International Celestial Reference System and Frame
The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to be "fixed" with respect to the stars. ICRS coordinates are approximately the same as equatorial coordinates, and the differences at J2000.0 are:
- The mean pole in the ICRS lies at 17.3±0.2 mas in the direction 12 h and 5.1±0.2 mas in the direction 18 h.
- The mean equinox is shifted from the ICRS right ascension origin by 78±10 mas (direct rotation around the polar axis).
The International Celestial Reference Frame (ICRF) is a realization of the International Celestial Reference System using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a reference frame is the physical realization of a reference system, i.e., the reference frame is the set of numerical coordinates of the reference sources, derived using the procedures spelled out by the ICRS.
The ICRF creates a quasi-inertial frame of reference centered at the barycenter of the Solar System, whose axes are defined by the measured positions of extragalactic sources (mainly quasars) observed using very long baseline interferometry. Although general relativity implies that there are no true inertial frames around gravitating bodies, the ICRF is important because it does not exhibit any measurable angular motion since the extragalactic sources used to define the ICRF are so far away. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects.
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 0.001 arcsecond or better.
At optical wavelengths, the ICRS is currently realized by the Hipparcos Celestial Reference Frame (HCRF), a subset of about 100,000 stars in the Hipparcos Catalogue. The Gaia-CRF2, based on 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."
ICRF3 is the latest version of the ICRF.
The ICRF, now called ICRF1, was adopted by the International Astronomical Union (IAU) on 1 January 1998. ICRF1 had an angular noise floor of approximately 250 microarcseconds (µas) and a reference axis stability of approximately 20 µas; this was an order-of-magnitude improvement over the previous reference frame derived from Fifth Fundamental Catalog (FK5). 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. The ICRF1 agrees with the orientation of the Fifth Fundamental Catalog (FK5) "J2000.0" frame to within the (lower) precision of the latter.
An updated reference frame ICRF2 was created in 2009. 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. Radio observations in both the S-band (2.3 GHz) and X-band (8.4 GHz) were recorded simultaneously to allow correction for ionospheric effects. The observations resulted in about 6.5 million group-delay measurements among pairs of telescopes. The group delays were processed with software that takes into account atmospheric and geophysical processes. The positions of the reference sources were treated as unknowns to be solved for by minimizing the mean squared error across group-delay measurements. The solution was constrained to be consistent with the International Terrestrial Reference Frame (ITRF2008) and earth orientation parameters (EOP) systems.
ICRF3 is the third major revision of the ICRF, and was adopted by the IAU in August 2018, to become 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 contains positions for 4536 extragalactic sources. Of these 303 have been identified as defining sources. ICRF3 also increases the number of defining sources in the southern sky.
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