Validity of the Rytov Approximation*

Abstract

The limitations on the applicability of the Rytov approximation are examined in this paper. It is shown that (1) the singular behavior of the perturbation series of which the Rytov approximation is the first term cannot be removed by adding a "constant" as suggested by the proponents of Rytov's method; (2) the validity of Rytov approximation is limited to situations in which the mean-square value of the logarithmic amplitude fluctuation is much smaller than unity; (3) the MTF data of Coulman do not provide a good test on the validity of the Rytov approximation; (4) experimental data on the range dependence of intensity fluctuations do not agree with the predictions of the Rytov approximation; and (5) in problems involving propagation near the zenith, the validity of the Rytov approximation is limited to the near field of the turbulent layers at the surface of the earth and near the tropopause.