About the radiometer, how brightness temperature is defined, and why 3 frequencies are used?

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About the radiometer, how brightness temperature is defined, and why 3 frequencies are used?

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The brigthness temperature of a surface is equal to the product of the emissivity of this surface by its physical temperature. The radiation measured by the radiometer depends on the ocean surface emissivity, its physical temperature and water vapour and cloud absorption in the atmosphere. If you want to know precisely the atmospheric water vapour contents, you have to substract surface and cloud contribution from the signal received by the radiometer. That’s why several frequencies (3 in most cases) are used, each one being more sensitive than the other to one of these contributions. By combining measurements done at each frequencies, you can extract the water vapor signal.