timecorrelation

Overview ¶

Example: Putting a real timestamp on telemetry

A spacecraft does not know what time it is. It counts ticks of its own oscillator, and that count drifts against UTC. Telemetry is stamped with the count, so before any of it can be compared with anything on the ground, the count has to be turned into a time.

That job is time correlation, and it is two separate problems:

Formats:
  1. Write an instant as a CCSDS time code and read it back
  2. Choose between CUC, CDS, CCS and ASCII, and see what each costs

Correlation:
  1. Collect pairs of on-board count and known ground time
  2. Fit an offset and a rate from them
  3. Apply the fit to every other timestamp in the pass

The second part is where the errors live. A perfectly decoded time code still reads the wrong instant if the clock model is wrong.