# 3.5.11 Conclusions

Author(s): Lennart Lindegren

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 ($G\lesssim 12$ 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).