Global astrometry with OSI

— The Orbiting Stellar Interferometer is a proposed space-based interferometer which makes very accurate wide-angle astrometric measurements on several thousand stars. As opposed to Hipparcos, OSI does not have a “basic angle” between two separate field-of-views but three separate interferometers and a highly accurate metrology system. The questions addressed in this paper are whether it is possible to perform global astrometry with such an instrument and what kind of results could then be expected. A time-independent observing scenario, leading to a solution for positions and ignoring effects of proper motion and parallax, as well as a global astrometric data analysis technique are presented and a covariance study is performed. We show that a gain of a factor of at least 4 on the final astrometric accuracy could be achieved if such a technique was used. With an initial single measurement precision of 10 micro-arcseconds, our solution leads to an astrometric accuracy of 2-3 micro-arcseconds.