Saturation of Scintillation Magnitude in Near-Earth Optical Propagation*

Abstract

Both a pulsed laser and a helium–neon laser have been used to examine the magnitude of scintillation as a function of range and turbulence strength over a near-earth, horizontal path. A photo-optical technique was utilized to record directly a 61-cm cross section of a received laser beam. Photographs of beam cross sections made at ranges of from 200 to 1500 m were used to compute log-irradiance variances. Simultaneous with the optical data, measurements of the index-structure coefficient Cn were made by use of a high-speed thermal technique. The data are used to test the spherical-wave equation that gives the log-amplitude variance as a function of range and Cn. The measurements indicate that the variance increases for ranges up to about 700 m, at which distance saturation occurs, i.e., no further growth of the variance is observed. The data are also compared with the saturation equations of Tatarski and deWolf. In addition, the effect of the transmitter-beam divergence on the magnitude of scintillation is examined. Finally, a modification of the inertial subrange of the Kolmogorov turbulence model is suggested to explain the occurrence of particular optical effects observed during temperature-inversion conditions.