Some Calculations on Coupling Between Satellite Communications and Terrestrial Radio-Relay Systems Due to Scattering by Rain

Sharing of microwave bands by terrestrial and satellite-communication systems has stimulated investigation of the factors which lead to mutual interference. Of course, antenna characteristics are of prime importance; very low immediate and far side lobes in the antenna 1491 1492 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1970 radiation patterns are mandatory in reducing coupling between stations of the two systems. However, even with ideal antennas, there are certain propagation mechanisms that can introduce coupling, one being scatter by precipitation, in particular, rain. Much knowledge of scattering by rain has been gained through studies with weather radar. 1 Relationships between the backscattering coefficient, the size of the raindrops, the rain rate, and the radio wavelength have been established. These commonly accepted relationships are used in our calculation. Attenuation by the rain is also germane to the calculation, especially at frequencies exceeding 10 GHz, because power from one station that is scattered by a particular volume of rain may well pass through additional precipitation before reaching the other station. The attenuation constants used here are based on measurements with terrestrial systems; 2 for high rain rates they agree with calculations using the Mie theory with a Laws-Parsons drop-size distribution. In these calculations, the satellite-communications antenna is assumed to be large in terms of wavelengths and to have a well-tapered quasigaussian illumination.