Statistics of differential rain attenuation on adjacent earth-space propagation paths

Forty hours of observations of the three-dimensional radar reflectivity structure of rain in the Montreal area were used to simulate the centimetre-wavelength attenuations occurring simultaneously over a multitude of earth-space propagation paths. The same data base had been used in a previous study which confirmed that the radar estimates of attenuation statistics are in good agreement with attenuation distributions measured independently by passive radiometers. Differences between the attenuations occurring simultaneously over pairs of paths to a common point at the ground were tabulated as a function of angular path separation, viewing direction, radio frequency, and system margin. These data are required for evaluating the possibility of interference at an earth station caused by a signal from an adjacent satellite. In this study, values of angular separation from 2 to 9 degrees, radio frequencies from 11 to 30 GHz, elevation angles from 5 to 30 degrees, and system margins from 2 to 10 dB were considered. It was found that the cumulative distributions of differential attenuation could be characterized effectively by the attenuation difference occurring during 1 % of the time that the carrier signal suffered some attenuation, but not an attenuation exceeding the system margin. It was also found that this characteristic differential attenuation could be expressed to good approximation as a simple analytical function of the system parameters.