radar cross section lectures
TRANSCRIPT
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Calhoun: The NPS Institutional Archive
DSpace Repository
Faculty and Researchers Faculty and Researchers Collection
1982
Radar cross section lectures
Fuhs, Allen E.
Monterey, California, Naval Postgraduate School
DTIC accession number: ADA125576
http://hdl.handle.net/10945/37502
Downloaded from NPS Archive: Calhoun
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III
1\1
RADAR CROSS SECTIONLECTURES
byDISTINGUISHED
PROFESSOR ALLEN E. FUHSDepartment of Aeronautics
Lui
NAVAL POSTGRADUATE SCHOOLC.:1 MONTEREY, CALIFORNIA
93940�0 -
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RADAR CROSS SECTION LECTURES
by
Distinguished Professor Allen E. FuhsDepartment of AeronauticsNaval Postgraduate School
Monteezy, CA 93940": ~(409)646-2948
AV 878-2948
MA-RM~AR 1 4 18 •
A
L . . . . . . . .4. . .
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INTRODUCTION
These notes were developed while the author was on Sabbatical at NASAAnes Research Center during FY 1982. The lectures were presented to engineersand scientists at NASA Ames in March-April 1982. In August 1982, the RCSlecturre, aere presented at General Dynamics Fort Worth Division..
T- thoroughly cover the content the following time schedule is required:
LECTURE .- HOURS LECTURE TIMEI
Il, "1.5,
II; "> 3.0
III V 1.5 0
/
--2--
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LECTURE I. INTRODUCTION TO ELECTROMAGNETIC SCATTERING
1. Level of Complexity
* 2. Features of EM Wave
3. What Is RCS?
4. Magnitude of Radar Cross Section
5. Polarization and Scattering Matrix
6. Inverse Scattering
7. Geometrical Versus Radar Cross Section
8. Polarization and RCS for Conducting Cylinder
9. Far Field vs Near Field
10. Influence of Diffraction on EM Waves
11. Relation of Gain to RCS
12. Antenna Geometry and Beam Pattern
13. Radar Cross Section of a Flat Plate
"14. Wavelength Regions
15. Rayleigh Region
16. Optical Region
17. Mie or Resonance Scattering
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(~A.'EFUHS1Before discussing RCS, a perspective is given on the conpexity of
problems to be encountered. A measure of complexity is the tool required
for numerical solution. The tools span from slide rule to CRAY computer.
Since these lectures are prepared mainly for the aerodynamicist,
typical aerodynamics problems are given along with classes of RCS problems.
The lectures provide sufficient information which allows back-of-envelope
calculations in the "Southwest" corner of the graph. The lectures discuss
in a descriptive way the scientific problems in the "Northeast" corner.
•::2-"
0
4-
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A:E.FUHS 2
FEATURES OF EN WAVE
0 Wavelength A " c/f
c - speed of EM wave- 3E8 =/sec
f - frequency, Hz
"0 Electric and Magnetic Fields*
- orientation related to antenna (source)-•.=zZ ; z (/)1/2 on-E-ohms
0 Polarization
- orientation of the electric vector E-9
- polarization may be important in determining magnitude of RCS
0 Energy and Power*
1 22 3energy density - energy/volume - i(eE + pH2 ) = Joules/m
flux of energy - power/area - - x - Watts/m2
power - (amplitude squared)
O Interference
field vectors add vectorially; may cause cancellation of waves
*J. C. Slater, Microwave Transmission, Dover, 1959.
6 =
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; A.E. FUHS
Wha t is RCS?
0 The RCS of any reflector may be thouht of as the projected area of
equivalent isotropic (same In all directions) reflector. The equivalent
reflector returns the same power per unit solid angle.
* 0 ECS is an area.
0 Meaning of RCS can be seen by arranging a in form:
-s " 1'- " 2
* al, - power intercepted and scattered by target, Watts
a•I/ 4 v - power scattered in 41T steradians solid angle, Watts/steradian
I rA - power into receiver of area Ar. Watts
2a - Ar/R - solid angle of receiver " seen fro target, steradian
I A i/1 - power reflected to receiver per unit solid angle, Watts/steradianr r
I rA ( /(A 2 - power reflected to receiver per unit solid angle,r\' Watts/steradian
0 Meaning of limit
R is distance from target to radar receiver.
H,* UI' and I1 are fixed.
E and H r vary as t/R in far field.
I varies as I/R' in far field.r
Hece, 0 has a limit as R *
S
S
a
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A. Es'FUHS 4
Magnitude of Radar Cross Section
0 RCS can be expressed in terms of area.
0 Since RCS Is an area, you can check your formulas for RC{S for din" 'on;
Sthe formulas should always have dimensions of Zenith squared.
0 The square meter is usually used as a reference to vxress a as a relative
value using decibels. An exmple of calculation
Given oa 28 db what is in ?
2 • 28db /10
2 310,j 631 a2
Given o - 0.34 m2, what is o in db ?an
a(db 3 ) - 10 log1 0 (0.34) - - 4.7 dba
0 Some typical values are shown for various objects. Also the mSagnitude
of creeping waves or traveling waves from an aircraft is shcAw.
When the RCS due to direct reflection is reduced, RCS from other wave
scattering phenomena may become important.
S
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* 4.- .. 4
17Z
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I"
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:,.~l .l,• " " ,...o---. • -,.-,- ..--•. ----.-.- ,.;,,- •; -• =<- . .rv- i.,----' ; • '.;- r, -r--r ' -:, '--- rrr; -."wj.."*' , w, .•".w '-" 'v I•, , ,,-, %
As Es FUHS 5
POLARIZATION AND SCATTERING MATRIX
0 The elements of scattering matrix have both phase and amplitude
a.E Ia., Iexp j
0 For monostatic radar (transmitter and receiving anrennas are colocated
or very close together)
". The expression is not true for bistatic radar.
0 Polarization of wave is specified by stating orientation of electric
field vector E.
o0 ro-s polarization occurs when target changes the polarization of
* reflected wave compared to incident wave.
0 Polarization may be specified by orientation of E relative to a long
distance of tsrget, e.g., a wire. In this case, the motation
a and u.-
is used.
0 Usr.lly 0 > .
13 .1•
4
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"A1 i2FUHS 6
( INVERSE SCATTERIG
0 To quote from P%-ofessor Xennaugh* on the subject of inverse scattering:
"One measure of electromagnetic scattering properties of an objectis the radar cross section (RC.S) or apparent size, In the earlydays of radar, it was found that rapid variation of RCS withaspect, radar polarization, and frequency complicate the relationbetween tk-ue and apparent sizes. As measurement capabilitiesImproved,. investigations of the variation of RCS with these parametersprovided the radar analyst with a plethora of data, but few insightsinto this relation. In the present context, such data are essentialin determining the physical features of a distant target, rather thanan annoying radar anomaly."
0 Inverse scattering provides a nonimaging method to determine t~arget
size, shape, etc.
- 0 By appropriately processing the backscattered waveforms or target signature
observed in radar receivers, different target shapes may be discriminated and
classified.
0 St.ýalth implies denial of detection; an expanded concept for stealth implies
control of backscattered waveform, thereby denying information about target
size, shape, etc.
*Edward M. Kennaugh, "Opening Remarks, Special Issue on Inverse Methods inElectromagnetics," IEEE Transactions on Antennas and Propagation, Vol. AP-29,March, 1981.
