Advanced Imaging Approaches for Detecting Obscured Objects

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Advanced Imaging Approaches for Detecting
Obscured Objects

• Sermsak Jaruwatanadilok
• Sumit Roy
• Yasuo Kuga
• Department of Electrical Engineering, University
  of Washington, Seattle, WA

BSI, Bellevue, WA, Feb 26, 2009
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Overview

• Goal and concepts
• Assets and capabilities
• Previous and on-going work

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Goal Concepts
GOAL Improve detection and imaging of objects in
obscuring and complex environments using
electromagnetic waves

• Concepts
• (1) Waveform design at transmitters to combat
  random media effects
• (2) Physics-based EM model of received signals
• (3) Signal processing at the receivers
• Exploit relationship among (1), (2), and (3)

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Assets and Capabilities

• Analytical formulations
• Angular / Frequency correlation functions of
  surface scattering
• Two frequency mutual coherence functions of waves
  in random media
• Numerical simulation tools
• Monte Carlo simulations
• Scattered waves in the presence of particle
  scatterings
• Full-wave simulation tools
• FDTD software
• COMSOL Multi-physics
• Experimental tools, equipments and facilities
• Array imaging system
• MMW systems
• Anechoic chamber

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Current and Previous Work Related to BSI

• I. MMW active imaging of concealed objects
• II. MMW passive imaging of concealed objects
• III. Microwave imaging using angular/frequency
  correlation methods
• IV. Time reversal method and time reversal
  imaging
• V. Coherent array imaging
• VI. Focused pulse beam imaging
• VII. Detection of vehicle and human movement
  using existing communication systems
• Combined use of the physics-based EM modeling and
• signal processing

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I. MMW Active Imaging of Concealed Objects
Simulated MMW Image Examples
Optical image

• Aperture radius 30 cm
• Distance 1 m
• Cloth material cotton
• Cloth thickness 8 mm
• Plastic explosive (C-4)

94 GHz simulated image 200 GHz simulated
image
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Simulated MMW Image Examples
Optical image

• Aperture radius 30 cm
• Distance 1 m
• Cloth material cotton
• Cloth thickness 1.2 mm
• Plastic explosive (C-4)

94 GHz simulated image 200 GHz simulated image
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Multi-layer Model

• ABCD matrix formulation

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Simulated MMW Pulse Imaging
94 GHz
Aperture radius 30 cm Distance 1 m Cloth
material cotton Cloth thickness 1.2 mm Plastic
explosive object Bandwidth 10 GHz
220 GHz
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II. MMW Passive Imaging of Concealed Objects

• 1 R. Appleby, (From previous slide)
• 2 National Academies, Assessment of
  Millimeter-Wave and Terahertz Technology for
  Detection and Identification of Concealed
  Explosives and Weapons, http//www.nap.edu/catalo
  g/11826.html, 2007

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Simulated Passive Imaging Examples
Optical image
94 GHz
220 GHz
OD 0.123
OD 0.288
Cloth thickness 1.2 mm
OD Optical depth
Metal object
OD 0.826
OD 1.9205
Cloth thickness 8 mm
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III. Angular Correlation Function / Frequency
Correlation (ACF / FCF)

• Correlation of waves with different angles and
  frequencies
• Exploit the difference of correlation
  characteristics when a target is presence
  compared to no target

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Experimental Studies of ACF/ FCF Memory Line

• Strong correlation on memory line

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Use of Angular and Frequency Correlation Function
(ACF/FCF) for Imaging

• beam 1 92 GHz 96 GHz 10 degree
• beam 2 78 GHz 12 degree
• Equivalent to imaging but this shows
• presence of particle scattering

Slope 5.9 radians / GHz
shrapnel
Slope 0.39 radians / GHz
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IV. Time-Reversal Method

• Concept of time-reversal imaging and focusing
• Send probing signals
• Obtain received signals (targets and surrounding)
• To focus re-transmit time-reversed signals
• To image process time-reversed signals

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Time-Reversal Focusing
Focusing improvement in random media (ODoptical
depth)
Geometry of the problem
Snapshots of wave field in random media. (a)
Gaussian pulse propagating through random media,
(b) Time-reversed pulse back-propagated in the
random medium. The energy focuses at the
original source location.
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Time-Reversal Imaging

• Multistatic data matrix
• Time reversal matrix
• How to model the time reversal
• matrix in the presence of random scattering
  media
• Time reversal imaging
• Time reversal MUSIC (multiple signal
  classification)

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Space-time transmitter-receiver 7-element array
with half wavelength spacing is located at, and a
point target is located at and in a random
medium. The left figure shows array and image.
Two figures on the right show images (in dB) in
the dotted expanded area for OD 0.1 and 0.5.
Space-time time reverse MUSIC images in free
space and random complex media at OD 0.1 and
0.5. (dB scale) Figs. 5 and Fig. 6 show the
result for identical physical problems. Note
that space-time time reversal MUSIC has superior
lateral resolution.
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V. Coherent Array (CA) Imaging and Detection of
Object in Random Media

• (a) SAR images is formed using backscattering
  signals. Received signal is a response of a
  single transmitter
• (b) CA method coherently combines responses from
  all receivers and transmitters

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• Numerical simulations (a) SAR images (b) CA
  images

CA method can mitigate effects from random
scattering and clutter, but suffers the reduction
in image resolution.
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VI. Focused Pulse Beam in Random Scattering Media

• Effects from random scattering media on the
  imaging two-frequency mutual coherence function
• Contribution from target and media

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Focused Beam Imaging
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VII. Detection of Vehicles and Human Movement
Using Existing Communication Systems Newly
Started Project in BSI
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Concept

• Range-Doppler image using digital correlator
• Angle-of-Arrival using MUSIC

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DETECTION SCHEME
Adaptive Cancellation - Remove direct signal
and clutter from surveillance channels to get
true echo signal - Adaptive filter uses a
lattice predictor structure
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• Cross Correlation
• Find Doppler shifts and time-delayed echoes of
  the targets.

• Drawbacks
• Excessive processing time for long input signals
• Decimation technique discard data at Doppler
  frequencies we know targets do not exist before
  Fourier Transform

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Time Delay ? Range
r1 r2 2a b2 a2-c2
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Adaptive Beamforming to Get Angular Resolution
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Spatial Subarray Smoothing

• For correlated signals

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Results from MUSIC AOA Estimation
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Some Simulation Results
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VIII. Array Imaging Systems

• Range angle imaging
• using step CW and angle
• of arrival processing

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MMW Radar for Imaging

• Frequency 30 GHz (to be extended to 100 GHz)
• Spotlight images using 2-D scan and stepped CW
  mode
• Doppler images using 2-D scan and short pulse

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Spotlight image using 2-D scan and stepped CW mode
Resolution Cross-range 2 degree (antenna
beamwidth) Down-range 3 cm 5 GHz bandwidth
Doppler images using 2-D scan and short pulse
With a known vibrating source at 20 Hz
(discrimination of an active source)
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On-going work

• Improving modeling of wave propagation in random
  scattering media and clutters
• Angular / Frequency correlation for detection and
  imaging of target
• Ultra wide band time reversal imaging and
  focusing
• Detection of vehicles and human movement using
  existing communication systems

Future work

• Collaborative imaging and detection from several
  receivers