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A Novel Wireless Internal Calibration Method of Spaceborne SAR
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摘要: 内定标利用雷达系统内部设备和定标通路来测量系统各部分幅度和相位在成像过程中的相对变化，是保证雷达图像辐射精度的重要手段。该文针对传统有线内定标方案定标通路未覆盖相控阵天线TR输出端至无源阵面路径、定标网络庞大且自身误差控制难等不足，提出了一种新颖的利用辅助天线的无线内定标方法，给出了定标原理和分析模型，推导了SAR天线TR通道幅相特性和系统传递函数的标定方法，并在典型星载SAR系统参数下对标定误差进行了仿真分析，仿真结果表明，辅助天线支撑杆位置引起的TR通道幅度标定误差在10–3 dB量级，可以忽略；引起相位标定误差与支撑杆位置偏差密切相关，可依据文中给出的仿真曲线得到。支持杆位置引起的系统传递函数幅度标定误差小于0.1 dB；引起的相位标定误差对支撑杆位置偏差不敏感。最后在实际相控阵天线上对无线内定标方法进行了验证，获取了TR通道幅相特性标定的实测结果，表明了该方案的可行性和有效性。Abstract: Internal calibration measures the changes in amplitude and phase of a system during imaging via the calibration loop built in Synthetic Aperture Radar (SAR). Internal calibration is also an important factor to improve the radiation accuracy of the radar. In this study, a novel wireless internal calibration method with auxiliary antenna is presented, considering that the traditional scheme is inefficient, as the calibration loop cannot cover the path from the TR (Transmitter-Receiver) output to the antenna radiator. The calibration loop also results in a complicated and heavy network. The principle and model of the new method is given, and the TR performance and system transfer function calibration approach are deduced. In addition, error analysis is conducted. The simulation results show that TR amplitude calibration errors caused by the rod of auxiliary antenna are at a 10–3 dB level, and phase calibration errors are obtained from the simulation curve in the paper; transfer function amplitude calibration errors are less than 0.1 dB, and phase calibration errors are not sensitive to the bias of the rod. Finally, the simulation results based on typical parameters of spaceborne SAR and the individual TR calibration experiments confirm the availability and feasibility of this novel method.
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图 1 传统星载SAR内定标原理
Figure 1. Diagram of traditional internal calibration in spaceborne SAR
图 5 位置误差引起的空间传输函数
$S\left( i \right)$ 的变化Figure 5.
$S\left( i \right)$ variation versus rod position errors
图 7
$\delta $ 函数幅相变化与位置误差关系Figure 7. Variation of function
$\delta $ versus rod position errors表 1 TR通道幅相特性标定误差源
Table 1. Error sources of TR amplitude and phase calibration
误差项 误差代号 幅度(dB) 相位(°) 空间传输函数误差 $\Delta S\left( i \right)$ $\Delta {A_S}\left( i \right)$ $\Delta {\varphi _S}\left( i \right)$ 移相器误差 $\Delta P\left( {i{\rm{,}}k} \right)$ $\Delta A\left( {i{\rm{,}}k} \right)$ $\Delta \varphi \left( {i{\rm{,}}k} \right)$ 
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表 2 系统传递函数标定误差源
Table 2. Error sources of transfer function calibration
误差项 误差代号 幅度(dB) 相位(°) 定标电缆的误差 $\Delta {C_L}$ $\Delta {A_{{\rm{CL}}}}$ $\Delta {\varphi _{{\rm{CL}}}}$ 内定标器误差 参考定标回路 $\Delta {\rm{C}}{{\rm{R}}_{\rm{1}}}$ $\Delta {A_{{\rm{CR1}}}}$ $\Delta {\varphi _{{\rm{CR1}}}}$ 发射定标回路 $\Delta {\rm{C}}{{\rm{R}}_{\rm{2}}}$ $\Delta {A_{{\rm{CR2}}}}$ $\Delta {\varphi _{{\rm{CR2}}}}$ 接收定标回路 $\Delta {\rm{C}}{{\rm{R}}_{\rm{3}}}$ $\Delta {A_{{\rm{CR3}}}}$ $\Delta {\varphi _{{\rm{CR3}}}}$ 传输函数矢量和误差 $\Delta \delta = \Delta \left\{ {\sum\nolimits_{i = 1}^N {{S_C}\left( i \right)S\left( i \right)} } \right\}$ $\Delta {A_\delta }$ $\Delta {\varphi _\delta }$
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表 3 仿真参数设置
Table 3. Parameters of simulation
参数名称 参数代号 参数值 工作频段 f X波段 天线长度 W 5 m 天线高度 H 1 m 支撑杆长度 L 1 m 方位向单元数 M 32 距离向单元数 N 16 支撑杆位置误差 $\beta $ 0°～0.10° 支撑杆旋转角 $\gamma $ 0°～360° 辅助天线形式 / 开口波导 辐射单元形式 / 波导缝隙 内定标器误差(幅度) / 0.4 dB 内定标器误差(相位) / 2.0° 定标电缆误差(幅度) / 0.1 dB 定标电缆误差(相位) / 1°
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表 4 系统传递函数标定误差
Table 4. Errors of system transfer function calibration
支撑杆位置误差 $\beta $(°) 传递函数标定误差 幅度(dB) 相位(°) 0 0.447 2.828 0.01 0.448 2.829 0.02 0.449 2.829 0.03 0.453 2.831 0.04 0.459 2.832 0.06 0.485 2.832 0.08 0.535 2.837 0.10 0.616 2.844
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