OCA/CERGA Grasse (France)
The Hipparcos mission has produced global astrometry at the level of 1 mas and 1 mas/yr for star of 8 magnitude, respectively for positions and proper motions. There is no hope to do better global astrometry from the ground due to the unmodelalable atmospheric paths at large angles, although very small field astrometry could be carried out at this level of precision.
We consider the various possibilities of making sub-miliarcsecond and micro-arcsecond global astrometry from space. The general principles and constraints are reviewed along with the science rationale for such missions. Prospects for the next two decades are illustrated from exisiting proposals at various levels of advancement such as FAME, OSI and GAIA.