Measurement of fine-scale atmospheric structure using an optical propagation technique

Abstract

A propagation experiment was performed with a coherent beam of optical-frequency radiation transmitted over a 10-meter path. The combination of short path length and short wavelength makes the experiment sensitive to the fine-scale structure of the turbulent atmosphere. From the propagation measuretnents, we are able to infer both the characteristic size of the small-scale eddies known as the microscale /o and the functional form of the spectrum of permittivity fluctuations at wave numbers in the region of dissipation (i.e., wave numbers of the order of the microscale). NATURE OF EXPERI1VIENTThe theoretical calculations pertinent to this study are based on the expression for C•(p) the covariance of the logarithmic amplitudes of signals received at two points spaced a distance p apart and located in a plane perpendicular to the direction of propagation of the trans-,mitred wave. The expression is found in Tartarski [1961]. As Strohbehn [1970] points out, the covariance 'function C•(p), a measurable property of a propagated wave, is related to the permiStivity spectrum (•,(•), a property of the medium. Furthermore, when the Fresnel zone appropriate to the path (XL) •/• is of the same order as the microscale lo, the shape of the covariance function C•(p) depends strongly on the magnitude of to and, to a lesser extent, on the shape of the permittivity spectrum in the vicinity of the microscale (in the region of viscous dissipation). For this reason, the integral expression for Cx(p) has been evaluated by assuming different models of the pennittivity spectrum and many different values of the microscale lo. The computations were performed for a radiation wavelength of 6328 A and a path length of 10 meters, thus giving a Fresnel zone, (XL) •/•', of 2.5 mm, of the order of the microscale. The resultant evaluations of the integral are shown in Fi'gures i and 2, where the • NOw with Bell Telephone Laboratories, Incorporated, Holmdel, New Jersey 07733.

Optical and millimeter line-of-sight propagation effects in the turbulent atmosphere

“…Gray and Waterman (1970) have used this technique over short paths to investigate lo, the inner scale of turbulence. A typical experimental case is shown in Figure 6.…”

Remote sensing of the troposphere

“…An additional complication arises for path lengths shorter than about 100 m as the size of the most effective irregularity approximates the inner scale of turbulence. Gray and Waterman (1970) have used this fact to measure, with short-path scintillations, the size of the inner scale and the slope of the spectrum in the dissipation subrange. Strohbehn (1970a) has calculated a number of scintillation covariance functions corresponding to various models of the turbulent spectrum, and he discusses the sensitivity of scintillation covariance measurements for the determination of spectral parameters of turbulence, particularly the slope of the spectrum in the inertial subrange.…”

Modern Topics in Microwave Propagation and Air-Sea Interaction

“…The natural turbulence provides a wind indicator (Gray et al 1970). The natural turbulence provides a wind indicator (Gray et al 1970).…”