LINKING THE HIPPARCOS CATALOG TO THE EXTRAGALACTIC REFERENCE SYSTEM

We present an overview of the methods currently planned to link the Hi pparcos reference system to the conventional extragalactic reference s ystem, 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 observation s 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 accu racy of the final link is on the level of 0.5 mas (milliarcsec) for th e positional system and 0.5 mas yr(-1) for the proper motion system. T he final Hipparcos Catalogue will be obtained by a rotational transfor mation from a provisional Hipparcos system to the extragalactic system . We derive the rigorous mathematical relations between the systems an d the linearised observation equations from which the link parameters can be determined. The synthesis of the link programmes will require c areful discussion of possible biases and the relative weights of the d ifferent categories of data.