浅海水下地形的SAR遥感仿真研究

Simulation study on SAR shallow water bathymetry

结合连续性方程和布拉格后向散射模型,在准一维简化浅海水下地形情况下,建立了浅海水下地形SAR海面相对后向散射强度仿真模型,将浅海水下地形区域的SAR海面后向散射强度的相对变化与大尺度背景流场、海面风场和雷达系统参数等联系起来。海上实验和研究结果表明,浅海水下地形的SAR成像主要由通过受水下地形影响的海表层流场对海表面风引起的微尺度波的水动力调制而获取浅海水下地形信息,其中潮流与水下地形的相互作用过程改变海表层流场,变化的海表层流与海表面微尺度波之间的相互作用改变海表面波的空间分布,雷达波与海表面波之间的相互作用决定雷达海面后向散射强度。因此SAR图像中浅海水下地形或水深信息量的多少不仅与海表层流场和海面风速有关,而且与雷达工作波段、雷达波束入射角和极化方式也密切相关。认为由水下地形变化引起的缓慢变化的表层流场中海表面定常微尺度波谱能量密度的变化满足波作用量谱平衡方程;而在波数空间中,海表面微尺度波谱的成长过程也可以用波数谱平衡方程描述,在此基础上,得出了海表面波高频谱(毛细-重力波)形式的解析表达式。众所周知,浅海水下地形信息是由于水下地形影响下SAR海面后向散射强度与背景海面后向散射强度的相对差异而在SAR图像上的呈现,从而在建立浅海水下地形SAR海面相对后向散射强度仿真模型的基础上,仿真计算了浅海水下地形SAR海面相对后向散射强度相对于海表层流场、海面风场等海况参数和SAR工作波段、SAR波束入射角、极化方式等雷达系统参数的数值仿真结果,分析得到了有关浅海水下地形SAR海面相对后向散射强度的特征和SAR浅海水下地形遥感的最佳海况参数与最佳雷达系统参数,为研究和开展SAR浅海水下地形遥感研究提供了有价值的参考。

Base on Synthetic Aperture Radar （SAR） imaging mechanism of the shallow water bathymetry and use the analytical expression for the high frequency ocean surface wave spectrum derived from wave number spectrum balance equation instead of surface wave action balance equation, combine with continuity equation and the first order Bragg backscattering theory, a simulating model for the shallow water bathymetry SAR relative normalized radar backscattering cross-section （NRCS） is developed under quasi-one-dimensional shallow water bathymetry conditions to link the relative change of the shallow water bathymetry SAR backscattering strength with the surface currents, sea surface winds and radar parameters et al. As far as we know, SAR cannot directly view shallow water bathymetry, and it images the bottom topography via surface effects induced by tidal flow variation over bottom topography under favorable meteorological and hydrodynamic conditions. Generally, shallow water bathymetry SAR imaging mechanism consists of three processes, the interaction between tidal flow and bottom topography which results in the modulation of surface flow velocity, the modulation by surface flow velocity causing variations of the wind-generated surface wave spectrum and finally the variation of the surface wave spectrum causing modulation of the level of radar backscatter. Thus the ability of SAR mapping shallow water bathymetry has close relationship with surface currents, sea surface winds and radar parameters, such as radar wave length, radar incidence angle and radar polarization et al. As for the evolution of the spectrum energy density of a wave packet traveling through the slowly varying surface current field is usually thought to obey the wave action balance equation, and in spectral domain, the wave number spectrum balance equation could also describe the evolution of sea surface wave spectrum and one analytical expression for the high frequency sea surface wave spectrum has been derived. It was also noted that the shallow water bathymetry is visible on SAR images due to the relative difference between the NRCS of SAR shallow water bathymetry images and the NRCS of the background sea surface, then the relationship of SAR relative NRCS with surface currents, sea surface winds, radar wave length, radar incidence angle and radar polarization has been simulated and analyzed, and the optimal conditions for shallow water bathymetry imaging by SAR was also concluded. Valuable help for the study on SAR shallow water bathymetry will be provided in this paper.