Optical-radio reference ties

Kovalevsky J.

In the course of the data reduction of the Hipparcos observations, the actual system of coordinates happens to be the result of complicated and partly random processes. It is therefore necessary to link it to a fixed - namely extragalactic - reference system ICRS and rotate the result in such a way that the catalogue be a realization of the ICRS.

Eleven different groups, using various methods, have provided a solution either for the fixed rotation vector, or the time- dependent rotation, or both. These methods will be described and the results discussed.

The relative weighting of these results and the final deter- mination of the best solution were performed by two different methods in CERGA and in Lund Observatory. They gave very similar results. The standard uncertainties achieved are +/- 0.6 mas in the rotation vector at epoch and +/- 0.25 mas per year in the time-dependent rotation.