Noncoherent 2-D and 3-D SAR
Reconstruction from
Wide Angle Measurements
Randy Moses and Lee Potter
Department of Electrical and Computer Engineering
The Ohio State University
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Motivation
•
Emerging technologies enable collection of radar backscatter
data over wide angular apertures
– Better INS
– UAV, or possibly multiple UAVs working in tandem
– Bi-static radar with close-in passive receiver
•
Operational use of SAR imagery limited by non-literal
visualization
–
–
–
–
Image analysis slow and requires extensive training
Image analysts discard SAR imagery
Specular scattering at millimeter waves
Humans prefer optical imagery
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Outline
• SAR processing of wide angle apertures
– Resolution and image properties
– Scattering persistence
• Data processing consequences
– Stable amplitude and phase for resolved reflectors
• Bandwidth extrapolation
• IFSAR on targets
• Polarization signatures on target sub-structures
– Limited persistence of reflectors
• Non-coherent combination of sub-apertures
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Scattering Physics
• At sufficiently high resolution, individual scattering
centers are isolated.
versus
– Phase, amplitude, polarization characteristics of isolated
scattering centers are stable and relate to object shape
• Most scattering responses have limited persistence
– Non-coherent combination of limited-aspect data
IFSAR from high-resolution image pairs can be used
to extract and non-coherently combine
3D scattering locations and features
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Wide Angle Image Response
Wide angle aperture , narrow bandwidth aperture
• sparse; high side-lobes
• -3 dB image resolution ~ circumscribed rectangle
• -24 dB image resolution area ~ 1/(aperture area)
0.3 meter resolution, Hamming
1 ft circle
0
-5
-10
-15
-20
0.1
0.4
0.05
0
-0.05
Crossrange, meters
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
-0.1
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Range, meters
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Image Response vs. Aperture
Frequency Support
Image
5°
0.3 meter resolution, Hamming
20°
0
-5
-10
-15
40°
-20
0.1
0.4
0.05
0.2
0
0
-0.05
Crossrange, meters
-0.1
-0.3
-0.2
-0.1
-0.4
Range, meters
110°
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
0.3
0.1
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Scattering Aspect Dependence
Frequency Support
Most target scattering has limited
response persistence: 20° or
Image
φc=−40°
less [Dudgeon et al., 1994]
‘Downrange’ resolution ≈ 1/BW
φc =−20°
‘Crossrange’ resolution depends
on scattering persistence
Image response is no
longer characterized by a
single impulse response
shape.
φc =0°
φc=20°
φc=40°
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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GLRT Image Formation
SAR Image: Image I(x,y) matched filter output to an isotropic
point scattering center at (projected) location (x,y)
[Rossi+Willsky].
GLRT Image: Image I(x,y) is GLRT output to a limitedpersistence scattering center at (projected) location (x,y).
I ( x, y ) = arg max R( x, y, f c , D f )
f c ,D f
R( x, y, f c , D f ) = std image with center f c , width D f
For fixed Δφ (=20°) and sampled φc , the GLRT image is
approximately the maximum over subimages.
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Wide-Angle Imaging
•
•
•
•
110° azimuth aperture, -10°-100°
500 MHz (1ft); 4GHz (1.5in)
10 GHz center frequency
30° elevation at center
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Coherent SAR 110° Image
4 GHz Bandwidth
500 MHz Bandwidth
Coherent wide-angle image is not well-matched to
limited persistence scattering behavior
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Composite GLRT Image
4 GHz Bandwidth
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
500 MHz Bandwidth
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GLRT Image with Angle Encoding
Color Denotes angle at which GLRT test is maximum
4 GHz Bandwidth
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
500 MHz Bandwidth
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Resolution Enhancement
• Use lp-norm image formation approach [Çetin and Karl,
2001]
• Seeks sparse solutions whose convolution with
imaging operator match measured image.
• Procedure:
– Apply resolution enhancement to each subimage using its
corresponding imaging operator A
– Non-coherently combine resolution-enhanced images in
GLRT
Joint work with Mujdat Çetin
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Resolution Enhanced; Color-Encoded
500 MHz Bandwidth
Res. Enhanced, p=0.8
Cetin p=1
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Wide-Angle IFSAR Processing
Use phase coherence to
obtain 3D reconstructions
via IFSAR processing.
Backscatter data
• 110° azimuth aperture, -10°-100°
• 30°, 30.05° elevations
IFSAR processing:
•
•
•
•
Form coherent image pairs from
sub-apertures
Select high amplitude pixels with a
single dominant scatterer
Height z in slant plane for each
image pixel estimated from relative
phase between the two images
Non-coherently combine points
with GLRT approach.
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Example: XpatchT Backhoe Data
• 3D reconstruction of IFSAR
points from many image
pairs, using top 40 dB pixel
RCS. Filter by ratio of pixel
amplitudes.
filtered
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
unfiltered
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Sparse Aperture 3D Imaging
Coherent IFSAR image pairs
•
1.5”x1.5” resolution
• 8-12 GHz
• 24° subapertures
•
•
•
20−50° el; 120−150° az
Three apertures; three platforms
31 total image pairs
Non-coherent combination of
‘IFSAR points’
5° el; -10−20° az
50° el; 20−110° az
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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3-Aperture IFSAR Reconstruction
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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3 Aperture IFSAR Reconstruction
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Visualizing Attributed Point Features
Point locations and attributes from each IFSAR pair:
1. Slant-plane (x,y) location and height
2. Polarization features from HH, HV, VH, VV pixel values
• Trihedral-dihedral mix parameter α
• Dihedral orientation angle θ
3. “Beam pattern” peak (φk,ψk)
Volume cloud visualization (e.g., Maya or Matlab)
–
–
RCS controls transparency
Color (hue, saturation) encodes attributes
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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IFSAR Pairs for Visualization
Coherent IFSAR image pairs
•
1.5”x1.5” resolution
• 8-12 GHz
• 24° aperture
•
Every 5° elevation
Δθ=0.05° elevation spacing
•
1296 total image pairs
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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RCS Only
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Polarization: Tri/Di and Polarization Angle
Trihedral
Dihedral
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Color Encodes Azimuth Angle
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Polarization
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Persistence Visualization: ±20º displayed
Odd-bounce
Even-bounce
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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Summary
•
Limited persistence of scattering centers suggests considering
alternatives to traditional coherent image formation.
– Non-coherent combination of subapertures
– GLRT interpretation
– 2D (image) and 3D (IFSAR)
•
At sufficiently high resolutions, scattering centers become isolated
and provide stability in phase and polarization
– Resolution enhancement
– IFSAR processing for targets
– Polarization features consistent with substructure shape and orientation
•
IFSAR points extracted from high-resolution images provide 3D
representations of targets, along with attributes
–
–
–
–
Sparse measurement aperture
“Compact” data representation
Polarization and angle features relate to target geometry
Good features for ATR?
Adaptive Sensor Array Processing Workshop (ASAP 2005), June 7-8, 2005
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