Sensor is a device that gathers energy (EMR or other), converts it into a signal and presents it in a form suitable for obtaining information about the target under investigation. These may be active or passive depending on the source of energy.
Sensors used for remote sensing can be broadly classified as those operating in Optical-Infrared (OIR) region and those operating in the microwave region. OIR and microwave sensors can further be subdivided into passive and active.
Active sensors use their own source of energy. Earth surface is illuminated through energy emitted by its own source, a part of it is reflected by the surface in the direction of the sensor, which is received to gather the information. Passive sensors receive solar electromagnetic energy reflected from the surface or energy emitted by the surface itself. These sensors do not have their own source of energy and can not be used at nighttime, except thermal sensors. Again, sensors (active or passive) could either be imaging, like camera or sensor, which acquire images of the area and non-imaging types like non-scanning radiometer or atmospheric sounders.
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