The instruments used for the remote sensing of NO₂ are passive spectrometers that measure the spectrum of light returning from the earth, the so-called 'earthshine'.

This earthshine is sunlight that has passed through the atmosphere and has potentially been subjected to a wide range of possible interactions in the various layers of the atmosphere and/or at the surface. One of these many interactions is the absorption by molecules of NO₂ in the atmosphere.

As they pass over the Earth these instruments scan the track beneath them from side to side and take measurements of the earthshine coming from discrete sections of the surface known as pixels.

This module describes the scanning techniques used by satellite-borne spectrometers and the interactions that can take place in the earthshine they measure.