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A. E# FUH5GEOMETRICAL V4RSUS RADAR CROSS SECTION
0 Sphere. The two areas are drawn to scale. For a sphere,
a - 'ira2 frdependent of wavelength in optical region. Solve for a:
a - SQR(a/7r) - 0.56 meter
' 0 Square Flat Plate. Consider a frequency of 8.5 GHz which corresponds
"to X - 0.035 a - 3.5 cm. The cross section for a flat plate is
2 2 24AjTA 4t[(0.lm) 1 2
7 (0.035)2
0 Aircraft Broadside. The aircraft may have a panel which is normal
to the wave vector k. A large RCS results due to reflection from the
panel.
0 Low RCS Aircraft Broadside. By a combination of RCS reduction methods, the
aircraft has a smaller RCS than projected area.
2'.
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"A*** Et" FUHS SPOLARIZATION AMD FCS FOR CONDUCTING CYLINDER
0 When A is smaller than a, polarization Is not important for magnitude of RCS.
0 The three regions based on relative size of X compared to a are shown.
In both Payleigh and optical regions, the RCS varies smoothly with clanging X.
In the Mie region, also known as resonance region, the RCS varies rapidly
2with changing 1. In optical region, 04 and a, converge to kaL
0 Cylinders with small ka are used for radar chaff.
0 A cylinder can be used as a model for estimating RCS of the leading edge of a
wing or rudder.
0 Mie region occurs where circumference of cylinder, i.e., 2ira, is nearly equal
to wavelength, A.
0 The values of ka for which cylinder diameter, d, equals A and for which
cylinder radius, a, equals A are shown in the graph.
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A. E; FUHS ,FAR FIELD vs NEAR FIELD
"O The symbol f refers to a fraction of a wavelength. In far field, the
variation in phase is small over a distance L.
0 In far field, the incident wave can be considered to be a plane wave.
0 The radiation from a dipole illustrates far field and near field for microwaves.
Nk3 _l-• NEAREe - -- exp[j (.t - kr)] sin e [ - + I FE LD
(k) (kr)
Ee - ep[j(wt -kr)] sin8eL FAR
O47r~ ir FIELD
,. - dipole moment
S.. - electric inductive capacity
k - 2wr/A
w - 2wf (f is frequency here)
e polar angle in polar coordinates
j- square root of minus one
S::
I I * I r l nl n• '- " '"- "
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• . • . . % .- o .,. •.• . • • • ,, • .• •-. ;.; ; • ;•, -,- --; • -. - 4-, . • , . , :-. ... .s ,a.. . - -• %-
. A;'I. FUHS 10
IWLUDIZ OF DIFFRACTION ON DI WAVES
"0 On the left-hand side is a barrier vith a WALU hole D. 7he waves are moving
to~wzd the right. The diffracted waves are nearly circular vith center at
hole.
0 A large value of ./D yields a beam which diverges.
0 On -the right-hand side, the hole D is much larger than a avelmgth. The
bem is tri MIeted through the barrier with little divergence.
0 A smal value of i/D yields a narrow bean fros an antenna.
-SJ
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ILL
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A; E.FPUHS
RELATION OF GAIN TO RCS
0 In the optical region, i.e., where ka is large, a formula can be written for
RCS involving gain and reflecting area. The formula is given in the viewgraph.
0 0 Gain is a rztio of tw solid angles. For a sphere, the solid angle is 4w
steradians. If the wave is cenfcned to a beam dxe to an antenna, the power is
concentrated in the beam. Gain indicates the extent the power is
i concentrated in the beam.
- 0 To find the solid angle of the beam, the relation e - CA/D is used.
C is a constant and usually has value 2/ir.
O The reflecting area is the surface area between two wavefronts spaced
AX apart. Surface area outside the volume defined by the two wavefronts does
not return radiation in a direction toward the radar antenna.
0 Derivation of the equation for gain:
SConsider the beam from an antenna to be a cone with half angle 0 - 2A/wD.
"At a range R, the cone has a bame with radius r. The value of r is given by
r -OR
2The area of the beam, A, at range R is lr. In terms of 0 and the
diffraction formula
24A D2
Ab il 2
The solid angle of the beam is
2R wrD'
By definition
G 411 4w 'rD 2 4rrAT 4
where A is the area of the antenna.
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4U:5j
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u A: E*FUHS
CL <
Q, ý: 1,<Z lq
09 'j-1r
C, 01
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A. E. FUHS 12
ANUMM GEOHETR MD BAM PATX3EZ
0 A circular antenna produces a circular beam of radius r.
0 An elliptical antenna produces an elliptical beam, Due to diffraction,
- the long dimension of the antenna, La, is at right angles to long dimension
of the beam, 8 R. The long dimensions of the antenna and beam are crossed.e
Note that 0 > % and L > L"'•e a a e
"0 The reason antennas are important to RCS ,s that the circular antenn, is
"equivalent to a circular disc. The circular antenna is the source for a
plane wave from an aperture in the form of a circle. Consider an incident
plane wave reflected from a circular disc. The result is a plane wave
from an aperture (the disc) in the form of a circle. Hence, the reflecting
area is equivalent to an antenna. Antennas have side lobes. The radiation
* -reflected by a flat plate or a disc has the same side lobes as an antenna of
same shape.
/6
6il
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4. . . . . .. . . ~ ... . . . . . . . .I-
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-- =S
13
RADAR CROSS SECTION OF A FLAT PLATE
- 0 The RCS of a flat plate is obtained from
(3 GA
where A equalE the plate area, Ap
' 0 Three different geometries leading to a series of wavefronts moving to the
right are illustrated. The beam in the far field is identical for the
three cases illustrated.
0 The shape of the flat plate does not influence the value of a so long as
the smallest dimension of the plate is much longer than a wavelength.
0 A test for whether or not the formula applies is accomplished by comparing
X and the square root of A . The result must be"p
p
4
t,
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-. 9.
-4 4C
tIJ -J
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.( A: EFUHS 14WAVELENGTH REGIONS
0 Recall k - 2wr/X; consequently
- Ja 2ira
a is a characteristic dimension of the body.
0 A complex shape such as an aircraft may have components spanning all three
regions. For example, the wing leading edge may be in the optical region
while a gun muzzle may be in the Rayleigh region.
- 0 The region where ka 1 is known as Mie region or as the resonance region.
Resonance may occur between creeping waves and the specular reflected waves.
The numerous wiggles characteristic of the resonance region-may be due to the
resonance.
0 The resonance region is difficult to analyze. In the Rayleigh region, series
expansions using ka as an expansion parameter can be accomplished. In the
optical region, the. expansion parameter is 1/ka. The series expansion
technique is not useful for the Mie region. For Rayleigh scattering,
the leading term in an expansion may be the electrostatic field.
-ai
-- (
4
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31
Nbl
u 9%
(u
cx~
At 6FU HS
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32.
•A Es FUHR 15
RAYLEIGH REGION
0 An oblate ellipsoid of revolution is shown in the figure. When the
distance in the axial direction is small, the oblate ellipsoid models a disc
like a penny. For this case, F is not near unity.
- 0 A prolate ellipsoid of revolution is shown in the vievgraph. When the
distance along the axis is emphasized, a wire can be modelled. In that case,
F is not near unity.
"" 0 For smooth bodies which do not deviate too much from a sphere, the RCS is
independent of polarization or aspect angle.
0 The formula for a can be tested for a sphere. For a sphere
a/na 2 0.1 when ka 0.33
3Assume F " 1.0. Then, since V - 41a /3
24 0~4 32 64 4 2
(Ta) a - (ka) va
Inserting the value for ka, one finds
-/•a2 - 0.084
which is close to the ac:urate value.
6
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33
44
ILl
'ACA
Q eQ
LL LL
K F~c
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-A E. FU H 16OPTICAL REGION
0 One can apply the formula a i fplP2 to a sphere. In that case P1 = p2 a.
S~Hence,
2•'. 0 - ira2
which is the anticipated result.
