THE INTERNATIONAL-CELESTIAL-REFERENCE-FRAME AS REALIZED BY VERY LONG-BASE-LINE INTERFEROMETRY

A quasi-inertial reference frame is defined based on the radio positio ns of 212 extragalactic sources distributed over the entire sky. The p ositional accuracy of these sources is better than about 1 mas in both coordinates. The radio positions are based upon a general solution fo r all applicable dual-frequency 2.3 and 8.4 GHz Mark III very long bas eline interferometry data available through the middle of 1995, consis ting of 1.6 million pairs of group delay and phase delay rate observat ions. Positions and details are also given for an additional 396 objec ts that either need further observation or are currently unsuitable fo r the definition of a high-accuracy reference frame. The final orienta tion of the frame axes has been obtained by a rotation of the position s into the system of the International Celestial Reference System and is consistent with the FK5 J2000.0 optical system, within the limits o f the link accuracy. The resulting International Celestial Reference F rame has been adopted by the International Astronomical Union as the f undamental celestial reference frame, replacing the FK5 optical frame as of 1998 January 1.