International Celestial Reference System and its realizations

Its origin is at the barycenter of the Solar System, with axes that are intended to "show no global rotation with respect to a set of distant extragalactic objects".

[1][2] This fixed reference system differs from previous reference systems, which had been based on Catalogues of Fundamental Stars that had published the positions of stars based on direct "observations of [their] equatorial coordinates, right ascension and declination"[3] and had adopted as "privileged axes ... the mean equator and the dynamical equinox" at a particular date and time.

[5] More specifically, the ICRF is an inertial barycentric reference frame whose axes are defined by the measured positions of extragalactic sources (mainly quasars) observed using very-long-baseline interferometry while the Gaia-CRF is an inertial barycentric reference frame defined by optically measured positions of extragalactic sources by the Gaia satellite and whose axes are rotated to conform to the ICRF.

Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured very accurately by Very Long Baseline Interferometry (VLBI).

[9] In August 1997, the International Astronomical Union resolved in Resolution B2 of its XXIIIrd General Assembly "that the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths.

The solution was constrained to be consistent with the International Terrestrial Reference Frame (ITRF2008) and earth orientation parameters (EOP) systems.

"[1] Using a variety of linking techniques, the coordinate axes defined by the Hipparcos catalogue were aligned with the extragalactic radio frame.

[20] In August 1997, the International Astronomical Union recognized in Resolution B2 of its XXIIIrd General Assembly "That the Hipparcos Catalogue was finalized in 1996 and that its coordinate frame is aligned to that of the frame of the extragalactic sources [ICRF1] with one sigma uncertainties of ±0.6 milliarcseconds (mas)" and resolved "that the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths.

As with the earlier Hipparcos and Gaia reference frames, the axes of Gaia-CRF3 were aligned to 3142 optical counterparts of ICRF-3 in the S/X frequency bands.

Consequently, the IAU decided that Gaia-CRF3 shall be "the fundamental realization of the International Celestial Reference System (ICRS) ... for the optical domain.