Attitude reconstruction for the Gaia spacecraft

Context. The Gaia mission will produce a stereoscopic map of the Milky Way by collecting highly accurate positions, parallaxes and proper motions for about 1 billion stars. These astrometric parameters will be determined through the astrometric core solution of the Gaia mission which will employ about 10$^{8}$ primary sources (a subset of the observed sources with the best astrometric properties). The attitude of the spacecraft is reconstructed as part of the astrometric solution and provides the reference frame relative to which the astrometric measurements are obtained. This implies extreme demands on the accuracy of the attitude reconstruction. Aims: This paper presents an analysis of the capabilities and limitations of the Gaia attitude reconstruction, focusing on the effects on the astrometry of bright (V {lsim} 11) stars and the implications of employing cubic B-splines in the modelling of the attitude measurements. Methods: We simulate the attitude of the spacecraft using a realistic and very detailed model that considers not only physical effects but also technical aspects like the control system and thruster noise. We include the effect of shorter integration times for the bright stars on the effective attitude and we estimate the residual modelling noise in the reconstruction of the attitude. Results: We provide an analysis of the dependency of the residual modelling noise in the reconstructed attitude with respect to the following parameters: integration time, B-spline knot interval, micro-propulsion system noise, and number of observations per second. Conclusions: The final noise in the attitude reconstruction for Gaia is estimated to be {ap}20 {$μ$}as, and the main source will be the micro-propulsion system. However its effect on the astrometric performance will be limited, adding up to 7 {$μ$}as rms to the parallax uncertainties. This is larger than the 4 {$μ$}as from previous estimations and would affect the performance for the brightest (V {lsim} 11) stars.