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Modeling the astrometric performance of the Space Interferometry Mission

The Space Interferometer Mission (SIM) will achieve important science objectives of NASA's Origins program by performing astrometric measurements of targets within and beyond our galaxy with a precision of the order of a few microarcseconds. This accuracy can only be attained by carefully balancing sources of astrometric error arising from the systematic accuracy of the interferometer, thermal distortions, vibration caused by spacecraft systems, and errors in knowledge of the spacecraft's altitude and velocity. A rigorous systems engineering approach must be applied to the conduct of trades between these different sources of error and the TRW SIM Study Team has developed a mathematical model of the performance of SIM to support these trades. This paper shows how the model is constructed using simple analytical relationships and then employed to determine the dependence of system performance on a wide range of configuration parameters, such as baseline length, aperture size, and attitude knowledge. These studies indicate where subsystem performance requirements are critical and where requirements may be relaxed with little degradation of overall astrometric accuracy.