Satellite sensors record radiation reflected, emitted and scattered from the Earth's surface
Depending on the source of the predominant source of electromagnetic energy in the remote sensing system, remote sensing can be classified into passive and active remote sensing.
Passive remote sensing depends on a natural source to provide energy. The sun is the most powerful and commonly used source of energy for passive remote sensing. The satellite sensor in this case records primarily the radiation that is reflected from the target. Remote sensing in the visible part of the electromagnetic spectrum is an example of passive (reflected) remote sensing.
A portion of the sunís radiation that is not reflected back to the sensor is absorbed by the target, raising the temperature of target material. The absorbed radiation is later emitted by the material at a different wavelength. Passive remote sensing can also be carried out in the absence of the sun. In this latter case, the source of energy is the target material itself and the sensor records primarily emitted radiation. Remote sensing in the thermal infrared portion of the electromagnetic spectrum is an example of passive (emitted) remote sensing.
Active remote sensing uses an artificial source for energy. For example the satellite itself can send a pulse of energy which can interact with the target. In active remote sensing, humans can control the nature (wavelength, power, duration) of the source energy. Remote sensing in the microwave region of the electromagnetic spectrum (radar remote sensing) is an example of active remote sensing. Active remote sensing can be carried out during day and night and in all weather conditions.