The Gaia Catalogue does not only produce a wealth of data, it also represents
a complex processing before a Catalogue can be issued.
The main data processing is being handled by three DPAC Coordination Units, CU3 for
the astrometric data, CU5 for the photometric data and CU6 for the spectroscopic data.
Then three Coordination Units analyse the processed data, CU4 for optical
or binary stars, solar system objects and extended objects, CU7 for variable stars,
and CU8 for classification.
Finally, CU9 takes care of the intermediate and final publication of the Gaia data.
For Gaia DR1, the situation has been simplified in the sense that CU4, CU6 and CU8
did not contribute to the first Catalogue.
At the last step several data fields may have been computed by several
Coordination Units (e.g. parallaxes computed by CU3, then again by CU4 with a fit
of an astrometric+binary model if the star happens to have a significant binary motion;
or a mean magnitude computed by CU5 may be superseded by another estimation
from CU7 if the stars happens to be a periodic variable; etc.),
in several Data Processing Centres, so an (a) homogeneous, (b) convenient,
(c) consistent Catalogue has to be built.
First, to a so-called CompleteSource is attached astrometric and photometric
information, then possible variability information is integrated,
producing an homogeneous Catalogue. Second, sources that do not meet some minimum
astrometric or photometric quality are filtered out.
The filters applied are described in Section 4 of Gaia Collaboration et al. (2016a)).
Third, while flat files are kept for further operations, the data is integrated
inside the Gaia Archive Core System (GACS) database; crossmatch with external
catalogues is also performed, providing the convenient access to the data.
Fourth, the consistency of the Catalogue is obtained through a dedicated
validation of its content. Sources that do not pass the validation criteria
are then filtered out.
This chapter describes these successive steps which are being followed
for the consolidation of the Catalogue.