星载合成孔径雷达(Synthetic Aperture Radar,SAR)是海洋常规观测的重要技术手段,然而在其数据处理和关键物理参数反演中,SAR图像方位角?IMG和卫星飞行方向角?SAT相混淆的情况普遍存在,影响SAR海面风速的反演精度.对此,根据SAR图像方位角的定义,结合空间卫星坐标系转换关系推导了?IMG,并进一步提出了基于SAR图像地面控制点的?IMG计算方法.通过对比分析 2016年全球近 70万景哨兵一号卫星波模式SAR图像,量化了?IMG与?SAT的系统偏差,发现该偏差与雷达入射角和轨道方向紧密相关.选用五次多项式拟合该偏差随纬度的变化规律,同时发现将卫星飞行方向角近似等于SAR图像方位角会引起海面风场反演误差,该误差空间分布不均,且升轨降轨有所差异,风速误差最大可达±0.5 m·s-1.研究结果表明,?IMG的正确计算和使用对于SAR地球科学研究具有重要意义,本文提出的?IMG计算方法对其他SAR卫星系列也具有实际参考价值.
The spaceborne Synthetic Aperture Radar(SAR)is a broadly acknowledged technique to monitor the vast open ocean.However,a frequent confusion between the SAR image azimuth angle ?IMG and the satellite flight direction ?SAT exists in the data processing and geophysical applications,which influences for instance the accuracy of SAR wind inversions.An overview of ?IMG derivation from the transformation of the spatial satellite coordinate systems is presented in this paper,along with a method for calculating ?IMG based on the Ground Control Points(GCP).By analyzing a set of 0.7 million Sen-tinel-1 SAR wave mode images acquired in 2016,we quantify the systematic bias between ?IMG and ?SAT and find out that this bias is related to the incidence angle and orbital direction.It can be accurately fit-ted using a fifth-order polynomial relative to latitude.The misuse of ?SAT is demonstrated to approxi-mate ?IMG can lead to nonnegligible wind retrieval errors.The spatial distribution of this wind error is non-uniform and differs between ascending and descending orbits,with a wind speed bias of±0.5 m·s-1.Results obtained in this study evidence that the precise estimate of ?IMG are significant to various SAR applications in relevant studies.The proposed method here also holds practical reference for other satel-lite SAR missions.