GPS block IIR rubidium frequency standard life test results

The life test of two Perkin-Elmer rubidium atomic frequency standards (RAFS) at the US Naval Research Laboratory (NRL) in a simulated space environment and flight configuration began 31 March 1997. These clocks were production clocks from the Global Positioning System (GPS) Block IIR build and especially provided for this test by the GPS Joint Program Office and the Block IIR satellite contractor team led by Lockheed Martin. This continuously running test was designed to provide information on long term performance, operation, and potential unique characteristics of these clocks. This included potential failure mechanisms, and day-to-day characteristics of the clocks and their internal monitors. GPS Block IIR was the first on-orbit use of this design and there was initially significant concern over their lack of actual space experience. This paper summarizes the test results to date and compares those results with the observed on-orbit performance of the Perkin-Elmer rubidium clocks in the GPS constellation. The utility of life testing in supporting operation and new space clock introduction into the system is highlighted.