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ATMOSPHERIC WINDOW,Source of Energy,Remote Sensing


In the range of visible light were the sun emits highest intensities, atmospheric transmittance is the highest (see figure). In the range of higher wavelengths transmittance is reduced to narrow bands. This includes the optical windows in the thermal infrared, where the Earth's surface emits radiation. In the range of microwaves the atmosphere is nearly transmissive, but the sun and earth's radiation are low; therefore, this range is used by active radar systems. Wavelengths smaller than the ultraviolet are nearly totally absorbed by the atmosphere and are therefore less relevant for remote sensing. Remote sensing concentrates on the transmissive ranges, the so called atmospheric windows.

In particular, the molecules of water, carbon dioxide, oxygen, and ozone in our atmosphere block solar radiation. The wavelength ranges in which theatmosphere is transparent are called atmospheric windowsRemote sensing projects must be conducted in wavelengths that occur withinatmospheric windows.

Source of Energy

As noted earlier in lesson 1, the first requirement for remote sensing is to have an energy source to illuminate the target. Just as our eyes need objects to be illuminated by light so that we can see them, sensors also need a source of energy to illuminate the earth’s surface. The sun is the natural source of energy. This energy is in the form of electromagnetic radiation (EMR). The following subsections explain the classification of remote sensing.

Depending on the predominant source of electromagnetic energy in the remote sensing system, the remote sensing can be passive or active.
Passive Remote Sensing depends on a natural source to provide energy. The sun is the most 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 sun’s 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.

Remote Sensing Classification 

Based on wavelength regions the remote sensing can be classified as:
1.       Visible and reflective infrared remote sensing.
2.       Thermal infrared or emitted remote sensing.
3.      Microwave remote sensing.
The energy source used in the visible and reflective infrared remote sensing is the sun. The sun radiates EM energy with a peak wavelength of about 0.5 μm. Remote sensing data obtained in the visible and reflective infrared regions mainly depends on the reflectance of objects on the ground surface. Therefore, information about objects can be obtained from the spectral reflectance. However laser radar is exceptional because it does not use the solar energy but the laser energy of the sensor.
The source of the radiant energy in the thermal infrared remote sensing is the object itself, because any object with a normal temperature of about 27oC will emit EM radiation with a peak at about 9.7 μ m.
In microwave region, there are two types of microwave remote sensing, passive microwave remote sensing and active microwave remote sensing. In passive microwave remote sensing, the microwave radiation emitted from an object is detected, while the back scattering co-efficient is detected in active microwave sensing.

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