Principles Of Satellite Remote Sensing

All objects emit electromagnetic radiation. The hotter the source, the greater is the intensity of emission. Substances which absorb all the radiation falling on them at every wavelength are called "black bodies". The coefficient of absorption is then unity. As per Kirchhoff's law, good absorbers are good emitters as well. Hence a black body also has an emissivity unity. At any wavelength it emits the maximum amount of radiation that is appropriate to its temperature.

Most substances, however, are not perfect black bodies. Their emissivity is less than unity. Figure 3 shows wavelengths of different types of radiation and the channels used for satellite imagery. It includes the spectra of solar radiation (at temperatures of about 6000 deg. K and also of terrestrial radiation of the earth and atmosphere at temperatures between 200 and 300 deg. K). Solar radiation is in shorter wavelengths and the terrestrial radiation is in longer wavelengths. Solar radiation of significant intensity occurs at wavelengths between 0.2 and 4.0 ^m, the peak intensity at about 0.5 ^m in the visible part of the spectrum. Terrestrial radiation is emitted at wavelengths between 3 and 100 ^m which falls entirely within the infrared region. The maximum intensity is around 11 ^m.

Unlike solids and liquids, gases are not black bodies. They only absorb or emit strongly at certain wavelengths. Water vapour (H2O), carbon dioxide (CO2) and ozone (O3) are such gases within the visible and infrared wave bands that are important in meteorology. Each of these gases is active in certain narrow absorption bands. There are other regions where the absorption by all the gases is so weak that the atmosphere is almost transparent. These regions are known as "windows" and are used for production of cloud imagery. Satellite imagery is obtained from radiometers that measure scattered electromagnetic radiation emitted from the sun, earth and the atmosphere.

6000 k

6000 k

0.2 0.4 0.6 0.81.0 2.04.0 6.0 8.010 ).1 Wave length (microns)

0.2 0.4 0.6 0.81.0 2.04.0 6.0 8.010 ).1 Wave length (microns)

Wave number (cm )

Figure 3 : Blackbody radiation emitted at temperatures corresponding to the Sun and the Earth

The satellite imageries in common operational use are :

a) Visible (VIS) - imagery derived from reflected sunlight at visible and near-infrared wavelength (0.4 - 1.1 ^.m).

b) Infrared (IR) imagery (Fig. 3) derived from emissions by the earth and its atmosphere at thermal infrared wavelengths (10-12 ^m)

c) Water Vapour (WV) imagery derived from water vapour emissions (6-7^m) and d) 3.7^m (often referred to as channel 3) imagery in the overlap region of solar and terrestrial radiation and hence sometimes called near IR.

e) Images from microwave radiometer such as Special Sensor Microwave/ Imager (SSM/I), and TRMM Microwave Imagers (TMI) can provide a lot of useful information. Microwave radiation is not affected by the presence of clouds and that is an important factor in the science of weather. Microwave observations are widely used for inferring sea surface temperature, sea surface wind speed and atmospheric water vapor content (over ocean surfaces), cloud liquid water content, rainfall, and the fraction of ice/snow particles within the raining systems.

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