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Published online by Cambridge University Press: 12 April 2016
The celestial frame of HIPPARCOS astrometric satellite will be materialized by the positions and proper motions of about 120000 stars relative to arbitrary origins. As the HIPPARCOS reference frame is not naturally related to fixed directions in space, it has to be linked to an inertial frame of similar quality. The technique of VLBI determines the coordinates of extragalactic radio sources precise at the milliarcsecond level in an equatorial frame. The precision expected for HIPPARCOS normal mission is 0.002ʺ for the positions, yearly proper motions and parallaxes.