THE HIPPARCOS-CATALOG AS A REALIZATION OF THE EXTRAGALACTIC REFERENCESYSTEM

The paper describes the methods and observations by which the Hipparco s Catalogue was linked to the International Celestial Reference System (ICRS). The contributions of several groups represented in the author ship of this paper, using a variety of techniques, were synthesised in order to determine the global orientation and rotation (spin) of the coordinate frame defined by the Hipparcos data with respect to extraga lactic sources. The following link techniques were used: interferometr ic observations of radio stars by VLBI networks, MERLIN and VLA; obser vations of quasars relative to Hipparcos stars by means of CCDs and ph otographic plates. and by the Hubble Space Telescope photographic prog rammes to determine stellar proper motions with respect to extragalact ic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of Earth orientation parameters obtained by VLBI and by ground-based o ptical observations of Hipparcos stars. Although vastly different in t erms of instruments, observational methods and objects involved, the v arious techniques generally agree to within 10 mas (milliarcsec) in th e orientation and 1 mas/yr in the spin of the system. Two different nu merical methods are described for the systematic comparison and synthe sis of the link observations. The methods give very similar solutions, and a mean value was adopted for the definition of the system of posi tions and proper motions in the Hipparcos Catalogue. As a result, the coordinate axes defined by the published catalogue are believed to be aligned with the extragalactic radio frame to within +/- 0.6 mas at th e epoch 1991.25, and non-rotating with respect to distant extragalacti c objects to within +/- 0.25 mas/yr.