The feasibility of detecting clear-air turbulence layers (CAT) by measuring the covariance function of the fluctuations in log amplitude of an optical signal propagated through the atmosphere is discussed. In particular, the problem of determining Cn2(z), the magnitude of the refractive-index fluctuations, is investigated. It is extremely unlikely that CAT can be detected from such measurements. Variations of the log-amplitude covariance function induced by CAT layers are so small that they will normally be masked by variations expected in other atmospheric parameters, such as changes of the spectral density of the refractive index or nonhomogeneity of the medium. There appears to be more promise in measurements that more directly measure the wind; further studies of this type are recommended.
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