Two methods are presented which can be applied to determine the rotation of the Hipparcos system of stars with respect to the FK5. These methods are presently tested in numerical simulation runs.

VLBI observations of bright radio stars have been initiated in an attempt to measure the positions and proper motions of their radio components in order to tie the future HIPPARCOS stellar frame to a VLBI extragalactic reference frame. Through VLBI observations of a sample of 20 known radio stars we have identified 11 stars that should be appropriate for both astrometric VLBI and HIPPARCOS observations. Our measurements indicate that the angular extent of their radio emitting regions is small, i.e. < 3 milliarcseconds for 7 of them. Most of these radio stars belong to the RS Canum Venaticorum class of binary systems.

A series of VLBI observations of planet-orbiting spacecraft and angularly nearby quasars has provided an estimate of the relative orientation of the JPL VLBI reference frame and the ephemeris frame of the inner planets. We find the two frames to be coincident in both right ascension and declination to less than 20 milliarcseconds (mas).

The automated astronomical system “Astrometrist” for the determination of the parameters of terrestrial rotation (DTRP) is intended for astrometric and meteorological data collection and on-line processing and feedback to the user by a computer.