Line-of-sight propagation through atmospheric turbulence near the ground

Abstract

Line-of-sight measurements of the log-amplitude and phase fluctuations of pure tones between 250 and 4000 Hz propagated over distances between 2 and 300 m in the turbulent atmosphere close to the ground are compared quantitatively with simple theory using simultaneously measured meteorological variables. The theory is based on the assumption of homogeneous and isotropic turbulence and approximates the availability of eddy sizes in the source region of turbulence by a Gaussian spectrum. In particular the transverse or mutual coherence function (the coherence in a plane perpendicular to the direction of propagation) and the coherence in the direction of propagation which we call the longitudinal coherence, are also calculated and compared with the measurements. When the measured mean square phase fluctuations are compared with the theory using the meteorological measurements, good agreement is obtained. However the measured mean square log-amplitude fluctuations are in general substantially smaller than predicted and, in addition, show clear evidence of saturation. The distance to saturation is shown to correspond to the longitudinal coherence length. Because of this behavior of the amplitude fluctuations both the transverse and longitudinal coherences are essentially a function of the phase variance only.