Attitude reconstruction for theGaiaspacecraft

Abstract

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 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 ≈20 μas, and the main source will be the micro-propulsion system. However its effect on the astrometric performance will be limited, adding up to 7 μas rms to the parallax uncertainties. This is larger than the 4 μas from previous estimations and would affect the performance for the brightest (V 11) stars.

GaiaData Release 1

“…5a) required the use of a longer knot interval (30 s) for the attitude model than foreseen in the final astrometric solution (<10 s; see the AGIS paper, Sect. 7.2.3, and Risquez et al 2013). Residual modelling errors contribute significant correlated noise in the present solution.…”

“…However, since the physical attitude angle is not directly observable, but only a moving average over the CCD integration time (the socalled effective attitude; Risquez et al 2013), the effect as seen in the astrometric residuals is not instantaneous but linear over the CCD integration time, which is 4.42 s for ungated observations, and shorter for gated observations. The top panels of Fig.…”

ASTROMETRIC EXOPLANET DETECTION WITHGAIA

“…Both are assumed constant over the field crossing; we adopt σ att = 20 μas (Risquez et al 2013), and similarly for σ cal . Evidently, both are provisional pending results from the global iterative solution.…”

Dynamical attitude model for Gaia

“…The expected accuracy for the reconstruction of the along scan attitude is of the order of 20 μ as [32,42]. This should in principle be sufficient to reach the best expected parallax error at the end of the mission (≈ 7 μas), as each star is observed several times: 9 times per field of view transit (because there are 9 consecutive CCDs in the focal plane), with ≈ 70 transits during the whole Gaia lifetime.…”