A highly accurate frame of reference for astrometric measurements adopted as the fundamental celestial reference frame at the start of 1998, replacing the FK5 optical frame. It is based on the positions of 212 quasars and radio galaxies measured by VLBI (more sources were later added, bringing the total to 608). Distant radio sources are used because they do not exhibit the proper motions that introduce long-term errors in the positions of nearby stars. At optical wavelengths, the positions of stars in the *Hipparcoscatalogue are tied to the ICRF.
Subjects: Astronomy and Astrophysics.