Integration of Spatial Heterodyne Spectroscopy with the Stratospheric Wind Interferometer For Transport studies (SWIFT)

The Doppler Michelson Interferometer (DMI) approach developed for atmospheric wind measurement was first flown as the WIND Imaging Interferometer (WINDII), launched on NASA's Upper Atmosphere Research Satellite in 1991. WINDII obtained global winds over the altitude range of 80–300 km, using airglow emission as a Doppler target. The same concept was proposed for the Stratospheric Wind Interferometer For Transport studies (SWIFT), destined for flight on the CSA Chinook mission. SWIFT was intended for measurement of winds in the altitude range of 20–45 km where the only available emission for Doppler measurement is thermal radiation from minor constituents. An ozone line at about 8.8 µm was adopted as the target. The challenge is to isolate this line from other lines in this complex spectrum to measure its phase with sufficient accuracy. The demands on the filter fabrication and the knowledge of its characteristics in flight, including thermal drifts, became a major consideration. Spatial Heterodyne Spectroscopy (SHS) also uses a Michelson interferometer but with the mirrors replaced by diffraction gratings; this produces a spatial interferogram across an imaging detector for a narrow spectral band. By using a longer path difference in one arm, as is done in DMI, in a configuration called DASH (Doppler Asymmetric Spatial Heterodyne), it is possible to measure the phases of each of these lines in the band separately, which significantly reduces the demands on filter fabrication and knowledge. A development model is now being implemented within the Centre for Research in Earth and Space Science at York University (CRESS); the concept is described and a report on progress is provided.