The International Celestial Reference System (ICRS)

The ICRS constitutes the set of prescriptions and conventions together with the modelling required to define at any time a triad of axes. The System is realised by VLBI estimates of equatorial coordinates of a set of extragalactic compact radio sources, the International Celestial Reference Frame (ICRF). The ICRS can be connected to the International Terrestrial Reference System (ITRS) by use of the IERS Earth Orientation Parameters (EOP).