Multifrequency coherences of short‐term fluctuations of line‐of‐sight signals—Electromagnetic and acoustic

Spherical wave weak‐scattering propagation theory is employed to show that the resolution of multifrequency atmospheric probing techniques is limited to inhomogeneity scale sizes that are larger than a certain cutoff scale size l c . The magnitude of l c depends on the frequency separation and the path geometry. Inhomogeneities smaller than l c cannot be resolved and inevitably contribute to measurement errors. Propagation experiments performed at radio and acoustic wavelengths confirm the existence of the predicted cutoff and are in good agreement with the theory. An example is given to illustrate how this cutoff limits the ultimate accuracy of a radio‐optical comparison measurement aimed at determining path‐averaged water vapor variations.