The ICRF1
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Note that the ICRF1 was called ICRF before its second realization in 2009.

The ICRS is materialized by precise equatorial coordinates of extragalactic radio sources observed in Very Long Baseline Interferometry (VLBI) programmes. This first realization of the ICRF, although officially adopted in 1998, was elaborated by the IAU Working Group on Reference Frame (WGRF) during 1995. The IERS, jointly with the IAU Working Group on the Reference System and the IVS are in charge of the maintenance of the conventional system and frame and of its ties to other frames. A complete description of the ICRF, as well as the procedures used for its elaboration, are given in the IERS Technical Note 23 (1997) and in Ma et al. (1998).

The ICRF1 contains J2000.0 VLBI coordinates of 608 extragalactic radio sources evenly distributed on the sky. The radio positions are based upon a general solution for all applicable dual-frequency Mark III VLBI data available through the middle of 1995. Because of different observing histories and astrometric suitability, the source positions estimated from the VLBI data analysis are of varying quality. Thus the objects in ICRF1 are classified in three categories: defining, candidate and other sources.

Defining sources : 212 high-astrometric-quality objects, as far as the period of observations analyzed to build the frame is concerned. They define the ICRF axes.

Candidate sources : 294 sources, some have insufficient observations or duration of observation to be defining, while others with many observations may have larger than expected differences in position between catalog. They are likely to climb up to the former category in the future.

Other sources : 102 sources with identified excessive position variation, either linear or random. They are included for the sake of densifying the frame or because they have contributed to the optical frame ties.