Space Interferometry Mission System testbed-3: external metrology inversion

The Space Interferometry Mission\´s System testbed-3 has recently integrated its precision support structure and spacecraft bus, or "backpack", on a pseudo free-free 0.5 Hz passive isolation system. The precision support structure holds a 3-baseline stellar interferometer instrument. The architecture of the instrument is based on the current SIM flight system design, and its main purpose is to demonstrate nanometer class fringe stabilization using the path length feed forward technique. This paper briefly describes the nanometer-class metrology system used in this testbed to estimate the length and orientation of the science baseline vector, which cannot be measured directly. The focus is on the mathematical inversion problem that results and its solution.