The AOCS determines the attitude of the XMM spacecraft while in orbit, based on the information from one of XMM's two star trackers (which are operated in cold redundancy) and its ``Fine Sun Sensors''. During slews and the post-slew phase (comprising attitude determination, trim, and settling of the spacecraft), entries are made into the Attitude History File (AHF) every 10 (TBC) seconds. Note that during slews the AHF will not contain attitudes reconstructed from actual AOCS telemetry, but the results of a slew time/path predictor, based on the actually observed slew start/end times and attitudes. The accuracy of the attitude reconstruction during slews is expected to be better than 1'.
In the ``stable pointing mode'' (i.e., after the slews and the post-slew phase), the conditions under which entries into the AHF are made are programmable parameters. An entry is made into the AHF only in case of Relative Pointing Errors (RPEs) exceeding the programmed limit. The minimal programmable limit (i.e., the smallest programmable deviation from the nominal boresight) is 2''. The minimal time resolution with which entries into the AHF can be made in case of such instantaneous excursions is 2 seconds. For a single nominal pointing entry, only a mean RPE will be provided.
The AOCS attitude information is independent of that from the OM's star tracking windows (§ 3.5.3).