These pages provide information about 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 microarc seconds (μas). 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).
The VLBI analysis which permits to calculate radio source coordinates also provide corrections to the conventional IAU models for precession and nutation, thus leading to the accurate estimation of the shift of the mean pole at J2000.0 relative to its conventional position, to which the pole of ICRS is attached. One can estimate that the pole at J2000.0 is shifted from the ICRS pole by 17.3 ± 0.2 mas in the direction 12 h and by 5.1 ± 0.2 mas in the direction 18 h (IERS 1997).
The pole of ICRS is also consistent with that of FK5. Using the Hipparcos catalogue which includes all the FK5 stars, Mignard & Froeschle (1997) have derived the FK5 pole in ICRF with an uncertainty of a few mas. Assuming that the error in the precession rate is absorbed by the proper motions of stars, the uncertainty in the FK5 pole position relative to the mean pole at J2000.0 can be estimated to ±50 mas. The ICRS celestial pole is consistent with that of FK5 within the uncertainty of the latter.
Fig. 1. Direction of the mean pole at J2000.0 in the ICRS, determined by VLBI and LLR with its uncertainty (0.1 mas on the pole, 10 mas on the equinox), and direction of the FK5 pole in the ICRS, determined by comparison with Hipparcos, with its uncertainty (2.3 mas at J2000.0). The original uncertainty is also shown (dotted circle, 50 mas).
The Ox axis of ICRS was implicitely defined in the initial realization of the IERS celestial reference frame (Arias et al. 1988) by adopting the mean J2000.0 right ascensions of 23 radio sources in a group of VLBI catalogues. These catalogues were compiled by fixing the right ascension of the quasar 3C273B to the usual conventional FK5 value (Hazard et al. 1971).
According to Mignard & Froeschle (1997) the FK5 origin of right ascensions is offset from the ICRS one by -22.9 mas. Considering the mean epoch of 1955 for the proper motions in right ascension, the uncertainty in the FK5 origin of right ascensions can be estimated in ±100 mas (Morrison et al. 1990, Lindegren et al. 1995). Comparing VLBI and LLR Earth orientation and terrestrial reference frames, Folkner et al. (1994) estimated the frame tie between the IERS celestial system and the JPL planetary ephemeris and concluded that the Ox axis of ICRS is offset from the mean equinox at J2000.0 by 78 ± 10 mas.
The IAU has charged the IERS with the responsibility of