Emitted radiation (depending on temperature according to Planck's law) interacts with atmospheric absorbing molecules or scattering particles. Molecular absorption occurs only at discrete wavelengths. In the thermal infrared domain, absorption is due rotation-vibration of molecules of H2O, CO2, O3, CH4, N2O and some other gas traces. Overall, there are more than 800.000 absorption lines, the position of which being well known. The other line features like intensity and width are generally also well chararacterized but it remains some uncertainties for some of them. These data are given and updated in spectroscopic databanks like GEISA or HITRAN.
To compute the radiances spectra as measured from IASI, it is needed to integrate the radiative transfer equations on the vertical scale and in the spectral domain. The spectral integration is quite tough since the spectral transmittance is a very discontinuous function. The line by line models allow to compute the radiances at the highest spectral resolution and then to integrate by convoluting with the instrument spectral response functions.
There are several line by line models currently used to simulate spectra. Among them the 4A/OP radiative transfer code developed by LMD and maintained by Noveltis. In the framework of IASI preparatory program, the main codes have been compared (Tjemkes et al, 2002 in IASI publications).
To simulate the spectra and jacobians to be used in the inversion or assimilation schemes, much faster radiative transfer codes are needed. These hyperfast codes (like e.g. RTIASI) are based on predictors and simple functions with coefficients computed by regression with line by line simulated radiances.