The space astrometry project: history, technical evolution and future prospects.

A plan to provide coordinates of 400,000 stars of magnitude 10 to 12 with uncertainties from 0.02 to 0.06 arcsec is outlined. The B and V magnitudes are estimated with uncertainties from 0.02 to 0.06 mag, color indices with uncertainties from 0.03 to 0.10 mag. Data are collected by the Hipparcos satellite, scanning the sky, under permanent active attitude control. The reference system is derived from angular measurements. Satellite attitude motion is deduced from transit time data of reference stars. Star position is derived from a combination of satellite attitude data and transit times. Data smoothing techniques and equipment modifications which improve accuracy are discussed.