Objective With the advantages of large capacity, low cost and high efficiency, vessels have become a crucial mode of transportation for the development of international trade and the global supply chain. Maritime electro-optical monitoring system can provide high-resolution optical images for target detection and identification. Therefore, it has been widely applied for maritime surveillance, traffic navigation and maritime rights protection. However, traditional electro-optical monitoring technologies face several limitations in acquiring far high-resolution optical images at night and low light level conditions, particularly under low visibility conditions such as rain and fog. In addition, they encounter challenges in accurately positioning targets without automatic identification system (AIS). To address these challenges, a method for maritime electro-optical monitoring and positioning at night based on laser range-gated three-dimensional imaging is proposed, which enables achieve high-resolution optical imaging and precise positioning of long-range vessels at night.

Methods The maritime electro-optical monitoring and positioning system (MEOMAPS) in Fig.3 consists of a laser range-gated imaging module, an inertial attitude stabilization control module and a Beidou Navigation Satellite System (BDS) module. The laser range gated imaging module is responsible for capturing near-infrared gated images of targets. The inertial attitude stabilization control module is composed of a stabilization gimbal and an inertial measurement unit (IMU). The stabilization gimbal controls the pointing direction of the laser range-gated imaging module and compensates for image jitter caused by attitude variations of the carrier platform such as ships and airplanes. The IMU monitors the platform’s attitude variations in real time. Additionally, the BDS module acquires satellite signals and provides real-time positional data. In Fig.1 the MEOMAPS works as follows. Firstly, the computer controls the stabilization gimbal to gradually scan the maritime area of interest. When detecting a target, the range-gated imaging module captures the A-frame and B-frame gated images, while the other modules simultaneously acquire the imaging system’s position and attitude data. Secondly, the gated images are reconstructed into depth images based on the range-intensity correlation 3D imaging algorithm (Fig.2). Finally, the target’s latitude and longitude coordinates are calculated based on the great-circle navigation method, which obtains the target’s range information from the depth image, the attitude data from the inertial attitude stabilization control module, and the coordinates of MEOMAPS.

Results and Discussions The proposed method has been validated in experiments. Firstly, the maritime target detection experimental results demonstrate that the laser range-gated imaging module can provide high-contrast, high-resolution two-dimensional and three-dimensional images of targets at night, with an effective detection range exceeding 12 km. Both the spatial resolution and the effective detection range of laser range-gated imaging significantly surpass those of visible-light and long-wave infrared imaging (Fig.4 and Fig.5). Secondly, the maritime target positioning experimental results demonstrate that the proposed method can achieve accurate positioning, with a maximum error of 14.46 m at a distance of 4.2 km (Fig.6 and Tab.1). Finally, the experiment on detection and positioning of yachts without AIS demonstrates that the proposed method can also achieve high-resolution optical images and high-precision positioning of targets without AIS at night. (Fig.7 and Tab.2).

Conclusions In this paper, a method for maritime electro-optical monitoring and positioning at night based on laser range-gated 3D imaging is proposed, which aims to enhance the detection and positioning capabilities of maritime targets at night. The method can suppress aerosol scattering noise based on laser range-gated imaging and acquires high-resolution near-infrared gated images of targets at night. Therefore, it facilitates the identification of textual information. Furthermore, the gated images are reconstructed into depth images by using the triangular range–intensity correlation 3D imaging algorithm. The latitude and longitude of targets can be calculated based on the depth images of targets, the coordinates of MEOMAPS, and the attitude data. The experimental results of monitoring and positioning maritime targets at night demonstrate that the proposed method enables high-quality optical imaging and high-precision positioning of targets under low-light nighttime conditions, with a positioning accuracy of ≤14.46 m at a distance of 4.2 km and a maximum detection range exceeding 12 km.