O Optical approximation has the greatest use to calculate specular returns
and the associated sidelobes.
"0 Optical approximation may fail when there is a surface singularity such as
2-. an edge, a shadow, or a discontinuity in slope or curvature. Surface
-� singularities may cause second-order effects which include creeping and
-• traveling waves. When specular returns are weak, the RCS may be
dominated by creeping or traveling waves.
0
0.
S'
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* .~&s~a. --
* 93
ti
-4--_ _ N4J
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CIL.v) N)
7Z U
4 0:
I-K
AV Eo FUH5
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36
A. E# FUHS ' 7
MIE OR RESONANCE SCATTERING
To satisfy electrical boundary conditions on a body, a grid with nodes spaced at a
small fraction of a wavelength, say X/6, is needed. For the optical region
where A < a, the number of grid points is very large. However, for the
reso-ance or Mie region, the value of X is near a dimension of the body. Fewer
grid points are needed in the resonance region than in the optical region.
°
-,
-q
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37
SI
left 4
V)
4-.
L~Lo
-4-.
ui LU
ICLuJi U.)L~
'4LL - I
As-. Eo FUH
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31
"LECTURE II. RADAR CROSS SECTION CALCULATIONS; RADAR RANGE EQUATION
1. Physical Optics
2. Radar Range Equation 1
3. Radar Range Equation 2
4. Radar Range Equation 3
5. Burnthrough Range
6. RCS for Simple Shapes
7. Calculation of "Flat Plate" Area
8. Why RCS for Sphere Does Not Depend on
9. Wavelength Dependence of Specular Reflection
* 10. Wavelength Dependence Using Flat Plate C7
11. Wavelength Dependence Using P. 0. Integral
12. Radar Cross Section of a Sphere
13. Addition of Specular and Creeping Waves
14. Derivation of a - TrplP 2
15. Radar Cross Section for Wires, Rods, Cylinders and Discs
16. Radar Cross Section of Circular Disc of Radius, a--Linear Scale
17. Radar Cross Section of Circular Disc of Radius, a-Decibel Scale (f - 12 GHz)
- 18. Radar Cross Section of Circular Disc of Radius, a-Decibel Scale (f a 2 CUz)
19. RCS of Dihedral
20. Determination of RCS for Dihedral
21. Sample Calculation of RCS for a Dihedral
22. Calculated Dihedral RCS for - 300 < 0 < 1200
23. Data for a Dihedral at 5.0 Cdz
24. Creeping Waves
25. Traveling Waves26. RCS of Cavities
27. Retroreflectors
28. RCS of Comon Trihedral Reflectors
29. Vector Sum for Radar Cross Section
30. Radar Cross Section for Two Spheres
31. Sample Output for Two-Spheres Model
32. Radar Cross Section and Antenas
4 A.E, I'UPb
•--4.
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AI.E. FUHS 1PRYSICAL OPTICS
0 The symbol I is irradilace, Watts/a2 .
0 The value of 10 is I at 6 , O.
limit I ke 2
0 Using the Kirchhoff Integral, which is a technique used in physical optics,
the radar cross section is obtained.
"0 A vocabulary guide is given sinme optics people use different words
than utcrowave people.
"References are as follows:
J. M. Stone, Radiation and 0ptics, lcGraw Hill, New York, 1963.
J. W. Crispin, Jr., and K. M. Siegel, Editors, Methods of Radar Cross SectionA.glysis. Academic Press, New York, 1968.
--
I
tL
I
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iL
-J
'Ua
(00> ~t4 Q Q
Q.
c~ LL~V
~- L
LL'
IZ3l
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A, E. FJUH 2
RADAR RANGE EQUATION 1
"0 By using the dimensions, one can understand the various steps in the
derivation.
0 Symbols have the following definitions:
P = radar transmitter power, Watts
G - antenna gain
U R - range, i.e., distance f:om radar to target, meters
- solid angle
0 Note that atmospheric attenuation is neglected.
• a.
IJ-
I'
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*3j
'a '4
~tI
II~
.1 w'(I -Q
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A;: E UHSRADAR RANGE EQUATION 2
o Note that ICS has units of an area and is used as the area which intercepts
outgoing radar power.
0 The signal, S, has units of power.
0 Note that IIR is (steradian/unit area).
A.4
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Iaj
LaL
- UJ4~ 4 tz
tz'
Q-J
ex~
Iji
uj <
Li,
Q-I~ 4c8
<'
6 Iix
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,-A.' , FUHS 4RADAR RANGE EQUATION 3
O Detection range does not necessarily equal RO.
0 Using logarithmic, differentiation, one can show that
AR0 1 A 4+.AA 1 Aa 1 AX 1 ANRo 4P + 2c 2A" 4 N
0 A 40 per cent reduction in a causes only a 10 per cent reduction in range.
0 The formula for relative detection ranges is useful. An example:
"Radar power is doubled. Howmuch does the reference range increase?
p 1/4R [_'] (2)1/4- 1.189R 1 p 1
Range increases by 19 per cent for a 100 per cent increase in power.
2...2.& I
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f7
I- I
-'U Ita -V
tjJ> "Ic
LLJ
r4.
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>:: A, E, FUH1• •
BIJRZTHROUGH RANGE
0 Symbols have definitions as follows:
2Ar radar antenna area, in
A i Jamer antenna area, m
A br area of radar beam at jammer, m2
-Ab area of jammer beam at radar, m2
angle of jammer beam, radians
e angle of radar beam, radians
S signal at radar due to Jammer, Watts
S r Signal at radar due to reflected power from target whichr is carrying jammer, Watts
.P radar power, Watts
"-P Jammer power, Watts
R range, meters
x wavelength, meters
2o RCS of target which is carrying jammer, m
0 Obviously both radar and Jammer must be on same X.
0 Note that S varies as R 2 .
0 Note that S varies as R4 .r
*0 A narrow beam for Jammer is not practical since this implies jammer must
be aimed. Hence A is small.
0 Note that R.b varies as SQR(o).
- 0 For penetrating aircraft, a small value of Rb is desired.
I.
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ASE* FUHS
4 Q
l<~
V ~ 'I *:
w C
-uj
ca)
It ~-it
-4 J.Q
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SA E. FUHb 6
RCS FOR SIMPLE SHAPES
"0 The direction of the incident wave is specified by • which is usually
parallel to an axis for the simple cases considered here.
0 The equations are valid only in optical region where ka >> 1.
0 The cone and paraboloid extend to infinity. a is due to scattering at the
tip for a cone and blunt nose for a paraboloid.
0 Compare the RCS for a sphere and a paraboloid. What do you notice?
0 The prolate (cigar shaped) ellipsoid of revolution has a RCS less than a
sphere of radius b. Rewrite formula for 0 as
a O (ub 2 )(b/a) 2 (RCS OF SPHERE OF RADIUS b)(b/a) 2
As ratio b/a decreases, the radius of curvature at the nose decreases;
a decreases. Interprete the result in terms of
a - iplP2
0 The circular ogive is tangent to a cylinder. The cylinder must extend to
infinity. Note RCS is same for a cone and an ogive. RCS is due to
scattering by the tip.
b'0
w
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Ul'
u-j
LL k
IA
It
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A; E. F'IJHS 7
CALCULATION OF "FIAT PLTATE" AREA
0 To use the formulaS•!i-4TTA 2
"" -
one must evaluate Ap
o The method for determtning A is shown for two cases, a sphere and a cylinder.I pThe quantity F is a small number, and FA is a small fraction of a wavelength.
