The earth surface radiates a little microwave energy as well as visible and infrared because of thermal radiation. The thermal radiation of a black body depends on Plank's law in the visible and infrared region, while the thermal radiation in the microwave region is given by the Rayleigh Jeans radiation law .
Real objects, the so called gray bodies are not identical to a black body but have constant emissivity which is less than a blackbody . The brightness temperature TB is expressed as follows.
TB = T
where T : physical temperature
: emissivity (0 << 1)
Emissivity of an object changes depending on the permittivity, surface roughness, frequency, polarization, incident angle, azimuth etc., which influence the brightness temperature.
Figure 3.3.1 shows the characteristics of emissivity for 3.5 % density salt water with respect to incident angle, polarization and frequency. Figure 3.3.2 shows the emissivity of horizontal polarization (eh) and vertical polarization (ev) for two clay soils with different soil moisture, and a sea water with respect to incident angle.
Table 3.3.1 shows the emissivity of typical land surface covers for two different grazing angles of 30 and 45 .
Most users would like to get the physical temperature T instead of brightness temperature TB , which is measured by microwave radiometers.
Therefore emissivity should be measured or algorithms should be developed to identify the component of atmospheric radiation.
In the case of the receiving antenna of the microwave radiometer, all radiation from various angles are input to the antenna, which needs a correction of the received temperature with respect to directional properties of the antenna. The corrected temperature is called the antenna temperature.
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