The celestial reference system and its role in the epoch of global geodetic technologies

The paper discusses the role of celestial reference systems in astrometry, space and satellite geodesy. A historical review of the fundamental systems and methods for implementing international celestial reference systems (ICRS), from classical astrometry to VLBI, are presented. The paper emphasizes the role of celestial reference frames as a realization of the inertial frame. The definition of the ICRS and its realisation ITRF are also briefly discussed. The accuracy of contemporary realizations of ICRF are considered.