0 The cross section for a sphere doesnot depend on wavelength.
0 The cross section for a cylinder decreases as X increases.
0 One can understand the dependence of a on X in terms of diffraction.
0 The reflecting area, A is much smaller than the projected area of the body.
4
-i
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ifj
LKL41'
IlkI
Il a- I k-
II4NI
S N
i
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A. E. FUHS 8
WHY RCS FOR SPHERE DOES NOT DE•PE) ON A
0 The symbols have the following ro-aning:
r - radius of reflecting area, meters
A - reflecting area, m
e - angle of reflected beam
S- solid angle of reflected beam
P - reflected power, Watts
0 In words the result, 02 - aV can be expressed as follows:
As A decreases, the reflected power decreases. However, the angle of the
reflected beam, which is due to diffraction, decreases also. The changes
in reflected power and solid angle of the reflected beam compensate for each
other. As wavelength decreases, reflected power decreases; however, the
reflected power is in a smaller reflected beam.
0 Exercise for the Motivated Reader.
Using viewgraphs 7 and 8, repeat the analysis for a cylinder. Show that
1 2
e
S
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4LC
OLJ
K1044 04 t4J ;
L7 QA
tL Ci
II
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A. E. FFJHU 9'WAVELENGTH DEFEMENCE OF STPECULAR REFLECTON
0 In the optical region, the RCS for various geometrical s~hapes varies with A.
The variation is due to specular reflection.
o The variation for a sphere was discussed by vieWraph 7.
0 The variation of a with I can be understood by using
.plP2
0 The flat plate, cylinder, and ellipsoid can be understood in terms of
47rA2
•2
0 The variation of A with X determines variation of X.p
S/
-- 4I
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LU 547
r4:242 -u
>~ 4;zUJw
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A' E. FUHS 1o
WAVELENGTH DEPENDENCE USING FLAT PLATE a
o For a flat plate, p1 * p2 P •" Hence A does not change with X.2 .p
0 For a cylinder, p1 ÷ and 2 is finite and nonzero.
0 For a sphere, p1 - p2 and both are finite and nonzero.2/
i- "
/
'0
S
ai
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U}vEL-EAIMT PEPEWIDrA/CE Vs/A16 jrL4T EAE(
WAVE PIQOA/ ~4'6Frl
- *WAvE- Fpuffrage
( CASE IAr
LDOES Ahr CA SE 3(b)DEPEPJD 61%) 1\5RFC
dOSw V41ZIES As A
or v X-2.AA
417 471
41r
I A. EV4 EH
A uW
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ll iA. Et FUWSWAVELENGTH DEPENDENCE USING P. 0. INTEGRAL
o P. O.- Physical Optics
0 The wedge, curved wedge, and cone have at least one zero value for radius
of curvature. These can be understood in terms of the formula for a
which is based on an integration of aA/3p along the direction of
incident wave motion.
0 In this viewgraph, p is the distance along the direction of incident
wave propagation.
0 The equation for a is somewhat analogous to the equation for supersonic
potential function. See page 237 of Liepmann and Roshko.*
*| *H. W. Liepmann and A. Roshko, Elements oC Gas Dynrmics, Wiley, New York, 1957.
- . 1....... .. .. . .....-- ' - -
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WA1VEelEN6TYz VPNEC USA6p.C. IA7-~gA
tcr k -jf 2ikf Ad1o
46E07OA7/a (1) P~qa. 2f~ CgCISP,,t 04d SIESE1. &w W
~zero7 OAe/~
dp ~ ~ - = .27Tr?.suAVAVE FpCi
Se CJO
g k {f2icZcI 2
4A
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-AE E. FUHE 12
RADAR CROSS SECTION OF A SPHERE
0 The formula valid in Rayleigh region
64-"- 64ira2 - 9 (ka)
comes from Lecture 1, Viewgraph 15.
0 In the optical region, co is independent of ka; subscript "o" refers to
optical region.
0 In the HIE or RESONANCE region, the cross section is the sum of two
contributions. Electric fields add vectorially; power does not add.
Hence the formula
a A, v0 c
applies only at maxima or minima of the curves. At other locations a phase
angle is required.
0 The meaning of the word "resonance" now becomes apparent. When the speculkr
and creeping waves have the correct relative phase, one gets "resonance" or an
addition of the two waves.
O At point 1, which is a maximum in Mie region, ka is nearly 1.0. Hence
--2 1.03* ,ra
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-4"
LI14
I tr
9cz-
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_ _.-o!
ADDITION OF SPECULAR AND CREEPING WAVES
0 One can calculate a for the maximum at point 1 of RCS wave
2/lra = 1.03
---a 0o/a2 = 1.00
/Va /77a 2 + /ahra] 2 4.06Ira
0
In terms of db, precise calculations show that the cross section is 5.7 db hig
at point 1. For the calculations here
adb - 10 logl0(4.06) - 6.09
which is close.
0 At the minimum at point 2 on the curve ka- 1.8
c 1.03(.8)- 0.237
22
_22S...2 - [,~i'--- O" ]2- 0.263
which is close to value of 0.28
0 In summary, the wiggles in the RCS curve in the Mie region are due to
"constructive or destructive interference between specular reflected and
-- -- I • creeping waves.
U
U
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CAJ
V.k to L U
Lw
- -j
4~~t C - .-
-l I- u
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A. E PUH~ 14
DERIVATION OF a -iplP
0 AS arclength along the reflecting surface
angle subtended by AS from center for radius R
AS arclength along the reflected wavefrontr
4..ki propagation vector for incident wave
ki propagation vector for reflected wave
R radius of curvature of the reflected wavefront
p radius of curvature of reflecting surface
a solid angle formed by reflected wavefront
Pi incident power, Watts
-Pr reflected power, Watts
0 The fact that the angle associated with p, i.e., A81/2, is one-half of the
angle associated with R, i.e., A6,, is an important fact.
O In the derivation of the equation, one uses the definition of RCS.
0!i
S_
Si
SJ
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467
. 4.
~D _A_, VA, 7,'0N 0 F T"r..'F? P,
"A= As
P r
77I P,
fPF;
c " 4-T S, 41-
i i ," E ,r '-i J
*Rp = ,DcIUS OF cuR,/,TVF•E
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A:" E. FUHS lRADAR CROSS SECTION FOR WIRES, RODS, CYLINDERS AND DISCS
0 The problem has three characteristics lengths, i.e., a, L, and A, and
two ratios, i.e., ka - 21a/X and LI/.
0 The values of L/X and ka determine the RCS.
0 Consider a reference square area which is X on each side. The various
geometrical figures have the A-square drawn to indicate relative sizes
--1 of ka and LIX.
O The (L/X) - (ka) plane has been divided into three regions. In the upper
left wiere L/A >> 1 and ka >> 1, the polarization of the wave is not
important. In the corner near the origin where L - a and ka << 1, polarization
is not important. In between these two regions, polarization is important,
and one needs both oa and a. to be complete.
6-
0O
6
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5--o
V-4V
LLU
luN
U LL NLL%. ~uzI -\ "
4 ~CZI u4h
4z 7Z I:
th / C
-N
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70
16
RADAR CROSS SECTION OF CIRCULAR DISC OF RADIUS, a
LINEAR SCALE
O The RCS of a circular disc has been calculated.
O To evaluate the formula, one needs to know frequency and disc radius.
7- These values are given.
S0 A disc with an area of
2 2 2A - rr2 - f(O.4572) 2 = 0.66 m
yields a cross section of almost 9000 m2 at 12 GHz.
