Radar terrain return measured at near-vertical incidence

An experimental program to investigate the reradiation properties of terrain at near-vertical incidence was carried out. Data were obtained at 415 and 3800 mc, using narrow-pulse type radar at altitudes of 2000 to 12,000 feet over many different target areas. At frequencies over 400 mc most terrain acts as a scatterer of energy even at near-vertical incidence with a backscattering "radiation pattern" that drops off rapidly as the angle of incidence is increased. An exception occurs for heavily wooded areas which appear as nearly isotropic scatterers. At 415 mc, the radar cross section per unit area at vertical incidence ranges between values of 0.7 for woods to approximately 4 for city targets, while at 3800 mc, the variation ranges between values of 0.8 for woods to about 18 for some city targets. If the ground were a lossless isotropic scatterer, the radar cross section per unit area would be 2 at vertical incidence. For a wide beam antenna, the fading range between the level exceeded by 95 per cent of the return pulses and the level exceeded by only 5 per cent of the return pulses is generally between 12 and 17 db, except for a few very smooth areas which give considerable specular (nonfading) type of return and have a smaller fading range.