The IERS extragalactic Celestial Reference Frame and its tie with HIPPARCOS.

Abstract

This Technical Note gives a presentation of analyses of VLBI extragalactic reference frames performed in the framework of IERS, in the course of the maintenance of its conventional celestial frame. In section 1 the definition of the IERS celestial reference system and its relationship with the 1991 IAU Resolution A4 on Reference Systems are described. Section 2 is devoted to the comparison of celestial frames obtained from independent observation programs by GSFC, JPL, NGS and USNO. Section 3 gives an estimation of the expected accuracy of the link of the HIPPARCOS frame to an extragalactic frame and its time evolution based on VLBI observations of radio stars.