  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.
