The various quality assessment and validation procedures give confidence in the
overall correctness of Gaia DR2 primary and secondary solutions, but also indicate many
weaknesses and problems that have to be addressed and removed in subsequent
releases. Compared with Gaia DR1 (Lindegren et al.2016) the second release
contains a vastly increased number of sources with full astrometric data, including
parallaxes and proper motions. For the bright ( mag) sources where
such data were included already in the first release, the present results are generally
more accurate and fully independent of the Hipparcos and Tycho catalogues.
In spite of these improvements it should be remembered that the astrometric
results in Gaia DR2 are based on less than two years of observations and
very preliminary calibrations that have not yet benefited from the iterative improvement
of the pre-processing of the CCD measurements. As a consequence, both random
and systematic errors are considerably higher than can be expected for the final
mission products. The detailed astrometry results for Gaia DR2 can be found in Lindegren et al. (2018).
A more independent catalogue consolidation and validation of the science results for
Gaia DR2 was also performed and are documented in Chapter 9 and will be
published in Arenou et al. (2018).