SIFTI : a Static Infrared Fourier Transform Interferometer dedicated to ozone and CO pollution monitoring

Measuring pollutant concentrations in the boundary layer of the atmosphere is a major challenge for air quality. Infrared sounding, providing vertically resolved profiles for several trace gases in the troposphere, is a must for pollution observation. In this framework, CNES is currently leading a phase-A study for SIFTI, a Static Infrared Fourier Transform Interferometer devoted principally to ozone and CO measurements in the thermal (TIR) and short-wave infrared (SWIR). We will first describe the high-level mission requirements, including orbital considerations like the revisit frequency or the need for cloud-free observations. Instrument specifications, like spectral band position, spectral resolution, radiometric noise, are then derived from the precision needed in CO and ozone profile retrievals. The sensitivity of the profile retrievals, given in terms of vertical sensitivity and errors, to instrument performances are studied using the optimal estimation theory. The instrument concept proposed by CNES (Centre National d’Etudes Spatiales, the French space agency), an interferometer with no moving part, based on scaled mirrors, is a simple and efficient solution to meet these requirements and obtain very high resolution spectra (better than 0.1 cm) with a high signal-to-noise ratio. Spectra are measured within two thermal infrared bands (for CO and O3) and one optional shortwave infrared band (for CO and CH4) for synergetic more accurate SWIR/TIR inversion of CO profile. Thanks to an intelligent pointing mechanism based on real-time analysis of observations from an imbedded infrared imager, the probability of clear sky is dramatically increased. An optimization of the instrument, based on an irregular but well-chosen sampling of the interferogram, opens the way to still higher quality profiles.