The USNO Astrometric Interferometer (USNOAI; a subarray of the Navy Prototype Optical Interferometer at Lowell Observatory) is presently under construction and expected to begin limited operations within a year. The main goal of the USNOAI observations is to provide a northern hemisphere catalog of several thousand stars with positions known to a few mas. In order to meet this requirement, a baseline laser metrology system must be employed to measure the three-dimensional motions of the baselines with an accuracy better than ~ 0.1 μm. The metrology scheme, as presently conceived, represents the largest and most complex high-resolution laser metrology system ever attempted.
