I. Introduction
Unmanned surface vehicles (USVs) have gone through considerable progress worldwide in the past few years with advanced computer, communication and control technologies. Due to their ability to perform high-risk missions in extreme environments, it can be foreseen that USVs will play an increasingly significant role in future marine activities, such as intelligence gathering, ocean resources exploration and territorial sea surveillance. Compared with a single USV, the cooperation of a group of USVs can greatly enhance efficiency, scalability and robustness to complete various marine tasks in complex marine environments. Therefore, as a crucial branch of cooperative control, formation control, aiming to drive a fleet of USVs to keep the desired formation configuration and track the predefined trajectory, has received more and more attention. Considering the intricate evolving ocean environment and the increasingly involuted marine tasks, this brings difficulties and challenges to the formation control problem. Undoubtedly, it is still a promising, intriguing, yet challenging problem with respect to feasible control strategies for USV formation, which is also the motivation of this paper.