The International Celestial Reference Frame (ICRF) is the realization of the International Celestial Reference System (ICRS). It comprises a catalog of precise equatorial coordinates of extragalactic radio sources observed by Very Long Baseline Interferometry (VLBI). For celestial reference frame, the typical preferred properties of radio sources are high flux density, compactness, i.e. little and stationary intrinsic structure, and continuum radiation, such as synchrotron radiation. Since the objects are extragalactic, they are all sufficiently distant, so that parallaxes and proper motions are negligible.
Historically, the ICRF has been observed in S/X bands. Therefore, the ICRF (S/X) is the conventional reference frame for Earth orientation and thus essential for geodesy. It is required for the definition of reference directions for satellite orbits and provides the basis for astrometry. The realizations at other radio wavelengths, i.e. K and X/Ka-bands, are currently considered as less precise and are thus referred to ICRF (S/X). Nevertheless, they represent independent realizations of ICRS that gradually improve in terms of number of observations and precision. Other celestial systems, such as the dynamic Lunar and planetary ephemerides, refer to ICRF (S/X). Catalogues of astrometric satellite missions, such as Hipparcos and Gaia of ESA, mark the access in optical wavelengths, but provide an orientation independent of the Earth surface. For accessing these data from ground or from orbiting platforms, they have to be referred to ICRF through a datum that fixes at least the orientation and spin degree of freedom. Gaia (DR2) for instance used a prototype of ICRF3 (S/X) for the datum definition. The Gaia mission is still ongoing. Future products will very likely outperform current ones.