SCIENCE CHINA Information Sciences, Volume 62, Issue 4: 042301(2019) https://doi.org/10.1007/s11432-017-9405-6

## A coupled convolutional neural network for small and densely clustered ship detection in SAR images

• AcceptedApr 2, 2018
• PublishedSep 19, 2018
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### Abstract

Ship detection from synthetic aperture radar (SAR) imagery plays a significant role in global marine surveillance. However, a desirable performance is rarely achieved when detecting small and densely clustered ship targets, and this problem is difficult to solve. Recently, convolutional neural networks (CNNs) have shown strong detection power in computer vision and are flexible in complex background conditions, whereas traditional methods have limited ability. However, CNNs struggle to detect small targets and densely clustered ones that exist widely in many SAR images. To address this problem while preserving the good properties for complex background conditions, we develop a coupled CNN for small and densely clustered SAR ship detection. The proposed method mainly consists of two subnetworks: an exhaustive ship proposal network (ESPN) for ship-like region generation from multiple layers with multiple receptive fields, and an accurate ship discrimination network (ASDN) for false alarm elimination by referring to the context information of each proposal generated by ESPN. The motivation in ESPN is to generate as many ship proposals as possible, and in ASDN, the goal is to obtain the final results accurately. Experiments are evaluated on two data sets. One is collected from 60 wide-swath Sentinel-1 images and the other is from 20 GaoFen-3 (GF-3) images. Both data sets contain many ships that are small and densely clustered. The quantitative comparison results illustrate the clear improvements of the new method in terms of average precision (AP) and $F$1 score by 0.4028 and 0.3045 for the Sentinel-1 data set compared with the multi-step constant false alarm rate (CFAR-MS) method. The values are verified as 0.2033 and 0.1522 for the GF-3 data set. In addition, the new method is demonstrated to be more efficient than CFAR-MS.

### Acknowledgment

This work was partially supported by National Natural Science Foundation of China (Grant No. 61331015) and China Postdoctoral Science Foundation (Grant No. 2015M581618). The authors are grateful to thank Prof. T. K. Truong for his helpful comments and suggestions that significantly improved this manuscript.

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• Figure 1

Overview of proposed method, mainly including an ESPN and an ASDN, both of which share convolutional layers for feature learning. In ASDN, “RoI" represents the regions of interest and “FC" layer indicates the fully connected layer.

• Figure 2

Ship proposal generation strategy of (a) RPN and (b) ESPN.

• Figure 3

Ship proposals and their contextual regions are both bypassed by the RoI pooling layer. The upper row and the lower row show an example of a ship proposal and its context region, respectively. Both of them are pooled by the RoI pooling operation. Then, the feature maps are concatenated into one fused feature map to improve their representation capability.

• Figure 4

Illustration of image cropping strategy. The white grids are blocks without overlap. The red rectangles represent divided blocks with $50$ pixel overlap. In both cases, blocks without ship targets are discarded and the remaining ones are used for training and testing.

• Figure 5

Performance curves over eight Sentinel-1 images. (a) Recall vs. IoU curve for each method; (b) precision vs. recall curve for each method.

• Figure 6

Ship detection results in offshore area of Sentinel-1 images. (a-1)–(a-4) exhibit the visualization results by using the proposed $\text{Coupled-CNN}\_\text{E}\_\text{A}$ method. (b-1)–(b-4) show the detection results of the CFAR-MS method. The green boxes indicate the correctly detected targets, the red ones indicate false alarms, and the blue ones represent the ground-truth.

• Figure 7

Ship detection results with (a) $\text{Coupled-CNN}\_\text{E}\_\text{A}$ and (b) CFAR-MS for an image block cropped from the wide-swath Sentinel-1 SAR imagery over the Strait of Malacca, Singapore. In both subfigures, two areas (highlighted by the yellow boxes) are enlarged to exhibit a clear visual effect. The green box indicates the correctly detected targets, the red indicates false alarms, and the blue represents the ground-truth.

• Figure 8

Performance curves over four GF-3 images. (a) Recall vs. IoU curve for each method; (b) precision vs. recall curve for each method.