0 The linear scale of a illustrates the big change in a as frequency increases.
The width of the reflected beam becomes much narrower as frequency increases.
i4
,o_ • .•. 1
4
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7,
16
RADAR CROSS SECTION OF CIRCULAR DISC OF RADIUS, a
LINEAR SCALE
10000
f, W 2 x 109 Hz J (2ka sin 0) 2
f2 " 12 x 109 Hz t = ra2 tan
k, - 41.9 1/m-
8000 k2 - 251.3 /m rn9
disc radius, a, 0.4572 m
kla - 19.151
k a - 114.907S26000
0
00
4000S..
12 GHz
2000
2 GHz
0-30 -20 -10 0 10 20 30
0, Angle Between Disc Normal, n, and Propagation Vector, k, DegreesI,
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p7z
:::A. -E. IJ 17RADAR CROSJ SECTION OF CIRCULAR DISC OF RADIUS, a
"DECIBEL SCALE
0 When dbsm is used as a value for RCS, the sidelobes become more apparent.
For f - 12 GHz, the main lobe of the beam is about 20 wide.
!
4
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73
17
RADAR CROSS SECTION OF CIRCULAR DISC OF RADIUS, a
DECIBEL SCALE
a - 36 inches - 0.457 meter f - 12 GHz
40
~Z~i~HS~
30
20 _ _ _ _ _ _ _ _ - -- _ _ -
0
-10 -- _
I _ _ _ _ _ _i
-20 -l I--30 -20 -10 0 10 20 30
0, Angle Between Disc Normal, n, and Propagation Vector, k, Degrees
K
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§ 74•
A;E. FU W4 18
RADAR CROSS SECTION OF CIRCULAR DISC OF RADIUS, a
DECIBEL SCALE
0 The side lobes at f - 2 GHz cannot be seen in the plot using a linear scale;
see viewgraph 16. However, with the decibel plot, the side lobes are evident,
At 2 GHz, the main lobe is almost 120 wide.
-- 4
4•
-- Il
4•
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18
RADAR CROSS SECTION OF CIRCULAR DISC OF RADIUS, a
DECIBEL SCALE
a- 36 inches - 0.457 meters f -2 GHz
40
A;E-F-UH-
30
20
0
al
( 0r4
_10 -
C,,
0
10
-20 -30 -20 -10 0 -0 20 ?0-30 -20 -10e0 10- 20 n
0, Angle Between Disc Normal, n, and Propagation Vector, k, Degrees
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A : F[.J19RCS OF DIHEDRAL
0 The radar cross section is due to different surfaces when viewing angle
changes. Starting at 8 - 00, the surfaces contributing to the RCS will be
noted.
o Near 00. The plate P2 and the edge El are the main contributors. Consider
perpendicular to edge E1. The RCS for E1 can be modelled as a wire using RCS
from viewgraph 15. The flat plate P 2 can be modelled using RCS from
vievgraph 1.
0 Between 00 and 90°. The dihedral forms a retroreflector. In this region,
use formula for the retroflector.
O Near 90°. Ditto for 00; however, use E2 and PI.
0 Between 900 and 1350. Both plates P 1 and P 2 contribute to RCS. Once
again, use RCS formula from viewgraph 1.
O Between 1350 and 1800. In this region the fact that the two plates P1 and P2
form a 90 0 -wedge becomes important. The symbol FW means use the finite
-dedge formulas with plate P1 in shadow.
O Near 1800. Plate P is (almost) normal to incident wave. A large RCS results2
due to flat plate P2.
0 Between 1800 and 2700. Use formulas for finite wedge with both surfaces of
wedge exposed. Subscript e means both surfaces are exposed.
0 Between 2700 and 3600. Already discussed due to symmetrically located regions.
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ý17
u.4
U-'U7=U
LU)
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"A.EFUHS 20
DETERMINATION OF RCS FOR DIHEDRAL
* :0 The largest RCS occurs when 0 is 450 as seen in left-hand side of viewgzdph.
" 0 One uses the flat plate formula to calculate RCS.
0 When e is not equal to 450, A can be found by a topological trick. Rotate
the dihedral about an axis parallel to k and passing through the dot on thecorner line. Area co-on to both the initial and rotated dihedral is A.
The angle 0 is identical to 0 used in the preceding viewgraph.
41
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~poil
"'I U
"LLN~
fN
v'I.
A: E.J
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. F PUHS 21
SAMPLE CALCULATION OF RCS FOR A DIHEDRAL
0 The cross section due to retroreflection from dihedral, i.e., 0(8), and
the cross section from flat plate, i.e., op, were added using the formula
shown. The formula implies both reflected waves have the same phase angle.
I.
i,
a
a
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U,1
[LIL
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*622-5 A. E'. PUH 22
CALCULATED DIHEDRAL RCS FOR 30° < e < 1,20,5 0o
0 The peak at -0 0 is due to flat plate P2 The peak at - 45 is
due to retroreflection by the dihedral. The peak at e - 900 is due to
flat plate Pl. For 900 < 6 < 1200, the cross section is due to plates
P1 and P2 .
0 This curve should be compared with the curve in the following viewgraph.
I
4
n
I
I- |
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22
(•JCULATED DIHEDRAL RCS FOR - 300 < e < 1200
-15
0 ..- 2
. -5 -- -- .
Q
*! 0 -10 -- - . ..-....
-20
S-40
* -3 0. ... . ..5,
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110..
0 degrees
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23
DATA FOR A DIHEDRAL AT 5.0 GHz
0 The simple model given in viewgraph 21 provides accurate results except for
the dip at 450*
C,
6
!1
I
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LI 41
% CA
NQ t.4 :z tJ9~~ ~-t4
Q.i -. it
0 wl
a i I N-4
ýj Luzwj u
tu tij uU.. _ _ _'
4t
-J -Jo
-r - X -t
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24
CREEPING WAVES
0 Creeping waves usually yield smaller RCS than specular reflection.In case of sphere, a was as large as the specular return for ka = 1.
C
0 Creeping waves are important for smooth blunt bodies such as spheres, cylinders,
and ellipsoids.
i
!.
I-o
I.
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ervie
'I-)
V) Uj~j
-~ -a
(.ýe-
tjo'4
1101q
-L C;L
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"25
TRAVELLG WAVES
o Body acts like a traveling wave antenna.
0 Formula for RCS due to wire for L - 39% and a - A/4.
Cl 8.E sine4A [1. n os sin[124.5(l - cos 8)1
8 - 0 is for t parallel to wire. At 0 80, the value of O/X2 attains a
value of about 10.
0 The conditions for excitation of traveling waves are noted, namely long,
thin bodies with near nose-on incidence of waves.
0 Bodies with dielectrics favor excitation of traveling waves.
_I.
I
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LLA
~~LI
NII
AVE g'Z C(41
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7026
RCS OF CAVITIES
0 The flat plate model gives an order-of-magnitude estimate of the inlet,
exhaust, or radar cavity.
0 Fenestrated radomes may be opaque at some radar frequencies avoiding problem
of transparent radome and exposure of radar cavity.
"C.
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-'4NZ
to zl7ýk
LuU
IL40
Qj.
'ZA
47( S ~ S
.LI
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27
RETROREFLECTORS
0 Use of retroreflectors
drones
sail boats
navigation buoys
0 Looking at retroreflectors, RR, on sail boats in Monterey Bay showed that
almost every one was installed wrong if the radar was on another ship.
One plate of the RR usually was mounted horizontally which is wrong.
0 Retroreflectors are inadvertently designed into a vehicle causing very
large RCS.
