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The numerical
system simulation
The performance of a radar system
includes a number of subtasks (Fig. 2, 3)
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antenna problem
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wave propagation and scattering problem
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radar system problem .
All these subtasks have to be taken
into account adequately. In the very most cases the antenna problem can
be treated as mutually uncoupled with the ground and the objects in question.
However, the antenna pattern (source) have to be treated as a complex 3D-pattern.
The 3D wave propagation is handled by a general asymptotic GTD/UTD-method
/2,3,4/. In special cases (Fig. 8) or in cases which are beyond the scope
of the GTD/UTD-method complementary hybrid methods have to be applied (Method
of Moments, Fig. 8, /5/) or the specialized transmission line method (Fig.
7). Other methods available are not generally applicable (Physical optics,
Kirchhoffs current integration method) or not applicable due to the electrical
size of the airport simulation problem (Finite element FE; Finite difference/integration
FI/FD).
The Fig. 7, 8 and 9 show 3 numerically
modeled examples where details have been numerically treated and optimized,
i.e. a
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multilayer structure (3 layers. glas,
air, glas) of basic dimensions in Fig. 7a
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the same structure, but optimized in
thickness for a minimum reflection in the direction –45° (Fig. 7b).
This process is a narrow band matching optimization and converts the reflected
lower part into an almost fully transmitted wave for some spatial range.
It has to be decided on the radar system level on the base of operational
implications if this is a solution for the given problem.
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descattering of a metallic body in a
certain direction by attached scattering wires (Fig. 8) in front of the
body. This process is narrow band and creates new maxima in other directions.
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Attached Salisbury screen type absorber
designed for the SSR-frequency 1030MHz and perpendicular incidence (Fig.
9a). It can be seen that this process is narrow band (Fig. 9b) and angular
selective (Fig. 9a).
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For grazing incidence (45°) of the
wave, Fig. 10 shows the cases for perpendicular and parallel polarisation
and an additional layer of water of different thickness. It can be clearly
seen that this absorber is sensitive for the polarisation and for an environmental
rain load.
Only in the latter Salisbury screen
case a relevant absorption process takes place. Due to the narrow band
behaviour of all cases an effective optimization for the modern ASR-radar
(2.7GHz) and the MSSR (1.03GHz) is not possible in real world.
It must be emphasized also, that the first case (simulation of thermal
glas) is calculated on the base of an infinite structure. In reality the
metallic window frames are not negligible (Fig. 6). The physical design
problem of a multilayer structure is identical with that for antenna radomes
/6/.
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