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Analysis and Experimental Validation of Key Technologies for Unmanned Aerial Vehicle-borne Bistatic Interferometric Synthetic Aperture Radar
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摘要: 双站干涉合成孔径雷达(SAR)技术可突破单站双天线干涉SAR的基线限制，获得更加灵活的基线构型，有利于提升干涉测量精度。无人机载双站干涉SAR机动性好、飞行成本低，具有很高的应用价值，也能为无人机载分布式干涉、三维成像等提供关键支撑，具有重要的研究意义。中国科学院空天信息创新研究院牵头设计研制了国内首套无人机载双站干涉SAR系统，并在内蒙古百灵机场开展了飞行实验。该文简要介绍了该系统方案设计、基本构成和主要性能，并重点介绍了该系统的空间、时间及相位同步和数据处理关键技术，然后介绍了首次飞行实验的方案和实施研究情况，最后给出了实验数据处理结果，验证了关键技术和该无人机载双站干涉SAR系统0.5 m高程测量精度等主要指标，为后续多无人机平台协同开展分布式干涉数据获取及处理研究提供了基础。Abstract: Bistatic interferometric Synthetic Aperture Radar (SAR) overcomes the baseline length limit of the configuration of single-station interferometric SAR with two antennas and has become the primary method of terrain mapping using spaceborne interferometric SAR. To reduce the cost of surveying and mapping while promoting the development and application of Unmanned Aerial Vehicle (UAV)-borne bistatic interferometric SAR, the Aerospace Information Research Institute, Chinese Academy of Sciences took the lead in designing and developing a UAV-borne bistatic interferometric SAR processing system and performed flight experiments at Bailing Airport in Inner Mongolia. Herein, the system design, composition, and performance are introduced, and the scheme and implementation of the first flight experiment, along with the preliminary data processing results, are presented. In addition, the key performance metrics of the system, such as 0.5 m-elevation measurement accuracy, are verified in this study. The system serves as a foundation for future research topics, such as distributed InSAR using a multiaviation platform and tomography data acquisition and processing.
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图 2 主站L波段全极化SAR和双向同步链集成
Figure 2. Master L-band full-polarization SAR and bidirectional synchronous chain integration
图 3 从站L波段双极化接收系统和双向同步链集成
Figure 3. Slave L-band dual-polarization receiving system and bidirectional synchronous chain integration
图 5 通过UWB技术实现双机协同定位技术
Figure 5. Dual-machine co-positioning technology is realized through UWB technology
图 7 UWB测距和差分GPS定位的融合处理流程图
Figure 7. Flowchart of the fusion processing of UWB ranging and differential GPS positioning
图 8 双站InSAR系统双向同步链示意图
Figure 8. Diagram of the bidirectional synchronous chain of bistatic InSAR system
图 10 从站雷达时间和相位同步后的测量结果
Figure 10. Measurements after time and phase synchronization of slave radar
图 11 无人机载双站InSAR成像处理流程图
Figure 11. Flow chart of UAV-borne bistatic InSAR imaging processing
图 13 无人机载双站InSAR飞行的典型航迹和姿态测量数据
Figure 13. Typical trajectory and attitude measurements of UAV-borne bistatic InSAR
图 18 某三面角成像结果的方位向曲线
Figure 18. Curve of trihedral corner reflector in azimuth in imaging results
图 19 干涉相位误差与距离向像素关系
Figure 19. Relation of interference phase error and pixel location in range direction
图 20 极化校正前后的主站Pauli图
Figure 20. Pauli diagrams of the master station before and after polarization correction
图 24 去平地和滤波后的相位图
Figure 24. Interference phase diagram after flat-earth phase removal and filtering
图 28 去平地及滤波后的干涉相位图
Figure 28. Interference phase diagram after flat-earth phase removal and filtering
