PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
  PUBLICATION_DATE    = 2003-04-01
  NOTE                = "Description of GRS clock drift"
END_OBJECT            = TEXT
END

General Clock Drift Description


We use a command called TIME_PACKET_CREATE to generate clock
correlations.  We used to send the command a couple times a day, but
now it is done once or twice a day, autonomously in the map_comm
block.  The clock correlation commands are handled in a special way by
the spacecraft and the clock correlation telemetry comes down
immediately and is marked on the ground with the received time.  The
clock drift is primarily due to small thermal variations at the
processor's clock crystal.

On the ground, the day is analyzed every week or so with an algorithm
that uses actual bit rate, One Way Light Time (OWLT), and a fixed
system delay.  The resulting error in time is tracked and when it
exceeds a certain threshold, a new SCLK_SCET file is generated.
Looking back on data since mapping started, it looks like we have
never had more than 20 milliseconds or error and most of the time we
are well below 10 milliseconds.