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
