A laser beam propagating through the lower atmosphere will be distorted by variations of the refractive index along the propagation path causing random fluctuations in the received signal flux. If the receiver aperture is finite (i.e., not a point), these fluctuations will in some sense be averaged. An aperture-averaging factor has been defined to specify this effect. Theoretical predictions of this factor in the region of weak fluctuations have existed in the literature for some time. Earlier experimental work, however, failed to show agreement with these calculations. Measurements of the aperture-averaging factor were made during the summer of 1972 near Boulder, Colo., over a horizontal (statistically homogeneous) path at distances of 200–1600 m. Good agreement with the theoretical calculations was observed in the region of weak fluctuations (i.e., at the shorter path lengths). Data in the region of strong fluctuations (i.e., at the longer path lengths) are inconclusive.
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