The retrieval of the quantity of NO₂ in the atmosphere using satellite measurements is based on absorption spectroscopy. When using the Differential Optical Absorption Spectroscopy (DOAS) method, a number of assumptions are made to simplify the complex radiative transfer equation to a system that can be solved quickly and with good accuracy.

The present section will show the main steps in this procedure, without, however, providing a rigorous derivation. These steps are:

1. The simplification of the light path
2. The relationship between irradiance and earthshine
3. The substitution of slant column densities
4. The substitution of a scattering polynomial
5. The derivation of the final DOAS radiative transfer equation