Abstract
To ensure the full capabilities of Gaia's measurements, a programme of daily observations with Earth-based telescopes of the satellite itself - called Ground Based Optical Tracking (GBOT) - was implemented since the beginning of the Gaia mission. The constraint of 20 mas on the tracking astrometric quality and the fact that Gaia is a faint and relatively fast moving target (its magnitude in a red passband is around 21 mag and its apparent speed around 0.04″/s), lead us to rigorously analyse the astrometric precision obtainable for CCD observations of faint moving objects.
