Effect of multiple scattering on line-of-sight propagation

This paper deals with the effect of small random variations in the atmosphere's refractive index upon electromagnetic waves propagated along a line-of-sight path. These fluctuations are considered to be due only to the turbulent character of the medium, and the effect of the earth and other bodies is taken to be negligible. Previous analyses of this problem have utilized the first Born or single-scattering approximation. This method, however, assumes that each part of the scattering region is illuminated by the incident wave, i.e., that any incident wave is scattered at most once. The applicability of this approximation in the microwave region is questionable since the mean free path for scattering at high frequencies decreases as the square of the frequency. If at microwave frequencies the transmission distance is large compared to the mean free path length multiple-scattering effects are of importance. It is these effects which are considered here. Multiple-scattering analysis is used to determine expressions for the mean value, mean-square value, and variance of the field at a single receiver. Also determined is the correlation function for zero time difference for the fields at two spaced receivers, as well as the mean value, mean square, and variance of the difference of the fields at two spaced receivers.