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Stimulated by the experimental acoustic radar work of McAllister, this paper reviews the potential usefulness of acoustic methods for the remote probing of the lower atmosphere. Starting with a comparison of the effects of temperature, wind, and humidity fluctuations upon the refractive index of air to electromagnetic and acoustic waves, it is shown that the fluctuations in acoustic refractive index may be expected to be about one thousand times stronger than in the radio case. Since the scattered power is proportional to the square of the refractive index fluctuations, the scatter of acoustic waves may be expected to be roughly one million times stronger than for radio waves. In addition, the millionfold ratio between the velocities of electromagnetic and acoustic waves results in an acoustic system requiring one million times less bandwidth to interrogate a given atmospheric volume. Since the ambient noise levels per cycle per second in the two types of receivers are likely to be approximately equal, this results in an overall reduction in interfering noise power for the acoustic case of about a factor of one million. The net result is that the acoustic signal-tonoise ratio from a given scattering region (for equal radiated powers, antennas and wavelengths), is likely-to be some twelve orders of magnitude stronger than in the radio case! 619 https://ntrs.nasa.gov/search.jsp?R=19720017734 2018-05-11T02:31:22+00:00Z ACOUSTIC METHODS OF REMOTE PROBING OF THE LOWER ATMOSPHEREThe system parameters required to achieve an effective acoustic radar are discussed, using the theoretical work of Kallistratova, and including the effect of absorption. It is concluded that the acoustic radar technique could be developed to provide continuous information on the profile of wind speed and direction, the profile of mechanical turbulence (and hence of atmospheric diffusion), the profile of temperature inhomogeneity, (and therefore of optical refractive index inhomogeneity), the existence, location, and intensity of temperature inversions, and the variation of humidity with height. Limitations to the acoustic radar technique include its limited range (up to about 1500 meters) and the probability of serious loss of sensitivity due to increased noise level during periods of strong wind or rain or hail.

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