• Figure 9

Ship detection results with (a) $\text{Coupled-CNN}\_\text{E}\_\text{A}$ and (b) CFAR-MS for an image block cropped from the GF-3 SAR imagery. In both subfigures, two areas (highlighted by the yellow boxes) are enlarged for a clear visual effect. The green boxes indicate the correctly detected targets, the red ones indicate false alarms, and the blue ones represents the ground-truth.

• Figure 10

Ship detection results in offshore areas of GF-3 image. (a-1)–(a-4) exhibit the visualization results by using the proposed $\text{Coupled-CNN}\_\text{E}\_\text{A}$ method. (b-1)–(b-4) shows the detection result of the CFAR-MS method. The green boxes indicate the correctly detected targets, the red ones indicate false alarms, and the blue ones represent the ground-truth.

• Table 1   Parameter configurations for three proposal branches in ESPN
 Layer name $\text{Conv}4\_3$ $\text{Conv}5\_3$ $\text{Conv}6\_1$ Filter size (pixel) $3\times3$ $5\times5$ $7\times7$ $3\times3$ $5\times5$ $7\times7$ $3\times3$ $5\times5$ $7\times7$ Anchor height (pixel) $10$ $16$ $22$ $28$ $34$ $40$ $46$ $52$ $58$ Height-to-width ratio 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1 1:2,1:1,2:1
• Table 2   Detailed structure, number of parameters, and MAC for each layer when using $\text{Coupled-CNN}\_\text{E}\_\text{A}$ method with $1024\times768$ input
 Part Name Type Stride Output #Params. MAC Shared CNN layers Conv1_1 $3\times3$ convolution 1 $1024\times768\times64$ $1.9$k $1359.0$M Conv1_2 $3\times3$ convolution 1 $1024\times768\times64$ $41.0$k $28991.0$M Pool1 $2\times2$ max pooling 2 $512\times384\times64$ Conv2_1 $3\times3$ convolution 1 $512\times384\times128$ $81.9$k $14495.5$M Conv2_2 $3\times3$ convolution 1 $512\times384\times128$ $163.8$k $28991.0$M Pool2 $2\times2$ max pooling 2 $256\times192\times128$ Conv3_1 $3\times3$ convolution 1 $256\times192\times256$ $327.7$k $14495.5$M Conv3_2 $3\times3$ convolution 1 $256\times192\times256$ $655.4$k $28991.0$M Conv3_3 $3\times3$ convolution 1 $256\times192\times256$ $655.4$k $28991.0$M Pool3 $2\times2$ max pooling 2 $128\times96\times256$ Conv4_1 $3\times3$ convolution 1 $128\times96\times512$ $1310.7$k $14495.5$M Conv4_2 $3\times3$ convolution 1 $128\times96\times512$ $2621.4$k $28991.0$M Conv4_3 $3\times3$ convolution 1 $128\times96\times512$ $2621.4$k $28991.0$M Pool4 $2\times2$ max pooling 2 $64\times48\times512$ Conv5_1 $3\times3$ convolution 1 $64\times48\times512$ $2621.4$k $7247.8$M Conv5_2 $3\times3$ convolution 1 $64\times48\times512$ $2621.4$k $7247.8$M Conv5_3 $3\times3$ convolution 1 $64\times48\times512$ $2621.4$k $7247.8$M Pool5 $2\times2$ max pooling 2 $32\times24\times512$ Conv6_1 $3\times3$ convolution 1 $32\times24\times512$ $2621.4$k $18119$M ESPN SPN4_3 $3\times3$ convolution 1 $128\times96\times6$ $30.7$k $339.7$M SPN4_5 $5\times5$ convolution 1 $128\times96\times6$ $79.9$k $943.7$M SPN4_7 $7\times7$ convolution 1 $128\times96\times6$ $153.6$k $1849.7$M SPN5_3 $3\times3$ convolution 1 $64\times48\times6$ $30.7$k $84.9$M SPN5_5 $5\times5$ convolution 1 $64\times48\times6$ $79.9$k $235.9$M SPN5_7 $7\times7$ convolution 1 $64\times48\times6$ $153.6$k $462.4$M SPN6_3 $3\times3$ convolution 1 $32\times24\times6$ $30.7$k $21.2$M SPN6_5 $5\times5$ convolution 1 $32\times24\times6$ $79.9$k $59.0$M SPN6_7 $7\times7$ convolution 1 $32\times24\times6$ $153.6$k $115.6$M ASDN RoIPooling1 $7\times7$ RoI pooling $7\times7\times512$ RoIPooling2 $7\times7$ RoI pooling $7\times7\times512$ RoI_concat $3\times3$ convolution $5\times5\times512$ $5242.9$k $118.0$M FC FC 4096 $52428.8$k $52.4$M FC_cls FC 2 $8.2$k $32.8$K FC_bbr FC 8 $32.8$k $32.8$K Total $\textbf{75.66M}$ $\textbf{256.77B}$
• Table 3   Comparison result of the number of parameters and the MAC for each part when using $\text{Coupled-CNN}\_\text{E}\_\text{A}$ method with $1024\times768$ input
 #Params. MAC Shared CNN layers ESPN ASDN Total Shared CNN layers ESPN ASDN Total $\text{Coupled-CNN}\_\text{E}\_\text{A}$ $\underline{18966.2{\rm~k}}$ $792.6$k $57712.7$k $\textbf{75.66M}$ $\underline{258653.9{\rm~M}}$ $4112.1$M $170.4$M $\textbf{256.77B}$
• Table 4   Information of Sentinel-1 imagery used in this study
 Satellite Imaging mode Band Polarization Product type Resolution Pixel spacing Average size per image (rg$\times$az) ($\mathrm{m}$) (rg$\times$az) ($\mathrm{m}$) (rg$\times$az) ($\mathrm{pixel}$) Sentinel-1 IW C VH GRD $20\times22$ $10~\times~10$ $25000\times18000$
• Table 5   Information of GF-3 imagery used in this study
 Satellite Imaging mode Band Polarization Pixel spacing Average size per image (rg$\times$az) ($\mathrm{m}$) (rg$\times$az) ($\mathrm{pixel}$) GF-3 NSC C VH $20\times5$ $8800\times21000$
• Table 6   Performance comparison of different methods for the Sentinel-1 data set$^{\rm~a)}$
Methods  Ground truth
 True positive
 False positive
Recall Precision  Average precision
$F1$ score  Average time (s) per image
CFAR-MS $6814$ $2710$ $751$$0.3977 0.7830 0.3123 0.5275 2550 FRCN 6814 4544 845$$0.6669$ $0.8432$ $0.5812$ $0.7447$ $105$
$\text{Coupled-CNN\_E}$ $6814$ $4843$ $560$$\underline{0.7107} \underline{0.8964} \underline{0.6519} \underline{0.7928} 113 \text{Coupled-CNN\_A} 6814 4656 823 0.6833 0.8498 0.6069 0.7575 108 \text{Coupled-CNN\_E}\_\text{A}$$6814$ $5260$ $570$$\textbf{0.7719}$ $\textbf{0.9022}$ $\textbf{0.7151}$ $\textbf{0.8320}$ $115$

