The IASI processing system generates calibrated atmospheric spectra from raw interferograms. These spectra are ready for assimilation by users and are processed on-board (within the DPS unit) and by the associate ground segment.
On board processing reduces the data rate from 45 Megabits per second to 1.5 Megabits per second. The algorithm used to process the raw interferograms basically correct non-linearities in the detection process, calculate the inverse optical Fourier transform and perform radiometric calibration of the spectra obtained. These spectra are downlinked from the satellite to the ground segment and from level 0 data.
Data are processed on the ground at three levels:
- level 1 A comprises decoding, spectral calibration, radiometric post-calibration, IASI/AVHRR/3 co-registration, location and dating
- level 1B consists in resampling the spectra
- level 1C comprises an apodization function and analysis of AVHRR/3 radiance levels in IASI pixels
All level 1 data are available to users.
Main level 2 products consists of:
- Temperature profiles
- Water vapour profiles
For instrument in-flight radiometric calibration the scan mirror positions its view towards the internal blackbody or the deep space view, which is at a known temperature. The deep space field of view is also used for long trend instrument degradation correction (due to ageing and contamination of the optical surface). Due to the instrument thermal fluctuation, the calibration occurs once every eight seconds.
The on-board data processing applies the numerical inverse Fourier transform and the radiometric calibration before data transmission to ground segment.