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摘要: 双基合成孔径雷达(SAR)通过收发分置、协同工作,不仅能对接收站飞行前方实现高分辨成像,还具备出色的隐蔽性和抗干扰能力等优势,在海洋监测、成像侦察等军民领域具有广阔的应用前景。然而,海面舰船目标由于受到海浪影响,存在复杂且未知的三维随机剧烈摆动,且该摆动与双基平台的运动均随时间变化,导致双基SAR舰船目标成像结果的视图与方位时间强相关,难以获得有效的目标特征信息。此外,目标的三维摆动与收发双站的分置运动相互耦合叠加,导致双基舰船回波多普勒存在非线性强空变,造成舰船目标图像出现严重散焦。针对此问题,该文提出了一种双基SAR舰船成像时段寻优的成像处理方法,获得了成像视图最优且聚焦良好的双基SAR舰船目标图像。首先,采用短时傅里叶变换,精确反演舰船目标强散射点的时频信息;然后,联合多散射点时频信息,最优估计舰船目标的三维旋转参数,从而获得成像投影平面的时变规律;最后,以成像投影平面最优为准则,选取双基SAR舰船目标成像视图最优的成像时刻,再以成像分辨率最优为准则,选取双基SAR舰船目标成像时长,从而完成双基SAR舰船目标成像时段寻优成像处理。仿真实验验证了该方法在不同双基构型和不同信噪比条件下目标转动参数估计的准确性、成像投影平面选取的有效性,解决了双基SAR舰船目标成像视图强时变和多普勒非线性强空变问题,实现了双基SAR舰船目标图像的良好聚焦且成像视图最优,极大地提升了舰船目标特征信息获取的准确性。Abstract: Bistatic Synthetic Aperture Radar (SAR), with the separated transmitter and receiver working in coordination, cannot only achieves high-resolution imaging in the forward-looking mode, but also possesses outstanding concealment and anti-interference capabilities. Therefore, bistatic SAR thrives in both civilian and military applications, such as ocean monitoring or reconnaissance imaging. However, ship targets are typically influenced by sea waves, generating unknown and complex three-dimensional oscillations. These random oscillations and radar motions vary with slow time, making the imaging view of bistatic SAR ship targets strongly time-dependent, so that it is extremely difficult to extract effective target features from final imaging results. Moreover, target oscillations are also coupled with the motion of bistatic platforms, which causes severe nonlinear spatial Doppler shifts in target echoes, and thus bistatic SAR images are usually defocused. To address these problems, this paper proposes an imaging method for bistatic SAR ship target by imaging time optimization, which generates well-focused bistatic SAR ship target images with the optimal views. Firstly, short-time Fourier transform is utilized to extract the time-frequency information of the ship. Secondly, based on this time-frequency information from multiple strong scatterers, the optimal three-dimensional rotation parameters are estimated, revealing the time-varying characteristics of the imaging projection plane. Then, the optimal imaging time centers are selected based on the optimal imaging projection planes, while the corresponding optimal imaging time intervals are chosen based on the optimal imaging resolutions. Finally, with the selected optimal imaging times, the desired images of the bistatic SAR ship target are produced. Simulation experiments verify the accuracy of target rotation parameter estimation under different bistatic configurations and noise conditions, as well as the effectiveness of imaging projection plane selection. In general, this method tackles with the issues of the time-varying imaging views of bistatic SAR ship targets and nonlinear spatial Doppler shifts, obtaining well-focused and optimally viewed target images, which significantly enhances the accuracy of subsequent target feature extraction.
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表 1 雷达参数与舰船运动参数
Table 1. Parameters of Bistatic SAR and the ship target
参数 符号 数值 载波频率 $ {f}_{\mathrm{c}} $ 9.6 GHz 带宽 B 200 MHz 脉冲重复频率 $ \mathrm{P}\mathrm{R}\mathrm{F} $ 1600 Hz 采样频率 $ {f}_{\mathrm{s}} $ 800 MHz 总观测时间 $ {T}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}} $ 20 s 发射平台位置 $ {\boldsymbol{r}}_{\mathrm{T}} $ [4000 , –2600, 2000]T m 发射平台速度 $ {\boldsymbol{v}}_{\mathrm{T}} $ [10, 57, 20]T m·s–1 接收平台位置 $ {\boldsymbol{r}}_{\mathrm{R}} $ [2000, –8000, 3000]T m 接收平台速度 $ {\boldsymbol{v}}_{\mathrm{R}} $ [57, 15, 15]T m·s–1 目标速度 $ {\boldsymbol{v}}_{\mathrm{p}} $ [20, 10, 0]T m·s–1 目标散射点位置 $ {\boldsymbol{r}}_{\mathrm{A}} $ [–78.0, 0.0, 13.0]T m $ {\boldsymbol{r}}_{\mathrm{B}} $ [–13.0, 0.0, 35.0]T m $ {\boldsymbol{r}}_{\mathrm{C}} $ [–47.0, –9.3, 8.0]T m 表 2 舰船旋转参数
