Passive remote sensing systems record electromagnetic energy that is reflected (e.g., blue, green, red, and infrared light) or emitted (e.g., thermal infrared radiation) from the surface of the Earth. Passive sensors therefore rely on an external energy source (e.g. sun illumination, Earth heat emission). Contrary to passive sensors, who detect naturally occurring radiation, active sensors emit radiation and collect and analyze the signal that is sent back by the Earth’s surface or atmosphere. Active remote sensing systems produce therefore their own electromagnetic energy. They transmit and receive the radiation that is reflected or backscattered from the illuminated target. They do not necessitate an external source of radiation (e.g. Sun or Earth). Contrary to most passive sensors that are bound to detecting either the reflected Sun radiation or emitted radiation by the Earth’s surface in ranges from the ultraviolet to the thermal infrared, active sensors can use any radiation from the electromagnetic spectrum, the only limitation being the transparency of the Earth’s atmosphere. They often use wavelengths that are not sufficiently provided by the Sun, e.g. microwaves.
Active systems can be categorized either according to their imaging capability, or according to the considered emitted wavelength, or also according to the way they use the returned signal. For the last category, it is generally distinguished between ranging systems, which use as principal information the time delay between transmission and reception of the electromagnetic radiation at the sensor, and scattering systems, which consider the strength (also called magnitude or intensity), of the returned signal. Some systems also register both information.
As active sensors produce their own radiation and do not rely on e.g. Sun radiation, they are daytime independent and can also retrieve information about the Earth’s surface by night. Furthermore, depending of the considered wavelength, active sensors are weather independent. For longer wavelengths of the microwave domain, clouds are transparent, as the transmitted wavelength is larger than the water particles constituting the cloud and do not interact with them.
Active sensors can control the direction of their illumination to a specific target to be investigated, but require in general more energy than passive sensors as they “actively” illuminate the Earth’s surface.