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Techniques of Remote Sensing.

Remote sensing is detecting and monitoring an area’s physical characteristics by measuring its reflected and emitted radiation at a distance. The earth is scanned by satellites or high-flying aircraft in order to obtain information about it.

Two main types of remote sensing exist and are classified according to the source of signal they use to explore the object, active and passive.

Active remote sensing instruments operate with their own source of emission or light.

Passive remote sensing relies on the reflected emission to obtain information. Reflected sunlight is the most common source of radiation measured by passive sensors.


Passive sensors can only detect energy when 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. The energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the energy is large enough to be recorded.

Active sensors, on the other hand, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor.

The advantages of 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 relatively large amount of energy to illuminate targets adequately.

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 the energy that is naturally available are called passive sensors.

A random/common example is using a camera in sunlight. During a bright sunny day, enough sunlight illuminates the targets and then reflects toward the camera lens. The camera records the radiation provided. This can be described as remote sensing in passive mode.

On a cloudy day or inside a room, there is often not enough sunlight for the camera to record the targets adequately. So, it uses its own energy source – a flash – to illuminate the targets and record the radiation reflected from them. This can be described as an active mode of remote sensing.