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Mechanism Study of Ionospheric Effects on Medium-Earth-Orbit SAR
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摘要:
中高轨道合成孔径雷达(Synthetic Aperture Radar, SAR)是下一代星载SAR重要发展方向之一，电离层影响分析是推动中高轨道SAR系统发展的关键技术之一。该文利用电离层实测数据分析了电离层时空变化特征，根据电离层时空变化特性，结合中高轨道SAR合成孔径时间长、测绘带宽、轨道高等特点，从SAR成像理论角度出发，分别从距离向和方位向阐述了电离层对中高轨SAR成像质量的影响因素和影响机理，通过分析揭示了中高轨SAR电离层影响与低轨SAR的不同之处。
Abstract:The Medium-Earth-Orbit SAR (MEOSAR) is one of the potential spaceborne SAR of next-generation owing to its excellent performance. Ionospheric effects analysis is one of the critical techniques for the development of MEOSAR. The spatio-temporal variability of the ionosphere is analyzed using the measured ionosphere data. The factors and the mechanism of ionospheric effects on MEOSAR are studied based on the spatio-temporal variability of the ionosphere and the characteristics of MEOSAR such as long synthetic aperture time, wide swath, and high orbit. The results reveal that there are many differences in the ionospheric effects between MEOSAR and LEOSAR.
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Key words:
- Ionospheric effect /
- Medium-Earth-Orbit SAR (MEOSAR)
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图 5 南加州中心区域TEC日变化图
Figure 5. Ionospheric TEC variation in a day (South California Area)
图 8 中高轨道/低轨道SAR距离向横跨电离层尺度示意图
Figure 8. Sketch of range beam on ionosphere for MEO/LEO SAR
图 9 不同轨道高度下电离层TEC校正后剩余误差导致的最大二次相位误差
Figure 9. Peak quadratic phase error induced by residual TEC after correction for various orbit
图 10 电磁波在随机介质中的传播几何
Figure 10. Propagation geometry of a signal propagating in ionosphere
图 11 SAR信号双程通过电离层示意图
Figure 11. Sketch of SAR pulses propagating through ionosphere twice
图 12 电离层不规则体引起的距离向分辨率的下降随轨道高度的变化
Figure 12. Degradation of resolution in range induced by irregularities vs. SAR orbit height
图 13 中高轨SAR条件下，电离层不规则体在距离向分布示意图
Figure 13. Sketch of range beam on ionospheric irregularity for MEO/LEO SAR
图 15 中高轨SAR和低轨SAR合成孔径在电离层上投影示意图
Figure 15. Projection on ionosphere of synthetic aperture for MEO and LEO SAR
图 16 电离层时变对方位向成像的影响
Figure 16. Effects on azimuth image induced by temporal-variability of ionosphere
图 18 不同轨道下电离层不规则体引入的相位方差(p=2.5)
Figure 18. Phase variance induced by ionospheric irregularity vs. orbit height
表 1 仿真参数
Table 1. Simulation parameters
参数 数值 工作频率(GHz) 1.25 带宽(MHz) 100 不规则体下边缘高度(km) 300 不规则体上边缘高度(km) 500 不规则体内尺度(m) 0.5 不规则体外尺度(m) 500 不规则体p指数 11/3 不规则体起伏方差 0.25 
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表 2 仿真参数
Table 2. Simulation parameters
参数 数值 轨道高度(km) 600 1300 3000 10000 工作频率(GHz) 1.25 1.25 1.25 1.25 方位分辨率(m) 5 5 5 5 合成孔径时间(s) 2.6 5.7 13.7 54.5 多普勒带宽(Hz) 1384 1198 890 400 斜视角(°) 0 0 0 0 地面入射角(°) 45 45 45 45 波束视角(°) 40.2 36.0 28.7 16.0 卫星速度vr(km/s) 7.23 6.57 5.39 3.08 TEC二次变化率(TECu/s2) 0.02 0.02 0.02 0.02 TEC三次变化率(TECu/s3) 0.002 0.002 0.002 0.002
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