斜视聚束合成孔径雷达成像算法研究
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摘要
合成孔径雷达能够实施全天候、全天时的观测,目前该雷达较成熟的工作模式有条带(Strip)、扫描(Scan)、干涉(INSAR)和聚束(Spotlight)工作模式。聚束式合成孔径雷达的优点是能够实现较小场景的方位向高分辨率成像。聚束工作模式通过控制雷达方位向天线波束指向,使其沿飞行路径连续照射同一块成像区域以增加其相干时间,此时聚束模式的方位向分辨率不受天线波束宽度的限制,因此为获得1m或小于1m分辨率的雷达图像开辟了一条可实现的途径。本文主要对聚束式合成孔径雷达距离徙动校正和降低高脉冲重复频率两个方面进行了深入的研究。
距离徙动是由雷达与目标之间的相对运动产生的。通常把距离徙动的线性部分称为距离走动,二次项部分称为距离弯曲。大部分聚束式合成孔径雷达成像算法是基于正侧视模型提出的,此时多普勒中心频率为零,不存在距离走动部分。而在斜视情况下随着多普勒中心频率的增加,距离走动在距离徙动中逐渐起主导作用,已有的主要针对距离弯曲的成像算法无法做到有效的距离徙动校正。
距离徙动算法是理论上最优的成像算法,其关键步骤是Stolt插值。对于实时成像或大数据量系统来说插值运算是不实际的。频率变标算法就是距离徙动算法较好的近似实现,通过对距离徙动算法的近似来精确校正距离徙动,避免了插值运算。但是基于正侧视模型提出的频率变标算法无法适用于斜视成像,其局限性主要体现在二次距离压缩的近似误差。
为了降低高脉冲重复频率,通常将长的聚束孔径分成多个子孔径来处理,即方位向子孔径处理。阶梯变换方法是较成熟的子孔径处理方法。算法中影响最后聚焦效果的主要因素有两个,一个是相邻子孔径间谱峰的相对位移要满足整数倍频率间隔的限制条件,另一个是粗分辨率傅立叶变换结果存在高阶误差相位。
针对以上几个问题,论文做了如下工作并提出了几个改进算法。
第一:分析了距离徙动算法Stolt插值前信号的波数谱支撑区范围,得出了斜视成像的理论结果和斜视模型成像质量评估的理论值,并以此作为评估其它算法的主要依据。此外还提出了视线插值的距离徙动算法并给出了实验仿真结果。
第二:针对频率变标算法的局限性,提出了适用于斜视处理的非线性频率变标算法,给出了非线性频率变标函数的最优解,将近似误差的二次项误差完全补偿,确保了大斜视角大场景成像的良好成像效果。
第三:将自聚焦技术和阶梯变换方法有效地结合起来,用最小熵准则的自适应阶数多项式滤波器估计并去除误差相位,保证了精分辨率傅立叶变换的精度。
本文最后给出了星载聚束式合成孔径雷达和滑动聚束式合成孔径雷达回波仿真及相应的成像处理结果,以及机载实际数据的处理图像。
Without the restriction of sunlight and weather, synthetic aperture radar (SAR) can perform all-weather and all-time observation. SAR systems have different modes, like the strip, the scan, and the spotlight modes. Different modes have different applications. The advantage of the spotlight mode SAR is that it can perform high azimuth resolution imaging for the illuminated scene. It is implemented by directing the beam of the antenna to the same area during the synthetic aperture time. In this situation, the azimuth resolution of the spotlight SAR does not depends on the width of the beam of antenna. Therefore, the spotlight mode SAR supplies a feasible way to obtain radar images of a 1-m or higher resolution.
In this dissertation, we will focus on how to correct the range migration and reduce high pulse repetition frequency (PRF)。
Range migration results from the relative motion between the radar and the target, and cannot be avoided in a SAR system. Usually, the linear part of range migration is called range walk, and the quadratic part is called range curve. Most of spotlight SAR processing algorithms were proposed for the broadside mode, and there is no range walk. For the more general mode, the squint mode, range walk dominates gradually with the increasing of the Doppler centroid. In this case, the imaging algorithms with only range curve considered cannot work well.
The range migration algorithm (RMA) is a promising spotlight SAR imaging algorithm and the key to the RMA is the Stolt interpolation. For an actual imaging system, due to the large-storage requirement and computation burden, the interpolation is not practical. The frequency scaling algorithm (FSA) is a preferable approximation of the RMA. It avoids the interpolation of the RMA by an accurate approximation. Unfortunately, the original FSA is based on the broadside mode and does not apply to the squint imaging. The approximation phase error caused by the secondary range compression restricts the performance of the FSA processing for the squint mode.
In order to reduce high PRF, the long spotlight aperture is usually divided into several sub-apertures to process. The step transform is an effective sub-aperture processing approach. The quality of the final image is limited by two factors. One is that the offset between two adjacent sub-apertures must be a multiple of the frequency interval. The other is that the high order phase error exists in the coarse resolution data.
Considering the aspects mentioned above, we have obtained following achievements.
First, the wave-number spectrum extension before the Stolt interpolation of the RMA is analyzed to derive the measure of image quality in the squint imaging. This can be used to evaluate results by other imaging algorithms. Moreover line-of-sight interpolation for the RMA is proposed.
Second, in order to compensate the phase error from secondary range compression accurately, we derive the optimal nonlinear frequency scaling function. The nonlinear frequency scaling algorithm works very well even for large squint angles and large imaging scenes.
