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.
Results from the comparisons are given in the Gaia DR2 astrometry paper,
Lindegren et al. (2018) and in the Gaia DR2 validation paper Arenou et al. (2018).