Index of /hsa/legacy/operations/moc/dft

	Name	Last modified	Size	Description
	Parent Directory		-
	data/	2016-11-09 08:28	-

Uplink Drift Files

This directory contains the uplink drift (DFT) files provided by MOC.

Commands that are uplinked to the spacecraft must be converted from UTC to On-Board Time (OBT), taking into account the drift of the onboard clock. The drift is a predicted value as commands are uplinked for execution at a time in the future.

The drift correction applied is always an integer number of seconds. This ensures that a sequence of commands are executed at the correct relative times, although observations as a whole may be displaced by up to a second in the worst case.

The correction value is incremented/decremented by one second when the accumulated drift has reached approximately one second (about every 16 days). This step jump is applied during the DTCP at a time when no observations are being executed, so that it has no impact on the science observations.

Details of the drift corrections applied was sent by MOC in the form of a Drift File (DFT). This listed the time at which each step occurred and the resulting integer number of seconds offset. The values decreased with each delivery because the onboard clock was running too slow by about 60 ms per day. An updated file was delivered each time a step was applied. The final delivery was version 66. Since this contained all previous steps, the earlier files are not included here.

Format/ICD

The file contains the following columns:

• UTC time at which step was applied
• The new value of the correction in seconds
• The version number of the file delivered to SOC (e.g. 66)

Known Issues

• An incorrect offset was used for approximately 50 minutes on 2010-09-12. This can be seen in the drift file.

• A leap second occurred during the mission on 2012-07-01. This resulted in a one-second jump in the drift. The drift correction was not updated until the next DTCP (OD 1145), just before the first observation, approximately 16 hours later.

• The on-board clock was set to TAI minus 34 seconds at launch (the number of leap-seconds at that time). This results in a further offset of -34 seconds throughout the mission which is not included in the drift file. This offset has a fixed value of -34 seconds, which does not change with the introduction of a leap second, because the clock is free-running.