Gaia's astrometric measurements are made using the global astrometry concept successfully demonstrated by Hipparcos. Gaia measures the relative separations of the thousands of stars simultaneously present in the combined fields of view. The astrometric field in the focal plane is sampled by an array of 62 CCDs, each read out in time-delayed integration mode synchronized to the scanning motion of the satellite.
The spacecraft operates in a continuously scanning motion, such that a constant stream of elative angular measurements is built up as the fields of view sweep across the sky. High angular resolution (and hence high positional precision) in the scanning direction is provided by the primary mirror of each telescope, of dimension 1.45 x 0.5 m2 (along scan x across scan). The wide-angle measurements provide the high rigidity of the resulting reference system.