SHIP DETECTION USING X BAND DUAL-POL SAR DATA

S. Angelliaume (ONERA)Ph. Durand, J.C. Souyris (CNES)

2

CNES study of Space System Conceptfor maritime surveillance.

• Allow observation of non-cooperative boats with temporal revisits compliant with the objectives of reactivity of maritime surveillance

• Concept proposed by CNES (French Space Agency) based onradar operating with a very wide swath,

• Implies grazing conditions of acquisition.

• Only few radar data on maritime surfaces available with this geometry:

• CNES decided to carry out a dedicated campaign, with ONERA airborne SAR sensor (February 2009 – Mediterranean sea)

Context

3

Context

Experimental Signal to Clutter Ratio (SCR) estimated over cooperative boat

Depending on :

• Sea state (wind direction / strength)

• Angles of observation (incidence and azimuth)

• Type / size of boat

• Polarization state

Questions :

• detection capability improved by the use of Polarization ?

• what is the best method : FP, DP, or sub-aperture ?

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Polarization for ship detection

0 45 90°Alpha angle

RADARSAT-2 Pauli basis

H / α classification

Classification Occurrences plane

FP data could be very efficient for ship detection

But : swath/revisit time is divided by 2 compared to a DP system

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Outline

• SAR facility / Campaign description

• Dual-Pol analysis :• Sea clutter measurements• Ship detection capability

• Sub-aperture analysis

• Conclusion / perspective

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ONERA SAR Team

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SAR facility / Campaign of acquisition

• Acquisition parameters :• Sensor -> ONERA / SETHI• Waveform -> X-band, Hh & Hv, B=150 MHz• Trajectory -> linear and circular

Internal looking: Ship RCS study External looking: Sea clutter study

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SAR facility / Campaign of acquisition

• Acquisition parameters :• Sensor -> ONERA / SETHI• Waveform -> X-band, Hh & Hv, B=150 MHz• Trajectory -> linear and circular

• Scenario :• February 2009• Mediterranean sea• Different sea states• 20m cooperative boat

X-Hh – Incidence 50°

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Dual-Pol analysis : sea clutter

X-band, incidence 50°, sea state 4 (wind -> 8-10 m/s)Analysis over the full 360° azimuth range

2 maximums : upwind (azimuth 45°) and downwind (225°)Hh and Hv : same behavior : Δ ~ 10 dBm2/m2Hv close to SETHI NESigma0

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-31

-29

-27

-25

-23

0 50 100 150 200 250 300 350

Azimuth angle (deg)

Sig

ma0

(d

Bm

2/m

2)

X-Hh

X-Hv

Azimuth = 0° -> Looking North

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Dual-Pol analysis : sea clutter

X-band, incidence 50°, Hh onlySea state 4 (wind -> 8-10 m/s) and 5 (wind -> 10-12 m/s)

2 maximums : upwind (azimuth 45°) and downwind (225°)Difference between max and min growing with sea state (2.5 -> 4 dB)

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-29

-28

-27

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-25

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-22

-21

-20

0 50 100 150 200 250 300

Azimuth angle (deg)

Sig

ma0

(d

Bm

2/m

2)

sea state 5

sea state 4

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Dual-Pol analysis : 20m boat signature

X-band, incidence 50°, precise analysis over the first 90° degrees

Hh -> maximum value when looking backward (15°)Hh and Hv -> not exactly the same behaviorΔ(Hh-Hv) -> from 5 to 12.5 dB

0

5

10

15

20

25

0 15 30 45 60 75 90

Azimuth angle (deg)

RC

S (

dB

m2

)

X-Hh

X-Hv

20m

Boat and sensor trajectories available

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Dual-Pol analysis : 20m boat detection

X-band, incidence 50°, sea state 4, 20m boat

Mono-polarization : detection using Hh or HvBest result using Hv but close to Hh

20m

0

2

4

6

8

10

12

14

16

18

0 15 30 45 60 75 90

Azimuth angle (deg)

Sh

ip t

o C

lutt

er R

atio

(d

B)

X-Hh

X-Hv

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Dual-Pol analysis : 20m boat detection

X-band, incidence 50°, sea state 4, 20m boat

Hh and Hv : hermitian product (un-normalized coherence)Gain is equal to 4 dB on average, and it could be greater than 6 dB

20m

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Dual-Pol analysis : hermitian product

Windows size influence : • The hermitian product is a biased measurement• Ratio measurement

Hermitian product HhHv*Sea clutter measurements

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-30

3x3 5x5 7x7 11x11

(dB

)

Hermitian product HhHv*Ship measurements

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-5

-4

-3

-2

3x3 5x5 7x7 11x11

(dB

)

Tanker 1

Tanker 2

Tanker 3

Hermitian product HhHv*Ship to clutter ratio

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27

28

29

30

3x3 5x5 7x7 11x11

(dB

)

Tanker 1

Tanker 2

Tanker 3

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Polarization / sub-aperture approach

Incidence 50° : Dual-Pol data provide better result than Single-Pol

However, the interest of cross-polarization is determined by the system ability to measure low backscattering levels (NESigma0), which may limit interest in a spaceborne context and/or at higher incidence angle.

Sub-aperture analysis :• Range and azimuth• Coherence measurement between the two sub-apertures• Various combinations

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Sub-aperture : what are the two best windows ?

Range & azimuth : same behaviorHh & Hv : same behavior

Best result : disjoint and contiguous windows

Ship to clutter ratio / X-band 87°Range subaperture - Hh polarization

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22

24

26

28

30

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38

40

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Delta F (MHz)

dB

Tanker 1

Tanker 2

Tanker 3

Tanker 4

Ship to clutter ratio / X-band 87°Range subaperture - Hv polarization

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20

22

24

26

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Delta F (MHz)

dB

Tanker 1

Tanker 2

Tanker 3

Tanker 4

No sub-apertureOverlapping subaperturesDisjoint subapertures

B = 75 MHz , ΔF = 0 MHz

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Sub-aperture performance results

The optimal choice : 2 disjoint and contiguous windows in the range direction at Hh polarization

Ship to clutter ratio / X-band 87°

10

20

30

40

Tanker 1 Tanker 2 Tanker 3 Tanker 4

dB

HhHv*

HhR1R2*

HvR1R2*

HhR1HvR2*

HhA1A2*

HvA1A2*

HhA1HvA2*

Hh

Hv

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Conclusion

Sea clutter : • Hh and Hv measurements• theoretical / experimental behavior as a function of the azimuth angle

Ship detection :• Deep angle (50°) : Hv-SCR greater than Hh-SCR• Hermitian product very efficient (but less at grazing angle)• Simulation for spatial concept : special care to NESigma0 level

• Grazing angle : • Sub-aperture more efficient than HhHv*• Optimal windows : 2 disjoint and contiguous windows in the range direction

at Hh polarization

• Sub-aperture not yet investigated at deep incidence angle

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On going work

Airborne campaign in the Atlantic Ocean in Nov / Dec 2011

• Wave-Form : X and L band simultaneously, full-polarimetric• More cooperative boats of different size• AIS information from ships of opportunity

Objectives :• Comparison between Mediterranean an Atlantic sea clutter backscattering• Full-polarimetric analysis :

• Sea reflectivity• Ship RCS

• Answering the question :

what is the best mode of acquisition -> SP, DP, FP ?

Thank you !

Sebastien.Angelliaume@onera.frONERA DEMR/RIM