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regions are dened depending on the target size in
terms of the incident wavelength as: low-frequency
region or Rayleigh region, resonance region or Mie
region, high-frequency region or optical region.
• Low-frequency region or Rayleigh (2π/λ< 1),
where the induced current in the body of the
target is approximately constant in amplitude
and phase.
• Resonance or Mie region (2π/λ≈1). e phase
variation of the current through the body of
the target is important and all parts contribute
to the dispersion. Generally, λ against 2π/L
will uctuate.
• High-frequency or Opticalregion (2π/λ>1).
ere are many cycles in the variation of the
current phase through the target body and,
consequently, the discrete eld will be a highly
dependent angle (Jenn D. C., 1995).
Methods to predict RCS
Methods are available to make RCS
predictions,such as methods of moments, nite
dierence, microwave optics, and method of
optical physics where many disadvantages can be
found in each of them in terms of high computation
requirements and increased runtime to perform
the prediction;described as the best option is
RCS prediction in the optical physics method
consuming the least amount of computational
resources and with less complexity (Garrido,
2000). It is necessary to validate, in any case, the
accuracy for ranges of frequencies of interest and
the size of the ships to consider.
Modeling and visualization of radar targets
Several types of modeling and visualization of
radar exist such as geometric modeling of solids,
modeling by borders, modeling by Curved Patches,
and Modeling by Facets, which are techniques for
predicting radar cross section that require a realistic
model of the target to impose boundary conditions
of the electromagnetic problem on its surface.
e most common method used today is modeling
by borders through at facets in which the modeled
surface is more similar to the real and results in
much higher prediction (Rius Casals, 1991).
e main drawback of modeling with at facets is
that the performance of the model is dicult when
the radar target presents very complex surface
shapes, as shown in Fig. 3, where the wings and
aircraft turbines have hundreds of facets to achieve
its real constitution.
To achieve a precise model, the facets should be
approximated to the actual surface as much as
necessary at the expense of raising the number
of facets used. e large number of facets, each
characterized by the spatial coordinates of its
vertices, constitutes an extremely large amount of
information to process, requiring a considerable
computational eort even with high-frequency
techniques, although much less than the level
required by discretization with low-frequency
methods.
Physical optical model
It is another of the techniques most commonly
used today to predict cross section of complex
radar targets and it is obtained by consistently
adding the contributions of each of the illuminated
areas (Garrido, 2000).
e method of modeling by facets allows easily
adding the diraction in edges by analyzing the
wedges that form adjacent facets. ese wedges
may correspond to the edges of the real model, such
as the trailing edge of the wings on an airplane or
articial edges due to the creation of facets of the
real curved surface.
e main limitation of the geometrical optics
approximation is to assume innite frequency.
e physical optics approximation introduces
Fig. 3. Boeing 727 modeled with at facets
(Rius Casals, 1991)
Díaz, Gómez
Ship Science & Technology - Vol. 6 - n.° 11 - (91-106) July 2012 - Cartagena (Colombia)
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