Radar Cross Section (RCS) - MATLAB Simulations for Radar Systems Design - page 498

Exact methods of RCS prediction are very complex even for simple shape
objects. This is because they require solving either differential or integral equa-
tions that describe the scattered waves from an object under the proper set of
boundary conditions. Such boundary conditions are governed by MaxwellÓs
equations. Even when exact solutions are achievable, they are often difficult to
interpret and to program using digital computers.

Due to the difficulties associated with the exact RCS prediction, approxi-
mate methods become the viable alternative. The majority of the approximate
methods are valid in the optical region, and each has its own strengths and lim-
itations. Most approximate methods can predict RCS within few dBs of the
truth. In general, such a variation is quite acceptable by radar engineers and
designers. Approximate methods are usually the main source for predicting
RCS of complex and extended targets such as aircrafts, ships, and missiles.
When experimental results are available, they can be used to validate and ver-
ify the approximations.

Some of the most commonly used approximate methods are Geometrical
Optics (GO), Physical Optics (PO), Geometrical Theory of Diffraction (GTD),
Physical Theory of Diffraction (PTD), and Method of Equivalent Currents
(MEC). Interested readers may consult Knott or Ruck (see bibliography) for
more details on these and other approximate methods.

11.3. Dependency on Aspect Angle and Frequency
Radar cross section fluctuates as a function of radar aspect angle and fre-
quency. For the purpose of illustration, isotropic point scatterers are consid-
ered. An isotropic scatterer is one that scatters incident waves equally in all
directions. Consider the geometry shown in Fig. 11.2 . In this case, two unity
( ) isotropic scatterers are aligned and placed along the radar line of sight
(zero aspect angle) at a far field range . The spacing between the two scatter-
ers is 1 meter. The radar aspect angle is then changed from zero to 180 degrees,
and the composite RCS of the two scatterers measured by the radar is com-
puted.

1 m 2

R

This composite RCS consists of the superposition of the two individual radar
cross sections. At zero aspect angle, the composite RCS is . Taking scat-
terer-1 as a phase reference, when the aspect angle is varied, the composite
RCS is modified by the phase that corresponds to the electrical spacing
between the two scatterers. For example, at aspect angle

2 m 2

10° , the electrical

spacing between the two scatterers is

2 .0 × ( × cos ( ° ) )

elec spacing – = --------------------------------------------------

(11.6)

λ

λ is the radar operating wavelength.