Design and on-orbit performance of the WINDII instrument's Michelson interferometer on UARS

The Wind Imaging Interferometer (WINDII), a joint Canada-France project, is one of ten instruments on NASA's Upper Atmosphere Research Satellite (UARS), which was launched on 12 September 1991. WINDII's primary task is to measure winds in the 80-300 km altitude region of the atmosphere by detecting Doppler shifts in the airglow emissions produced there. It consists of a limb-viewing CCD camera which observes the airglow emission layers in profile through a field-widened Michelson interferometer and a selection of narrow-band filters. Several images are recorded as one Michelson fringe is scanned piezoelectrically in order to determine its phase and visibility. Doppler shifts are detected as changes in phase and, after correction for the spacecraft motion and earth rotation, images of the radial component of the wind are produced. Complete wind vectors are determined by combining measurements of approximately the same volume of air from two points on the orbit, as seen in WINDII's two orthogonal fields of view. The design of the interferometer features achromatic field widening over the range 550-770 nm, mechanical stability, thermal compensation and suppression of secondary fringes. In its first eight months of operation, WINDII has produced about 3,000,000 individual images, or 300,000 complete measurement sets. The interferometer is functioning as planned, with no known problems. This paper describes the design of the Michelson interferometer and evaluates its performance in space.