Theoretical expressions for the spectral density of amplitude, amplitude-difference, and phase-difference fluctuations of waves propagating over a line-of-sight path in a weakly scattering turbulent medium are derived. Experimental observations made at radio (35 GHz) and acoustic (3 kHz) frequencies are in good agreement with the theoretical predictions under a variety of meteorological conditions. Comparison of experimental and theoretical spectra yields a measure of the average across-the-path wind velocity and the average refractiveindex structure constant C,•. In general, wind speeds and refractive-index structure constants inferred from simultaneous meteorological measurements agree with those obtained from propagation data. rived a theoretical expression for the power spectrum of amplitude fluctuations of a plane wave propagating through a turbulent atmosphere. Later, Tatarskii [1971] extends these results to phase and phasedifference variations; however, he still considers only the plane-wave case. Skrypnik [1966] examines similar spectral expressions for the case of nonuniform wind along the propagation path. Temporal frequency spectra for a spherical wave are obtained by Clifford [1971] and Ishimaru [1972] in the form of double integrals; they also give asymptotic expressions for large and small fluctuation frequencies. Mandics [1971, pp. 17-22] numerically evaluates amplitude and phase spectra for a specific geometry. Numerous workers have calculated fluctuation spectra from measured scintillations of optical, microwave, and acoustic frequencies (e.g., Tatarski [1961], HOhn [1966], Janes et al. [1970], and Golitsyn et al. [1960]). Due to lack of an adequate theoretical framework, however, it has become possible only recently to compare experimentally obtained spectra with the appropriate theory (see Clifford et al. [1971], Ishimaru [1972], and Mandics [1971, pp. 59-74]).The present study will give the results of line-ofsight propagation experiments performed at mm-wave and acoustic frequencies. These experiments are unique in that the incident wavefront is sampled (nearly) simultaneously at several points in space and at a very rapid rate. The sampling rate is set high enough in each case to include all atmosphere-185 of this assumption will have to be examined more closely.Assuming that scatterers of importance lie within the inertial subrange of ,the turbulence spectrum it is possible to express •(•) in the following form [Tatarski, 1961, p. 58] ß ,,(g) = 0.033C,,=(s)t• -•/a 2r/Lo < t• < 2r/go (3) where it is realized that the refractive-index structure constant, C,, may be a function of position SHORT-TERM SIGNAL-FLUCTUATION SPECTRA 157 For the moment, however, the -11/3 exponent indicated in equation 3 is assumed to be valid. It is convenient to normalize the amplitude covariance expression in the following manner' along the path. (Here, l0 and L0 are the inner and that is, it is possible to express the time-lagged autoouter scales of turbulence, respectively.) In addi-covariance function, B•(r), in...

Experiments have been carried out in order to measure the spatial and temporal nature of amplitude fluctuations observed on a 35 GHz, 28-km line-of-sight transmission path. The fluctuations are typically 1 dB in amplitude, with spectral widths of up to 1 Hz, and apparently drift horizontally past the several receiving antennas. The velocity of this drift, as determined by cross-correlation analysis of the fluctuations at several points, correlates well with the spectral width of the fluctuations, and is also correlated with observed windspeeds, indicating that the observed drift of the amplitude fluctuations is the result of the gross motions of the atmosphere. No corresponding drift was observed for vertical antenna separation. The spatial scale length of the fluctuations remained approximately the same for all conditions encountered, in both the horizontal and vertical planes. It is concluded that the temporal characteristics of the received signal are largely the result of the horizontal drift of a relativel_y fixed spatial distribution of amplitude, rather than of temporal changes in this distribution.