双基地前视合成孔径雷达运动补偿
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摘要
在自主着陆、自主导航等应用领域,需要掌握飞行器正前方区域的地形地貌及目标情况,解决长期困扰雷达正前视高分辨成像的技术难题。双基地合成孔径雷达(SAR),通过收发平台分置,在飞行器正前方区域能够形成等距离线和等多普勒线的近似正交分割,具备前视高分辨成像的潜力,是目前雷达领域的研究热点。
高效、高精度的运动补偿是实际应用中实现双基地前视SAR成像的关键环节。本文围绕双基地前视SAR运动补偿所涉及的信号特性、误差建模、参数估计及补偿成像等问题,主要开展了以下工作：
1.分析了距离徙动特性、多普勒特性及高阶相位误差特性,导出了运动误差条件下的点目标二维频谱,建立了运动误差与多普勒参数的关系模型,为参数估计和运动补偿奠定了理论基础。
2.提出了基于改进Radon变换的多普勒质心估计方法，通过传感器信息利用、灰度数据稀疏化及粗精两步Radon变换等步骤,实现了双基地前视SAR多普勒质心的快速无模糊估计；提出了基于相位迭代校正的多普勒质心估计方法,以波形熵为衡量标准,通过迭代搜索距离走动斜率,同时实现了双基地前视SAR距离走动校正和多普勒质心估计。
3.提出了结合时间-调频斜率分布与改进Radon变换的多普勒参数估计方法,通过在时间-调频斜率域检测直线的斜率,实现了双基地前视SAR高阶相位误差估计和补偿。
4.提出了低对比度场景下的参数估计策略,通过首先检测时频域直线的斜率得到高对比度条件下的多普勒调频率,然后根据各距离单元之间参数的关系,实现了全场景的多普勒调频率参数估计。
5.提出了基于双曲等效模型的扩展波数域成像算法,克服了常规波数域成像算法难以嵌入二阶运动补偿的局限性,可以实现具有大距离徙动特征的双基地前视SAR成像。
采用仿真与实测数据对上述方法和方案进行了验证,结果表明,这些方法能够满足双基地前视SAR参数估计与误差补偿精度需求,有效提高成像质量。
To get knowledge of topography and targets in front of aircraft, and solve the technical challenge which troubles forward-looking high-resolution imaging for a long time period, is an urgent issue in the field of autonomous landing and autonomous navigation. By setting the transmitter and receiver on different platforms, the orthogonal partition between equi-distance line and equi-DoppIer line can be formed in front of aircraft, thus bistatic synthetic aperture radar (SAR) has the potential of forward-looking high-resolution imaging. Hence, bistatic forward-looking SAR (BFSAR) is the focus of current research.
Motion compensation with high efficiency and high precision is of great significance to guarantee high quality image of BFSAR. In this paper, following works involving signal properties, error model, parameter estimation and compensation methods of BFSAR are carried out,
1. The range migration properties and Doppler properties of BFSAR are analyzed as well as high-order phase error. The point target spectrum for BFSAR with motion error is derived, and the relationships between motion error and Doppler parameter are established. The work of this part will lay theotical foundation for parameter estimation and motion compensation.
2. Using prior information of sensors, converting intensity data into binary data and dividing Radon transform into coarse and fine Radon transform, an improved Radon-transform-based Doppler centroid estimation scheme is proposed, thus Doppler centroid without ambiguity for BFSAR can be obtained with rapid speed. By searching for range walk slope based on the minimum entropy, an iterative scheme of Doppler centroid estimation is proposed, thus the Doppler centroid can be estimated for BFSAR. Meanwhile, the range walk is corrected.
3. Combining time-Doppler rate distribution with improved Radon transform, the high-order Doppler parameter estimation scheme is proposed. By detecting slope in time-Doppler rate domain, high-order phase error of BFSAR can be estimated and compensated.
4. The estimation strategy of Doppler rate in low-contrast scene is proposed. According to the strategy, the Doppler rate in high-contrast scene is estimated firstly by detecting slope in time-frequency domain, and then Doppler rate in full scene can be acquired using the relationship of Doppler rates in different range bins.
5. To overcome the problem that conventional wavenumber-domain imaging can not be combined with second-order motion compensation, the extended wavenumber domain imaging based on hyperbolic-equivalent model is proposed, and then high-resolution imaging of BFSAR with large range migration can be realized.
