The unmanned surface vessel (USV) trajectory with spatial and temporal information plays an important role in its positioning and navigation. Unlike traditional trajectory reconstruction methods, this paper proposes a novel method based on the automatic identification system (AIS) for USV. Aside from the AIS data applied for restoring the USV’s trajectory, the proposed method considers the constraints of the vessel’s navigation state, maneuvering factors and time stamps. This method consists of three steps: AIS data restoration, Empirical Mode Decomposition (EMD) denoising and Fermat’s spiral fitting. Firstly, AIS data restoration is applied to eliminate abnormal data. Next, the EMD noise reduction algorithm is used to reduce the jitters and interference of Speed over Ground (SOG) and Course over Ground (COG) components, and the denoised latitude and longitude positions are calculated using the kinematics model of the vessel. Finally, a curve fitting process based on Fermat’s spiral is employed to reconstruct a smoother trajectory. Several experiments illustrate that the low-cost AIS equipment is compatible with the navigation system of the USV. The reference trajectory is determined by Global Position System (GPS) and Inertial Measurement Unit (IMU) modules in USV. Compared to conventional methods, the residual errors of the proposed method is smaller. The results show that the novel trajectory reconstruction method is effective and can be applied to USV’s positioning and navigation.

In order to improve the accuracy of the moving object detection algorithm, a combination of improved three frame difference and background subtraction based on Gaussian mixture model algorithm is proposed in this paper. On the basis of the traditional three frame difference method, the "XOR" and "OR" operations are used instead of the "AND" operation to extract the foreground to solve the hole problem in the extraction of the foreground. But it will cause problems such as elongated target, blurred outline and noise. The background subtraction of Gaussian mixture model extracts the whole foreground but will detect the shadow. We consider combining two foregrounds by using the "AND" operation to obtain a comprehensive foreground image. The experimental results show that the algorithm can extract moving objects quickly, accurately and completely.

This brief addresses the dynamic positioning (DP) control problem of surface vessels with input saturation and external disturbances, and a simple DP control law is proposed. First, a saturation handling model and a dynamic compensation system are designed to deal with input saturation. Then, an exponentially convergent disturbance observer is constructed to approximate the external disturbances. Moreover, to guarantee the asymptotic stability of the DP closed-loop system and reduce the design complexity of the DP control law, a simple auxiliary control law is designed by combining the hyperbolic tangent function. Notably, combined with LaSalle’s invariance principle, a DP system stability analysis strategy is constructed, which ensures that the DP system can be asymptotically stable, even if the derivative of the scalar function proving the stability of the DP system is semi-negative definite. Finally, the numerical simulation results demonstrate the effectiveness of the proposed DP control law.

In this paper, a variable gain prescribed performance control law is proposed for dynamic positioning (DP) of ships with positioning error constraints, input saturation and unknown external disturbances. The error performance index functions are designed to preset the prescribed performance bounds and the error mapping functions are constructed to incorporate the prescribed performance bounds into the DP control design. The variable gain technique is used to limit the output amplitude of the control law to avoid input saturation of the system by dynamically adjusting the control gain of the DP control law according to the positioning errors, and the error mapping function replaces the positioning error as a recursive sliding-mode surface to realize the prescribed performance control of the system and guarantee the stability of the closed-loop system with variable control gains. It has been proved that the proposed DP control law can make the uniformly ultimately boundedness of all signals in the DP closed-loop control system. The numerical simulation results illustrate that the proposed control law can make the ship’s position and heading maintain at the desired value with positioning error constraints, enhance the non-fragility of the DP control law to the perturbation of system’s parameters and improve the system’s rejection ability to external disturbances.