mo4.l09 - potential and limitations of forward-looking bistatic sar

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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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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

mo4.l09 - potential and limitations of forward-looking bistatic sar

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