Table 2. Rotation parameters of the ship target
维度 摆动幅度$ {{A}}_{{i}} $ (rad) 角频率$ {{\varOmega }}_{{i}} $ (rad·s–1) U轴(横滚) 0.3351 0.5150 V轴(俯仰) 0.0297 0.9378 W轴(偏航) 0.0332 0.4425 表 3 舰船旋转参数估计结果与误差
Table 3. Estimation results and errors of rotation parameters of the ship target
维度 摆动幅度$ {{A}}_{{i}} $(rad) 误差(%) 角频率$ {{\varOmega }}_{{i}} $(rad·s–1) 误差(%) U轴(横滚) $ 0.3354 $ $ +0.08 $ $ 0.5150 $ $ +0.00 $ V轴(俯仰) $ 0.0294 $ $ -0.88 $ $ 0.9378 $ $ +0.00 $ W轴(偏航) $ 0.0331 $ $ -0.16 $ $ 0.4426 $ $ +0.03 $ 表 4 强散射点位置参数估计结果与误差
Table 4. Estimation results and errors of locations of strong scatterers
散射点 坐标(m) 误差(m) $ \mathrm{A} $ $ {\left[-78.2718, 0.2639, 12.8119\right]}^{\rm{T}} $ $ {\left[-0.2718, 0.2639, -0.1881\right]}^{\rm{T}} $ $ \mathrm{B} $ $ {\left[-13.2415,\mathrm{ }0.1169,\mathrm{ }34.8537\right]}^{\rm{T}} $ $ {\left[-0.2415,\mathrm{ }0.1169,\mathrm{ }-0.1463\right]}^{\rm{T}} $ $ \mathrm{C} $ $ {\left[46.9313, -9.2144, 7.9404\right]}^{\rm{T}} $ $ {\left[-0.0687, 0.0856, -0.0596\right]}^{\rm{T}} $ 表 5 不同双基SAR构型的参数
Table 5. Radar parameters of different bistatic SAR configurations
雷达构型 $ {\boldsymbol{r}}_{\mathrm{T}} $(m) $ {\boldsymbol{r}}_{\mathrm{R}} $(m) $ {\boldsymbol{v}}_{\mathrm{T}} $(m·s–1) $ {\boldsymbol{v}}_{\mathrm{R}} $(m·s–1) 1 $ {\left[-1000,-2600,\mathrm{ }3000\right]}^{\rm{T}} $ $ {\left[-2000,\mathrm{ }2000,\mathrm{ }3000\right]}^{\rm{T}} $ $ {\left[10, 57, 20\right]}^{\rm{T}} $ $ {\left[57, 15, 15\right]}^{\rm{T}} $ 2 $ {\left[4000,-2600,\mathrm{ }4000\right]}^{\rm{T}} $ $ {\left[0,\mathrm{ }-8000,\mathrm{ }3000\right]}^{\rm{T}} $ $ {\left[10, 57, 20\right]}^{\rm{T}} $ $ {\left[57, 15, 15\right]}^{\rm{T}} $ 3 $ {\left[4000, -2600, 2000\right]}^{\rm{T}} $ $ {\left[2000, -8000, 3000\right]}^{\rm{T}} $ $ {\left[30, 30, 10\right]}^{\rm{T}} $ $ {\left[40, 40, 0\right]}^{\rm{T}} $ 4 $ {\left[4000, -2600, 2000\right]}^{\rm{T}} $ $ {\left[2000, -8000, 3000\right]}^{\rm{T}} $ $ {\left[30, -10, 0\right]}^{\rm{T}} $ $ {\left[30, -20, 0\right]}^{\rm{T}} $ 5 $ {\left[3000,-\mathrm{2600,3000}\right]}^{\rm{T}} $ $ {\left[4000,-\mathrm{8000,3000}\right]}^{\rm{T}} $ $ {\left[-10,\mathrm{ }50,\mathrm{ }0\right]}^{\rm{T}} $ $ {\left[50,-20,0\right]}^{\rm{T}} $ 6 $ {\left[4000,-\mathrm{2600,2000}\right]}^{\rm{T}} $ $ {\left[-4000,-\mathrm{8000,2000}\right]}^{\rm{T}} $ $ {\left[25,-58,\mathrm{ }0\right]}^{\rm{T}} $ $ {\left[-33,-25,\mathrm{ }2\right]}^{\rm{T}} $ 表 6 各中心时刻成像指标
Table 6. Image indices of imaging results at different time centers
中心时刻 方位分辨率(m) 图像对比度 图像锐度 图像熵 归一化互信息(NMI) $ {\eta }_{1} $ $ 0.3456 $ $ 73.4218 $ $ 1.1672\times {10}^{11} $ $ 2.0656 $ $ 0.3561 $ $ {\eta }_{2} $ $ 0.6558 $ $ 62.8259 $ $ 1.0108\times {10}^{11} $ $ 2.4770 $ $ 0.4961 $ $ {\eta }_{3} $ $ 1.8712 $ $ 63.7224 $ $ 7.7467\times {10}^{10} $ $ 2.3724 $ $ 0.6292 $ 表 7 俯视图各中心时刻成像指标
Table 7. Image indices of imaging results at different time centers for top view
中心时刻 归一化互信息(NMI) 图像对比度 图像熵 $ {\eta }_{\mathrm{t}\mathrm{o}\mathrm{p}1} $ 0.3778 18.5044 3.0481 $ {\eta }_{\mathrm{t}\mathrm{o}\mathrm{p}2} $ 0.3161 24.4196 2.3900 $ {\eta }_{\mathrm{t}\mathrm{o}\mathrm{p}3} $ 0.1492 14.3753 2.6636 表 8 侧视图中心时刻成像指标
Table 8. Image indices of imaging results at different time centers for side view
中心时刻 归一化互信息(NMI) 图像对比度 图像熵 $ {\eta }_{\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}1} $ 0.4119 14.8815 2.0307 $ {\eta }_{\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}2} $ 0.4812 16.2234 1.9778 -
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