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Poster Session
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Microwave radiometer and its application
to snow measurement*
Zhang Junrong, Zhao Renyu, Guo Fenglian,
Zhao Kai, and Hu Xuewei
Changchun Institute of Geography,
Academia Sinica
China
Abstract
The microwave radioactive property of snow is briefly summarized. From 1986 to 1989, experiments were carried out the Changchun Jingyue Tan Remote Sensing Site, using self developed microwave radiometers to study the microwave radioactive property of snow. The relation between brightness temperature and observation angle was obtained with both vertical and horizontal polarizations. The curves corresponding brightness temperature and snow depth were also drawn out for different frequency band sand physical conditions. The results demonstrate the application potential of the microwave radiometer in this area.
Principile of snow measurement using microwave radiometer
All substances at a finite temperature radiate electromagnetic energy. In the microwave region, the Rayleigh-Jeans'approximation satisfied the Plank's formula which leads to
B ( l, T) = (2KT ( q, j) / l2) [ w. m-2 . s-1r. H1z] ............... (1)
where
l -------- wave length (m);
K -------- Boltzman constant [1.38 X 10-23 W. S. K-1];
T [ q, j] ----------Physical temperature of black body (K),
with [ q, j] denotes the angular dependence.
Radiation of real materials is less than black body. A equivalent radiometer temperature T B ( q, j) called brightness temperature is relatively lower than T( q, j)
TB = e. T ............... (2)
e is called the emissivity of the substance, 0 [1]
PR = K. D f.(òòò TB (q,j) . D(q,j) . D· (q,j) . dW / òòò D (q,j) . D· (q,j) . dW
= K. TA . Df ............... (3)
where
TA = (òòò TB (q,j) . D(q,j) . D· (q,j) . dW / òòò D (q,j) . D· (q,j) . dW
The brightness temperature is averaged by the weighting function
D(q,j) which is the antenna pattern.
TA=TB if the medium is isotropical or the antenna beam width is
sufficiently narrow. The PR is linearly related to TB when the
radiometer is calibrated. and independent of. Df We conclude from the above discussion that the power intensity which is received by the microwave radiometer is determined by. the emissivity of the material. Different material have different emissivities. The emiccivity of snow depends on the ice particle size, wetness, polarization and observation angle. Therefore, it's possible to detect the depth and the state of snow accordingly. For lossy material, the emissivity and the reflectivity is related by
e = 1 -r ............... (4)
For either smooth or rough surface, r is mainly determined by the dielectric constant of the material.
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* The project is supported by the National Natural Science Foundation
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