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An Improved Bilateral CFAR Ship Detection Algorithm for SAR Image in Complex Environment
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摘要: 双边恒虚警率(BCFAR)检测算法通过高斯核密度估计器计算出合成孔径雷达(SAR)图像的空间信息，并将它与图像的强度信息相结合得到联合图像以进行目标检测。相较于只使用强度信息来进行目标检测的经典CFAR检测算法，双边CFAR有着更好的检测性能和鲁棒性。然而，在复杂环境下出现连片的高强度异质点时(例如防波堤、方位模糊和幻影等)，核密度估计器计算出的空间信息会出现较多误差，这会导致检测结果中出现大量虚警。此外，当遇到相邻像素点间相似度较低的弱目标时，双边CFAR会发生漏检。为了有效改善这些问题，该文设计一种复杂环境下改进的SAR图像双边CFAR舰船检测算法(IB-CFAR)。该文所提IB-CFAR主要分为3个阶段来实现，分别为基于非均匀量化法的强度层级划分、强度-空间域信息融合、杂波截断后的参数估计。基于非均匀量化法的强度层级划分可以提升弱目标的相似度和对比度信息，从而提升舰船检测率。强度-空间域信息融合在于将空间相似度、距离向和强度等信息进行融合，在进一步提升检测率的同时对舰船的结构信息进行精细化描述。杂波截断后的参数估计可以去除背景窗口中连片的高强度异质点，最大限度地保留真实海杂波样本，使参数估计更加精确。最后，根据估计出的参数建立精确的海杂波统计模型以进行CFAR检测。该文使用高分3号和TerraSAR-X数据来验证该算法的有效性和鲁棒性。实验结果表明，所提出的算法在包含较多密集分布的弱目标环境下表现良好，在此类环境下能获得97.85%的检测率和3.52%的虚警率，相比于现有的检测算法，检测率提升了5%，并且虚警率降低了10%，但在弱目标个数较少且背景十分复杂的环境下，则会出现少量虚警。Abstract: The Bilateral Constant False Alarm Rate (BCFAR) detection algorithm calculates the spatial information of Synthetic Aperture Radar (SAR) image by the Gaussian kernel density estimator, and combines it with the intensity information of image to obtain the joint image for target detection. Compared with the classical CFAR detection algorithm which uses only intensity information for target detection, bilateral CFAR has better detection performance and robustness. However, with continuous high-intensity heterogeneous points (such as breakwater, azimuth ambiguity and phantom) in a complex environment, spatial information calculated by kernel density estimator will have more errors, which will lead to many false alarms in detection results. In addition, when it comes to a weak target with less similarity between adjacent pixels, it will miss detection. To effectively improve these problems, this paper designs an Improved Bilateral CFAR (IB-CFAR) algorithm in complex environment. The IB-CFAR proposed in this paper is mainly divided into three stages: intensity level division based on the nonuniform quantization method, intensity spatial domain information fusion and parameter estimation after clutter truncation. The intensity level division based on the nonuniform quantization method can improve the similarity and contrast information of weak targets, leading to improved ship detection rate. The information fusion of strength spatial domain is to fuse the spatial similarity, distance direction and strength information, which can further improve the detection rate and describe the ship structure information. Parameter estimation after clutter truncation can remove continuous high-intensity heterogeneous points in the background window and retain the real sea clutter samples to the maximum extent, which makes parameter estimation more accurate. Finally, according to the estimated parameters, an accurate sea clutter statistical model is established for CFAR detection. In this paper, the effectiveness and robustness of the proposed algorithm are verified by using GaoFen-3 and TerraSAR-X data.The experimental results show that the proposed algorithm performs well in the environment with more dense distribution of weak targets, and can obtain 97.85% detection rate and 3.52% false alarm rate in such environment. Compared with the existing detection algorithms, the detection rate increased by 5% and the false alarm rate reduced by 10%. However, when the number of weak targets is small and the background is very complex, few false alarms will appear.
