EHF attenuation derived from emission temperatures in light rain

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 interpreting 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 is that 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 better than about 1 dB provided that the effective medium temperature in the radiometric formula is a function of frequency and ground temperature.