Realization of the International Celestial Reference Frame from VLBI Astrometry

The International Celestial Reference Frame (ICRF) has now been realized with compact extragalactic objects whose radio-frequency positions are measured at the submilliarcsecond level by VLBI (Very Long Baseline Interferometry). The ICRF replaces the FK5 as the defining realization of the celestial reference system as of 1 Jan 1998 according the resolution adopted by the IAU General Assembly in Kyoto. The ICRF catalog includes 212 defining sources, 294 candidate sources, and 102 other sources, the sum uniformly populating the celestial sphere. The majority are quasars with compact cores and many have optical counterparts. Because of the nature of the VLBI observations (precise angular measurements over a large portion of the sky in a 24-hr interval) and the absence of measurable real transverse motion of the objects, the ICRF can be constructed rigidly and with great precision using data of considerable temporal and geographic extent. The positions and uncertainties, typically <0.5 mas, represent the distillation of 1.6 million dual-frequency Mark III VLBI delay and delay rate observations acquired between August 1979 and July 1995 from various geodetic and astrometric observing programs using radio observatories on every continent. Following IAU resolutions, individual ICRF positions will evolve as more observations and improved models become available, but the orientation (axes) of the ICRF will be maintained by a statistical no-net-rotation condition between the catalog positions of successive realizations. Celestial positions are no longer referred to a system associated with the equator or the ecliptic. Further information about the ICRF can be found in Technical Note 23 of the International Earth Rotation Service.