ESA GNC Conference Papers Repository

This paper details subtle attitude disturbances of the Gaia Spacecraft introduced by star tracker measurement inaccuracies, which have surfaced due to Gaia's unprecedented dynamic stability. Tasked with mapping 1 billion stars to unprecedented precision (to the micro-arc-second level, comparable to the width of a coin on the Moon as viewed from Earth), ESA's Science cornerstone mission is expected to also discover and chart 100,000's of new objects including near Earth Asteroids (NEA), exoplanets, brown dwarfs and Quasars. After a flawless launch on 19 Dec 2013, Gaia was brought the circa 1.5 million km into L2 via a sequence of technically demanding orbit transfer manoeuvres using on-board thrusters in thrust vectoring mode. Starting in parallel to this and lasting 6 months, the spacecraft was fully commissioned and brought gradually up to the highest operational mode. The commissioning phase involved detailed checks of the relatively high number of bespoke units, including the 106 CDD focal plane assembly, telescope-in-the-loop AOCS control and micro-propulsion. The unprecedented rate stability (the rate error on Gaia is equivalent to one rotation every 420 years) occasionally displays tiny, temporary, divergences from the reference scan law, which have been observed to occur when particular stars were inside the field of view of the star tracker whose catalogue (note the star tracker catalogue is based on a subset of sources from the Hipparcos catalogue). Further investigation showed that neighbouring stars not included in the Hipparcos catalogue were consistently shifting the calculated centroid of the problematic stars. This paper focuses on the characterisation and mitigation of these star tracker induced attitude errors.