SVN 20 End-of-Life Frequency Standard Test Results

Reeves E. Bower, Gary L. Dieter, M.J. Van Melle

Abstract: In early 1989 the first Block II Global Positioning System (GPS) satellites were launched, making the beginning of the second phase of the GPS program. The satellites were designed for a seven-and-one-half year lifespan and a six-year mean mission duration. Launched in March of 1998, SVN 20 was the seventh of the Block II GPS satellites. On 11 May 1996, SVN 20 was set permanently unhealthy to users after being operations for just over six years. Due to degradation of the Attitude Velocity and Control Subsystem (AVCS), SVN 20 was no longer capable of maintaining the stable, earth-pointing platform necessary to perform the GPS navigation mission. However, through intensive maintenance by personnel at the Master Control Station (MCS) at Falcon Air Force Base, it was possible to stabilize the vehicle long enough to perform extensive end-of-life testing on the four on-board atomic frequency standards. These tests on both the cesium and rubidium frequency standards varied in duration and complexity. Of the four frequency standards, one had been operations for five months, two have been used previously but were since off-line for two years and four years, respectively, and one had ever been initialized. Areas of interest for the rubidium clock included the determination of the temperature coefficient, voltage-controlled crystal oscillator (VCXO) open-loop operations, and measurement of the C-field and VCXO tune ranges. For the cesium frequency standards, the tests focused on Ramsey pattern generation, gain determination through loop time constant measurement, and C-field and VCXO performance degradation. All four clocks were still operational at various levels of performance and accuracy. The results of these tests were encouraging in that they supported conclusions made during the SVN 9 and SVN 10 (Clock I) end-of-life testing. It is hoped that these results will be useful to the GPS clock community as observations of performance characteristics of space-based atomic frequency standards.
Published in: Proceedings of the 29th Annual Precise Time and Time Interval Systems and Applications Meeting
December 2 - 4, 1997
Sheraton Long Beach Hotel
Long Beach, California
Pages: 181 - 192
Cite this article: Bower, Reeves E., Dieter, Gary L., Van Melle, M.J., "SVN 20 End-of-Life Frequency Standard Test Results," Proceedings of the 29th Annual Precise Time and Time Interval Systems and Applications Meeting, Long Beach, California, December 1997, pp. 181-192.
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