LINKING THE HIPPARCOS CATALOG TO THE EXTRAGALACTIC REFERENCE SYSTEM

We present an overview of the methods currently planned to link the Hipparcos reference system to the conventional extragalactic reference system, as defined by VLBI observations. The link programme consists of VLBI and connected element interferometry of radio stars, separation measurements with the Hubble Space Telescope, photographic observations of optical counterparts of radio sources, proper motion surveys with respect to background galaxies, and a comparison of Earth Orientation Parameters as determined by VLBI and optical means. The expected accuracy of the final link is on the level of 0.5 mas (milliarcsec) for the positional system and 0.5 mas yr(-1) for the proper motion system. The final Hipparcos Catalogue will be obtained by a rotational transformation from a provisional Hipparcos system to the extragalactic system. We derive the rigorous mathematical relations between the systems and the linearised observation equations from which the link parameters can be determined. The synthesis of the link programmes will require careful discussion of possible biases and the relative weights of the different categories of data.