With the term 'atmospheric windows' we refer to the regions of the electromagnetic spectrum where the interaction of the atmosphere with the electromagnetic radiation is minimized. There are three main ‘windows’ in the Earth's atmosphere. The first of these includes the visible and near-infrared (VNIR) parts of the spectrum, between wavelengths of about 0.38 μm and 3.5 μm, although it does also contain a number of opaque regions. The second is a rather narrow region between about 8 μm and 15 μm, in which is found the bulk of the thermal infrared (TIR) radiation from objects at typical terrestrial temperatures. The third more or less corresponds to the microwave region, between wavelengths of a few millimetres and a few metres. Thus we can expect that any active system designed to penetrate the Earth’s atmosphere will operate in one of these three ‘window’ regions.