Objective   Ship wake laser detection and tracking is a new method for underwater vehicles to detect, identify and track ships. Due to the cavitation effect of the propeller, the breaking of the sea waves and the large amount of air involved in the waterline part of the ship during navigation, the air curtain belt containing a large number of bubbles, namely the ship wake, has formed at the ship's tail, which has very different optical characteristics from the surrounding water environment. Through the study of the laser characteristics of the ship wake, the characteristics of the ship's navigation path and speed in the ocean can be further judged, and then the precise guidance and damage attack of underwater vehicles such as the detection system can be realized. Ship wake is a dynamic changing environment, and the distribution characteristics of ship wake and bubble target characteristics are different in different ships and different environments. To achieve accurate attack on ships, it is necessary to study the distribution characteristics of ship wake and bubble target characteristics. By simulating the changing trend of echo signals under different ship wake conditions, it provides theoretical and simulation support for the ship wake in lakes, ocean and other outfield tracking and detection.

  Methods   The simulation environment is established based on the ship wake distribution characteristics and bubble target characteristics (Fig.1). The Monte Carlo method is used to simulate the multi-scale, wide-number density and large-thickness ship wake bubble group. Through the analysis of the ship wake backscattering echo signal under different conditions, the real state of the detection system under the characteristics of the ship wake target can be effectively simulated. The signal changing trend of the detection system in the search and tracking phase and the echo signal change intensity of different target ships are obtained (Fig.4-7). The experiment of laser tracking and detection of ship wake in lake environment is carried out, and the simulation results are verified (Fig.9-12).

  Results and Discussions   Through the in-depth study of ship wake distribution characteristics and bubble target characteristics, the laser backscattering echo characteristics of different bubble size, bubble number density, bubble layer thickness, and bubble distance are verified by simulation (Tab.1,2). Based on the horizontal/vertical distribution characteristics of bubbles in the wake of ships with different tonnage and speed, the detection ability and tracking method of underwater vehicles at different distances from the wake are studied (Fig.3). The outfield lake test of the laser detection prototype is carried out (Fig.8). The detection device is arranged at different depths to detect the wake of large sand carriers and yachts, which realizes the detection of the wake target under dynamic conditions, and verifies the system detection ability of the underwater vehicle at different distances from the wake (Fig. 9-12).

  Conclusions   Based on the engineering application of laser detection of ship wakes, the manuscript establishes a simulation environment based on the distribution characteristics of ship wakes and target characteristics, and uses the Monte Carlo simulation method to simulate the multi-scale, wide-number density, and large-thickness ship wakes bubble groups. By summarizing and analyzing the backscattering echo signals of ship wakes under corresponding conditions, the real state of the detection system under the characteristics of ship wakes target can be effectively simulated. It is obtained that when the laser detection system is located under the wake, the bubble echo amplitude of large ships slowly rises, the bubble pulse width significantly broadens, and the closer to the ship target, the more obvious the bubble echo changes. When the laser detection system is in the wake, the bubble echo amplitude gradually decreases and the pulse width gradually narrows. When the laser detection system is under the wake and the detection system is in the wake, the signal changes are opposite. The signal changing trend of small ships is basically consistent with that of large ships, but the echo intensity of wake laser detection is lower. An outfield laser backscattering echo experimental system is built to verify that when the detection system is under the wake, the bubble echo signal changes to a slow increase in bubble amplitude and a significant broadening of bubble pulse width. When the detection system is in the wake, the bubble echo amplitude gradually decreases and the pulse width gradually narrows. It can provide support for ship wake detection in practical engineering applications.