时-空变化的背景电离层对星载合成孔径雷达方位向成像的影响分析

对于星载合成孔径雷达(SAR)成像，方位向信号的相关性可能会因时-空变化的背景电离层而遭到破坏，特别是对于低波段系统。该文将孔径内方位时变的斜距电子总量(STEC)归结于3个因素：垂向电子总量(VTEC)的时间变化、空间变化以及电磁波传播路径的变化，分别分析了每个因素造成的时变STEC各阶系数。该文建立了统一的分析模型，即时变STEC影响下的SAR方位向信号3阶泰勒展开模型，推导了方位向偏移和相位误差解析表达式，并基于此得到了不同星载SAR系统的时变STEC各阶系数容限。利用实测的VTEC数据以及国际参考电离层(IRI)模型，开展了信号级仿真。数值分析和信号级仿真的结果表明，对于低轨P波段SAR系统，空变VTEC与传播路径变化是导致方位时变STEC的主要因素；而对于中高轨SAR系统，时变VTEC是导致方位时变STEC的主要因素。随着载频的下降与合成孔径时间的增加，方位向成像性能更加容易受到方位时变STEC的影响。

Research on Background Ionospheric Impacts Imposed by Spatio-temporal Variations on Spaceborne Synthetic Aperture Radar Azimuth Imaging

In spaceborne Synthetic Aperture Radar (SAR) imaging, the coherence of azimuth signals might be degraded by spatio-temporal variations of the background ionosphere, especially for low-frequency systems. In this paper, the azimuth temporal-varying Slant Total Electron Content (STEC) is attributed to three factors: the spatial- and temporal-varying Vertical Total Electron Content (VTEC), and the propagation path variation. Derivative of each order of the azimuth-varying STEC is analyzed as to each factor. A uniform analytical model is established, that is the third-order Taylor expansion model of SAR azimuth signals influenced by the temporal-varying STEC. The analytical expressions of the azimuth shift and phase errors are derived. Based on this model, thresholds of the varying STEC derivatives are derived for different spaceborne SAR systems. Signal-level simulations are performed by means of the VTEC real data and the International Reference Ionosphere (IRI) model. Numerical analyses and signal-level simulations indicate that the spatial-varying VTEC and the propagation path variation highlight in the low-orbit P-band SAR system, while the temporal-varying VTEC becomes a predominant factor that results in the azimuth temporal-varying STEC in the medium- or high-orbit SAR system. As the carrier frequency decreases and the synthetic aperture time increases, the azimuth imaging performance is more susceptible to the azimuth temporal-varying STEC.