On Inverse Problem of Strongly Nonlinear SAR Imaging of Sea Waves
Chernyshov, Pavel¹; Ivonin, Dmitry^2
1P.P.Shirshov Institute of Oceanology, Southern Branch, RAS, RUSSIAN FEDERATION; ²P.P.Shirshov Institute of Oceanology, RUSSIAN FEDERATION

The problem of correct treatment of SAR images of wind seas is revisited. It is well known that conventional SAR imaging mechanism misinterprets the orbital motions of the sea waves as the azimuthal offset of the facets. Because of the latter mechanism the azimuthal cutoff for the sea waves spectrum is typically 125 m for fully-developed wind sea at moderate winds of 5 m/s or (188 m for 7.5 m/s) [Janssen and Alpers, 2006]. It makes inadequate estimates of spectrum of waves traveling in oblique direction to the range. Moreover, validity of estimates of grouping waves parameters and freak waves occurrence probability using conventional SAR images remains very questionable and unreasonable [Janssen and Alpers, 2006]. This problem arises due to strongly nonlinear imaging mechanism of sea waves by SAR [Alpers, 1981; Hasselmann et al., 1985] that is not taken into account by currently available methods. Another side of the problem is impossibility to use SAR images for wind wave spectra evaluation at closed and semi-closed seas where typical length of energy waves are shorter 70 m.

We address problem of finding a solution of the inverse problem for retrieval wind wave spectra from SAR data for waves shorter 70 m. Numerical simulations were performed for the forming SAR images from various wind wave realizations which fitted the conventional parameterizations of wave spectra for moderate seas (e.g. JONSWAP). We use the mathematical approach proposed by Kanevsky (2009). The strongly nonlinear imaging mode parameters are used, typical for Terrasar-X and Radarsat-2. The way for solution of the strongly nonlinear inverse problem was found and proposed in some cases. On this base we retrieved several key parameters for reconstruction the real wind wave spectrum. Also, it is possible to propose the way how to improve the quality of the ASAR Wave Mode data for the case of asimuthal short waves.

Case studies have been carried out for Terrasar-X and Radarsat-2 images got in 2009 and 2012 over the Oil Stones in the Caspian Sea for the wind velocity range from 4 m/s to 7 m/s.

This work was funded by Russian Government (Grant No.11.G34.31.0078) for research under supervision of the leading scientist at Russian State Hydrometeorological University, Program of the Presidium RAS N 23 ''Sea wave dynamics in remote sensing of the sea surface problems at near-shore zone'', RFBR grants N 12-05-31498, 12-05031463, N 12-05-92004-HHC_a.

1. Hasselmann, K., R. K. Raney, W. J. Plant, W. Alpers, R. A. Shuchman, D. R. Lyzenga, C. L. Rufenach, and M. J. Tucker, ''Theory of synthetic aperture radar ocean imaging: A MARSEN view,'' J. Geophys. Res, 90, 4659- 4686, 1985.
2. Janssen P., and W. Alpers. Why SAR wave mode data of ERS and Envisat are inadequate for giving the probability of occurrence of freak waves // Proceedings of the SEASAR 2006 workshop, 23-26 January 2006, Frascati, Italy, http://earth.esa.int/seasar06/proceedings/
3. Alpers W.. On the Detectability of Ocean Surface Waves by Real and Synthetic Aperture Radar // Journal of Geophysical Research, Vol. 86, No. C7, pp. 6481-6498, 1981.
4. Kanevsky M. B. Radar Imaging of the Ocean Waves // Elsevier. 2009. 209 pp.