The satellite-based instruments used for tropospheric NO₂ retrieval measure the solar irradiance falling on the Earth, together with the earthshine, the radiance from the Earth. As such they are known as passive instruments, because they are not using their own light source.

The principal instruments are spectrometers, which can determine the absorption, reflection and scattering characteristics of the atmosphere.

The ratio of extra-terrestrial irradiance and the upwelling radiance provides information about the amounts and the distribution of important atmospheric constituents which absorb or scatter light, as well as the spectral reflectance (or albedo) of the Earth's surface. This, in turn, allows us to deduce information about the composition of the Earth's atmosphere.

Characteristics of satellite platforms

Although the details of satellite technology are not directly relevant for a study of the techniques of remote sensing, it is useful to understand some basic facts about the satellite platforms that carry the instruments used in remote sensing. Two satellite-borne instruments are of interest for the measurement of NO₂ in the Earth's atmosphere: GOME on the ERS-2 satellite and SCIAMACHY on the ENVISAT satellite. Both these satellites are in sun-synchronous, retrograde, near-polar orbits.

The following sections explain the geometry and characteristics of such orbits for the remote sensing of trace gas species such as NO₂.