The International Celestial Reference System: ICRS


At its 23rd General Assembly in August 1997, the International Astronomical Union (IAU) decided that, as from 1 January 1998, the IAU celestial reference system shall be the International Celestial Reference System (ICRS), in replacement of the FK5 (Fricke et al. 1988). The consequences of this new situation for accuracy needs more stringent than 0.05" are summarized by Feissel & Mignard (1997).

By Reference System is meant the set of prescriptions and conventions together with the modelling required to define at any time a triad of axes.

The ICRS complies with the conditions specified by the 1991 IAU Recommendations. Its origin is located at the barycenter of the solar system through appropriate modeling of VLBI observations in the framework of general relativity. Its pole is in the direction defined by the conventional IAU models for precession (Lieske et al. 1977) and nutation (Seidelmann 1982). Its origin of right ascensions was implicitly defined by fixing the right ascension of 3C273B to the Hazard et al. (1971) FK5 value transferred at J2000.0. See Arias et al. (1995) for more detail.

The Hipparcos star positions and proper motions and the JPL Solar System ephemerides are expressed in the ICRS.

The directions of the ICRS pole and right ascensions origin are maintained fixed relative to the quasars within ±20 microarcseconds. The ICRS is accessible by means of coordinates of reference extragalactic radio sources. It is realized by VLBI estimates of equatorial coordinates of a set of extragalactic compact radio sources, the International Celestial Reference Frame (ICRF). The IERS is in charge of its maintenance.

The ICRS can be connected to the International Terrestrial Reference System by use of the IERS Earth Orientation Parameters (EOP).