The International Celestial Reference Frame (ICRF3)

The practical realisation of the International Celestial Reference System — a catalogue of precise positions for several thousand extragalactic radio sources, mostly quasars, measured by very-long-baseline interferometry. Its third realisation, ICRF3, was adopted in 2018; because quasars are effectively fixed, it provides the quasi-inertial grid to which optical frames such as Gaia's are aligned.