表 1 无人机载双站InSAR系统误差分配
Table 1. Systematic error distribution of UAV-borne bistatic InSAR
参数名称 参数值 飞行高度(相对于地面) 2 km 入射角 45° 基线角 0° 有效基线长度 30 m 水平基线长度 42.43 m 航迹控制精度(分配值) 1 m 时间同步引入的包络对齐误差 0.1个采样单元 运补残余误差导致的干涉相位误差 1° 同步链的干涉相位误差(分配值) 5° 去相干引入的相位误差 2.54° 基线测量误差(分配值) 4 mm 基线角误差(分配值) 0.003° 高程误差(理论分析值) 0.46 m 
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表 2 无人机载双站InSAR系统总体构成
Table 2. Overall composition of the UAV-borne bistatic InSAR system
名称 说明 L-SAR(主站) 由L波段全极化SAR和双向同步链组成，其中，L波段全极化SAR由信号产生与发射通道、功放、双极化接收通道、天线及天线支架、开关组合、数据采集存储模块等组成 L-接收系统(从站) 由L波段双极化接收系统和双向同步链组成，其中L波段双极化接收系统由双极化接收通道、天线及天线支架、数据采集存储模块等组成 无人机双机编队 由两架固定翼HC-140无人机和两套协同控制器组成，其中，单架无人机最大作业载荷30 kg、最大翼展尺寸5.4 m，最大飞行速度45 m/s 位姿测量系统 由差分GPS模块和微型惯性测量组成；测量数据更新频率为200 Hz，地面后处理后，航迹测量精度0.05 m，偏航角测量精度0.02°，横滚和俯仰角测量精度0.015°
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表 3 L-SAR载荷参数
Table 3. L-SAR load parameters
名称 参数名称 参数值 系统 波段 L波段 系统同步误差 ≤5° 噪声等效后向散射系数(NEσ0) ≤40 dB 主站 分辨率 ≤0.5 m 最大信号带宽 400 MHz 最大作用距离 ≥6 km 最大测绘带宽 ≥3 km 主站/从站 极化方式 全极化 极化隔离度 ≥25 dBc 极化通道相位不平衡度 ≤8° 幅度不平衡度 ≤0.3 dB
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表 4 双站InSAR构型参数
Table 4. Configuration parameters of bistatic InSAR
模式 飞行高度(m) 基线(m) 说明 模式1 1000 30 双站角0.87o，基线长度探底双机协同飞行安全距离的下限 模式2 2000 50 双站角0.72o，各项参数均比较合适
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表 5 分辨率测试结果
Table 5. Results of resolution test
分辨率 距离向(m) 方位向(m) 主站 0.3747 0.4534 从站 0.3747 0.4860
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表 6 主站数据极化定标前后的三面角极化质量评价
Table 6. Polarization quality evaluation of master data before and after polarization calibration based on trihedral corners
状态 三面角编号 隔离度(dB) 幅度不平衡(dB) 相位不平衡(°) 极化校正前 J2* 29.3217 0.21983 99.8529 J3 36.6858 0.44359 97.3340 J4 28.0391 –0.05674 98.3664 极化校正后 J2* 322.3542 5.786E–15 –7.2788E–15 J3 32.2467 0.15738 –3.2333 J4 37.7835 –0.32067 –1.2498 注：表中带*的J2为定标中使用的三面角，其余定标中未用
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表 7 从站数据极化定标前后的三面角极化质量评价
Table 7. Polarization quality evaluation of slave data before and after polarization calibration based on trihedral corners
状态 三面角编号 隔离度(dB) 幅度不平衡(dB) 相位不平衡(°) 极化校正前 J2* 28.4590 –0.23950 –90.2117 J3 26.8158 –0.08019 –87.2605 J4 27.9540 –0.71433 –86.5173 极化校正后 J2* 311.252 –2.893E–15 3.317E–14 J3 31.8702 0.065833 2.3704 J4 33.8167 –0.493960 3.3900 注：表中带*的J2为定标中使用的三面角，其余定标中未用
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表 8 双站SAR干涉反演高度误差表
Table 8. Table of elevation inversion error of bistatic interferometric SAR
定标点 相干系数 反演高度(m) 实测高度(m) 误差(m) J1 0.97 1385.89 1385.71 0.18 J2 0.89 1383.39 1383.23 0.17 J3 0.93 1385.14 1384.86 0.28 J4 0.97 1382.56 1383.68 –1.12 J5 0.86 1383.23 1384.45 –1.21 J6 0.98 1383.81 1384.38 –0.56 J7 0.99 1382.40 1382.11 0.29 J8 0.96 1387.60 1387.05 0.55 J9 0.92 1383.41 1384.28 –0.87 J10 0.87 1380.68 1383.05 –2.37 J11 0.98 1382.76 1383.38 –0.61 J12 0.96 1383.26 1383.61 –0.35 J13 0.98 1383.29 1382.98 0.31 J14 0.96 1385.93 1386.03 –0.09 J15 0.66 1386.80 1387.79 –1.00 J16 0.89 1388.00 1387.68 0.32 J17 0.48 1385.26 1387.25 –1.99 J18 0.88 1386.49 1386.65 –0.17 J19 0.97 1385.82 1386.09 –0.27 J20 0.93 1385.39 1385.57 –0.18 J21 0.98 1385.53 1385.23 0.30
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