The accuracy of the future international celestial reference frame obtained from astrometric satellites: SIM and GAIA are given.

The conditions required to construct, a quasi-inertial celestial reference frame are discussed, and several cases, in which if the ground-based observation can satisfy, arc enumerated.

The heliotropic, great-circle and Hipparcos telescope reference frames are explicitly defined in the light of the scanning law of Hipparcos satellite.