OBSS vastly surpasses ground-based astrometric programs. The best wide-field accuracies from the ground are achieved by optical interferometers, which can reach 1 milliarcsecond (mas) accuracy for about 50 stars per night. There are no astrometric optical interferometers in the southern hemisphere. Narrow field accuracies of hundreds of microarcseconds can be achieved by interferometers and special instruments on very large telescopes for only a small number of stars. Ground-based survey instruments using charge coupled devices (CCDs) can at best achieve 25 mas relative accuracies. Thus, there is no current means of achieving a full-sky survey of millions of stars at microarcseconds accuracies other than by a space instrument.
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