Active & Passive Remote Sensing
Various references have been made to the sun as a source of energy or radiation in remote sensing. The sun provides a very convenient source of energy for remote sensing. The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then re-emitted, as it is for thermal infrared wavelengths.
Remote sensing systems which measure energy that is naturally available are called Passive Sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth.
There is no reflected energy available from the sun at night. Energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded.
Active sensors, on the other hand, transmit short bursts or 'pulses' of electromagnetic energy in the direction of interest and record the origin and strength of the backscatter received from objects within the system's field of view. Passive systems sense low level microwave radiation given off by all objects in the natural environment.
Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Active sensors can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated.
However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and a synthetic aperture radar (SAR)
Radar imaging systems such as ERS (European Remote Sensing Satellite), JERS (Japan Earth Resources Satellite), and RADARSAT-1 are active systems. They both transmit and receive energy. Microwave scanning radiometers only receive microwave energy. The Japanese MOS (Marine Observation Satellite) and JERS satellite systems employ microwave scanning units. Since the source is a very low level of electromagnetic energy, this type of data is prone to noise.
One advantage of active radar sensing systems is that, since they provide their own source of energy, they can collect data at any time of the day or night. Passive sensors - optical, thermal, and microwave - rely on receiving the naturally emitted or the sun's reflected energy from the Earth's surface and can therefore image only during the daytime.
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