The 4A/OP software package includes the radiative transfer model 4A (Automatized Atmospheric Absorption Atlas), initially developed at LMD (Scott & Chédin, 1981). The 4A model pioneered the idea of bypassing the time-consuming line-by-line process by calculating once and for all Look-Up-Tables (LUT) of compressed monochromatic optical depths («the Atlases»). Originally designed for the Earth atmosphere, the 4A concept has easily and successfully been extended to the Jovian atmosphere in the frame of the Voyager experiment.

• 4A allows accurate computations: The Atlases are created by the line-by-line and layer-by-layer model, STRANSAC (Scott, 1974), with state-of-the-art physics (line mixing and pressure shift for several species) based on inputs from the latest version of the GEISA spectroscopic database (or from any equivalent spectroscopic database);

• 4A allows fast computation of monochromatic or ISRF-convolved transmittances, radiances, and Jacobians for any geometrical/optical path or observation level, according to the user’s choice;

• 4A allows fast analytical computation of the Jacobians (Chéruy et al, 1995): these partial derivatives of the radiances with respect to temperature, gas mixing ratio, emissivity or albedo, aerosol optical depth and surface pressure are the backbone of online inversion algorithms for the retrieval of surface or atmospheric properties.