3.2 Attenuation of Microwave

Attenuation results from absorption by atmospheric molecules or scattering by aerosols in the atmosphere between the microwave sensor on board a spacecraft or aircraft and the target to be measured. The attenuation of the microwave will take place as a function of the exponential with respect to the transmitted distance mainly due to absorption and scattering. Therefore the attenuation will increase in proportion to the distance, under homogeneous atmospheric conditions. The attenuation per unit of distance is called specific attenuation. Usually the loss due to attenuation can be expressed in the units of dB (decibel) as follows.

dB = K e B dr

where Ke: specific attenuation (dBkm)
B : brightness temperative ( Wm sr ) in the distance of dr
dr : incremental distance

Figure 3.2.1 shows the attenuation characteristics of atmospheric molecules with respect to frequency. From this figure it can be seen that the influence of atmospheric attenuation occurs in the region greater than 10GHz. The intensity of attenuation depends on the specific frequency (absorption spectrum) of the corresponding molecule. This is the reason why the energy of the microwave is absorbed by the molecular motion of the atmospheric constituent. However, if proper frequencies are carefully selected, the attenuation can be minimized because the composition of the atmospheric constituent is almost homogeneous.

In the case of satellite observation, the optical path is usually long in distance, so that attenuation can be influenced by the change in atmospheric conditions. Particularly because the attenuation of vapor (H2O) is very strong in the specific frequencies, the change of vapor can be detected by a microwave radiometer.

The most remarkable scattering in the atmosphere is due to rain drops. Figure 3.2.2 shows the attenuation characteristics due to scattering of rain drops and mist. The attenuation increases if the intensity of rainfall increases, and the frequency increases until about 40 GHz. However, over 40 GHz the attenuation does not depend on the frequency.

Remarks 1) dB is 1 / 10 bel.
"bel" is logarithmic ratio of two powers P1 and P2 .
N = log10 ( P1 / P2 ) [bel] or
n = 10 log10 ( P1 / P2 ) [dB]

2) Specific attenuation Ke is originally expressed as Np m or neepers m. But K e is converted to dB km for convenience by multiplying 10 log e = 4.34 x 10 by Np m.

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