Infrared atmospheric sounding interferometer
Although primarily intended to provide information in near real-time on atmospheric temperature and water vapour to support weather forecasting, the concentrations of various trace gases can also be retrieved from the spectra.
IASI belongs to the thermal infrared (TIR) class of spaceborne instruments, which are devoted to tropospheric remote sensing.
Thus, it blends the demands imposed by both meteorology - high spatial coverage, and atmospheric chemistry - accuracy and vertical information for trace gases.
[3] Designed by the Centre national d'Études Spatiales, it now combines a good horizontal coverage and a moderate spectral resolution.
Under an agreement between CNES and EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites), the former was responsible for developing the instrument and data processing software.
Each such element consists of a 2×2 circular pixel matrix of what is called instantaneous fields of view (IFOV).
The first two levels are dedicated to transforming the interferograms into spectra that are fully calibrated and independent of the state of the instrument at any given time.
[7] A digital processing subsystem executes a radiometric calibration and an inverse Fourier transform in order to obtain the raw spectra.
[7] The central objective of the Level 0 processing is to reduce the transmission rate by calibrating the spectra in terms of radiometry and merging the spectral bands.
[7] This level is concerned with deriving geophysical parameters from the radiance measurements:[1] The processes here are performed synergically with the ATOVS instrument suite, AVHRR and forecast data from numerical weather prediction.
The choice of algorithm is guided by knowledge of these limitations, the resources available and the specific features of the atmosphere that wish to be investigated.
The process is repeated again and again, the aim being to adjust the amount of contaminants such that simulated spectrum resembles the measured one as closely as possible.
IASI's main structure comprises 6 sandwich panels that have an aluminium honeycomb core and carbon cyanate skins.
This determines a very good thermal stability for the optics of the interferometer: the temporal and spatial gradients are less than 1 °C, which is important for the radiometric calibration performance.
[9] This was preferred to a cryogenic machine due to the fact that the vibration levels of the latter can potential cause the degradation of the spectral quality.