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图 1 弱目标在双边CFAR联合图像中的值与在原始图像中的强度值之间的对比
Figure 1. The comparison between the value of weak target in the joint image of bilateral CFAR and the intensity value in the original image
图 3 非均匀强度分级原理图，黄色区域代表第
${L_N}$ 级Figure 3. Schematic diagram of non-uniform strength grading, which yellow area represents the level N
图 4 各项性能指标在不同
${L_N}$ 等级下的变化曲线Figure 4. Variation curves of various performance indicators at different levels
图 5 非均匀量化方法增强弱目标内部的相似度信息
Figure 5. Non-uniform quantization method enhances similarity information within weak targets
图 6 在包含强相干斑噪声环境下双边CFAR对杂波的抑制效果
Figure 6. Clutter suppression effect of bilateral CFAR in the presence of strong speckle noise
图 7 双边CFAR和本文所提IB-CFAR在高强度异质点分布密集的背景下处理得到的融合图像对比图
Figure 7. The comparison of the fusion images of bilateral CFAR and IB-CFAR under the background of dense distribution of high-intensity heterogeneous points
图 8 在包含强相干斑噪声环境下所提出的IB-CFAR对杂波的抑制效果
Figure 8. Clutter suppression effect of IB-CFAR in the presence of strong speckle noise
图 9 OR-CFAR和本文所提IB-CFAR在高强度异质点环境下对异质点进行杂波截断的性能评估
Figure 9. The high-intensity outliers elimination performance evaluation of OR-CFAR and the proposed IB-CFAR
图 10 2017年2月25日，高分3号在UFS模式下获得的上海港附近海域的SAR图像
Figure 10. Gaofen-3 test image of the homogeneous sea area near Shanghai harbor acquired by UFS mode on February 25, 2017
图 11 2009年7月31日，TerraSAR-X在SM模式下所获得的巴拿马运河地区的高分辨率、多视、HH极化SAR图像
Figure 11. High-resolution, multi-look, HH polarized SAR image of the Panama Canal region acquired by the X-band TerraSAR SM mode on July 31, 2009
图 12 2018年9月1日，高分3号在FSI模式下获得的长江入海口非均匀海况下的SAR图像
Figure 12. Gaofen-3 test image of the heterogeneous open sea of Yangtze River Estuary acquired by the FSI mode on September 1, 2018
图 13 复杂环境下的检测结果比较，其中包含密集分布的30个目标，并且图像中还存在重影和防波堤
Figure 13. Comparison of detection results in complex environment, which contains 30 targets with dense distribution, and there are ghost and anti wave in the image
图 14 原强度域图像和融合域图像在相同坐标点下的对比图
Figure 14. Comparison of original intensity domain image and fusion domain image at the same coordinate point
表 1 实验中所使用到的SAR图像的详细信息
Table 1. Details of SAR images used in the experiment
Name Gaofen-3 Terra SAR Gaofen-3 Acquisition date 2017-02-25 2009-07-31 2018-09-01 Acquired region The sea area near Shanghai harbour Panama Canal Sea of Yangtze River Estuary Location E121.9, N31.0 W79.55, N8.93 E121.3, N31.5 Imaging mode Ultra Fine Stripmap (UFS) StripMap (SM) Fine Stripmap I (FSI) Band C X C Polarization VH HH HH Resolution 3 m 3 m 5 m Number of looks 1 50 1 
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表 2 各CFAR检测器的性能分析
Table 2. Performance analysis of CFAR detectors
各CFAR检测器 Fig. 13(a) Fig. 15(a) ${D_{\rm{r}}}$[%] Fom FAR[%] Precision Time[s] ${D_{\rm{r}}}$[%] Fom FAR[%] Precision Time[s] CA-CFAR 9.35 0.0342 2.71 0.0854 6.25 3.54 0.0354 0 1.0000 1.62 TP-CFAR 92.90 0.8342 13.51 0.8747 12.03 85.23 0.3425 38.23 0.3720 3.15 LN-CFAR 82.67 0.5813 3.84 0.6235 15.32 73.42 0.2314 3.22 0.3321 5.36 K-CFAR 89.24 0.5442 11.23 0.5442 180.25 68.53 0.2552 15.21 0.2566 27.28 双边CFAR 52.65 0.2002 68.26 0.2301 17.63 52.43 0.1002 58.25 0.1567 6.67 IS-CFAR 91.28 0.7986 32.85 0.7986 25.12 38.46 0.3342 47.36 0.4342 10.23 IB-CFAR 97.85 0.9223 3.52 0.9243 27.52 98.24 0.5894 13.28 0.6074 11.09
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