Gaia Data Release 1: Astrometry - one billion positions, two million
  proper motions and parallaxes
  Gaia Data Release 1 (Gaia DR1) contains astrometric results for more than 1
billion stars brighter than magnitude 20.7 based on observations collected by
the Gaia satellite during the first 14 months of its operational phase. We give
a brief overview of the astrometric content of the data release and of the
model assumptions, data processing, and validation of the results. For stars in
common with the Hipparcos and Tycho-2 catalogues, complete astrometric
single-star solutions are obtained by incorporating positional information from
the earlier catalogues. For other stars only their positions are obtained by
neglecting their proper motions and parallaxes. The results are validated by an
analysis of the residuals, through special validation runs, and by comparison
with external data. Results. For about two million of the brighter stars (down
to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to
Hipparcos-type precision or better. For these stars, systematic errors
depending e.g. on position and colour are at a level of 0.3 milliarcsecond
(mas). For the remaining stars we obtain positions at epoch J2015.0 accurate to
~10 mas. Positions and proper motions are given in a reference frame that is
aligned with the International Celestial Reference Frame (ICRF) to better than
0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03
mas/yr. The Hipparcos reference frame is found to rotate with respect to the
Gaia DR1 frame at a rate of 0.24 mas/yr. Based on less than a quarter of the
nominal mission length and on very provisional and incomplete calibrations, the
quality and completeness of the astrometric data in Gaia DR1 are far from what
is expected for the final mission products. The results nevertheless represent
a huge improvement in the available fundamental stellar data and practical
definition of the optical reference frame.
