| description abstract | The theory of the back-scatter from invisible atmospheric targets which are considered to be likely sources of ?angel? echoes is reviewed. Special attention is given to planes and bubbles and vapor sheaths of large radius of curvature with cross sections which cannot be obtained from the conventional far-zone radar theory. The cross sections of such surfaces approximate reported ?angel? cross sections if the surface reflection coefficient and corresponding refractive index gradient is large but not inconceivable. The segments of these surfaces need only be approximately as large as the first Fresnel zone in order to behave essentially like the entire surface. In the microwave band, the required sizes are of the order of a few meters at short ranges. Partial focusing by surfaces concave toward the radar may enhance the cross sections further. However, surface roughness may reduce the cross sections significantly. While it may be difficult to visualize essentially smooth atmospheric surfaces as large as 5 to 10 m, the alternative solution, which assumes bubbles or eddies of small radius of curvature, requires extremely large and questionable refractive index gradients. Although the question is left open for future validation, the author believes, on the basis of limited observations, that the first alternative is the more likely one. This is indicated in several important cases of ?angel? activity. | |
