RESUMO
The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ~10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.
RESUMO
Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index sigmaI(2) and the atmospheric structure constant Cn(2) are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of Cn(2) or of sigmaI(2) akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment.
