A ISAR Imaging Method for Space Targets Based on Fast Estimation of Joint Motion Parameters
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摘要: 逆合成孔径雷达(ISAR)是空间目标成像和监测的重要手段之一,大转角下空间目标成像结果的越分辨单元徙动(MTRC)现象加剧,严重影响ISAR成像的性能。为快速估计和补偿空间目标运动产生的回波相位误差,结合BFGS优化算法效率高与极坐标格式变换算法(PFA)补偿精度高的优势,该文提出了一种基于联合运动参数快速估计的空间目标ISAR成像方法。所提方法建立了目标平动和转动参数联合估计的最小化图像熵优化模型;为降低优化陷入局部最优的可能,设计了目标参数粗估计和精估计的高效BFGS求解子步骤,实现了目标转动参数的快速估计与大转角情况下MTRC的补偿。点目标仿真和实测民航客机数据成像结果表明,相比PSO-PFA算法,所提方法在低信噪比条件下的运动参数估计精度更高,运算时间缩短为原来的五分之一,具有显著优势。Abstract: Inverse Synthetic Aperture Radar (ISAR) is an important tool for imaging and monitoring space targets. The large rotation angle of space targets can exacerbate the phenomenon of Migration Through Resolution Cells (MTRC), seriously affecting the ISAR imaging performance. For the fast estimation and compensation of echo phase errors caused by the motion of space targets, this paper proposes an ISAR space-target imaging method based on the rapid estimation of joint motion parameters. This method combines the advantages of the high efficiency of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization algorithm and the high compensation accuracy of the Polar Format Algorithm (PFA) algorithm. The proposed method formulates an image entropy minimization model considering the joint estimation of the translation and rotation parameters of the target. To reduce the possibility of optimization falling into local optima, the proposed method solves sub-steps, which comprise rough and fine estimations of the target motion parameters, using the BFGS optimization algorithm. The proposed method rapidly estimates target rotation parameters and performs quick MTRC compensation under large rotation angles. The simulation of point targets and imaging results of actual civil aircraft data show that compared with the Particle Swarm Optimization-Polar Format Algorithm (PSO–PFA) algorithm, the proposed method estimates motion parameters with a higher accuracy under low signal-to-noise ratio conditions. Further, the computational efficiency is improved by more than five times, which is significantly advantageous.
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1 BFGS算法伪代码
1. Pseudocode for the BFGS algorithm.
输入:待估计值$ {k_i} $,目标函数$ {H_z}{(}{k_i}{)} $ 初始化:粗估计值$ {k^0_i} $,黑塞矩阵$ {H^0}{ = }{\bf{I}} $,步长$ {\alpha ^j} $,迭代次数$ j = 0 $。 开始迭代: 1. 更新变量: $ k_i^{j + 1}{ = }k_i^j - {\alpha ^j}{H^j}\dfrac{{\partial {H_z}}}{{\partial k_i^j}} $ (33) $ {x^j}{ = }k_i^{j + 1}{ - }k_i^j $ (34) $ {y^j}{ = }\dfrac{{\partial {H_z}}}{{\partial k_i^{j + 1}}}{ - }\dfrac{{\partial {H_z}}}{{\partial k_i^j}} $ (35) 2. 更新黑塞矩阵: $ {\rho ^j}{ = }\dfrac{1}{{{{{(}{x^j}{)}}^{\mathrm{T}}}{y^j}}} $ (36) $ {H^{j + 1}}{ = [}{\bf{I}}{ - }{\rho ^j}{y^j}{{(}{x^j}{)}^{\mathrm{T}}}{{]}^{\mathrm{T}}}{H^j}{[}{\bf{I}}{ - }{\rho ^j}{y^j}{{(}{x^j}{)}^{\mathrm{T}}}{] + }{\rho ^j}{x^j}{{(}{x^j}{)}^{\mathrm{T}}} $ (37) $ j{ = }j{ + }1 $ (38) 3. 判断是否达到阈值 输出:完成迭代后,输出估计值 表 1 仿真参数表
Table 1. Simulation parameter table
仿真参数 数值 中心频率 20 GHz 脉冲宽度 200 μs 带宽 4 GHz 脉冲重复频率 100 Hz 去斜后距离向采样率 20 MHz 表 2 不同信噪比下各算法运算时间和转动参数估计质量比较
Table 2. Comparison of algorithm calculation time and rotation parameter estimation quality under different SNR
信噪比(dB) 算法 运算时间(s) 转动速度
(rad/s)转动速度估计误差
(rad/s)转动加速度
(rad/s2)转动加速度
估计误差(rad/s2)理论值 0.0800 0.0100 5 PSO-PFA算法 48.09 0.0779 0.0021 0.0096 0.0004 所提算法 10.40 0.0784 0.0016 0.0094 0.0006 0 PSO-PFA算法 49.85 0.0750 0.0050 0.0095 0.0005 所提算法 10.88 0.0780 0.0020 0.0093 0.0007 –5 PSO-PFA算法 59.64 0.0696 0.0104 0.0082 0.0018 所提算法 11.01 0.0771 0.0029 0.0092 0.0008 –10 PSO-PFA算法 32.75 0.0714 0.0086 0.0083 0.0017 所提算法 12.25 0.0781 0.0019 0.0093 0.0007 表 3 点目标成像结果性能对比
Table 3. Comparison of imaging results in point
方向 算法 ISLR (dB) PSLR (dB) IRW(采样点) 距离向 RD算法 – 12.6383 – 13.3929 2.0172 PSO-PFA算法 – 7.9669 – 13.0756 1.0804 所提算法 – 8.3111 – 13.1223 1.0031 方位向 RD算法 – 16.0624 – 16.2671 2.9232 PSO-PFA算法 – 14.5834 – 16.2696 1.4372 所提算法 – 16.4953 – 16.9167 1.3865 表 4 实测实验参数表
Table 4. Experimental parameter table
参数 数值 中心频率 14.6 GHz 脉冲宽度 500 μs 带宽 600 MHz 脉冲重复频率 2000 Hz 去斜后距离向采样率 25 MHz 表 5 实测数据成像结果指标
Table 5. Indicators for imaging results based on actual measurement data
算法 运算时间(s) 图像熵 图像对比度 RD算法 5.5604 22.3463 PSO-PFA算法 296.38 5.1896 26.7782 所提算法 41.11 5.1950 26.1366 表 6 点P3成像指标
Table 6. Comparison of imaging results in point P3
方向 算法 ISLR (dB) PSLR (dB) IRW(采样点) 距离向 RD算法 – 12.5781 – 15.7136 1.3516 PSO-PFA算法 – 15.1908 – 18.1396 1.1771 所提算法 – 15.1643 – 18.1401 1.1788 方位向 RD算法 – 8.2897 – 12.5406 1.2609 PSO-PFA算法 – 14.1262 – 15.5056 1.1882 所提算法 – 14.1223 – 15.4902 1.1891 -
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