0 Retroreflectors were left on the moon by the astronauts.
I
I
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I(l 27
~~3LL~LL
-JL~Li
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A. •. F tJH* 28
RCS OF COMMON TRIHEDRAL REFLECTORS
0 Consider the corner to form x,y,z coordinate system.
The angle of a symmetrically located vector can be found by
Cos82 + cos20 + cos 2 - 1.0x y z
-8 - e -=x y z
8e arccos(I/r3) - 54.7360
4
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Is.J
ui Qi
urn
u-i
-Z4
'mU
A'II
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A:~FluH!, 29
VECTOR SUM FOR RADAR CROSS SECTION
"0 The radar cross section for multiple scatterers can be found using the
equation. In the equation, the sum is from first to m-th scattering object.
0 The equation contains phase information which may lead to cancellation.
0 The symbols have the following definitions:
radar cross section of k-th object
dk distance from k-th object to radar receiver
I wavelength of radar
E electric field in reflected wave
E_- electric field in reflected wave due to k-th objectrK
0 Since radar waves make round trip, a difference Ad - X/4 gives a phase
change of X/2 which is 1800 phase. A 1800 phase causes cancellation. A
difference Ad - X/2 yields a phase of A; the reflected waves are in phase
and add.
--- 0
I
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29
-..
04 ti
LLLq
1 4 -
--
C-
•TN
A.: •.-IHU i,
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30
RLADAR CROSS SECTION FOR TWO SPHEREXS
O A variety of features of RCS can be illustrated with the two-sphere model.
0 Vector addition of E
Assume f -1 and e 900; then a -4a1
The cross section is 4 times that of one spheral
O Influence of a-paci, I relative to X. Assur.e f - 1.
87rk-- cos Gm 0or 2nvr a maximum occurs
87TL.cos 6 (2n 1.l).t; n i- ,2,3,49 ... a minimum (zero) occurs
As Z/X increases, the number of maxima increases.
O Influence of unequal RCS for scatterIng centers (i.e., spheres not same size)
f 1/4 E-2 1 N 1,l!4
E Il+l1/4 5/4 E I 11~4 3/4maxmi
C max (5/4)2 1.56 Emi (.75 0.56
O Influence when Alk or Z/X are not integers
- interference still occurs
- angular location of i~nterference peaks are shifted
-large a at e 00 is modified
0 0 When spheres are broadside to wave, the greatest sensitivity of RCS to a change
in 0 occurs, i.e.
-~is largest
'. 4.O When spheres are on a line parallel to k, the least sensitivity of RCS to a
change in 0 occurs, i.e.
"is smallest
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A.. ic. FJHS 3
tuu
4D4
CAA
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31
SAMPLE OUTPUT FOR TWO-SPHERES MODEL
0 The RCS due to two spheres was calculated. Plotted is a/a as a function of
6. When e is 900, the spheres are broadside to the waves. The spheres are
oriented as shown in the drawing. A non-integer spacing was selected;
2Z - 0.714X.
0 The figure on the left-hand side is for f - 1, i.e., both spheres are the
same size. Broadside to the incident waves, a/cI is equal to 4. When 6 - 0,
the RCS does not decrease to zero. At e - 00, the phase angle between the
electric vectors is
(2)(0.71)(360) - 360 - 151.20
0Since the phase angle is not 180 , the RCS does not vanish.
0 The figure on the right side has the same spacing for the spheres. However,
the relative sphere size has been changed since f - 1/2. In fact, the
RCS of one sphere is only f2 or 1/4 as large. The RCS does not vanish at
any value of 8 due to destructive interference.
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f-FKN
luii
ftt
V)h
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- °
32
RADAR CROSS SECTION AND ANTENNAS
o When the flat plate is illuminated at an angle a off the normal, the
(monostatic) radar does not see the main lobe.
o The (monostatic) radar receives the N - 3 sidelobe for the case illustrated.
O Note that the angle off the normal a is one-half of the angle of the N - 3
lobe from the main lobe, i.e.
22a
4
I
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ZgqDAR C1 OSJ SEC770AI 4'AIb AA/T7DVAS 32
ITE I) IF FPFFCT WAVE
PLATE~
waV( F~qR F(L
~F'-~CT~J pj gcT ~ ~NAV3
iifrFIELA441057RI 0SE S 4'3 0,4
TA7 11 A: k 714f6C 5-s0M -S
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*•. IcC•
LECTURE III. AIRCRAFT DESIGN AND RADAR CROSS SECTION
1. Origin of Electromagnetic Wave Scattering
2. Cintributors to Aircraft Radar Cross Section
3. Relative Size of Contributors to RCS
4. Aircraft at Visible and Microwave Frequencies
5. Fire Fox HIG-31
6. Plan Form Fire Fox MIG-31
7. Gross Features of RCS for Fire Fox MIG-31
8. Antenna Scattering
9. Radar Cross Section Reduction
10. Impedance Loading
11. Shaping to Reduce Radar Cross Section
12. Do's and Don't's for Shaping to Achieve Low RCS
13. Radar Absorbing Material, RAM
i s. Practical Aspects of L4M
15. Construction Materials
16. Radar "Hot Spots"
17. Payoff of Reduced Radar Cross Section
A; E. FUH5
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I06
ORIGIN OF ELECTRC1AMTIC WAVE SCATTERNG
SPECULAR. Mirror-like reflection. Lobes occur due to diffraction. Main
contribution occurs when k -n - kit i.e., wvefronts are tangent to
surface. ti is wave propagation vector which in normal to incident
wavefront.
DIFFRACTION. A discontinuity occurs, and electromagnetic (E4) boundary
conditiona must be satisfied. The scattered wave is necessary to satisfy
the boundary conditions.
TRAVELING WAVE. A long thin body with near nose-on incidence may cause
traveling waves. Along the body, EM scattering may occur due to
surface discontinuity
change in material, e.g., metal to plastic
end of body
CREEPING WAVE. Waves which propagate in the shadow region of smooth
bodies are creeping waves.
CHANGES IN EM BOUNDARY CONDITIONS. As the incident wave propagates along the
surface of the body, the EM boundary conditions are satisfied by currents in
body. Whenever a change in EM boundary conditions occurs, scattering results.
Examples are:
gaps and edges
surface discontinuities in slope, curvature. etc.
change in surface raterials
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/07
c 4 INC
lla
tu lU3
Ita
lu
/ 1
LU
(0
4i 2
-zz
A. E.6 FU H.5
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/og
2
CONTRIBUTORS TO AIRCRAFT RADAR CROSS SECTION
(1) RADC*. If radome is transparent, then radar wave "sees" inside the
cavity containing A/C radar. Black boxes inside may form retroreflectors.
If radome is opaque, then tip diffraction may occur.
(2) A mooth rounded surface may have a creeping wave for the shown.
(3) Cockpit is a cavity and may be a large contributor to RCS.
(4) The propagation vector ki is about tangent to surface. The incident
wave encounters an edge which is a scattering device.
(5) Multiple reflections may occur. This may be more important for bistatic radar
(6) Large flat areas may cause glints. "'Flat" is in quotes because a surface may
have p >> X and appear to be flat. p is radius of curvature.
(7) Ordnance and drop tanks contribute to RCS.
(8) Edge diffraction (like a wedge) occurs at sharp leading edges and trailing
edges. Sharp is p << 1. Blunt is p = or p > 1. Blunt edges use a
cylinder as model.
(9) Inlet cavities my give very large RCS.
(10) The -udder and elevator may form a right angle dihedral which acts as
retrord lec tor.
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tog
'JJ
kUJ
-4 t
%J U.T,-
~~ic
UU
cIi
to ,
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3
RELATIVE SIZE OF CONTRIBUTORS TO RCS
ORDNANCE. Missile may have own radar which can have large RCS.
RUDDER-ELEVATOR DIHEDRAL may be big due to action of retroreflector.
EXHAUST. Waves can propagate within the cavity and reflect from internal parts.
