Special Features of the Hipparcos Mission
Several features combine to make the Hipparcos mission unique and the corresponding results so dramatic. The satellite was able to observe the entire celestial sphere from its location in space. This advantage was combined with the absence of a perturbing atmosphere, and the instrumental stability brought about by the absence of gravitational instrumental flexure as well as an actively controlled thermal environment. Differential angular measurements were made over large angles, at many different orientations, and at many different epochs. The parallaxes are consequently absolute, and regional or systematic errors in positions and annual proper motions are expected to be well below the milliarcsec level. A rotation-free, or 'quasi-inertial', system is represented by relating the final catalogue to observations of extragalactic objects, and the link to the radio reference frame by direct or indirect observations of radio stars or quasars observed by VLBI.