Modelling the Hipparcos Attitude

A detailed study of the Hipparcos attitude exposed events in the satellite’s motion (micro-meteorite hits and clanks) that require special adaptations in the attitude reconstruction modelling. Identifying these events in the data stream, and incorporating them in a specially designed dynamical attitude modelling, has led to an overall reduction in the attitude reconstruction noise from 3 mas down to 0.6 mas. Considering that the main noise contribution on the astrometric data for stars brighter than magnitude 8 in the Hipparcos catalogue originated from the attitude reconstruction, and that this noise led to correlated parallax errors for neighbouring stars, it was decided to redo the astrometric reductions of the Hipparcos data. This was completed in 2007, and resulted in the publication of a new catalogue. The lesson to be learnt for Gaia is, that incorporating a detailed understanding of the satellite’s motions in the attitude modelling can have a significant impact on the overall reliability of the final astrometric data produced by the mission