Showing papers on "Scintillometer published in 1979"

Turbulence of the upper atmosphere and isoplanatism.

Abstract: We report the results of 1,049 measurements of the vertical profile of optical turbulence as recorded by a scintillometer above a site at White Sands Missile Range. The distributional law for these measurements is shown to be approximately log normal and examples of monthly to hourly variations in profile structure are presented. An estimate is formed for the isoplanatic angle for wave propagation through each profile by calculating its five-third moment. The ensemble of these calculations is found to be log normally distributed with a mean of 7.2 microrad at a wavelength of 0.5 microm. A strong temporal correction is observed between the size of the isoplanatic angle and the intensity of scintillations. We develop a theory based upon aperture averaging to account for this phenomenon and propose the use of scintillometry to make direct measurements of isoplanatism.

Laser beam propagation in atmospheric turbulence

Abstract: The optical effects of atmospheric turbulence on the propagation of low power laser beams are reviewed in this paper. The optical effects are produced by the temperature fluctuations which result in fluctuations of the refractive index of air. The commonly-used models of index-of-refraction fluctuations are presented. Laser beams experience fluctuations of beam size, beam position, and intensity distribution within the beam due to refractive turbulence. Some of the observed effects are qualitatively explained by treating the turbulent atmosphere as a collection of moving gaseous lenses of various sizes. Analytical results and experimental verifications of the variance, covariance and probability distribution of intensity fluctuations in weak turbulence are presented. For stronger turbulence, a saturation of the optical scintillations is observed. The saturation of scintillations involves a progressive break-up of the beam into multiple patches; the beam loses some of its lateral coherence. Heterodyne systems operating in a turbulent atmosphere experience a loss of heterodyne signal due to the destruction of coherence.

Optical propagation through turbulent air

Abstract: Turbulent air causes optical rays to deviate from otherwise straight paths by irregular slight undulations, and it also changes the phase velocities irregularly. These fluctuations cause a host of unpleasant degradations to communications signals. This survey circumvents the Scylla of propagation theory and Charybdis of meteorological input by introducing simple physical concepts to explain the underlying interaction of random fluctuations of the refractive index typical of turbulence with basic electromagnetic waves.