be used as a consistency check of the TGAS/HTPM solution. ]{}

Introduction
============

The ESA astrometry satellite [*Gaia*]{} was launched in December 2013 with the aim of mapping more than a billion stars ($V\la 20$) in our Galaxy . For stars brighter than $V=15$ mag, it is expected to yield positions, parallaxes and annual proper motions at an accuracy level of 5–25 $\mu$as. This accuracy can only be achieved after a global reduction of observations collected over an extended period of time (nominally five years), during which each star is seen crossing the focal plane of [*Gaia*]{} on average about 70 times. The multiple observations of a given star over several years are crucial for a successful disentanglement of the effects of stellar parallax and proper motion. A certain redundancy of observations is also required to estimate the additional parameters for the spacecraft attitude and calibration.

While the final [*Gaia*]{} results are thus expected post-2020, intermediate (provisional and less accurate) releases of astrometric data will be made; the first one is expected in mid-2016. Being based on a much shorter stretch of observations, it is envisaged that this first release will only give the mean positions of the stars, as 