The so-called ``Absolute Measurement Accuracy'' (AMA), i.e., the error on the XMM boresight, after application of all corrections on-ground, is anticipated to be ca. 3''-4'' (on average). The absolute pointing error (APE), i.e., the offset between actual and required pointing direction, is expected to be about 15'' (95% radius). The relative pointing error (RPE) during an exposure, after correction for spacecraft drift (leaving mostly spacecraft jitter), will be about 2'' (rms; likely better).
Independent pointing information will come from the Attitude and Orbital Control System (AOCS) and the OM, when operating. The OM information is expected to provide about the same AMA as the AOCS data. Thus, if OM data are available, these can be used to cross-check the AOCS attitude solution. The necessary functionality to analyse OM data will be contained in the SAS (§ 6.1).