The cross-identification between the SIMBAD database and the ESA Gaia mission Data Release 2 has been successfully performed on June 30, 2018, on about 4.5 million objects.
The following data have been upgraded in SIMBAD :
4.5 million coordinates, Gaia DR2 identifiers, G magnitudes
3.8 million parallaxes
3.8 million proper motions
2.0 million radial velocities
The criteria used for the cross-identification :
Objects must have had sub-arcsecond coordinates already, corresponding to quality A, B, and C in SIMBAD. Objects with insufficient astrometry, quality D or E, have been completely ignored.
There were 6.5 million potential targets for the cross-identification.
Coordinates of Gaia objects were calculated at epoch 2000.0 taking into account the proper motions in order to be compared to the positions in SIMBAD.
310,000 objects have been discarded because they have a neighbour in SIMBAD closer than 3''. This value corresponds to the resolution of 2MASS. As there are many 2MASS identifiers in SIMBAD, we did not risk to add Gaia identifiers on 2MASS sources which would actually be a blend of two or more objects in Gaia.
273,000 objects have been discarded because they have a neighbour at less than 3'' in Gaia DR2, with a difference of magnitude smaller than 3 mag, the brighter source being at less than 1'' from SIMBAD position.
Finally 4.5 million objects have been cross-identified with a Gaia DR2 source with a positional difference smaller than 1.0''.
About 1.4 million objects had no counterpart in Gaia DR2 at less than 1''. They are mostly (very) faint objects.
The data recorded in the SIMBAD Collection of Measurements :
Parallaxes in "PLX". Note that negative parallaxes have not been added in SIMBAD.
Distances in "distance". Distance and uncertainty have been calculated using the classical formulae, BUT ONLY IF the parallax uncertainty was less than 10