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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

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Page 1: RCS Measurements - Faculty

AY2011 1

RCS Measurements (Chapter 8)

EC4630 Radar and Laser Cross Section

Fall 2011

Prof. D. Jenn [email protected]

www.nps.navy.mil/jenn

Page 2: RCS Measurements - Faculty

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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

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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

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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.

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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

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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

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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.

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Sample Plot From Pt. Mugu

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Sample Plot From Pt. Mugu

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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

+ + −=

− + +− +

=+ −

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Compact Range

Compact range using an offset reflector Point Mugu test range

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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

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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

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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

=

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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

− − −

=

= +

=

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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.

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Diagnostic Imaging Radar (1)

(Courtesy of Lockheed)

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Diagnostic Imaging Radar (2)

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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)

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

F-15 Image Using XPATCH (2)