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• Hafidz Baswedan

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Radar Systems Engineering Lecture 7 – Part 1 Radar Cross Section Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer

IEEE New Hampshire Section Radar Systems Course 1 Radar Cross Section 1/1/2010

IEEE AES Society

Block Diagram of Radar System This lecture Transmitter

Propagation Medium Target Radar Cross Section

Power Amplifier

Waveform Generation

T/R Switch Antenna Receiver

Signal Processor Computer A/D Converter

Pulse Compression

Clutter Rejection (Doppler Filtering)

User Displays and Radar Control

General Purpose Computer

Tracking

Parameter Estimation

Thresholding

Detection

Data Recording Photo Image Courtesy of US Air Force Used with permission. Radar Systems Course 2 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Definition - Radar Cross Section (RCS or σ)

Figure by MIT OCW.

Radar Cross Section (RCS) is the hypothetical area, that would intercept the incident power at the target, which if scattered isotropically, would produce the same echo power at the radar, as the actual target. Radar Systems Course 3 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Factors Determining RCS

Figure by MIT OCW.

Radar Systems Course 4 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Threat’s View of the Radar Range Equation Antenna Gain G Transmit Power PT

Transmitted Pulse Target Cross Section σ

Received Pulse Figure by MIT OCW.

R Distance from Radar to Target

Cannot Control

Radar Range Equation Can Control Pt G2 λ2 σ

S = N (4π)3 R4 k TS Bn L Cannot Control Radar Systems Course 5 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline

•

Radar cross section (RCS) of typical targets – Variation with frequency, type of target, etc.

•

Physical scattering mechanisms and contributors to the RCS of a target

•

Prediction of a target’s radar cross section – Measurement – Theoretical Calculation

Radar Systems Course 6 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Radar Cross Section of Artillery Shell RCS vs. Aspect Angle of an Artillery Shell 0 Typical Artillery Shell Radar Cross Section (dBsm)

-10 -20 -30 -40 -50 -60

0

Courtesy US Marine Corps

20

40

60

80

100

120

Aspect Angle (degrees) Radar Systems Course 7 Radar Cross Section 1/1/2010

140

160

180

M107 Shell for 155mm Howitzer

IEEE New Hampshire Section IEEE AES Society

Radar Cross Section of Cessna 150L Measured at RATSCAT (6585th Test Group) Holloman AFB for FAA

Radar Cross Section (dBsm)

40

S Band VV Polarization

20

0

-20 Courtesy of Federal Aviation Administration

0

90

180 Aspect Angle (degrees) Cessna 150L (in takeoff)

Scott Studio Photography with permission

Radar Systems Course 8 Radar Cross Section 1/1/2010

270

360

Cessna 150L (in flight)

Scott Studio Photography with permission

IEEE New Hampshire Section IEEE AES Society

Aspect Angle Dependence of RCS Cone Sphere Re-entry Vehicle (RV) Example

Figure by MIT OCW.

Radar Systems Course 9 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Examples of Radar Cross Sections Square meters Conventional winged missile 0.1 Small, single engine aircraft, or jet fighter 1 Four passenger jet 2 Large fighter 6 Medium jet airliner 40 Jumbo jet 100 Helicopter 3 Small open boat Small pleasure boat (20-30 ft) Cabin cruiser (40-50 ft) Ship (5,000 tons displacement, L Band)

0.02 2 10 10,000

Automobile / Small truck Bicycle Man Birds (large -> medium) Insects (locust -> fly)

100 - 200 2 1 10-2 - 10-3 10-4 - 10-5 Adapted from Skolnik, Reference 2

Radar Cross Sections of Targets Span at least 50 dB Radar Systems Course 10 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline

•

Radar cross section (RCS) of typical targets – Variation with frequency, type of target, etc.

•

Physical scattering mechanisms and contributors to the RCS of a target

•

Prediction of a target’s radar cross section – Measurement – Theoretical Calculation

Radar Systems Course 11 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

RCS Target Contributors

Inlet Body Shape Exhaust

Seeker

Control Surfaces

•

Altimeter

Types of RCS Contributors – Structural (Body shape, Control surfaces, etc.) – Avionics (Altimeter, Seeker, GPS, etc.) – Propulsion (Engine inlets and exhausts, etc.)

