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
Overview
• Goal and concepts
• Assets and capabilities
• Previous and on-going work
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)**
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
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
I. MMW Active Imaging of Concealed Objects
Simulated MMW Image Examples
Optical image
•
•
•
•
•
94 GHz simulated image
Aperture radius = 30 cm
Distance = 1 m
Cloth material: cotton
Cloth thickness = 8 mm
Plastic explosive (C-4)
200 GHz simulated image
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
Multi-layer Model
air
Incident
wave
cloth
c
plastic
p
Human
skin
h
air
Incident
wave
cloth
 A2
C
 2
plastic
B2   A3
D2  C3
h2
Human
skin
B3 
D3 
h3
• ABCD matrix formulation
A  B / Z 4  Z1  C  D / Z 4 
Rs 
,
A  B / Z 4  Z1  C  D / Z 4 
 A B   A2
C D   C

  2
Ts 
2
A  B / Z 4  Z1  C  D / Z 4 
B2   A3
D2  C3
B3 
D3 
Am  Dm  cos  qm hm  , Bm  jZ m sin  qm hm  , Cm  j sin  qm hm  Z m
m
Zm 
, qm   m  j m
qm
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
II. MMW Passive Imaging of Concealed Objects
Aperture
D
Ta
T1
T2
T3
T4
T5
T6
T7
Cloth
Air gap
or
explosive
Human
skin
Target
T8
Ta  ambient or sky temperature = 50o ~100o K outdoor, ~295o K indoor
T1  reflection from cloth, T8  medium emission  Tm 1  exp   o  
T2  reflection from human skin, T3  medium to skin,T4  body emission   bTb exp   o 
T5  reflection from target, T6  medium to target, T7  target emission   t Tt exp   o 
 t  target emissibility,  b  body emissibility ,  o  optical depth, resolution 

D
Imaging is done by the difference T2 + T3 + T4 and T5 + T6 + T7
[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/catalog/11826.html, 2007
L
Simulated Passive Imaging Examples
Optical image
94 GHz
220 GHz
Cloth thickness = 1.2 mm
OD = 0.123
OD = 0.288
Metal object
Cloth thickness = 8 mm
OD = 0.826
OD = Optical depth
OD = 1.9205
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
Experimental Studies of ACF/ FCF Memory Line
• Strong correlation on ‘memory line’
• q1 = 19o , q2 = 20o
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
#1 Tx / Rx
1
no explosive
explosive present
3
Phase (rad)
2
1
0
-1
-2
-3
-4
13
14
15
16
17
Frequency difference (GHz)
Human
skin
2
Slope = 5.9 radians / GHz
Phase of FCF/ACF
Air gap
or
explosive
Angular and
Frequency Correlation
(ACF/FCF)
#2 Tx / Rx
4
Cloth
18
Slope = 0.39 radians / GHz
shrapnel
IV. Time-Reversal Method
Concept of time-reversal imaging and focusing
(1) Send probing signals
(2) Obtain received signals (targets and surrounding)
(3) To focus: re-transmit time-reversed signals
To image: process time-reversed signals
Time-Reversal Focusing
Geometry of the problem
Focusing improvement in random media
(OD=optical depth)
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.
Time-Reversal Imaging
• Multistatic data matrix
M
K   m g m g m
m 1
g m  G  rm , r1  G  rm , r2 
G  rm , rN 
T
• Time reversal matrix
M
M
T  K K    m*  m g*mg*mg m g m
*
m 1 m 1
• How to model the time reversal
matrix in the presence of random scattering media
• Time reversal imaging
• Time reversal MUSIC (multiple signal classification)
SAR
Random
complex
medium
Array
X
X
X
X
X
X
X
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.
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
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.
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
Focused Beam Imaging
VII. Detection of Vehicles and Human
Movement Using Existing Communication
Systems
Newly Started Project in BSI
Concept
• Range-Doppler image
using digital correlator
• Angle-of-Arrival using
MUSIC
Source
Objects
clutter
clutter
Reference
signal
Passive array
system
Reference
antenna
Adaptive
Beamforming
Adaptive
Cancellation
Cross
Correlation &
Doppler
Processing
Array antenna
Adaptive clutter
mitigation
Angle of arrival
estimation
2-D target
imaging
DETECTION
SCHEME
Adaptive Cancellation
- Remove direct signal and clutter from surveillance channels to get true echo signal
- Adaptive filter uses a lattice predictor structure
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
Time Delay  Range
r1 + r2 = 2a
b2 = a2-c2
D1
( D1  D 2)  D3
td 
c
D1  D 2  td  c  D3
D3
D2
Adaptive Beamforming to Get Angular
Resolution
Source
s(t)
Antenna
Array
ym(t)
f(x,t)
Array
Processor
ˆ
R
1
MUSIC
MUSIC

MUSIC
z(t)

M
H
v
v
 i i
i  Nz 1


1
1
ˆ
 e R MUSICe
H
Spatial Subarray Smoothing
For correlated signals:
Results from MUSIC AOA Estimation
0
X: -9.216
Y: 0
-20
X: 69.32
Y: -7.283
X: 33.42
Y: -16.79
Amplitude - dB
-40
-60
-80
-100
-120
-100
-80
-60
-40
-20
0
20
Angle of Arrival - Deg
40
60
80
100
Some Simulation Results
Target - AOA
150
1 GHz
Source
100
y (m)
100 m
Target 1
50
0
Passive
array
xxxxxxx
Target 2
-50
100 m
-50
0
x (m)
50
100
VIII. Array Imaging Systems
• Range – angle imaging
using step CW and angle
of arrival processing
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
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)
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