扁率摄动对地球同步轨道SAR成像聚焦的影响分析

Impact of Earth's Oblateness Perturbations on Geosynchronous SAR Data Focusing

针对地球同步轨道星载合成孔径雷达(Geosynchronous Synthetic Aperture Radar,GEOSAR)长合成孔径成像受地球扁率摄动影响的问题,推导了卫星不同轨道根数受摄动所导致的雷达回波多普勒调频率和2次相位公式,通过分析扁率摄动对成像的影响,得到结论:地球扁率摄动使雷达回波产生附加的2次相位调制,相位调制的主导成分是卫星轨道长半轴受摄分量,相位调制幅度与成像所采用的轨道弧段有关,2次相位调制总量经过分钟量级的长合成孔径累积几乎在卫星运动全周期超过45°的容限。雷达成像聚焦不能简单忽略扁率摄动的影响,必须采取相应的补偿措施,否则会造成图像散焦。

In this study, we focus on the ultra-long integration of orbital perturbations of geosynchronous Synthetic Aperture Radar （SAR） for imaging. By deriving mathematical expressions for the Doppler rate and quadratic phase from orbital elements perturbated by oblateness or the J2 term of the non-spherical gravitational force of the Earth, we analyze the impact on SAR data focusing. Based on our results, we conclude that the quadratic phase will exceed 45＆#176;, which is the defocusing threshold for imaging, after accumulation during a long integration time at the minute level. Because the potential for defocusing exists throughout nearly the entire satellite motion cycle, the SAR processor must carefully manage and compensate for the quadratic phase to avoid image degradations.