Radar Systems Course 12 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Single and Multiple Frequency RCS Calculations with the FD-FD Technique •

RCS Calculations for a Single Frequency – – –

•

Illuminate target with incident sinusoidal wave Sequentially in time, update the electric and magnetic fields, until steady state conditions are met The scattered wave’s amplitude and phase can the be calculated

RCS Calculations for a Multiple Frequencies – – –

Illuminate target with incident Gaussian pulse Calculate the transient response Calculate to Fourier transforms of both: Incident Gaussian pulse, and Transient response

–

Radar Systems Course 13 Radar Cross Section 1/1/2010

RCS at multiple frequencies is calculated from the ratios of these two quantities

IEEE New Hampshire Section IEEE AES Society

Scattering Mechanisms for an Arbitrary Target Diffraction at Corner Gap, Seam, or Discontinuity Echo Multiple Reflection

Curvature Discontinuity Return

Backscatter from Creeping Wave

o ch E ve Wa

m f ro

v Tra

Specular Surface Reflection

ave W g elin

Edge Diffraction

Tip Diffraction at Aircraft Nose

Radar Systems Course 14 Radar Cross Section 1/1/2010

Return From Engine Cavity

Tip Diffraction from Fuel Tank

IEEE New Hampshire Section IEEE AES Society

Measured RCS of C-29 Aircraft Model 1/12 Scale Model Measurement

Full Scale C-29 BAE Hawker 125-800 Courtesy of Arpingstone

20 Fuselage Specular

RCS (dBsm)

10

Wing Leading Edge

X-Band HH Polarization Waterline Cut

0 -10 -20 -30 0

Radar Systems Course 15 Radar Cross Section 1/1/2010

60

120 180 240 Aspect Angle (degrees)

300

360 Adapted from Atkins, Reference 5 Courtesy of MIT Lincoln Laboratory

IEEE New Hampshire Section IEEE AES Society

Outline

•

Radar cross section (RCS) of typical targets – Variation with frequency, type of target, etc.

•

Physical scattering mechanisms and contributors to the RCS of a target

•

Prediction of a target’s radar cross section – Measurement – Theoretical Calculation

Radar Systems Course 16 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Techniques for RCS Analysis

Full Scale Measurements

Theoretical Prediction Scaled Model Measurements Courtesy of MIT Lincoln Laboratory Used with Permission

Radar Systems Course 17 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Full Scale Measurements Courtesy of MIT Lincoln Laboratory Used with Permission

•

Target on Support

Foam column mounting – Dielectric properties of Styrofoam close to those of free space

•

Metal pylon mounting – Metal pylon shaped to reduce radar reflections – Background subtraction can be used Derived from: http://www.af.mil/shared/media/photodb/photos/050805-F-0000S-003.jpg

Radar Systems Course 18 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Full Scale Measurement of Johnson Generic Aircraft Model (JGAM) Courtesy of MIT Lincoln Laboratory Used with Permission

Radar Systems Course 19 Radar Cross Section 1/1/2010

RATSCAT Outdoor Measurement Facility at Holloman AFB

IEEE New Hampshire Section IEEE AES Society

Compact Range RCS Measurement Radar Reflectivity Laboratory (Pt. Mugu) / AFRL Compact Range (WPAFB)

Courtesy of U. S. Navy.

Main Reflector

Target

Plane Wave

Low RCS Pylon Feed Antenna Radar Systems Course 20 Radar Cross Section 1/1/2010

Sub-Reflector IEEE New Hampshire Section IEEE AES Society

Scale Model Measurement MQM-107 Drone in 0.29, 0.034, and 0.01 scaled sizes

Full Scale Measure at frequency f Courtesy of MIT Lincoln Laboratory Used with Permission Radar Systems Course 21 Radar Cross Section 1/1/2010

Scale Factor S (Reduced Size)

Subscale Measure at frequency S x F

IEEE New Hampshire Section IEEE AES Society

Scaling of RCS of Targets Scale Factor S

Quantity

Full Scale

Subscale

Length

L

L´ = L / S

Wavelength

λ

λ´ = λ / S

Frequency

f

f´ = S f

Time

t

t´ = t / S

Permittivity

ε

ε´ = ε

Permeability

μ

μ´ = μ

Conductivity

g

g´ = S g

Radar Cross Section

σ

σ´ = σ / S2

Radar Systems Course 22 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Outline

•

Radar cross section (RCS) of typical targets – Variation with frequency, type of target, etc.

•

Physical scattering mechanisms and contributors to the RCS of a target

•

Prediction of a target’s radar cross section – Measurement – Theoretical Calculation

Radar Systems Course 23 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Radar Cross Section Calculation Methods

•

Introduction – A look at the few simple problems

•

RCS prediction – Exact Techniques Finite Difference- Time Domain Technique (FD-TD) Method of Moments (MOM)

– Approximate Techniques Geometrical Optics (GO) Physical Optics (PO) Geometrical Theory of Diffraction (GTD) Physical Theory of Diffraction (PTD)

•

Comparison of different methodologies

Radar Systems Course 24 Radar Cross Section 1/1/2010

IEEE New Hampshire Section IEEE AES Society

Radar Cross Section of Sphere

Radar Cross Section / πa2

10

Higher Wavelengths

Lower Wavelengths

Rayleigh Region λ >> a σ = k / λ4

Optical Region

1

λ > a

10-1

Mie or Resonance Region Oscillations Backscattered wave interferes with creeping wave

Resonance or Mie Region

Rayleigh Region

Optical Region λ > a) (λ ~ a) (λ > a) (λ ~ a) (λ