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International Earth Rotation and Reference Systems Service

The International Celestial Reference Frame (ICRF)

The International Celestial Reference Frame (ICRF) is the official realization of the International Celestial Reference System (ICRS). It is defined by the precisely determined equatorial coordinates of compact extragalactic radio sources, mostly quasars, observed with Very Long Baseline Interferometry (VLBI). As the sources are extragalactic, their parallaxes and proper motions can be regarded as negligible, allowing them to serve as quasi-inertial reference points on the celestial sphere.

The first realization, ICRF1, was adopted in 1997 and later refined by two extensions (ICRF1-Ext.1 and ICRF1-Ext.2). It was replaced in 2009 by ICRF2, which improved accuracy and increased the number of defining sources. The current realization, ICRF3, was adopted by the IAU in 2018 (Resolution B2) and approved by the IAG in 2019 (Resolution 2). ICRF3 contains 4536 radio sources, of which 303 are designated as defining sources that establish the orientation and long-term stability of the frame. ICRF3 provides realizations in S/X, K, and X/Ka frequency bands, with the S/X-band remaining the conventional reference for geodesy, Earth orientation, and satellite orbit determination.

In the optical domain, the initial realization of the ICRS was provided by the Hipparcos Celestial Reference Frame (HCRF), derived from the Hipparcos Catalogue, which incorporated the FK5 reference stars and was astrometrically aligned to the first realization of the ICRF. The HCRF was formally adopted by the IAU in 1997 (Resolution B2) as the primary optical realization of the ICRS. Following IAU Resolution B3, effective 1 January 2022, the Gaia Celestial Reference Frame (Gaia-CRF3), derived from observations of the ESA Gaia mission, constitutes the fundamental optical realization of the ICRS, complementing ICRF3 as its radio-domain realization.

The IERS ICRS Centre disseminates the official realizations and ensures consistency within the broader system of Earth orientation parameters and terrestrial reference frames.