An attitude model for the spacecraft of the ESA mission Gaia

The coming European astrometry mission Gaia will set new standards for the accuracy of position and velocity of up to a billion objects in the universe. This implies an extensive effort in the processing of the measurement data at the highest possible level of physical fidelity. Studies related to the precursor Hipparcos have shown that the fully-dynamical modelling of the spacecraft's attitude leads to significant improvements in the understanding of the spacecraft's real behaviour in space and eventually in the high quality of the scientific results. As a consequence, the principle goal for Gaia is to get a thorough knowledge of the characteristics of the dynamic behaviour of the spacecraft and related parameters. By establishing a software for attitude and disturbance torque modelling, this knowledge will incorporate in the modelling of the attitude—to be ready well in advance of the launch.The Gaia ‘Dynamical Attitude Model’ (DAM) focuses on the development of a simulation software environment based on physical principles and effects, complemented by information from onboard software and hardware components to control the spacecraft's motion in space. DAM contains accurate models for all essential parts of a spacecraft simulator to gain sufficient knowledge of the Gaia attitude characteristics in order to aim for a level of knowledge that would ultimately lead to an improvement in the attitude modelling and thus in the science results. This paper presents the major parts of the DAM and shows the steps in the development of a high-precision tool for simulating Gaia's attitude.

► The future Esa mission Gaia will chart a 3D map of the Milky Way with an unprecedented accuracy in astrometric parameters.
► The determination and best possible control of the spacecraft's state, e.g. its attitude and rate, is a key element for the success of the mission.
► Simulations on the state help to identify and quantify peculiarities in the behaviour of the spacecraft.
► This paper gives an overview of the GAM software: a simulation tool to study the attitude and rate of the Gaia spacecraft under the influence of disturbances caused by external and internal sources.