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

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 dAvignon LSEET, UMR
CNRS-Université de Toulon et du Var
2
Introduction

• Doppler spectrum
• Ocean surface is time varying h(r,t)
• Harmonic w0 -gt s(w0dw)
• 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 -gt
  UHF
• Need for advanced hydrodynamic and
  electromagnetic models

3
Electromagnetic scattering
Time-harmonic scattering
Scattered field
Air
2D surface
Skin depth d ltlt l
Sea water
Curved surface impedance approximation
MFIE operators EFIE
Single non-singular integral equation
4
Small Slope Integral Equation
Meecham Lysanov approximation
Interaction distance rr
Height hsd
Slope s
Horizontal distance dd
Validity khsltlt1.
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
Doppler computation
Time-harmonic scattering and time-varying surface
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
f01.2GHz
8
Small Perturbation Method
2. Contribution of non-linear wave interactions
1. SPM2 applied to doppler spectrum
9
Hydrodynamic Non-Linearities
10
UNDRESSED SPECTRUM

1. Start from a dressed (experimental) spectrum,
2. Generate linear waves,
3. Generate 2nd order,
4. Undress the spectrum()
5. Generate linear waves,
6. Generate 2nd order.

() Elfouhaily and al. ,CRAS B, vol.13, 314-333,
2003
11
(Hz)
Doppler
f01.2GHz
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 lEquipe Projet Multi-Laboratoires
 Télédétection Active Océanique 
15
Creamer2undressed doubling the Doppler
frequency range