Abstract
The detection of small fishing ships is very important for maritime fishery supervision. However, it is difficult to detect small ships using synthetic aperture radar (SAR), due to the weak target scattering and very small number of pixels. Polarimetric synthetic aperture radar (PolSAR) has been widely used in maritime ship detection due to its abundant target scattering information. In the present paper, a new ship detector, named ΛM, is developed based on the analysis of polarization scattering differences between ship and sea, then combined with the two-parameter constant false alarm rate method (TP-CFAR) algorithm to conduct ship detection. The goals of the detector construction are to fully consider the ship’s depolarization effect, and further amplify it through sliding window processing. First, the signal-to-clutter ratio (SCR) enhancement performance of ΛM for ships with different lengths ranging from 8 to 230 m under 90 different combinations of windows are analyzed in detail using three set of RADARSAT-2 quad-polarization data, then the appropriate window size is determined. In addition, the SCR enlargement between ΛM and some typical polarization features is compared. Among these, for ships of length greater than 35 m, the average contrast of ΛM is 33.7 dB, which is 20 dB greater than that of the HV channel. For small vessels of length less than 16 m, the average contrast of ΛM is 16 dB higher than that of HV channel on average. Finally, the RADARSAT-2 data including nonmetallic small vessels are used to perform ship detection tests, and the detection ability for conventional and small ships of some classic algorithms are compared and analyzed. For large vessels of length greater than 35 m, the method proposed in this paper is able to obtain a superior detection result, maintain the ship contour well, and suppress false alarms caused by the cross side lobe in the SAR image. For small vessels of length less than 16 m, the method proposed in this paper can reduce the number of missed targets, while also obtaining superior detection results, especially for small nonmetallic vessels.
Highlights
The monitoring of ships plays a significant role in shipping safety, fishery monitoring, maritime law enforcement, and combating illegal smuggling
In the research on synthetic aperture radar (SAR) small ship detection, Arnaud proposed to detect small ship targets by using interferometric phase coherent graph [20], while Tello et al proposed a method based on wavelet transform [21], and Ouchi et al developed a method based on sub-look correlation [22]
Gao proposed a target detection method using polarization notch filter for constant false alarm rate (CFAR) detection in heterogeneous sea clutter, which was further extended to the two-channel Along Track Interferometric SAR (ATI-SAR) mode
Summary
The monitoring of ships plays a significant role in shipping safety, fishery monitoring, maritime law enforcement, and combating illegal smuggling. In the research on SAR small ship detection, Arnaud proposed to detect small ship targets by using interferometric phase coherent graph [20], while Tello et al proposed a method based on wavelet transform [21], and Ouchi et al developed a method based on sub-look correlation [22] Both the wavelet and sub-look correlation methods require time-frequency decomposition, which will reduce the resolution of the image and weaken the small ship targets that only possess a small number of pixels, and render the detection more difficult. Gao proposed a target detection method using polarization notch filter for CFAR detection in heterogeneous sea clutter, which was further extended to the two-channel Along Track Interferometric SAR (ATI-SAR) mode. These methods were able to detect smaller ship targets [23,24].
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