EHF attenuation derived from emission temperatures in light rain

Abstract : Atmospheric emission measurements have routinely been used to determine the total attenuation on an earth-space path at the centimeter and millimeter wavelengths. In the presence of rain, scattering effects have to be taken into account at frequencies 20 GHz when one interprets emission data to derive the total attenuation. If one uses the radiometric relation to derive the attenuation, assuming an absorptive medium, one finds that the predicted attenuation underestimates the true attenuation. The reason for this the rain scatters more emission energy out of the direct ray path than into it. Calculations of the atmospheric emission temperature and attenuation in light-to-moderate rain at different frequencies and rain rates are performed, taking into account molecular absorption, and varying cloud cover and ground temperature. The results show that for typically encountered meteorological conditions, the radiometric formula can be used to derive the total attenuation ( 10 dB) from emission measurements with accuracies of the order less than 1 dB, provided that the empirical constant in the radiometric formula, dependent on the ambient temperature, is a function of frequency and ground temperature.