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Analyse de spectres Doppler de la surface de la mer en bande L

Analyse de spectres Doppler de la surface de la mer en bande L. G. Soriano*, M. Joelson**, P. Forget # , M. Saillard # * Institut Fresnel, UMR CNRS-Université Aix-Marseille III ** LCSE, UMR CNRS-Université d’Avignon # LSEET, UMR CNRS-Université de Toulon et du Var. Introduction.

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Analyse de spectres Doppler de la surface de la mer en bande L

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  1. Analyse de spectres Doppler de la surface de la mer en bande L G. Soriano*, M. Joelson**, P. Forget#, M. Saillard# * Institut Fresnel, UMR CNRS-Université Aix-Marseille III ** LCSE, UMR CNRS-Université d’Avignon # LSEET, UMR CNRS-Université de Toulon et du Var

  2. Introduction • Doppler spectrum • Ocean surface is time varying: h(r,t) • Harmonic w0 -> s(w0+dw) • More information in backscattering radar configuration • HF and VHF radars provide current maps at km resolution • Coastal zone : UHF? • In coastal zone, environmental parameters change faster • Smaller wavelength provides better resolution -> UHF • Need for advanced hydrodynamic and electromagnetic models

  3. Exponential attenuation Time-harmonic scattering Electromagnetic scattering Scattered field Air 2D surface Skin depth d << l Sea water Curved surface impedance approximation MFIE operators EFIE Single non-singular integral equation

  4. Matrices associated to operators M and E are 2D Toeplitz Storage : 2N instead of N2 Product : 2Nlog2N instead of N2 Meecham – Lysanov approximation Small Slope Integral Equation Interaction distance r=|r| Height h=sd Slope s Horizontal distance d=|d| Validity : khs<<1. 1st order

  5. Ocean Surface Deep water - Open ocean - Irrotational motion Linear surface: Simulation by spectral method Random gravity waves (ignore surface tension) Electromagnetic wavelength: 25cm Pierson-Moskowitz height spectrum Free waves

  6. Time-harmonic scattering and time-varying surface Doppler computation At a given time step Then FT 1. generate the surface 2. Solve the scattering problem with SSIE 4. Compute deterministic Doppler complex amplitude Finally Monte Carlo 3. Store the backscattered complex amplitude 5. Statistical result by averaging Doppler intensity spectrum

  7. (Hz) Doppler f0=1.2GHz

  8. Small Perturbation Method 2. Contribution of non-linear wave interactions 1. SPM2 applied to doppler spectrum

  9. Hydrodynamic Non-Linearities

  10. UNDRESSED SPECTRUM • Start from a dressed (experimental) spectrum, • Generate linear waves, • Generate 2nd order, • Undressthe spectrum(*) • Generate linear waves, • Generate 2nd order. (*) Elfouhaily and al. ,CRAS B, vol.13, 314-333, 2003

  11. (Hz) Doppler f0=1.2GHz

  12. Experiments Toulon + december 03 and 04 + height: 90 m + VV, HH, VH, HV + 2 azimuts + in situ measurements: - omnidirectional spectrum - surface currents - wind 5 km Batterie de la Renardière East wind Cap Sicié Mistral

  13. Comparaison 14/12/2004 de 12h à 15h Vent de 2 à 3 m/s Orienté 10 à 50° par rapport au faisceau incident VV VH

  14. Conclusion • At UHF, the ocean surface height spectrum needs to be undressed before introducing hydrodynamic interactions • At low winds, some Doppler spectrum features can still be interpreted with SPM. Some new characteristics: broadened side peaks, significant cross-polarization. Perspectives • Find a more systematic way to undress the spectrum • Study the influence of the hydrodynamic model • Use more realistic (experimental) directional spectrum • Improve SSIE for grazing angles (80, 85°) Remerciements au Dept. STIC du CNRS pour son soutien à l’Equipe Projet Multi-Laboratoires « Télédétection Active Océanique »

  15. Creamer2undressed : doubling the Doppler frequency range

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