a) The bold numbers denote the optimal values in each column. The underlined numbers denote the suboptimal

• Table 7   Performance comparison of different methods on GF-3 data set$^{\rm~a)}$
Methods  Ground truth
 True positive
 False positive
Recall Precision  Average precision
$F1$ score  Average time (s) per image
$\text{CFAR-MS}$ $1757$ $981$ $316$ $0.5582$ $0.7562$ $0.4832$ $0.6423$ $1630$
$\text{FRCN}$ $1757$ $1179$ $373$ $0.6710$ $0.7597$ $0.5772$ $0.7126$ $85$
$\text{Coupled-CNN\_E}$ $1757$ $1210$ $364$ $\underline{0.7433}$ $\underline{0.7906}$ $\underline{0.6784}$ $\underline{0.7662}$ $86$
$\text{Coupled-CNN\_A}$ $1757$ $1306$ $346$ $0.6887$ $0.7687$ $0.5997$ $0.7265$ $89$
$\text{Coupled-CNN\_E}\_\text{A}$ $1757$ $1324$ $252$ $\textbf{0.7536}$ $\textbf{0.8401}$ $\textbf{0.6865}$ $\textbf{0.7945}$ $90$

a) The bold numbers denote the optimal values in each column. The underlined numbers denote the suboptimal

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