RUDDERS. RCS is small except for glint at broadside.
*'. WING. RCS is small except when viewed so as to see "flat" area.
INLET FOR APU. The inlets for APU, air conditioning ducts, and gun exhaust
gas ports can be large in certain direction.
COCKPIT. Big contributor to RCS.
GUN MUZZLE. Scattering is due to surface discontinuities.
RADOME. Big anLenna inside acts like a cat's eye in the dark.
FUSELAGE. Recall a - rP0P2 . Usually p1 and P2 are small compared to p of wing
upper or lotr surface.
t"t
141
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.. ~ ~ ~ ~ s E 6' F U- te .-.- ' 7
I"I
'CS
Ll
uiu
KIlk
4~ U
V) ) L*1
u
~ .4
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DIV,
4
AIRCRAFT AT VISIBLE AND MICR0WAVE FREQUENCIES
0 The E( boundary conditions and the wave equations are shown. The equation
for free space is
0 +
I For propagation inside dielectrics, change k0 to k,
C Three major cavities are illustrated:
aircraft radar cavity
engine inlet cavity
cockpit cavity
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AICA~ r VlslUE: Af0mIccou),q~
A. E.FUHS
13,C rdie leety ov yebaI
k a Ve.Ufor cl e/lez Aicz2+ I"' WA'E, Wroc 1
IY71e*j C';vi-y
Waves fisia'e caVity
V E~k - -o/ i
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FIRE FOX MIG-31
0 As an example of estimating RCS for an aircraft, the MIG-31 Fire Fox
will be used.
0 Some of the gross features of the RCS for the aircraft can be obtained
*:) from formulas discussed earlier.
O The Fire Fox was designed, not for Mach 6, but for movie audiences.
Low RCS and good L/D were not requirements. The design requirement was
to look "tmean."
Note: The comments for vievgraph 6 start here.
PLAN FORM FIRE FOX MIG-31
The assumed plan form is shown. The plan form can be verified now that theMIG-31 is in U. S. hands.
Assume the following:wing span, 30 m length, 28 m radar frequency, 12 GHz Wavelength, 0.025 u
The RCS will bi estimated in the plane of the plan form. One views the aircraft from
nose-on (6 - 0 ) moving clockwise to starboard wing tip (8 - 900). The variousSU scattering components are identified.
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* .**~.* *..* * - - , .- . *- * "
1VIP
uiS
ILL
u*JJU-m E~
V.m'F-m
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.. -- r_. _ .. .-. .*
PLAN FORM FIRE FOX MIG-31 (Continued from Previous Page) 6
NOSE-ON e 0=Tip Diffraction: The tip of the fuselage appears to be a wedge. Formulas forfinite wedges are quite complex and are outside the realm of a back-of-an-envelope calculation. Based on calculations for tip diffraction for cones,a. may not be too important. However, this should be verified.
Engine Inlets: There are four engine inlets which will be modelled as flatplates with size 0.7 m x 1.8 m for each.
a1 -4irA 2 41(0.7 x 1.8)2 2. I 2 (.025)2- ... 32000 m2 - 45 db-- 1 (.025)2
One can estimate the width of the main lobe from
a = [sin(ka sin D- 2
a0. kas in e>1
The first zero occurs when ka sin 6 - n.
"e - arcsin(ir/ka) 1 1.020
The lobe is very narrow.To add the four inlets, use the equation from Lecture 2, viewgraph 29. If phase
- •angles for the waves returned from the four inlets are all zero, then theaddition formula becomes
- 4= 2 16a
The RCS for all four inlets is 57 db.
.-LEADING EDGE OF STARBOARD CANARDThe LE has a sweep of 20'. The LE can be modelled as a wire. The assumeddimensions are L - 3.8 m (length of LE on MIG-31) and radius of a - 0.01 m."For these valuesS9 (.82(4
4 n = (3.8)•(251.3 x 0.01)
2- 4072 m - 36 db
The preceding equation appears in viewgraph 15 of Lecture 2. To estimate* g width of main lobe, one can use the same formula as used for the inlet. Wires
have a lobe structure similar to a flat plate.
C [sin(kL sin 8)
G 0 Usin 0
The first zero in RCS occurs when
kL sin 0 u
or where 0 - 0.20 when L - 3.8 m. The second peak occurs at
_ - arcsin(t - 0.28
The value of the second peak is 34 db
The other co-ent is that the lobes are very, very narrow.
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/1.. . .. . ..7
* /j?7
tLd
* 0
LL ~~~
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7°..GROSS FEATURES OF RC FOR FIRE FOX HIG-31
Continuing the calculations, the other leading edges were evaluated as follows:
2Sweep L, m a,m ,3 o, db330 3.5 0.02 55300 47
-, 40°40 10 0.02 450000 56
70 9 0.02 366000 55
90° (wing tip) 1.2 0.01 406 26
FUSELAGE
The fuselage has a normal vector at an angle of 6 85. Assumepl"3uand 2 - 10 m. rom
So-M P - 94 m2 -20 db
one finds the RCS for fuselage.
* TRAILING EDGE OF PORT WING
The TE of port wing is normal to the incident waves from 0 - 1700.Modelling the TE as a wire with L - 12 m and a - 0.01 m, the following resultswere obtained
a a 404000 m 2 . 46 db
"""8The large RCS is due to high radar frequency.
4-ENGINE EXHAUSTS AT 8 - 1800
The calculation for inlet was repeated with assumed dimensions of 0.8 m x 2.0 m.The result is
2o - 823550 m2 - 47 db
sm
The RCS have been plotted. The various ak should be added using the formulagiven in Lecture 2, viewgraph 29.
NOTE
A note about values of RCS is appropriate. The leading edges scale as
a 0
a1 2
Since the wavelength is small, the value of RCS is large. At X - 1.0 m, the2cross sections would be reduced by a factor of (0.025/1.0) - 6.25E-4 or - 32 db.
One could subtract 32 db from kmah va'lue shown for LE or TE if ý were 1.0 m.sm
O
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-. ~~~~~ --- ' 1r7
~111 T- I
it ell
-'---K- - -7
-L -1-l-
_ -I
~~i44- LV 44- -f-- -
-I-I-
IU T:±. WZ ____ --! zr++--+t{9- 1~
II 7-
K CA
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...................................--....,.• __ _ ,'''• • • " '•-......... .............. . 4....--
8
AINNA SCATTERING
0 0 Structural Scatterin Term is due to currents iniduced iu the antenna surface
'. and is independent of antenna load impedance.
0 Antenna Scattering Term is due to current induced at the antenna load terminals.
• .An antenna launuhes a plane wave from the antenna focus when radiating. The
power moves outward from focal point to beam. When radar illuminates the antenna
with plane waves, the power moves to the focus. The antenna fecd system has a
certain impedance. Depending on the impedance of the feed, the waves may or may
not be re-radiated.
0 The RCS of an antenna is given by! i•]2
o - [ ;-+ rd- expixp8 e
where a iU RCS of structural scattering term and a is the effective ecdo area
of antenna. The phase angle between a and a is J. The value for a is, forS e e
certain specific conditious,,2^ G2
e 4n
where G is &atenna gain at A. As an estimate, one can use
22
IIT
for antenna RCS.
0 As an example, consider an antenna with a gain G of 100 at A 0.5 m. The RCS of
the antenna is eatimated to be
0.) 2100) 8 2
IT
4
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741
Q~"4
IX
- ljj
Il
A. E V UH
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S.° . 9
"RADAR CROSS SECTION REDUCTION
- 0 SHAPING implies control of geometry so as tc reduce RCS.
* 0 RAM is material used to maLch rave impedance of free space or to absorb the
EH wave energy.
