SHIP DETECTION USING X BAND DUAL-POL SAR DATA S. Angelliaume (ONERA) Ph. Durand, J.C. Souyris (CNES)

2 CNES study of Space System Concept for 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 on radar 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 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 ?

4 Polarization for ship detection ° Alpha angle RADARSAT-2 Pauli basis H / α classification ClassificationOccurrences plane FP data could be very efficient for ship detection But : swath/revisit time is divided by 2 compared to a DP system

5 Outline SAR facility / Campaign description Dual-Pol analysis : Sea clutter measurements Ship detection capability Sub-aperture analysis Conclusion / perspective

6 ONERA SAR Team

7 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

8 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°

9 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/m2 Hv close to SETHI NESigma0 Azimuth = 0° -> Looking North

10 Dual-Pol analysis : sea clutter X-band, incidence 50°, Hh only Sea state 4 (wind -> 8-10 m/s) and 5 (wind -> m/s) 2 maximums : upwind (azimuth 45°) and downwind (225°) Difference between max and min growing with sea state (2.5 -> 4 dB)

11 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 behavior Δ(Hh-Hv) -> from 5 to 12.5 dB 20m Boat and sensor trajectories available

12 Dual-Pol analysis : 20m boat detection X-band, incidence 50°, sea state 4, 20m boat Mono-polarization : detection using Hh or Hv Best result using Hv but close to Hh 20m

13 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

14 Dual-Pol analysis : hermitian product Windows size influence : The hermitian product is a biased measurement Ratio measurement

15 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

16 Sub-aperture : what are the two best windows ? Range & azimuth : same behavior Hh & Hv : same behavior Best result : disjoint and contiguous windows No sub-aperture Overlapping subapertures Disjoint subapertures B = 75 MHz, ΔF = 0 MHz

17 Sub-aperture performance results The optimal choice : 2 disjoint and contiguous windows in the range direction at Hh polarization

18 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

19 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 ! ONERA DEMR/RIM