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GPS Block IIR Rubidium Atomic Frequency Standard Life Test

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Conference paper

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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.S. Naval Research Laboratory NRL, in cooperation with the Global Positioning System Joint Program Office GPS-JPO, the GPS control Segment, Lockheed Martin, ITT , and ECG, is conducting a three year life-test of two flight qualfied EGG 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 clocks 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. 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 satellites 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.

Subject Categories:

  • Test Facilities, Equipment and Methods
  • Navigation and Guidance

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