Measurements and analysis of a blue-green pulsed propagation through fog have identified three distinct regions for energy transport. Region I small number of attenuation lengths tau in the path (0 32): the direct beam and the forwardscattered beam have decayed to the point where the diffusion type multiple-scattered radiation is the dominant energy received. This component does not decay exponentially but results in large spatial, angular, and temporal spreading. This paper presents quantitative data on Region II.
The first blue-green laser propagation measurements through clouds that simulate the geometry of a satellite-to-ground communication link were made. The time history of large diameter (approximately 6-km) pulses illuminating cloud tops was recorded as a function of receiver field of view (FOV). The maximum pulse stretching observed for nanosecond laser pulses was 20 microsec for clouds of 1.5-km thickness. It was shown that the pulses could in general be represented by a linear combination of two modified gamma functions: One, a slowly decaying term, represents the power from the diffusion type of multiple scattering. The other, a much faster decaying term, represents the power from a direct nonscattered portion of the beam or from a lower order of multiple scattering. For very dense clouds, the only component measured was the diffusion type. Data of FOV scans are presented for various values of optical thickness.
An experimental-theoretical program aimed at characterizing over-the-horizon (OTH) propagation via marine atmospheric aerosols is described. Pathloss measurements as a function of transmitter-receiver azimuth-elevation angles for 1.06 microm and 0.53 microm over two OTH links are reported. The first link involves an overwater range of 63 km with 19-km and 40-km horizons; the second involves an overwater range of 128 km with a 19-km horizon. Two theoretical models, one based on particulate single scatter and the other based on particulate multiple scatter, are presented and compared with the pathloss measurements obtained over the 63-km link. Good agreement between both models and experiments is shown. No quantitative comparison between the models and the 128-km experiment is made because of the lack of accurate meteorological data over this extended path.
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