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AY2011 1
RCS Measurements (Chapter 8)
EC4630 Radar and Laser Cross Section
Fall 2011
Prof. D. Jenn [email protected]
www.nps.navy.mil/jenn
AY2011 2
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
RCS Measurements
• Measurement facilities o Indoor
offers privacy, controlled environment, security
limited in size o Outdoor
• Instrumentation o Frequency domain (FD)
continuous wave (CW) gated (pulsed CW) other waveforms (e.g.,
ultra-wideband monocycle) o Time domain (TD)
(The Fourier transform relates the time and frequency domains)
• Data analysis and presentation o RCS versus aspect angles for a fixed
frequency o RCS versus frequency for a fixed
aspect angle o color contour plots and 3-D surfaces o target imaging for diagnostics
(“troubleshooting”)
Basic measurement technique
AY2011 3
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Indoor Chamber Configurations
Far field chamber: Antennas must be far enough from the target so that a good plane wave condition exists Tapered chamber: Effective at low frequencies. The tapered region acts like a horn antenna Compact range: Plane wave is reflected from the offset reflector surface. It allows much smaller antenna-target distances
AY2011 4
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Far Field Condition
∆
TARGET
TRANSMITSOURCE
SPHERICALWAVEFRONT
R L
If R>>L then
2
2
2 2max 8
max 4
( / 2)
2
kLR
LR
k k R L R
k πλ
∆ = + − ≈ ∆ ≈
If the maximum allowable error is π/8
( )
2
min2
4 84
min
LR
LR
π πλ
λ
≡
=
Example: L=1 m and f = 6 GHz, then
min 320R = m. This turns out to be overly restrictive.
AY2011 5
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Measures of Chamber Performance
• Quite zone: cross sectional dimension over with the amplitude and phase are planar
• Noise floor: Minimum RCS level that can be accurately measured is several dB above the noise floor
• Repeatability o alignment accuracy o chamber conditions o instrumentation stability
• Pattern symmetry o good indication of chamber errors
• Instrumentation capability • Data processing and display/presentation
capabilities • Target size
o pedestal/mount weight limit o quiet zone size
AY2011 6
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Sources of Measurement Error
• Chamber sources o transmit-receive leakage o wall reflections o reflections from the mount
• Other sources o Calibration target misalignment o target misalignment o interactions between the target and
chamber
PARABOLOID
HYPERBOLOIDFOCUS (FEED)
TARGET
QUIET ZONE
FLOOR AND WALLS LINED WITH ABSORBER
MOUNT
EDGE SCATTERING
WALL SCATTERING
MOUNT SCATTERING
AY2011 7
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Measurement Procedure
In order to reduce measurement error the technique of background vector subtraction is often used.
1. Calibrate the receiver using a known target (usually a sphere or plate). 2. Remove the calibration target and measure the background signal at all measurement
frequencies and angles. Store the magnitudes and phases. 3. Measure the RCS with the target installed at all measurement frequencies and angles.
Store the magnitudes and phases. 4. Subtract the background signal from the total signal to get an accurate estimate of the
target only RCS.
Subtraction does not include target interactions with the chamber, but practically they are small.
AY2011 8
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Sample Plot From Pt. Mugu
AY2011 9
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Sample Plot From Pt. Mugu
AY2011 10
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Absorber for Chambers
WEDGEABSORBER
p
d
y
x
Pyramidal absorber:
• Faceted faces cause dispersion of the incident plane wave
• Faces cause reflections and diffractions into other wedges
• Reflectivity of 50 dB with thicknesses of 10λ or greater.
Other shapes and arrangements (Prob. 8.7):
Foam with dielectric constant aε that fills a volume fraction of space g gives an effective dielectric constant that is bounded by
1 eff 2ε ε ε≤ ≤ where
1
2
(1 ) (1 )(1 ) (1 )(2 )
(2 )
a oo
a oo a
ao a
g gg gg g
g g
ε εε εε εε εε ε
ε ε
+ + −=
− + +− +
=+ −
AY2011 11
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Compact Range
Compact range using an offset reflector Point Mugu test range
AY2011 12
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Imaging Using ISAR
Inverse synthetic aperture radar (ISAR) is a technique used to image a target. ISAR transmits a pulse and obtains the downrange distance (i.e. range to a location on the target) from the time delay. ISAR uses the target’s Doppler due to rotation to measure the crossrange. Scattering intensity is plotted for each downrange and crossrange cell. The cells form a pixel of size x∆ by y∆ in an image. ISAR Geometry ISAR Image
DOWNRANGE
CRO
SSR
AN
GE
TARGETTRANSMIT/ RECEIVE ANTENNA
RESOLUTION CELL
∆x∆y
y
x
Ro
AY2011 13
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Time Domain Ranging
• A single pulse is transmitted • The scattered signal is plotted vs. time • Large returns correspond to large scattering sources on the target • The downrange distance is found from a measurement of the round trip time delay RT .
For monostatic RCS 2
RcTR =
Time
RCS
Time
RCS
AY2011 14
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
ISAR Crossrange Formulas
2cτ
o BR θ
oR
RADAR
TARGET
RESOLUTIONCELL
R∆
rω vd
PLANEWAVE
TARGET
CENTER OFROTATION
Range to a pixel:
2 2 2 2 cos( )2 cos( )1
cos( )
o o
oo
o o
r R d R ddR R
RR d R d
π φφ
φ
= + − −
= +
≈ + >>
Scattered field is of the form:
( ) cos( 2 2 cos )s oE t t kR kdω φ− − Doppler shift:
1 ( 2 2 cos )22 2sin
d o
r r
df kR kddtd y
φπω ωφλ λ
= − −
= =
or simply use 2 rd
vfλ
=
AY2011 15
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Measuring Reflections From Materials
A free space arch (“NRL arch”) measures the reflection from material samples
AbsorbingFloor
Surface
MaterialSample
Support ArchTx
Rx
AbsorbingFloor
Surface
MaterialSample
Support ArchTx
Rx
Waveguide simulators measure reflections from inserts (“coupons”)
MATERIAL SAMPLEMATERIAL SAMPLE
TE10 mode is the sum of two plane waves incident at angle θ:
( )( )
( / ) ( / )
1
cos
2
tan /
zz o
x a z x a zo
xE E ea
E e e
a
β
π β π β
π
θ π β
−
− − −
−
=
= +
=
AY2011 16
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Maintenance, Testing & Validation
Special maintenance and test procedures must be used because of the unique materials and fabrication techniques used with LO platforms.
A diagnostic system is required to:
• In the field to determine when maintenance is needed • Provide a “quick look” before missions • Verify the integrity of repairs (either minor repairs in the field or major repairs at the
depot)
For localized repairs, “mini-arches” are used to test the reflection and transmission characteristics of materials.
Diagnostic imaging radar (DIR) is used to test the integrity of LO targets in the field and at a repair depot. When used in the field the system must be transportable, easy to set up and operate, and accurate enough to detect flaws.
AY2011 17
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Diagnostic Imaging Radar (1)
(Courtesy of Lockheed)
AY2011 18
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Diagnostic Imaging Radar (2)
AY2011 19
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
Diagnostic Imaging Radar (1)
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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
F-15 Image Using XPATCH (1)
AY2011 21
Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California
F-15 Image Using XPATCH (2)