test conditions are monitored and recorded with high resolution. Output from one of the two units has been used as input to the NRL Time Keeping System Simulator (TKSS). The TKSS was originally 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 from the TKSS have been used to evaluate TKS on-orbit performance and serve as an analysis reference. RAFS data from the beginning of the test on 31 March 1997 to August 1998 and representative TKSS data will be presented. Frequency stability results of the RAFS have routinely shown Allan deviation performance in parts in 10~(-14) 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 clocks is being added to the database of life data analysis as satellites are put into service.
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