Remote sensing process depends on source of energy, it can be natural in the form of sun to illuminate object. Sun light has to pass many obstacle before reaching and interaction with object. This obstacle can be atmosphere, which absorb or scatter sun light at different wavelengths.
The quality of satellite image may affected by these obstacles and quality for correct interpretation and accurate classification, one need to improve satellite images by enhancement process.
Electro-Magnetic Radiation (EMR) propagates through the Earth’s atmosphere, however atmosphere may influence
- Speed of radiation
- Spectral Distribution
- Change in Direction due to refraction
So EMR may refracted, scattered and absorbed by atmosphere before reaching to ground/Earth’s surface
Refraction may take place, when EMR come across substances of different density, like air and water. Refraction is bending of light when it passes through one medium to another medium.
Refraction is taking place due to change in speed to EMR in media of different density.
Major errors can occur in location due to refraction as images formed from energy detected at high altitudes or at acute angles.
Scattering is different from refraction as the direction of scattering is unpredictable. Scattering is unpredictable diffusion of EMR by particles present in the atmosphere.
Scattering depends upon wavelength of EMR and diameter of particles. There are different types of scattering, broadly defined as selective and non-selective scattering.
Selective further defined as
It occurs when diameter of particles or gases molecules is less than the wave length of EMR. Eg. Blue wavelength is scattered to a much degree than green and red wavelength of visible spectrum, due to this scattering of blue wavelength sky appears blue.
This scattering take place when diameter of molecules or particles is approximately equals to the wave length of EMR. Aerosols such as dust, smoke and pollen grains contribute to Mie scattering.
This type of scattering occurs when diameter of particles is bigger than the wave length of EMR. The size of particle is sometime ten times bigger than the wave length. It scatters all wavelengths of visible part of EMR with equal efficiency; because of this clouds appear white.
Absorption is the process by which radiant energy is absorbed by atmospheric constituents. Absorption is range of wavelengths in the electromagnetic spectrum in which the radiated electromagnetic energy is absorbed by a substance.
Atmosphere contains molecules of nitrous oxide (N2O), H2O, CO2, Ozone (O3), and Oxygen (O2), which absorbs incident EMR of different wavelengths.
Combined effect of absorption, scattering and refraction can dramatically decrease the amount of EMR reaching the Earth’s surface, which is important part of remote sensing process.
Due to loss of EMR by refraction, scattering and absorption in incident energy on ground object. It is required to avoid wave lengths which have impact of refraction, scattering and absorption.
Therefore an atmospheric window is a term introduced in remote sensing.
It is defined as part of the electromagnetic spectrum in which atmosphere transmits radiant energy effectively and very less or no impact of refraction, scattering and absorption are known as atmospheric windows
Three main atmospheric constituents that absorb radiation are
- Ozone (absorbs UV rays from sun)
- Carbon dioxide (absorbs TIR portion of spectrum)
- Water vapor (absorbs incoming long-wave of IR and shortwave microwave radiations)
So an absorption band is a range of wavelength in electromagnetic spectrum, which is absorbed by a substance while reaching to the earth’s surface.
In the remote sensing technology, satellite data acquisition is limited to these non-blocked wavelength called atmospheric windows.
The regions of the EM spectrum which are not influenced by atmosphere are used for remote sensing and called atmospheric windows.
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