The Gaia Project - technique, performance and status
  Gaia is a satellite mission of the ESA, aiming at absolute astrometric measurements of about one billion stars (all stars down to 20th magnitude, with unprecedented accuracy. Additionally, magnitudes and colors will be obtained for all these stars, while radial-velocities and spectral properties will be determined only for bright objects (V<17.5). At 15th magnitude Gaia aims at an angular accuracy of 20 microarcseconds (muas). This goal can only be reached if the geometry of the telescopes, the detectors, and the pointing of Gaia at each moment ("attitude") can be inferred from the Gaia measurements itself with muas accuracy. 
Stefan Jordan