4.5.4 Attitude verification

Author(s): David Hobbs

Attitude telemetry from Gaia arrives daily over the mission lifetime. The main processing of Gaia data proceeds over six month cycles. In each cycle new data in blocks of roughly one day must be added to the previous data. At first sight this may look straightforward, however, complications arise as there will be overlaps and gaps between existing blocks of attitude data. Additionally, care may have to be taken when attitude data with different accuracies is combined.

The attitude processing consists of a number of different attitudes with increasing degrees of accuracy as discussed in Section 3.4.5.

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  The commanded (requested) attitude.

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  The on-board star tracker measured raw attitude contained in telemetry.

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  IOGA – The Initial on-ground attitude which is a spline fit to the available raw attitude.

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  OGA1 – A Kalman filter designed to smooth the attitude (IOGA) and to improve its accuracy to the order of 50 milli-arcseconds (mas) (see Section 3.4.5).

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  OGA2 – A First Look (FL) process does a direct astrometric solution on a single days worth of data resulting in B-splines and quaternions, ODAS (see Section 3.4.5).

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  OGA3 – AGIS is the final step in the attitude improvement where all the available observations for primary stars are used together with the available attitude and calibration parameters to iteratively arrive at the final attitude solution with a targeted accuracy of $\sim$10 of $\mu$as (see Section 4.3.5).

Generally, OGA1 and OGA2 data arrive in batches of 1 day but may contain data gaps and may or may not overlap with existing data. Initially, it was proposed to use OGA1 (supplemented by IOGA or OGA2 if OGA1 was not available) as the starting values for the AGIS attitude improvement. However, after some experimentation and due to the patched nature of OGA1-IOGA-OGA2 it was decided to start AGIS from the crude commanded attitude. The convergence of AGIS proved to be very good provided data gaps (see Section 4.3.5) were handled correctly and only a small number of iterations were needed to improve the solution to be better than OGA1.

This also made the verification of the AGIS attitude very simple by comparing the AGIS attitude with commanded attitude in angles and rates about the SRS axes. Checking for range out of bounds (extreme values) and checking for attitude spikes (manual inspection). RSE values of the differences between the starting values and the current values are plotted in the AGIS monitor and the updates in the attitude are also plotted. Sampling of about 1/5 of a knot interval is needed. Additionally, manual checks of the attitude around dead times and manual checks on extra knots could be performed.