unit 8 radar crossection
TRANSCRIPT
8/12/2019 Unit 8 Radar Crossection
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RADAR TARGETS
INTRODUCTION TO RADAR
CROSS SECTION
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RADAR CROSS SECTION OF TARGETS
• The Radar cross section σ is the property of a
scattering object, or target, that is included in
the Radar equation to represent the magnitude
of the echo signal returned to the radar by
the target.
• The radar cross section of a target is the
(fictional) area intercepting that amount ofpower which when scattered equally in all
directions, produces an echo at the radar equal
to that from the target.
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Reradiated power density back at the radar
• = P t G * σ
4π R2 4π R2
A definition of the radar cross section found
in electromagnetic scattering is
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Radar cross section
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Radar cross section
• In this equation (2.36), it is assumed that
the target is far enough from the radar so
that the incident wave can be considered
to be planar rather than spherical.
• Radar cross section depends on the
characteristics dimensions of the
objects compared to the radar
wavelength.
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• For most common types of radar targets such
as aircraft, ships, and terrain ( ground ,landscape, mountain) , the radar cross section
does not necessarily bear a simple relationship
to the physical area, except that the larger thetarget size, the larger the cross section is
likely to be.
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Scatter ing and d i f fract ion • Scatter ing and di f f ract ion are variat ions of the
same physica l process.
• When an ob ject scatters an electromagnetic
wave, the scattered field is defined as the
difference between the total field in the presence
of the object and the field that would exist if theobject were absent (but with the sources unchanged).
• The diffracted field is the total field in the presence
of the object.
• With radar backscatter, the two fields are the same,
and one may talk about scattering and diffraction
interchangeably.
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• When the wavelength is large compared to the
object’s dimensions, scattering is said to be inthe Rayleigh region.
• The radar cross section in the Rayleigh regionis proportional to the fourth power of the
frequency , and is determined more by the
volume of the scatterer than by its shape.
• At radar frequencies , the echo from rain is
usually described by Rayleigh scattering.
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• At the other extreme, where the wavelength is
small compared to the object’s dimensions, is
the OPTICAL region.
• Here radar scattering from a complex objectsuch as an aircraft is characterized by
significant changes in the cross section
when there is a change in frequency or aspect
angle at which the object is viewed.
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• Scattering from aircraft or ships at microwave
frequencies generally is in the optical region.
• In the optical region, the radar cross section is
affected more by the shape of the object thanby its projected area.
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• In between the Rayleigh and the optical regions
is the resonance region where the radarwavelength is comparable to the object’s
dimensions.
• For many objects, the radar cross section is
larger in the resonance region than in the
other two regions.
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• The radar cross section of a simple sphere is
shown in Fig. 2.9 as a function of its
circumference measured in wavelengths
•2 π a/ λ
• where “a” is the radius of the sphere and is λ
wavelength).
• The region where the size of the sphere is smallcompared with the wavelength (2 π a/ λ « 1) is
called the Rayleigh Region.
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• Since the cross section of objects within the Rayleigh
region varies as λ -4, rain and clouds are essentially
invisible to radars which operate at relatively longwavelengths (low frequencies).
• The usual radar targets are much larger than
raindrops or cloud particles, and lowering the radar
frequency to the point where rain or cloud echoes are
negligibly small will not seriously reduce the cross
section of the larger desired targets.
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(2 π a/ λ « 1)
Size of the sphere issmall compared with
the wavelength
(2πa / λ » 1).
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• At the other extreme from the Rayleigh region is the
“optical region”, where dimensions of the sphere
are large compared with the wavelength• (2πa / λ » 1).
• For large 2πa / λ, the radar cross section
approaches the optical cross section π a2.
• In between the optical and region is the Mie, or
“resonance region”.
• The cross section is oscillatory with frequency
within this region.• The maximum value is 5.6 dB greater than the
optical value, while of first null is 5.5 dB below the
optical value.
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• Since the sphere is a sphere no matter from
what aspect it is viewed, its cross section
will not be aspect-sensitive.
• The cross section of other objects, however,
will depend upon the direction as viewed by the
radar.