Optical architecture of the new generation infrared atmospheric sounder interferometer (IASI-NG)

The Infrared Atmospheric Sounder Interferometer (IASI) is a Fourier Transform Spectrometer (FTS) working in the [3.6μm, 15.5μm] range, dedicated to Numerical Weather Prediction (NWP), atmospheric chemistry and climate monitoring. The second flight model (2 out of 3) is now in orbit and operational, as a payload of the MetOp-B satellite. A new generation of instrument (IASI-NG) to continue the IASI mission with increased performances is currently investigated by the French Space Agency (CNES). The performance objective is mainly a spectral resolution and a radiometric error divided by two compared with the IASI ones. Many different concepts of FTS were studied to try to fulfill these challenging requirements. This paper presents the different envisaged optical architecture and associated trade off. The major issue of the concept is to manage the so-called self-apodization of the interferogram and the associated degradation of the spectral resolution induced by the wider Field of View (FoV) and the longer Optical Path Difference (OPD). Increasing these two quantities have very constraining consequences on the optical architecture. Another critical point is the control of straylight which is quite severe and which has been taken into account early in the optical design. To assess the performances of the interferometer, different optical models were built combining analytical approach with ray tracing technics. We will describe the impacts of the demanding spectral requirements on the optical components and our analyses based on these models will be presented.