Our star-count Galaxy model (Chen 1997) has been updated and used to estimate the number of stars of different galactic populations (disk, thick disc, halo) that can be observed by Gaia, and to assess the observing conditions (background and crowding). Statistical distributions are presented, and the important role of interstellar extinction is discussed. We use the procedure of Lindegren (1998a) to compute the Gaia astrometric precision of parallaxes and proper motions as a function of position and magnitude of the star. Then we can estimate the distribution of the expected errors on the measured distances and tangential velocities.
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Torra et al. (1999) studied this question.