Accession Number : ADP009128

Title :   Ground Control System for the Midcourse Space Experiment UTC Clock,

Corporate Author : JOHNS HOPKINS UNIV LAUREL MD APPLIED PHYSICS LAB

Personal Author(s) : Dragonette, Richard

Report Date : 02 DEC 1993

Pagination or Media Count : 6

Abstract : One goat of the Midcourse Space Experiment (MSX) spacecraft Operations Planning Center is to maintain the onboard satellite UTC clock UTC(MSX) to within 1 millisecond of UTC(APL) (the program requirement is 10 msec). The UTC(MSX) clock employs as its timebase an APL built 5 MHz quartz oscillator, which is expected to have frequency instabilities (aging rate + drift rate + frequency off-set) that will cause the clock to drift approximately two to ten milliseconds per day. The UTC(MSX) clock can be advanced or retarded by the APL MSX satellite ground control center by integer multiples of 1 millisecond. The MSX Operations Planning Center is developing software which records the drift of UTC(MSX) relative to UTC(APL) and which schedules the time of day and magnitude of UTC(MSX) clock updates up to 48 hours in advance. Because of the manner in which MSX spacecraft activities are scheduled, MSX clock updates are planned 24 to 48 hours in advance, and stored in the satellite's computer controller for later execution. Data will be collected on the drift of UTC(MSX) relative to UTC(APL) over a three to five day period. Approximately 6 times per day the time offset between UTC(MSX) and UTC(APL) will be measured by APL with a resolution of less than 100 microseconds. From this data a second order analytical model of the clock's drift will be derived. This model will be used to extrapolate the offset of the MSX clock in time from the present to 48 hours in the future. MSX clock updates will be placed on the spacecrafts daily schedule whenever the predicted clock offset exceeds 0.5 milliseconds. The paper includes a discussion of how the empirical model of the MSX clock is derived from satellite telemetry data, as well as the algorithm used to schedule MSX clock updates based on the model.

Descriptors :   *CLOCKS, *CALIBRATION, *SPACECRAFT COMPONENTS, ALGORITHMS, ARTIFICIAL SATELLITES, CONTROL CENTERS, DRIFT, MODELS, ONBOARD, OPERATION, OSCILLATORS, PLANNING, RATES, REQUIREMENTS, TIME, FREQUENCY STANDARDS, HIGH RESOLUTION.

Subject Categories : Test Facilities, Equipment and Methods
      Unmanned Spacecraft

Distribution Statement : APPROVED FOR PUBLIC RELEASE