Accession Number : ADP009126

Title :   Time Maintenance System for the BMDO MSX Spacecraft,

Corporate Author : JOHNS HOPKINS UNIV LAUREL MD APPLIED PHYSICS LAB

Personal Author(s) : Hermes, Martin J.

Report Date : 02 DEC 1993

Pagination or Media Count : 11

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 + or - 10 ms with a + or - 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 resulting Time Maintenance system is a combination of Offset Measurement, Command/Telemetry, and Mission Planning hardware and computing assets. All 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 and measurement hardware controlled by a Macintosh computer. Simulations show that the system estimate the MSX clock offset to less than + or - 33 ps.

Descriptors :   *CLOCKS, *CALIBRATION, *MILITARY SATELLITES, *ANTIMISSILE DEFENSE SYSTEMS, ACCURACY, COMPUTERS, CONTROL, CORRECTIONS, DRIFT, ESTIMATES, GUIDED MISSILES, INPUT, MAINTENANCE, MEASUREMENT, PLANNING, PRECISION, SIMULATION, TIME STANDARDS.

Subject Categories : Unmanned Spacecraft
      Antimissile Defense Systems

Distribution Statement : APPROVED FOR PUBLIC RELEASE