Time Maintenance System for the BMDO MSX Spacecraft

Martin J. Hermes

Abstract:	The Johns Hopkins University Applied Physics Laboratory (APL) is responsible for designing and implementing a clock maintenance system for the Ballistic Missile Defense Organizations (BMDO) Midcourse Space Experiment (MSX) spacecraft. The MSX spacecraft has an on-board clock that will be used to control execution of time-dependent commands and to timetag all science and housekeeping data received from the spacecraft. MSX mission objectives have dictated that this spacecraft time, UTC(MSX), maintain a required accuracy with respect to UTC(USNO) of +/- 10 ms with a +/- 1 ms desired accuracy. APL's atomic time standards and the downlinked spacecraft time were used to develop a Time Maintenance system that will estimate the current MSX clock time offset during an APL pass and make estimates of the clock's drift and aging using the offset estimates from many passes. Using this information, the clocks accuracy will be maintained by uplinking periodic clock correction commands. The resulking Time Maintenance system is a combination of Offset Measurement, Command/Telemetry, and Mission Planning hardware and computing assets. AU assets provide necessary inputs for deciding when corrections to the MSX spacecraft clock must be made to maintain its required accuracy without inhibiting other mission objectives. This paper describes the MSX Time Maintenance system as a whole and details the clock Offset Measurement subsystem, a unique combination of precision time maintenance andmeasurement hardware controlled by a Macintosh computer. Simulations show that the system estimate the MSX clock offset to less than +/- 33 ps.
Published in:	Proceedings of the 25th Annual Precise Time and Time Interval Systems and Applications Meeting November 29 - 2, 1993 Ritz-Carlton Hotel Marina Del Rey, California
Pages:	433 - 444
Cite this article:	Hermes, Martin J., "Time Maintenance System for the BMDO MSX Spacecraft," Proceedings of the 25th Annual Precise Time and Time Interval Systems and Applications Meeting, Marina Del Rey, California, November 1993, pp. 433-444.
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