A smooth horizontal surface, such as a calm water body, acts like a mirror to produce a specular reflection. These surface will appear dark on the imagery because there is little to no incident energy being reflected back. Be aware that as the radar wavelength increases, features that once appeared rough may begin to appear smooth.
Dihedral (Cornor) Reflectors: Buildings in populated areas usually look bright in radar images because of their shape.
The planar building sides intersect the surrounding ground at a right angle, creating a corner reflector. As shown in the illustration to the right, the radar signal bounces off of both planar surfaces and is reflected directly back toward the antenna, regardless of the depression angle.
Dielectric Constant (Moisture Content): The presence of moisture increases a material's complex dielectric constant. The dielectric constant influences the ability of a material to absorb, reflect and transmit microwave energy. The moisture content of a material can change its electrical properties. This affects how a material appears on the radar image. Identical materials can vary in appearance at different times or different locations according to the amount of moisture they contain. The reflectivity, and hence image brightness, of most natural vegetation and surfaces is increased with increasing moisture content. Microwaves may penetrate very dry materials, such as desert sand. The scattering which results, is affected by both surface and subsurface properties. In general, the longer the radar wavelength, the deeper into the material the energy will penetrate