SWIFT-DASH: Spatial Heterodyne Spectroscopy Approach to Stratospheric Wind and Ozone Measurement

Passive wind measurements using Doppler shifts from atmospheric emissions were well demonstrated by the Wind Imaging Interferometer (WINDII) and the High Resolution Doppler Imager (HRDI) instruments on the National Aeronautics and Space Administration's (NASA's) Upper Atmosphere Research Satellite, operated from 1991 to 2005. For WINDII these emissions were from visible region upper atmospheric airglow in the altitude range from 80 to 300 km. Application of the same technique in the stratosphere requires using thermal emission from a minor constituent, and an ozone line near 1133 cm−1 (about 8.8 μm) has been identified as a suitable target line. The WINDII method employed a Doppler Michelson Interferometer, in which the wind is measured from phase shifts of a single spectral line. Isolating a single ozone spectral line is a major challenge but using Spatial Heterodyne Spectroscopy (SHS) offers a way to resolve a number of interferogram spectral components (fringes) within a narrow spectral range. The instrument is a Michelson interferometer similar to WINDII but one in which the two mirrors are replaced by diffraction gratings. A developmental instrument capable of measuring the phase shifts from several ozone lines within a spectral range of 4 cm−1 has been designed, built, and operated in the laboratory. Simulated retrievals using the measurement parameters of this instrument demonstrate the capability of wind measurement with an accuracy better than 3 m s−1 over an altitude range of 24 to 60 km. The retrieval employs four spectral lines for wind and three fringe frequencies for ozone concentration (of about 30 possible), each of which provides an optimal measurement for a particular altitude range. Ozone concentrations are also provided with an accuracy better than 10% from 20 to 50 km. Further detailed tests of this instrument are planned for the future. This work is supported by the Canadian Space Agency.