September 27, 2013 - CPF update for Offset Following Safehold

Precision attitude control was recovered and imaging operations resumed on day of year 264 (September 21, 2013). Post-anomaly calibration checks detected a systematic change in the alignment between the OLI and TIRS sensors. No change in the OLI alignment to the spacecraft attitude control system was apparent so the shift is being attributed to the TIRS instrument.

The magnitude of the change – a pitch axis offset of approximately 30 microradians (about 0.2 TIRS pixels) – is large enough to threaten the thermal-to-reflective band registration requirements. Band registration tests confirm that the along-track TIRS-to-OLI band registration performance has been degraded since the safe hold event.

A calibration parameter file (CPF) update to the ACS_to_TIRS_Rotation_Matrix parameter was developed to correct this offset and was put into production on September 27, 2013. This update is effective from September 21, 2013, so a subdivision of the 3rd quarter CPF effective date range will be required.  The new parameters will also be used for the upcoming 4th quarter CPF unless the ongoing monitoring of TIRS-to-OLI alignment indicates that further changes are require

No direct cause for the alignment change is known but it may be related to the change in the spacecraft thermal environment during the safe hold period. If this is the case, it is possible that, over time, the alignment will drift back closer to its earlier position.