An apparatus and method for relating echoes from a surface or object produced by irradiation, such as by sonar or radar, from different grazing angles according to the relationship: f s = cf x 2 ⁢ cos ⁢ ⁢ θ where fx is the spatial frequency corresponding to the size of the surface or object, θ the grazing angle, and fs the temporal frequency. In this manner one can relate to one another echoes from the same surface or object generated at different grazing angles, permitting good correlation between the echoes.