Observational Model for Precision Astrometry with the Space Interferometry Mission

The basic astrometric observable of the Space Interferometry Mission (SIM) instrument is the pathlength delay. This measurement is made by a combination of internal metrology measurements that determine the distance the starlight travels through the two arms of the interferometer and a measurement of the white light stellar fringe to nd the point of equal pathlength. Because this operation requires a nonnegligible integration time, the interferometer baseline vector is not stationary over this time period, as its absolute length and orientation are time-varying. This paper addresses how the time-varying baseline can be \regularized" so that it may act as a single baseline vector for multiple stars, as required for the solution of the astrometric equations.