SGPS SPACECRAFT SYSTEM CLOCK SYNCHRONISATION

A GPS receiver onboard a satellite can provide precise time for all of the satellite subsystems and can can provide precise time interval measurement, all in an autonomous mode.

Benefits of GPS:

Inexpensive and simple precise spacecraft timing.

Methodology:

At the time of GPS signal acquisition by a GPS receiver, the crystal oscillator (usually a TXCO-based receiver clock) is synchronised with GPS time by means of pseudorange measurements and the GPS navigation message. A spacecraft bus architecture can be used to supply GPS-derived time to synchronise all spacecraft subsystem clocks. In this manner, daily clock updates from ground stations will not be required and precise time-tagging of downloaded satellite data will be possible. Once receiving GPS signals, a GPS receiver is capable of a conservative GPS time clock synchronisation accuracy of approximately 300 nanoseconds with SA engaged. This accuracy is quite sufficient for many satellite missions.

GPS time transfer to a local clock:

Sychronisation is usually accomplished through the use of a feedback loop and an appropriate filter. A computer tracks the difference between GPS time (or UTC) and the local clock, and uses these data to steer the local clock by means of e.g., a phase microstepper.

Mission examples:

- The GADFLY mission to be launched on the SSTI-Lewis spacecraft requires a precise timing reference of 1 millisecond.

- The planned Gravity Probe B mission will have a clock sychronisation with GPS time in the order of 100 nanoseconds.

Other Internet information:

- The United States Naval Observatory (USNO) GPS Time Transfer site contains information about GPS time, time transfer data, and time transfer techniques.

References:

Due to the rapid developments in the field of spaceborne GPS and the aerospace industry in general, any comments, information or corrections pertaining to information on this site are welcome and encouraged.