# Monocular Vision Guidance for Unmanned Surface Vehicle Recovery
## 4 Conclusions  
The recovery and guidance of USVs is a crucial technology for their operation. In this regard, the accurate localization of USVs within a short range where GPS positioning is not precise becomes a challenge. To address this issue, this paper proposes a monocular vision-based localization method to guide USVs back to the mother ship. The proposed method achieves localization and guidance by recognizing artificial markers installed on the mother ship based on their structural and feature relationships. Firstly, in terms of artificial marker recognition, this paper applies a color segmentation method to obtain the position of the marker in the image. Then, it uses least squares fitting to approximate the elliptical shape of the marker, allowing for the extraction of the center coordinate. Furthermore, to address the misalignment caused by perspective distortion, a center point calibration method based on circular ring structure and cross-ratio invariance has been proposed. This method obtains the pixel coordinates of the unique center points of each circle on the marker in the imaging plane. Finally, the distance and angle are obtained by utilizing the coordinate transformation relationship in the PNP method.  
The results demonstrate that the proposed method can effectively recognize artificial markers and achieve accurate distance and angle measurements. Additionally, the center-point correction method presented in this paper successfully resolves the issue of misalignment between center points. The evaluation indicates that the distance error within 10 m is within 40 mm, and the angle error within \(\pm\)60\({}^{\circ}\) is within 5\({}^{\circ}\). These results validate the effectiveness of the proposed method, which can achieve precise localization and guide the USV back to the mother ship.  
The proposed method in this paper aims to achieve precise recovery guidance for short-range targets, while for long-range target guidance, it is necessary to complement this approach with other methods such as GPS positioning. In addition, this paper designs a specific artificial marker to locate and guide unmanned surface vehicles (USVs) back to the mother ship. In the future, other markers on the mother ship can be utilized for visual localization. For example, the recovery hatch on the mother ship can be used to guide the USV by leveraging its structural dimensions and features.  
Conceptualization, Z.S., Z.L. and Q.X.; methodology, Z.S. and Q.X.; visualization, Z.S. and Q.X.; investigation, Z.L. and Q.X.; experiments, Z.S. and Q.X.; writing--original draft preparation, Z.S.; validation, Z.S.; project administration, Z.L.; funding acquisition, Z.L. and Q.W. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the Key Research and Development Program of Jiangsu Province, grant number BE202062; the Zhangjiagang Science and Technology Planning Project, grant number ZKYY2314; the Doctoral Scientific Research Start-up Fund Project of Nantong Institute of Technology, grant number 2023 XK(B)02.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Actual Angle**} & \multicolumn{3}{c}{**Average Angle Error**} \\  & & \multicolumn{2}{c}{(\({}^{\circ}\))} \\ \cline{2-4}  & **Distance = 1000 mm** & **Distance = 2000 mm** & **Distance = 3000 mm** \\ \hline \(-60\) & \(-2\) & \(2\) & \(-2\) \\ \(-40\) & \(2\) & \(2\) & \(2\) \\ \(-20\) & \(2\) & \(-3\) & \(4\) \\ \(0\) & \(0\) & \(1\) & \(-1\) \\ \(20\) & \(1\) & \(2\) & \(2\) \\ \(40\) & \(-2\)