32
3 Radar Targets and Its Reflecting Properties
Herewith, generally, there is a possibility to conduct a signal measuring only proportional to S 11 . It is clear that in such situation, a target is described only by one number
S 11 . It was mentioned before that S 11 links fields measured in different points (one
is near a target and another is near an antenna); consequently, it is obvious that S 11
depends on range (distance) that causes certain inconvenience.
3.4 Scattering Cross Section of Simple Point Targets
Let us find RCS of simple point targets which both cannot be as models of some real
targets and be as its component parts.
Ball RCS Scattering and diffraction problems of flat electromagnetic wave on a
sphere are examined to the fullest extent possible comparing to all other bodies of
simple and complex forms. The special meaning of this problem implies for radar
location, since a sphere is one of the bodies of a simple form for which a string
solution is available. For this reason, metal spheres are widely used as RCS standard
sample. Besides, a sphere has a unique property: This is a single-body scattering
of energy in all directions uniformly. In other words, a sphere is an all-directional
reflector as in case when receiving antenna coincides with transmitting one (socalled single-positioned radiolocation) and when receiving and transmitting antennas
are located in different positions (two-positioned radiolocation). Omnidirectional
property of a sphere at single-positioned radiolocation is obvious and does not require
explanations. As for omnidirectivity of a sphere at two-positioned radiolocation, it
puzzles sometimes and needs in proof, which we perform in a form given in V.O.
Kobak “Radar reflectors” monography.
Let a flat wave incidents on perfectly conducting sphere, the radius of which ρ
λ, along negative direction of OZ axis, herewith the power flow density equals to
rad (Fig. 3.10).
Define a power flow density scattered by a sphere at an angle to direction of
incident. For this purpose, describe around a sphere a second auxiliary concentric
sphere of R 0 ρ radius. Highlight a strip corresponding to specularly reflected
ray scattering at β and β + dβ angles. Based on geometry, a strip will have ρ sin
(β/2) radius and (ρ/2)dβ width. The full power for a strip equals to rad d S 1 , where
d S 1 =
1
2
πρ
2 sin βdβ—a strip projected area to incident wave front. After reflection,
an energy distributes on a surface of a second sphere in the same way along an
annular strip, an area of which at R 0 ρ condition equals to d S r = 2π R
2
0 sin βdβ.
From here, we determine a power flow density of scattered wave sct = rad
d S 1
d S 2
=
rad
ρ
2
4R
2
0
.
Therefore, in fact, a power flow density of scattered wave does not depend on β
angle and is a constant value in all directions. Exclusion for this is β = π direction,
where application of optical zoom is impossible.
3 Radar Targets and Its Reflecting Properties
Herewith, generally, there is a possibility to conduct a signal measuring only proportional to S 11 . It is clear that in such situation, a target is described only by one number
S 11 . It was mentioned before that S 11 links fields measured in different points (one
is near a target and another is near an antenna); consequently, it is obvious that S 11
depends on range (distance) that causes certain inconvenience.
3.4 Scattering Cross Section of Simple Point Targets
Let us find RCS of simple point targets which both cannot be as models of some real
targets and be as its component parts.
Ball RCS Scattering and diffraction problems of flat electromagnetic wave on a
sphere are examined to the fullest extent possible comparing to all other bodies of
simple and complex forms. The special meaning of this problem implies for radar
location, since a sphere is one of the bodies of a simple form for which a string
solution is available. For this reason, metal spheres are widely used as RCS standard
sample. Besides, a sphere has a unique property: This is a single-body scattering
of energy in all directions uniformly. In other words, a sphere is an all-directional
reflector as in case when receiving antenna coincides with transmitting one (socalled single-positioned radiolocation) and when receiving and transmitting antennas
are located in different positions (two-positioned radiolocation). Omnidirectional
property of a sphere at single-positioned radiolocation is obvious and does not require
explanations. As for omnidirectivity of a sphere at two-positioned radiolocation, it
puzzles sometimes and needs in proof, which we perform in a form given in V.O.
Kobak “Radar reflectors” monography.
Let a flat wave incidents on perfectly conducting sphere, the radius of which ρ
λ, along negative direction of OZ axis, herewith the power flow density equals to
rad (Fig. 3.10).
Define a power flow density scattered by a sphere at an angle to direction of
incident. For this purpose, describe around a sphere a second auxiliary concentric
sphere of R 0 ρ radius. Highlight a strip corresponding to specularly reflected
ray scattering at β and β + dβ angles. Based on geometry, a strip will have ρ sin
(β/2) radius and (ρ/2)dβ width. The full power for a strip equals to rad d S 1 , where
d S 1 =
1
2
πρ
2 sin βdβ—a strip projected area to incident wave front. After reflection,
an energy distributes on a surface of a second sphere in the same way along an
annular strip, an area of which at R 0 ρ condition equals to d S r = 2π R
2
0 sin βdβ.
From here, we determine a power flow density of scattered wave sct = rad
d S 1
d S 2
=
rad
ρ
2
4R
2
0
.
Therefore, in fact, a power flow density of scattered wave does not depend on β
angle and is a constant value in all directions. Exclusion for this is β = π direction,
where application of optical zoom is impossible.