ICRF193

ICRF193 is a designation that may be encountered as a label for a release or subset of the International Celestial Reference Frame (ICRF). The ICRF is the standard quasi-inertial reference frame used for precise celestial coordinates in astronomy and for spacecraft navigation. It is realized by very long baseline interferometry (VLBI) observations of distant extragalactic radio sources, such as quasars and active galactic nuclei, whose positions are effectively fixed on the sky.

The official realizations of the ICRF to date are ICRF1, ICRF2, and ICRF3, maintained by the International Astronomical Union (IAU) and the International Earth Rotation and Reference Systems Service (IERS). Each realization provides a catalog of source positions anchored to the International Celestial Reference System (ICRS) and aims to maintain a stable, nearly inertial frame with milliarcsecond accuracy. The catalogs include coordinates of many radio sources and, in later realizations, a subset of defining or core sources used to stabilize the frame.

ICRF sources are determined from VLBI data gathered over many years, with continual updates as new observations refine positions and mitigate systematic effects such as source structure. The frame underpins precise astrometry, Earth orientation parameters, and deep-space navigation, and it is tied to the broader ICRS, which serves as a common celestial reference for both radio and optical astronomy.

If ICRF193 appears in a dataset, it is likely an internal version label, a non-official release, or a historical data product rather than the widely adopted official ICRF realization. Verification against IAU/IERS documentation is recommended. See also ICRS, VLBI, and the ICRF realizations (ICRF1–ICRF3).