The ICRF is based on hundreds of
extra-galactic radio sources, mostly
quasars, distributed around the entire sky. 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). The positions of most are known to 1
milliarcsecond (mas) or better. 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." In August 2021 the International Astronomical Union decided in Resolution B3 of its XXXIst General Assembly "that as from 1 January 2022, the fundamental realization of the International Celestial Reference System (ICRS) shall comprise the Third Realization of the International Celestial Reference Frame (ICRF3) for the radio domain and the Gaia-CRF3 for the optical domain."
Radio wavelengths (ICRF) ICRF1 The ICRF, now called ICRF1, was adopted by the
International Astronomical Union (IAU) as of 1 January 1998. The ICRF1 contains 212 defining sources and also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue. ICRF1 agrees with the orientation of the Fifth Fundamental Catalog (FK5) "
J2000.0" frame to within the (lower) precision of the latter. The update was a joint collaboration of the
International Astronomical Union, the
International Earth Rotation and Reference Systems Service, and the
International VLBI Service for Geodesy and Astrometry. ICRF2 is defined by the position of 295 compact radio sources (97 of which also define ICRF1). Alignment of ICRF2 with ICRF1-Ext2, the second extension of ICRF1, was made with 138 sources common to both reference frames. Including non-defining sources, it comprises 3414 sources measured using
very-long-baseline interferometry. The ICRF2 has a noise floor of approximately 40 μas and an axis stability of approximately 10 μas. Maintenance of the ICRF2 will be accomplished by a set of 295 sources that have especially good positional stability and unambiguous spatial structure. The data used to derive the reference frame come from approximately 30 years of VLBI observations, from 1979 to 2009.
ICRF3 ICRF3 is the third major revision of the ICRF, and was adopted by the IAU in August 2018 and became effective 1 January 2019. The modeling incorporates the effect of the
galactocentric acceleration of the solar system, a new feature over and above ICRF2. ICRF3 also includes measurements at three frequency bands, providing three independent, and slightly different, realizations of the ICRS: dual frequency measurements at 8.4 GHz (
X band) and 2.3 GHz (
S band) for 4536 sources; measurements of 824 sources at 24 GHz (
K band), and dual frequency measurements at 32 GHz (
Ka band) and 8.4 GHz (
X band) for 678 sources. Of these, 303 sources, uniformly distributed on the sky, are identified as "defining sources" which fix the axes of the frame. ICRF3 also increases the number of defining sources in the southern sky.
Optical wavelengths Hipparcos Celestial Reference Frame (HCRF) In 1991 the
International Astronomical Union recommended "that observing programmes be undertaken or continued in order to ... determine the relationship between catalogues of extragalactic source positions and ... the [stars of the]
FK5 and
Hipparcos catalogues." Using a variety of linking techniques, the coordinate axes defined by the
Hipparcos catalogue were aligned with the extragalactic radio frame. 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."
Second Gaia celestial reference frame (Gaia–CRF2) The second
Gaia celestial reference frame (
Gaia–CRF2), based on 22 months of observations of over half a million extragalactic sources by the
Gaia spacecraft, appeared in 2018 and has been described as "the first full-fledged optical realisation of the ICRS, that is to say, an optical reference frame built only on extragalactic sources." The axes of
Gaia-CRF2 were aligned to a prototype version of the forthcoming ICRF3 using 2820 objects common to
Gaia-CRF2 and to the ICRF3 prototype.
Third Gaia celestial reference frame (Gaia–CRF3) The third
Gaia celestial reference frame (
Gaia–CRF3) is based on 33 months of observations of 1,614,173 extragalactic sources. 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. In August 2021 the International Astronomical Union noted that the
Gaia-CRF3 had "largely superseded the Hipparcos Catalogue" and was "de facto the optical realization of the Celestial Reference Frame within the astronomical community." Consequently, the IAU decided that
Gaia-CRF3 shall be "the fundamental realization of the International Celestial Reference System (ICRS) ... for the optical domain." ==See also==