Aperture Averaging of Scintillation

The relationship between the statistics of log-amplitude fluctuations and irradiance fluctuations due to atmospheric turbulence is derived. This is used to evaluate the effect of use of a large aperture diameter in reducing the variance of a fluctuating signal. Curves for the reduction factor are presented. From these, an irradiance-fluctuation correlation distance is evaluated. This distance, unlike the correlation distance for log-amplitude fluctuations, is found to be a function of the log-amplitude variance. A particular example of the application of these results to a space-to-ground communications systems performance is worked out.