POSITIONS AND PARALLAXES FROM THE HIPPARCOS SATELLITE - A 1ST ATTEMPT AT A GLOBAL ASTROMETRIC SOLUTION

The last stage of the basic astrometric reduction of Hipparcos data is to combine angular measurements along great-circle scans into a globally coherent system of positions, proper motions and trigonometric parallaxes. We report the results of a first attempt at such a 'sphere solution' of positions and parallaxes using great-circle results calculated from a selection of the data collected between November 1989 and February 1991. The data, which were made available to the reduction consortia for test and validation purposes, constitute less than a fifth of the total data collected during that period. The solution included some 20000 stars, and for at least 10000 stars we have determined the positions at mid-epoch with an accuracy of about 5 milli-arcsec and the trigonometric parallaxes with an average external standard error of 7 milli-arcsec. The positions reveal systematic errors in the Hipparcos Input Catalogue of up to 0.2 arcsec, some of which can probably be traced back to zonal errors in the FK4 and/or FK5 system.