Abstract: The proposed New Millennium Interferometer consists of three spacecraft separated by up to several kilometers. A heterodyne laser metrology system is proposed to measure the relative distances between the spacecraft. Because diffraction losses for a round-trip measurement are prohibitively large, a two-laser metrology system has been suggested in which each spacecraft has both a laser and a receiver. The system has been successfully demonstrated with a one meter baseline and verified by a conventional single- laser system in a laboratory experiment. The precision was limited by thermal effects in the room environment for time scales greater than one minute. The single-laser system obtained a precision of 3 nm for integration times up to 0.5 seconds. The two-laser system obtained a precision of 20 and was limited by self-interference and electronics noise. The resolution of the two-laser metrology system was $lambda@30.!9
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