Surface and Volume Clutter - MATLAB Simulations for Radar Systems Design - page 277

6.3. Volume Clutter
Volume clutter has large extents and includes rain (weather), chaff, birds,
and insects. The volume clutter coefficient is normally expressed in square
meters (RCS per resolution volume). Birds, insects, and other flying particles
are often referred to as angle clutter or biological clutter.

As mentioned earlier, chaff is used as an ECM technique by hostile forces. It
consists of a large number of dipole reflectors with large RCS values. Histori-
cally, chaff was made of aluminum foil; however, in recent years most chaff is
made of the more rigid fiberglass with conductive coating. The maximum chaff
RCS occurs when the dipole length

L is one half the radar wavelength.

Weather or rain clutter is easier to suppress than chaff, since rain droplets
can be viewed as perfect small spheres. We can use the Rayleigh approxima-
tion of a perfect sphere to estimate the rain dropletsÓ RCS. The Rayleigh
approximation, without regard to the propagation medium index of refraction
is:

9π r 2 k ( 4

σ = r λ «

(6.29)

where k = 2πλ ⁄ , and r is radius of a rain droplet.

Electromagnetic waves when reflected from a perfect sphere become
strongly co-polarized (have the same polarization as the incident waves). Con-
sequently, if the radar transmits, for example, a right-hand-circular (RHC)
polarized wave, then the received waves are left-hand-circular (LHC) polar-
ized, because they are propagating in the opposite direction. Therefore, the
back-scattered energy from rain droplets retains the same wave rotation (polar-
ization) as the incident wave, but has a reversed direction of propagation. It
follows that radars can suppress rain clutter by co-polarizing the radar transmit
and receive antennas.

Denote η as RCS per unit resolution volume V W . It is computed as the sum

of all individual scatterers RCS within the volume,

N
∑

η = σ i

(6.30)

i = 1

where is the total number of scatterers within the resolution volume. Thus,
the total RCS of a single resolution volume is

N

N
∑

σ W = σ i V W

(6.31)

i = 1