Metrology for the Space Interferometry Mission

The space interferometry mission, SIM PlanetQuest, is a mission to use the tool of astrometry to investigate a broad range of topics in astronomy and astrophysics. SIM requires that the baseline vector lengths also be measured at the few micrometer level of precision. This is called absolute metrology, and is a capability built into the external metrology subsystem of SIM. SIM uses a heterodyne technique for its metrology system. Stabilized laser light is frequency shifted twice, once for a 'local oscillator' and once for a measurement beam. In external metrology, a compact interferometer bench called a beam launcher is interposed between two corner cube fiducials linked by six metrology beams.