GROUND CLUTTER

frequency (longer wavelength), the more the electromagnetic wave is diffracted. Sidelobes are a direct result of diffraction occurring near the edges of a radar antenna. GROUND CLUTTER Another factor affecting radar performance is ground clutter. Ground clutter is an unavoidable form of radar contamination. It occurs when fixed objects, such as buildings, trees, or terrain, obstruct the radar beam and produce non-meteorological echoes. Echoes resulting from ground clutter are usually exaggerated in both size and intensity and may cause radar systems to overestimate precipitation intensity near the radar. Clutter is normally found close to the antenna where the radar beam is nearest to the ground. Further out, the beam points gently skyward and overshoots most obstacles. Under certain circumstances, however, clutter may exist far away. A tall mountain range would be a good example of this. The key to dealing with ground clutter is operator awareness and experience. OTHER FACTORS AFFECTING EM ENERGY As energy travels through space, it interacts with millions of scatterers. This interaction causes a significant amount of energy to be lost or attenuated. Attenuation occurs in the form of scattering or absorption, but in either case, it reduces radar performance. The degree of attenuation is dependent on several factors, particularly radar frequency and atmospheric water vapor content. Scattering Scattering occurs when energy strikes a target and is deflected in all directions (fig. 2-17). Forward scattering is re-radiation of energy away from the antenna, and all of this energy is permanently lost to the radar. Not all forms of scattering are attenuation. Backscattering, for example, occurs when energy is reflected back toward the antenna resulting in a net gain. Larger targets tend to backscatter significantly more energy than smaller ones do, and therefore, result in stronger returns. Without backscattering, no targets would be detected and no echoes would be plotted. The bottom line is that scattering may or may not be attenuation. The type and amount of scattering present determines radar performance. The degree of scattering is not only dependent on particle size and composition, but also wavelength. Wavelengths used in weather radar are selected to minimize scattering by particles smaller than the radar was designed to detect (i.e. clouds and precipitation). Figure 2-17.—Scattering of radar energy. 2-15