Measuring the trigonometric parallax for a star yields the only fully reliable way of measuring distances in the local Universe. Some of the earliest recorded astrometry measurements date back more than 2000 years. Despite the huge progress made since then, ground-based measurements are limited by several factors (for example, fluctuating atmosphere, instrument flexure, limited sky coverage per observing site) which ultimately limit the measurement accuracy which can be achieved.

The first space-based astrometry mission, ESA's Hipparcos satellite, demonstrated that milliarcsecond accuracy was achievable by means of a continuously scanning satellite which observed two directions simultaneously. In addition, the global nature of the measurements, the fact that the positions and changes in positions caused by proper motion and parallax are determined in a reference system consistently defined over the whole sky, leads to the determination of absolute parallax measurements. (In ground-based parallax measurements the transformation of relative parallaxes to absolute distances is a non-trivial problem.)

Global astrometry has many intrinsic advantages over pointed observations: a global instrument calibration can be performed in parallel with the observations; and the interconnection of observations over the celestial sphere provides the rigidity and reference system needed for the kinematical interpretation of the observations themselves. Another important feature of global astrometric data is the capability of determining the astrometric parameters of double and multiple systems, including extra-solar planetary systems and brown dwarfs.

Gaia builds on the proven techniques established by Hipparcos to bring astrometry into the 21st century. When combined with newly-developed technology, the same measurement principle employed by Hipparcos can be used to gain, simultaneously, a factor of more than 100 improvement in accuracy, a factor 1000 improvement in limiting magnitude, and a factor of 10000 in the numbers of stars observed.