mo4.l09 - potential and limitations of forward-looking bistatic sar
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TRANSCRIPT
© Fraunhofer FHR
Ingo Walterscheid, Thomas Espeter, Jens Klare, Andreas Brenner, Joachim Ender
POTENTIAL AND LIMITATIONS OFFORWARD-LOOKING BISTATIC SAR
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OUTLINE
Introduction
Bistatic forward-looking SAR
Geometry
Iso-range and Iso-Doppler contours
Resolution
Experiment with TerraSAR-X and PAMIR
Experimental results
Summary
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IntroductionMonostatic SAR
Independent of weather and time of day High azimuth resolution Widely used for surveillance and remote sensing applications
Monostatic synthetic aperture radar typically operates with a side-looking
antenna to obtain high resolution images
Solutions using one radar platform: Doppler beam sharpening
using a rotating reflector antenna
Linear array antenna with one Tx and multiple Rx antennas
Limitation: Imaging in forward- and
backward-looking direction Left/right ambiguities Poor Doppler resolution
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IntroductionBistatic SAR
Advantages:
Additional information about the target (bistatic RCS)
Reduction of dynamic range (di- and polyhedral effects in urban areas)
Single-track interferometry with large baselines (across- and along-track)
Coherent and incoherent combination of bi- and monostatic signatures
Reduction of vulnerability in military systems
Imaging in flight direction or backwards
Bistatic synthetic aperture radaroperates with spatially separated
transmitand receive antennas that are mounted
on separated platforms
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Bistatic forward-looking SARGeometry and applications
Applications:
Observation, autonomous navigation
Landing assistance under low-visibility conditions (flight safety)
Identification of obstacles in flight direction (collision warning system)
Compact, low-cost and lightweight receive-only radar imaging system for small aircrafts
Geometry:
Platform velocities v1 and v2
LOS vectors u1 and u2
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Bistatic forward-looking SAR Iso-range and iso-Doppler contours (Monostatic case)
Doppler cone
Ran
ge s
pher
eIso-Range
Iso-Doppler
Nadir
Flight path
Ground track
v
For monostatic radars it is quite simple:
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Bistatic forward-looking SAR Iso-range and iso-Doppler contours (Monostatic case)
Tx/Rx
Side-looking
Forward-looking
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Bistatic forward-looking SAR Iso-range contours (Bistatic case)
Bistatic geometry
y
x
z1v
2v
TX
RX
R1()
R2()r
Bistatic range history
rr
rr
rRrRr
22
22
22
21
21
21
21;
v
v
R
Sum of two hyperbolas!
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Bistatic forward-looking SAR Iso-range contours (Bistatic case)
The set of equal bistatic range
is an ellipsoid with its focus pointsat Tx and Rx (Iso-range surface).
RRrM r
~;:);( rr
The cut with the earth surface is an ellipse (Iso-range-line).
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Bistatic forward-looking SAR Iso-range-rate (Doppler) contours (Bistatic case)
ruvr
rrrr
;;
;;;; 21
itiri
rrr
v
vvRv
Set of equal contribution vi is a cone surface with axis = flight direction and corner at platform
Iso-range-rate surface is the union over all cuts between the cones with the same sum of radial velocities
iriiiv vvvM rr ;:);(
);();();( 22
11
21
vMvMvM vvvv
vrv
r
Radial velocities
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Bistatic forward-looking SAR Iso-range-rate (Doppler) contours (Bistatic case)
Black rings are cuts between the range-rate cones Union of these rings for equal range-rate sum form the Iso-range-rate
surface Cut of this surface with the earth surface forms the Iso-range-rate contours
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Bistatic forward-looking SARIso-range and iso-Doppler contours
Bistatic geometry
Red = Iso-range lines, Blue = Iso-Doppler lines
Rx
Tx
Monostatic geometry
Tx/Rx
Non degenerated image grid in flight direction
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Bistatic forward-looking SAR Resolution in range and cross-range
Ground range resolution
Ground Doppler resolution
Ground cross-range resolution
with
c, Velocity of light, wavelengthB Signal bandwidthui Unit direction vector (LOS)xy Projector onto x-y-planeTint Integration timei Angular speed vector Angle between gradient of iso-range
and iso-Doppler lines
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Experiment with TerraSAR-X and PAMIRSensor parameters
TerraSAR-X
X-Band SAR satellite Centre frequency: 9.65 GHz Bandwidth: 300 MHz Active phased array antenna Azimuth scan range: +/- 0.75° Altitude: 515 km Velocity: 7600 m/s
PAMIR
SAR/GMTI System, Transall C-160 Centre frequency: 9.45 GHz Bandwidth: 1820 MHz Active phased array antenna Azimuth scan range: +/- 45° Altitude: 0.6 – 4 km Velocity: 100 m/s
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Experiment with TerraSAR-X and PAMIRBistatic configuration
TerraSAR-X
High-resolution spotlight mode(right-side looking)
Incidence angle: 24°
PRF ≈ 4.5 kHz
Altitude: 515 km
Velocity: 7600 m/s
PAMIR
Flight direction orthogonal to TerraSAR-X trajectory
Stripmap (backward-looking)
Incidence angle: 60°
PRF ≈ 1.5 kHz
Altitude: 1500 m
Velocity: 100 m/s
y
x
z1v
2v
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Experiment with TerraSAR-X and PAMIRData acquisition
Bistatic signal acquisition
Standard gain horn on the aircraft‘s loading ramp
Azimuth/Elevation beamwidth of 27°
PRFRX = PRFTX/3
Direct signal acquisition
Receiving of direct signal for synchronization purposes
Additional antenna on the top of aircraft‘s fuselage
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Experiment with TerraSAR-X and PAMIR Pulse synchronization (I)
PRF triggering
Hardware synchronization
Pulsed acquisition mode
Direct signal
Timing controller of PAMIRis triggered
Software synchronization
Remaining shift of subsequent rangelines
Compensation during a pre-processing step
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Experiment with TerraSAR-X and PAMIR Expected ground range and cross-range resolution
Ground range resolution
Cross-range resolution
x / m
y /
m
Ground range resolution at -3 dB (rectangular window)
-3000 -2000 -1000 0 1000 2000 3000-3000
-2000
-1000
0
1000
2000
3000
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
x / m
y /
m
Ground cross-range resolution at -3 dB (rectangular window)
-3000 -2000 -1000 0 1000 2000 3000-3000
-2000
-1000
0
1000
2000
3000
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
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Experiment with TerraSAR-X and PAMIR Iso-range and iso-Doppler contours (backward-looking)
Tx Rx
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Experimental resultsRaw data and range compressed data
Amplitude of raw data Range compressed data
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Experimental resultsGoogle Earth image of the scene
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Experimental resultsOptical image Bistatic SAR image
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Summary
Film
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Summary
Imaging in forward-looking direction using bistatic SAR
Bistatic geometry and resolution
Iso-range and iso-range-rate contours in the bistatic
case
Bistatic experiment with TerraSAR-X and PAMIR to
demonstrate the feasibility to image in forward and
backward direction
Experimental results
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Thank you very much!