DYNAMIC MODELLING OF THE HIPPARCOS ATTITUDE FLOOR VAN LEEUWEN 1 and ELENA FANTINO 2 1 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U.K. (e-mail: fvl@ast.cam.ac.uk) 2 Institute of Astronomy, Madingley Road, Cambridge, U.K. and On leave from: CISAS “G.Colombo”, Università di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy Abstract. We present a new method for a high-accuracy reconstruction of the attitude for a slowly spinning satellite. This method, referred to as the fully-dynamic approach, explores the possibility to describe the satellite’s attitude as that of a rigid body subject to continuous external torques. The method is tried out on the Hipparcos data and is shown to reduce the noise for the along-scan attitude reconstruction for that mission by about a factor two to three. The dynamic modelling is expected to give a more accurate representation of the satellite’s attitude than was obtained with a pure mathematical modelling. As such, it decreases the degrees of freedom in the a posteriori recon- struction. Some of the decrease is obtained through accumulating and subsequently implementing information on high frequency components in the solar radiation torques, which show to be system- atic and predictable. This could be expected, as they are primarily linked to the external geometry and optical properties of the satellite. In the context of an astrometric mission, the methods presented here can only be applied as a final iteration step: the star positions that are used to reconstruct the attitude are also part of the scientific objectives of the mission. An estimate for the potential of a re-reduction of the Hipparcos data using the fully-dynamic model for the attitude reconstruction was obtained from test reductions of the first 24 months of mission data. Improvement of the accuracies of the astrometric parameters for all stars brighter than Hp = 9.0 appears possible. The noise on the astrometric parameters for these stars was affected significantly by the along-scan attitude noise, which dominated for stars brighter than Hp = 4.5. The possible improvement for stars brighter than about Hp = 4.5 may, after iterations, be as much as a factor three. The reduced noise levels also allow a more accurate calibration and monitoring of instrument parameters, leading potentially to a better understanding of the instrument and the scientific data obtained with it. Key words: astrometry, data analysis, Hipparcos, instruments Abbreviations: CSS – Centro di Studi sui Sistemi; FAST – Fundamental Astronomy by Space Techniques; FDA – Fully-Dynamic Approach; FFOV –Following Field of View; GCR – Great- Circle Reduction; IDT – Image Dissector Tube; mas – milli arcsecond; NDAC – Northern Data Analysis Consortium; PFOV – Preceding Field of View; RGC – Reference Great Circle; RGO – Royal Greenwich Observatory; RTAD – Real Time Attitude Determination; SM – Star Mapper; TB – Tait-Brian; TFI – Thruster-Firing Interval. Nomenclature: KAP – Kluwer Academic Publishers; compuscript – Electronically submitted article Space Science Reviews 108: 537–576, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.