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IASI Level 0 and 1 processing algorithms description B. Tournier D. Blumstein

IASI Level 0 and 1 processing algorithms description B. Tournier D. Blumstein F. Cayla. Fourier transform spectroscopy. Physical measurement is not a spectrum Lots of transformations are necessary to retrieve the spectrum Interferogram raw spectrum computation

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IASI Level 0 and 1 processing algorithms description B. Tournier D. Blumstein

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  1. IASI Level 0 and 1 processing algorithms description B. Tournier D. Blumstein F. Cayla

  2. Fourier transform spectroscopy • Physical measurement is not a spectrum • Lots of transformations are necessary to retrieve the spectrum • Interferogram • raw spectrum computation • calibrations ... Michelson interferometer

  3. Radiometric calibration • Non linearity correction • Radiometric calibration done in the complex space using • cold calibration view SCS • hot calibration view SBB • First step done on board • Post-calibration on ground (level 1 processing) Radiometric calibration

  4. Field of View effects and spectral properties • Basic fact • Wave number n at an angle q from the axis of the instrument • Wave number n cos q on the axis give the same raw spectrum (first order only) • Consequences • angle between pixel and interferometric axis must be taken into account in the spectral calibration (off axis pixel) • broadening and deformation of the Instrument Spectral Response Function (ISRF) due to the extension of the IASI pixel (approx. 0.8 degree wide) • non uniform distribution of radiance in the field of view of the instrument must be taken into account at user level

  5. Instrument spectral response function • Level 1b ISRF • is not the same for the 4 IASI pixels • is dependant of the wave number • depends on a lot of possibly varying instrument characteristics • Level 1c ISRF removes this complexity (is uniq and constant) Level 1b ISRF Level 1c ISRF

  6. Level 1 processing architecture

  7. Input to level 1 processing • System parameters are computed off-line by the IASI Technical Expertise Center (TEC) based in Toulouse • Parameters used by the level 1 processing are mainly • Spectral data base (ISRF, apodisation functions, spectral calibration functions) • Input for radiometric post-calibration (reflectivity of scan mirror, emissivity of reference black body) • Spectrum coding/decoding tables • Geometry information for localisation of IASI products • Co-registration Imager/Sounder information • Input for NRT spectral calibration algorithm • Limits of spectral bands • NB: Noise and noise covariance matrix are provided for higher level processing

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