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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 and Mignard (1997).

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

The ICRS is accessible by means of coordinates of reference extragalactic radio sources, the International Celestial Reference Frame (ICRF).

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 modelling 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 3C 273B to the Hazard et al. (1971) FK5 value transferred at J2000.0. See Arias et al. (1995) for more details.

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.
Thanks to the fact that the Hipparcos catalogue includes all the FK5 stars, the location of the FK5 pole and right ascensions origin is known with
an uncertainty of a few mas (Mignard and Froeschlé 1997). Using a state of the art precession-nutation model, the analysis of long VLBI series
of the observed motion of the celestial pole allows to derive the coordinates of the mean pole at J2000.0 in the ICRS: 17.3 +/- 0.2 mas in the direction 12 h and 5.1 +/- 0.2 mas in the direction 18 h.(IERS 1997). 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 mean equinox of J2000.0 is shifted from the ICRS right ascension origin by 78 +/- 10 mas (direct rotation around the polar axis).

The ICRS is realized by VLBI estimates of equatorial coordinates of a set of extragalactic compact radio sources, the International Celestial Reference Frame (ICRF).

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


Arias E. F., Charlot P., Feissel M., Lestrade J.-F., 1995: The Extragalactic Reference System of the International Earth Rotation Service, ICRS, Astron. Astrophys., 303, pp. 604-608.

Feissel M., Mignard F.: The adoption of ICRS on 1 January 1998: Meaning and Consequences, Astron. Astrophys., 1998. To appear as Letter
to the Editor.

Folkner, W. M., Charlot, P., Finger, M. H., Williams, J. G., Sovers, O. J., Newhall, X X, and Standish, E. M., 1994: Determination of the extragalactic-planetary frame tie from joint analysis of radio interferometric and lunar laser ranging measurements, Astron. Astrophys., 287, pp. 279-289.

Fricke, W., Schwan, H., and Lederle, T., 1988: Fifth Fundamental Catalogue, Part I. Veröff. Astron. Rechen Inst., Heidelberg.

Hazard, C., Sutton, J., Argue, A. N., Kenworthy, C. N., Morrison, L. V., and Murray, C. A., 1971: `Accurate radio and optical positions of 3C273B, Nature Phys. Sci., 233, p. 89.

IERS, 1997: 1996 Annual Report, Observatoire de Paris, p. II-73.

Lieske, J. H., Lederle, T., Fricke, W., and Morando, B., 1977: Expression for the Precession Quantities Based upon the IAU (1976) System of Astronomical Constants, Astron. Astrophys., 58, pp. 1-16.

Mignard, F., Froeschlé, M., 1997: Linking the FK5 to the ICRF, JD12, 23rd IAU General Assembly (August 1997). Highlights in Astronomy.

Seidelmann, P. K., 1982: 1980 IAU Nutation: The Final Report of the IAU Working Group on Nutation, Celest. Mech., 27, pp. 79-106.

Standish, E.M., 1997: Linking the Dynamical Reference Frame to the ICRF, JD7, 23rd IAU General Assembly (August 1997). Highlights in Astronomy.

Text provided by the former Central Bureau.

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