Passive and Active Sensors

Active sensor (left), passive sensor (right)

If a sensor is designed to record radiation from the sun, as reflected by the Earth, then such a "passive" sensor can only record images while observing the sun-lit side of the Earth. Other passive sensors which record radiation emitted by the Earth, or an active sensor which provides its own illumination, can make observations on both the sun-lit, as well as the dark side of the Earth.