The solar spectrum is not an ideal source for remote sensing. It is a complex and variable spectrum with features such as Fraunhofer lines. The first stage of the DOAS procedure is a normalisation process that removes from the earthshine spectrum the unwanted features that are present in the spectrum of solar irradiance.

As a first step in the normalisation, the natural logarithms of the values of the earthshine and solar irradiance spectra are taken.