Third, the step transform is combined with autofocus to improve the sub-aperture processing. The minimum-entropy filter using an adaptive order polynomial model is designed to estimate and remove the phase error.
Finally, the simulation and processing results for the space-borne spotlight mode SAR system and the space-borne sliding spotlight mode SAR system are given. In addition, some practical airborne data are processed by the chirp scaling algorithm, and the results confirm the validity of the spotlight SAR imaging algorithm presented in this paper.
距离徙动是由雷达与目标之间的相对运动产生的。通常把距离徙动的线性部分称为距离走动,二次项部分称为距离弯曲。大部分聚束式合成孔径雷达成像算法是基于正侧视模型提出的,此时多普勒中心频率为零,不存在距离走动部分。而在斜视情况下随着多普勒中心频率的增加,距离走动在距离徙动中逐渐起主导作用,已有的主要针对距离弯曲的成像算法无法做到有效的距离徙动校正。
距离徙动算法是理论上最优的成像算法,其关键步骤是Stolt插值。对于实时成像或大数据量系统来说插值运算是不实际的。频率变标算法就是距离徙动算法较好的近似实现,通过对距离徙动算法的近似来精确校正距离徙动,避免了插值运算。但是基于正侧视模型提出的频率变标算法无法适用于斜视成像,其局限性主要体现在二次距离压缩的近似误差。
为了降低高脉冲重复频率,通常将长的聚束孔径分成多个子孔径来处理,即方位向子孔径处理。阶梯变换方法是较成熟的子孔径处理方法。算法中影响最后聚焦效果的主要因素有两个,一个是相邻子孔径间谱峰的相对位移要满足整数倍频率间隔的限制条件,另一个是粗分辨率傅立叶变换结果存在高阶误差相位。
针对以上几个问题,论文做了如下工作并提出了几个改进算法。
第一:分析了距离徙动算法Stolt插值前信号的波数谱支撑区范围,得出了斜视成像的理论结果和斜视模型成像质量评估的理论值,并以此作为评估其它算法的主要依据。此外还提出了视线插值的距离徙动算法并给出了实验仿真结果。
第二:针对频率变标算法的局限性,提出了适用于斜视处理的非线性频率变标算法,给出了非线性频率变标函数的最优解,将近似误差的二次项误差完全补偿,确保了大斜视角大场景成像的良好成像效果。
第三:将自聚焦技术和阶梯变换方法有效地结合起来,用最小熵准则的自适应阶数多项式滤波器估计并去除误差相位,保证了精分辨率傅立叶变换的精度。
本文最后给出了星载聚束式合成孔径雷达和滑动聚束式合成孔径雷达回波仿真及相应的成像处理结果,以及机载实际数据的处理图像。
Without the restriction of sunlight and weather, synthetic aperture radar (SAR) can perform all-weather and all-time observation. SAR systems have different modes, like the strip, the scan, and the spotlight modes. Different modes have different applications. The advantage of the spotlight mode SAR is that it can perform high azimuth resolution imaging for the illuminated scene. It is implemented by directing the beam of the antenna to the same area during the synthetic aperture time. In this situation, the azimuth resolution of the spotlight SAR does not depends on the width of the beam of antenna. Therefore, the spotlight mode SAR supplies a feasible way to obtain radar images of a 1-m or higher resolution.
In this dissertation, we will focus on how to correct the range migration and reduce high pulse repetition frequency (PRF)。
Range migration results from the relative motion between the radar and the target, and cannot be avoided in a SAR system. Usually, the linear part of range migration is called range walk, and the quadratic part is called range curve. Most of spotlight SAR processing algorithms were proposed for the broadside mode, and there is no range walk. For the more general mode, the squint mode, range walk dominates gradually with the increasing of the Doppler centroid. In this case, the imaging algorithms with only range curve considered cannot work well.
The range migration algorithm (RMA) is a promising spotlight SAR imaging algorithm and the key to the RMA is the Stolt interpolation. For an actual imaging system, due to the large-storage requirement and computation burden, the interpolation is not practical. The frequency scaling algorithm (FSA) is a preferable approximation of the RMA. It avoids the interpolation of the RMA by an accurate approximation. Unfortunately, the original FSA is based on the broadside mode and does not apply to the squint imaging. The approximation phase error caused by the secondary range compression restricts the performance of the FSA processing for the squint mode.
In order to reduce high PRF, the long spotlight aperture is usually divided into several sub-apertures to process. The step transform is an effective sub-aperture processing approach. The quality of the final image is limited by two factors. One is that the offset between two adjacent sub-apertures must be a multiple of the frequency interval. The other is that the high order phase error exists in the coarse resolution data.
Considering the aspects mentioned above, we have obtained following achievements.
First, the wave-number spectrum extension before the Stolt interpolation of the RMA is analyzed to derive the measure of image quality in the squint imaging. This can be used to evaluate results by other imaging algorithms. Moreover line-of-sight interpolation for the RMA is proposed.
Second, in order to compensate the phase error from secondary range compression accurately, we derive the optimal nonlinear frequency scaling function. The nonlinear frequency scaling algorithm works very well even for large squint angles and large imaging scenes.
Third, the step transform is combined with autofocus to improve the sub-aperture processing. The minimum-entropy filter using an adaptive order polynomial model is designed to estimate and remove the phase error.
Finally, the simulation and processing results for the space-borne spotlight mode SAR system and the space-borne sliding spotlight mode SAR system are given. In addition, some practical airborne data are processed by the chirp scaling algorithm, and the results confirm the validity of the spotlight SAR imaging algorithm presented in this paper.
引文
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