0 IMEDAUCE LOADING consists o5 passive or active elements added at appropriate
locations to control RCS.
0o
S
0
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A:E* FUHS
Laj
tJz
ItIA
W U
LIP) 4
cZZý
uj *N.
QUd4 'ý0
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10
I14PEDAI4CE LOADING
0 On the left-hand side is the case of a loaded pair of cylinders. By correct
choice of ZL the load impedance, the RCS is reduced by 35 db.
0 On the right-hand side is the case of a pair of circular oglives back-to-back.
The incident waves are arriving from the left when 6 - 00. A wire with
length X/4 is added to the tail end of the body. The wire causes a major
reduction in RCS for 0 < 8 < 45°. This is an example of scattering by
traveling waves when 0 is small. A relatively mi-ir change makes a very
large change in RCS.
O For simple cases, one may be able to exploit the me~thod. Application of
impedance loading to complex shapes may not be obvious.
6q
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i 11
SHAPING TO REDUCE RADAR CROSS SECTION
0 The direction of incident radar waves is an important consideration. If theaircraft will be illuminated from below, put engines on top of the wing.
(!) SHIELD INLETS. The inlets can be shielded by the fuselage. Locating the engineson top when radar is below A/C will help. If engine performance permits the useof wire mesh over the inlet, the RCS can be reduced. Mesh spacing is small fractionof X.
(2) CANT RUDDERS INWAIS. The4surface normal vector n is moved upward. The big RCSwhich occurs when k and n are parallel will occur only when radar is above the A/C.Also when a rudder-Ilevator combination is used, the retroreflector of the dihedralis avoided.
(3) SHIELD NOZZLES. The comments for inlets apply.
(4) ROUND WING TIPS. Use the formula a - vplP2 . A rounded wing tip has small P1 and p2 .
(5) CANT ýUSELAGE SIDES. This tips the surface normal n upward. For low RCS, do nothave n pointing toward the radar!
(6) BLEND COMPONENTS. Waves are scattered by discontinuities in slope, curvature, etc.Blending minimizes the geometrical discontinuities.
(7) MINIMIZE BREAKS AND CORNERS. Any shape resembling a retroreflector is bad.As shown in viewgraph 1, gaps scatter EK waves.
(8) PUT ORDNANCE LOAD INSIDE AIRCRAFT. This would make both the aerodynamicists and radarengineers happy. However, internal storage may not be possible. Drag equals qCDA.Internal storage may give large A.
(9) ELIMINATE BUMPS AND PROTRUSIONS. The comments of items (6) and (7) apply here.Use retractable covers over gun parts.
(10) USE BANDPASS RADOME. An opaque radome at the search radar wavelength eliminatesthis problem.
(11) USE LOW PROFILE CANOPY. Ever since the SPAD, aviators want to see. Dog-fights requiregood visibility. Having said that, a low profile canopy with gold plating will havemuch lower RCS. The thin layer of gold (or other metal) plated on the canopyscreens out microwaves.
(12) SWEEP LE. The A/C is frequently illuminated by a search radar from nose-on aspect.A swept LE is one way to reduce RCS. There are two philosophies in regard to LEshape. A straight LE concentrates a big RCS in a narrow lobe. If the search radaris never in that lobe, the A/C cannot be detected because of LE return. A curvedLE spreads a smaller RCS over a wide angle. Although RCS is spread over a largeangle, RCS is small.
FINAL NOTE: Think of A/C as a porcupine with surface normal vectors n as quills.Don't have any quills pointing toward radar!
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12"DO's AND DON'T's FOR SHAPING TO ACHIEVE LOW RCS
0 The aircraft designer may not be able to heed all the advice.
0 Ship superstructures are classic examples of built-in retroreflectors.
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"RADAR ABSORBING MATERIAL, RAM
0 The probability of achieving the stated goal for RAM is rather remote.
0 The book of Ruck, et al.*, provides a detailed discussion of RAM.
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*George T. Ruck, Editor, Radat Cross Section Handbook, Volume 2, n•.nu=1%, '
"Now York, 1970.
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S.1
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PRACTICAL ASPECTS OF RAW
In addition to electromagnetic properties, RAM must have other favorable
physical properties.
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"15
CONSTRUCTION MATERIALS
" EMP is electromagnetic pulse from exoatmospheric nuclear.explosions.
- The attrition rate for Mosquito bomber was low.
Factors, such as speed and twin-engines as well as low RCS, may have
contributed to the low rate.
0 The trend is toward composite materials. Mixed structures, such as wings
UI with composite skins and aluminum spars, may have high RCS due to
reflection from spars.
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16
RADAR "HOT SPOTS"
0 Many of the sources of "hot spots" have been discussed already.
0 Estimate the RCS of the trihedral corner reflector in tha cockpit. Use
the formula from Lecture 2, viewgraph 28:
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Assume X- 0.2 m and A - 0.5 m.
0 12 (0"2)-742 m2 -29 db
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17
% PAYOFF OF REDUCED RADAR CROSS SECTION
- 0 An air search radar may have a specification to be able to search
so-many m /sec of space. If the RCS is reduced, the volume search rate
.* decays rapidly.
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LECTURE IV. SOLUTION TECHNIQUES
1. Maxwell's Equations
2. Solution for Scattered Field
3. Solutions to Maxwell's Equations
S•4. Separation of Variables
5. Geometric Optics
6. Geometrical Theory of Diffraction
7. Physical Optics
8. Impulse Approximation
9. Numerical Methods for Radar Cross Section
10. Applicability of Sum-of-Components Model for RCS Calculation
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141
-MA --E'S EQUATIONS
"The symbols have the following definitions:
E electric field intensity, volts/m
H magnetic field intensity, ampere-turn/m
D electric displacement, coulomb/m 2
B magnetic induction, webers/m2
p charge density, coulomb/rn
•..J current density, amperes/m2
- 0 permittivity of free space, farad/m
permeability of free space, henry/m
k 0 free space propagation constant, 1/m
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2
SOLUTION FOR SCATTEREX FIELD
0 The symbols have the following definitions:
q a surface charge density, coulomb/mr2
K surface current density, amperes/m
O Additional boundary conditions apply at infinity.
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3
SOLUTIONS TO MAXWELL' S EQUATIONS
0 Scattering of electromagnetic waves is a haven for the applied
mathematician!
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4
SEPARATION OF VARIABLES
0 More than a dozen orthogonal coordinate systems have been discovered.
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GEOMETIC OPTICS
0 Geometrical optics accounts for transmission through radomes by
using Snell's laws.
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6
GEOMETRICAL TIEORY OF DIFFRACTION
0 By introducing phase angle as well as amplitude, the features of
diffraction can be incorporated into the theory.
0 Geometrical theory of diffraction is an ad hoc method without firm
theoretical foundation; it does work, however.
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7
PHYSICAL OPTICS
0 Physical optics involves integrals. The solutions are in terms of
integrals.
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WIPULSE APPROXIM(ATION
o Impulse approximation is important to the proble=n of inverse
scattering.
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NUM•ERICAL METHODS FOR RADAR CROSS SECTION
0 Numerical Methods are either in primary role or in secondary role.
0 The most common approach to machine calculation of RCS is the
"Sum-of-Components.
0 Direct solution of Maxell's equations is handicapped by computer
capability.
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APPLICABILITY OF SUM-OF-COMPONENTS MODEL FOR RCS CALCULATION
- 0 Even though kL >> 1.0, where L is for overall aircraft size, for parts
of the aircraft k = 1.0 or kZ << 1.0 may occur. Hence, solutions-- • must span all three frequency ranges. t is the size of a subcomponent
of the aircraft.
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