HIPPARCOS GREAT-CIRCLE REDUCTION - THEORY, RESULTS AND INTERCOMPARISONS

In this paper some of the results of the great-circle reduction in the two data reduction consortia (NDAC and FAST) are presented. It is shown that the root-mean-square (rms) error of the great-circle abscissa of stars brighter than 10 mag is between 3-5 milli-arcsec. This is as was expected before launch, even in spite of the fact that the data-sets are shorter in the revised mission. There is a large positive correlation between the abscissa of stars separated by a multiple of the basic angle. Moreover, the great-circle results are affected by systematic errors at the beginning of the reductions. This is confirmed by intercomparisons between the two consortia. Finally, it is shown that the along-scan attitude of the Hipparcos satellite can be described with a few hundred parameters with milli-arcsec accuracy.