ICRF

The ICRF (for International Celestial Reference Frame) is a celestial reference frame developed by the IAU, in its current version (ICRF3) based upon the radio position of a set of 303 quasars in the celestial sphere, providing a basis for celestial coordinate systems. The coordinates of such systems can (effectively) be defined according to their coordinates for these quasars, and the coordinates of an astronomical object can be determined most accurately and precisely by basing them upon the angular distances from these quasars, minimizing sources of error. The quasar positions are determined using very-long-baseline interferometry (VLBI) to a milli-arcsecond or better. A superset of 4536 objects is also specified, their positions carefully measured in relation to the 303 quasars, offering additional well-known positions with which to determine an object's coordinates. ICRF3 is the reference frame used by the International Celestial Reference System (ICRS) for non-optical wavelengths.

ICRF1 was adopted in 1998, ICRF2 in 2009, and ICRF3 in 2019. Earlier celestial reference frames were versions of the Catalogues of Fundamental Stars. Since quasars' distances are at least a million times greater than those of Milky Way stars, their proper motion is virtually zero, a major advantage.