Both the simulated and real data are used to verify the proposed methods, and the results show that these methods can be used to estimate Doppler parameter and compensate phase error of BFSAR, and the image quality is improved greatly.
高效、高精度的运动补偿是实际应用中实现双基地前视SAR成像的关键环节。本文围绕双基地前视SAR运动补偿所涉及的信号特性、误差建模、参数估计及补偿成像等问题,主要开展了以下工作：
1.分析了距离徙动特性、多普勒特性及高阶相位误差特性,导出了运动误差条件下的点目标二维频谱,建立了运动误差与多普勒参数的关系模型,为参数估计和运动补偿奠定了理论基础。
2.提出了基于改进Radon变换的多普勒质心估计方法，通过传感器信息利用、灰度数据稀疏化及粗精两步Radon变换等步骤,实现了双基地前视SAR多普勒质心的快速无模糊估计；提出了基于相位迭代校正的多普勒质心估计方法,以波形熵为衡量标准,通过迭代搜索距离走动斜率,同时实现了双基地前视SAR距离走动校正和多普勒质心估计。
3.提出了结合时间-调频斜率分布与改进Radon变换的多普勒参数估计方法,通过在时间-调频斜率域检测直线的斜率,实现了双基地前视SAR高阶相位误差估计和补偿。
4.提出了低对比度场景下的参数估计策略,通过首先检测时频域直线的斜率得到高对比度条件下的多普勒调频率,然后根据各距离单元之间参数的关系,实现了全场景的多普勒调频率参数估计。
5.提出了基于双曲等效模型的扩展波数域成像算法,克服了常规波数域成像算法难以嵌入二阶运动补偿的局限性,可以实现具有大距离徙动特征的双基地前视SAR成像。
采用仿真与实测数据对上述方法和方案进行了验证,结果表明,这些方法能够满足双基地前视SAR参数估计与误差补偿精度需求,有效提高成像质量。
To get knowledge of topography and targets in front of aircraft, and solve the technical challenge which troubles forward-looking high-resolution imaging for a long time period, is an urgent issue in the field of autonomous landing and autonomous navigation. By setting the transmitter and receiver on different platforms, the orthogonal partition between equi-distance line and equi-DoppIer line can be formed in front of aircraft, thus bistatic synthetic aperture radar (SAR) has the potential of forward-looking high-resolution imaging. Hence, bistatic forward-looking SAR (BFSAR) is the focus of current research.
Motion compensation with high efficiency and high precision is of great significance to guarantee high quality image of BFSAR. In this paper, following works involving signal properties, error model, parameter estimation and compensation methods of BFSAR are carried out,
1. The range migration properties and Doppler properties of BFSAR are analyzed as well as high-order phase error. The point target spectrum for BFSAR with motion error is derived, and the relationships between motion error and Doppler parameter are established. The work of this part will lay theotical foundation for parameter estimation and motion compensation.
2. Using prior information of sensors, converting intensity data into binary data and dividing Radon transform into coarse and fine Radon transform, an improved Radon-transform-based Doppler centroid estimation scheme is proposed, thus Doppler centroid without ambiguity for BFSAR can be obtained with rapid speed. By searching for range walk slope based on the minimum entropy, an iterative scheme of Doppler centroid estimation is proposed, thus the Doppler centroid can be estimated for BFSAR. Meanwhile, the range walk is corrected.
3. Combining time-Doppler rate distribution with improved Radon transform, the high-order Doppler parameter estimation scheme is proposed. By detecting slope in time-Doppler rate domain, high-order phase error of BFSAR can be estimated and compensated.
4. The estimation strategy of Doppler rate in low-contrast scene is proposed. According to the strategy, the Doppler rate in high-contrast scene is estimated firstly by detecting slope in time-frequency domain, and then Doppler rate in full scene can be acquired using the relationship of Doppler rates in different range bins.
5. To overcome the problem that conventional wavenumber-domain imaging can not be combined with second-order motion compensation, the extended wavenumber domain imaging based on hyperbolic-equivalent model is proposed, and then high-resolution imaging of BFSAR with large range migration can be realized.
Both the simulated and real data are used to verify the proposed methods, and the results show that these methods can be used to estimate Doppler parameter and compensate phase error of BFSAR, and the image quality is improved greatly.
引文
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