The results of the astrometric solution were compared with ground-based astrometric data.
As expected, this essentially showed the precision and the systematic errors of the
ground-based data, rather than contributing to the validation of the Gaia astrometry.
The sole exception are the VLBI positions of quasars, which both gave a confirmation of the
quality of the Gaia solution, as well as a first mas-level information on the
typical offset between the optical and the radio centres of light for quasars.