Attitude determination in the Hipparcos revised mission

An accurate knowledge of the attitude as determined by the observation of star transits through the star mapper is an essential factor in the final accuracy of the Hipparcos reductions. In comparison with the nominal mission, several features tend to degrade the precision of the attitude determination: an irregular increase of the background noise, an increased number of data recovery interruptions, and large domain of variation of the gravity gradient torque. It is shown in this paper how these difficulties have been overcome. A precision assessment of the attitude determination is given for the first treatment showing that the effects of the remaining errors (30 to 100 milli-arcsec rms) on the reduction is within acceptable limits. In addition, the software permits an improvement of the along-scan positions of the observed stars. The mean precision of the input Catalogue can be assessed from the results: 0.25 arcsec rms.