unit 8 radar crossection

8/12/2019 Unit 8 Radar Crossection

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RADAR TARGETS

INTRODUCTION TO RADAR

CROSS SECTION



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.



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



Radar cross section



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.



• 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.



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.



• 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.



• 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.



• 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.



• 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.



• 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.



• 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.



(2 π a/ λ « 1)

Size of the sphere issmall compared with

the wavelength

(2πa / λ » 1).



• 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.



• 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.

unit 8 radar crossection

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