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Aiming to address the problems of arbitrary orientations, large aspect ratios, and dense arrangements in ship detection, an arbitrary-oriented ship detection method based on RetinaNet is proposed. Our proposed method includes a rotated RetinaNet, a refined network, a feature alignment module, and an improved loss function. First, the rotated RetinaNet achieves rotation detection by using a feature pyramid network, rotated anchors, the skew intersection-over-union (IoU), and skew non-maximum suppression (NMS). Then, the refined network and feature alignment module are introduced to achieve better detection accuracy. Finally, to address the boundary discontinuity, the loss function is improved by introducing the IoU constant factor. Considering the problems with the HRSC2016 dataset, we establish a new dataset with more accurate labels and more images and object samples. Through an ablation study, we thoroughly analyze the validity of the proposed rotated RetinaNet, feature alignment module, and improved loss function. The experimental results show that our method is superior to other state-of-the-art methods.

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Ship detection in synthetic aperture radar (SAR) images has been widely applied in maritime management and surveillance. However, some issues still exist in SAR ship detection due to the complex surroundings, scattering interferences, and diversity of the scales. To address these issues, an improved anchor-free method based on FCOS + ATSS is proposed for ship detection in SAR images. First, FCOS + ATSS is applied as the baseline to detect ships pixel by pixel, which can eliminate the effect of anchors and avoid missing detections. Then, an improved residual module (IRM) and a deformable convolution (Dconv) are embedded into the feature extraction network (FEN) to improve accuracy. Next, a joint representation of the classification score and localization quality is used to address the inconsistent classification and localization of the FCOS + ATSS network. Finally, the detection head is redesigned to improve positioning performance. Experimental simulation results show that the proposed method achieves 68.5% average precision (AP), which outperforms other methods, such as single shot multibox detector (SSD), faster region CNN (Faster R-CNN), RetinaNet, representative points (RepPoints), and FoveaBox. In addition, the proposed method achieves 60.8 frames per second (FPS), which meets the real-time requirement.

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Ship detection in large-scale synthetic aperture radar (SAR) images has achieved breakthroughs as a result of the improvement of SAR imaging technology. However, there still exist some issues due to the scattering interference, sparsity of ships, and dim and small ships. To address these issues, an anchor-free method is proposed for dim and small ship detection in large-scale SAR images. First, fully convolutional one-stage object detection (FCOS) as the baseline is applied to detecting ships pixel by pixel, which can eliminate the effect of anchors and avoid the missing detection of small ships. Then, considering the particularity of SAR ships, the sample definition is redesigned based on the statistical characteristics of ships. Next, the feature extraction is redesigned to improve the feature representation for dim and small ships. Finally, the classification and regression are redesigned by introducing an improved focal loss and regression refinement with complete intersection over union (CIoU) loss. Experimental simulation results show that the proposed R-FCOS method can detect dim and small ships in large-scale SAR images with higher accuracy compared with other methods.