Indirect linking of the Hipparcos catalog to extragalactic reference frame via Earth orientation parameters

Abstract

The indirect method of linking the Hipparcos reference frame to the frame defined by extragalactic sources is described. To this end, two independent time series of Earth orientation parameters observed by two different techniques with respect to the two reference frames are used: a) Optical astrometry observations (referred to Hipparcos stars), b) VLBI observations (referred to extragalactic objects). The parallel use of both techniques during the last decade enables to determine the orientation of the two reference frames at a fixed epoch and their mutual slow rotation with precision of at least 1mas and 1mas/year, respectively. In order not to raise confusion, the potentiality of the method is demonstrated on the example based on the star catalogues originally used at the participating observatories, not on any of the existing preliminary versions of the Hipparcos catalog.