# Unmanned Surface Vehicle Using a Leader-Follower Swarm Control Algorithm
## 1 Introduction  
The marine environment occupies about 71% of the Earth's surface and has various resources such as oil, natural gas, rare earth elements, and aquatic resources [1]. As the importance of securing such resources has emerged, various countries are developing offshore platforms for seabed surveys and maintenance of submarine structures such as gas pipelines and submarine cables [2, 3]. Until now, most of these tasks, such as seafloor surveying and submarine structure maintenance and repair, have been carried out in a way that requires divers to dive directly to and work at the site [4]. However, even if a human wears professional equipment, continuously working for more than an hour at a depth of 30 m or more is difficult. Further, because professional skills are required, there is a disadvantage of high cost if a human is working in deep marine locations. Therefore, several studies on the investigation of the seabed and the inspection of damaged areas of subsea structures using acoustic image sensors such as sonar and multi-beam in unmanned surface vehicles (USVs) are in progress [5, 6].  
The multi-object USV system has a wider range of applications than the single-unit USV system for missions such as exploration and environmental investigation. In addition,the use of several objects simultaneously provides various advantages such as improved mission performance, increased efficiency, and energy saving. Thus, studies related to the multi-object USV system to which the swarm control algorithm is applied are receiving a lot of attention [7; 8; 9; 10].  
There are three major swarm control methods: behavior-based swarm control, virtual structure swarm control, and leader-follower swarm control [11]. Although each swarm control method has different advantages and disadvantages, the double leader-follower swarm control is easy to understand and express mathematically. Further, theoretically, the number of followers can be increased without limit. Thus, a multi-object USV system is advantageous to construct [12; 13; 14]. Communication methods used in leader-follower swarm control include centralization and decentralization methods. In the centralization method, the implementation of the swarm control algorithm is easier than in the decentralization method. However, in the decentralization method, information sharing time is shorter than in the centralization method and it is difficult to build infrastructure such as server rooms and use them in environments where it is impossible to fix the location of the land operation console. Therefore, the decentralization method is more suitable for the marine mission environment [8].  
In this study, a multi-object USV system was constructed using a leader-follower swarm control algorithm using a distributed method that allows the movement of a land operation console according to the characteristics of the marine environment. To this end, a land operation console and an operation program were developed. In addition, to verify the performance of the leader-follower swarm control algorithm proposed herein, a USV capable of three-degree-of-freedom (DOF) motion was designed and developed. To enable smooth route control in a marine environment with many disturbances, a controller was designed considering cross-track error (CTE), and its performance was verified. In addition, a sensor verification experiment was conducted to confirm the reliability of the sensor used in the study. Finally, the designed leader-follower swarm control algorithm was installed in the USV, and its performance was verified by experimenting in an actual sea area.  
However, despite the importance of developing such a multi-object system, the level of development of multi-object systems is low [15]. Therefore, in this study, USVs equipped with a leader-follower swarm control algorithm were developed, and their performance was verified.