利用多普勒频移反演ASAR海表面流速

Using the Doppler Shift Method to Retrieve the ASAR Sea Surface Velocity

为研究中小尺度的海流,以美国西海岸附近海域为研究区域,采用欧空局ENVISAT ASAR IMS数据,首先利用频谱拟合法计算得到影像的多普勒质心;再通过影像的多普勒系数和斜距时间去除卫星与地球之间相对运动产生的多普勒频移,最终得到海表面多普勒质心异常。同时利用布拉格散射模型去除了布拉格波的影响,利用CMOD5模型计算海表面风场,根据风场与雷达视向速度的经验关系去除海面风场的影响,经空间投影,最终得到雷达径向的海表面流速,反演的流速主要分布于-0.3～0.6 m/s。利用研究区域附近的岸基高频地波雷达(HF-radar)数据验证反演结果精度,结果表明,在HF-radar流速与ASAR反演流速-0.2～0.2 m/s差异范围内的标准误差和相关系数分别为0.1 m/s和0.81,反演结果与真实流速较为接近。ASAR反演流速和HF-radar流速结果均表明,在研究区域存在自西向东的沿岸流,最大径向流速达到0.6 m/s。因此,本文所使用的海表面流速反演方法准确有效,适合于中小尺度海流流速的反演。

In order to study the medium or small scale ocean currents, the surface current velocity is retrieved from ESA ENVISAT ASAR IMS image, taking the west coast of the United States as the study area.Firstly, the Doppler centroid of the image is calculated with the spectrum fitting method.The sea surface Doppler centroid anomaly is then obtained by removing the Doppler shift of the relative motion between the satellite and the earth with the Doppler coefficient and the slant range time of the image.The influence of Bragg wave on the current velocity retrieval is further removed by the Bragg scattering model.The influence of sea surface wind is also removed according to the empirical relationship between wind field and radar apparent velocity, where the sea surface wind is calculated by CMOD5 model.After spatial projection, the radial surface velocity of the radar is finally obtained.The inversed current velocity ranges from -0.3 to 0.6 m/s, the accuracy of which is verified by the shore-based HF-radar.The validation results show the inversion results are close to the HF-radar measurements with the standard deviation and correlation coefficient of 0.1 m/s and 0.81, respectively.Both the ASAR and the HF-radar data indicate that a coastal current exists in the study area, with the maximum radial velocity of 0.6 m/s.The results indicated that the sea surface current velocity inversion method adopted in this paper is accurate and effective, and is suitable for the inversion of small and medium-scale ocean current velocity.