The Hipparcos Catalogue as a realisation of the extragalactic reference system

International audienceThe paper describes the methods and observations by which the Hipparcos Catalogue was linked to the International Celestial Reference System (ICRS). The contributions of several groups represented in the authorship 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 extragalac-tic sources. The following link techniques were used: interfero-metric observations of radio stars by VLBI networks, MERLIN and VLA; observations of quasars relative to Hipparcos stars by means of CCDs and photographic plates, and by the Hubble Space Telescope; photographic programmes to determine stellar proper motions with respect to extragalactic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of Earth orientation parameters obtained by VLBI and by ground-based optical observations of Hipparcos stars. Although vastly different in terms of instruments, observational methods and objects involved, the various techniques generally agree to within 10 mas (milliarcsec) in the orientation and 1 mas/yr in the spin of the system. Two different numerical methods are described for the systematic comparison and synthesis of the link observations. The methods give very similar solutions, and a mean value was adopted for the definition of the system of positions 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 the epoch 1991.25, and non-rotating with respect to distant extragalactic objects to within ±0.25 mas/yr