One of the most common ways to classify remote sensing systems consists in distinguishing them into the passive systems, which detect naturally occurring radiation, and the active systems, which emit radiation and analyse what is sent back to them. The passive systems can be further subdivided into those that detect radiation emitted by the Sun (this radiation consists mostly of ultraviolet, visible and near-infrared radiation), and those that detect the thermal radiation that is emitted by all objects that are not at absolute zero (i.e. all objects). For objects at typical terrestrial temperatures, this thermal emission occurs mostly in the infrared part of the spectrum, at wavelengths of the order of 10 μm (the so called thermal infrared region), although measurable quantities of radiation also occur at longer wavelengths, as far as the microwave part of the spectrum. Active systems can, in principle, use any type of electromagnetic radiation. In practice, however, they are restricted by the transparency of the Earth’s atmosphere.