Fiducial reference for the Hipparcos reference system

Means of determining the Hipparcos satellite instrumental system rotation and for tying Hipparcos system coordinates to a fundamental coordinate system are discussed. Observational characteristics requirements for radio sources and stars, for a Hipparcos-Space Telescope reference tie are given. Rotation of the Hipparcos system with respect to the extragalactic system can be derived from observations of the relative radial positions and motions of selected Hipparcos stars and quasi-stellar objects.