Space astrometry projects.

Two space astrometry projects are presented which are specially designed for obtaining a very precise, coherent and complete stellar reference system. Both projects are based on a reference angle given by a complex mirror and on the accurate photoelectric measurement of relative positions in a telescope field while stars are scanned on a grid. The first project utilizes a Spacelab borne 40 cm-diameter telescope. Precision on the parallaxes can be obtained from two years of scanning, and a scan program of 40,000 stars could be envisaged. The second project utilizes a simpler guidance device, and could be flown on board a free flyer similar to the TD-1 satellite. The automatic sky scanning will enable the positions of 300,000 stars up to magnitude 11 to be determined with random errors in positions and parallaxes.