## 1 Introduction
In [10] authors present ASV/AUV coordination to achieve docking based on a visual estimate, while in [11] visual-based pose estimation techniques are used to dock a work-class remotely operated vehicle (ROV) with both static and dynamic docking stations. Mechanical design that complements the docking function or even a specific vehicle shape is necessary in real working conditions. Due to standard AUV hull shapes, this often means a funnel or cone-shaped system [12]. In [13], a system combining existing static cabled ocean observatory network structures and AUVs is described, in which the long-term mission capabilities of the AUV are enhanced. The authors of [14] describe an underwater wireless charging system consisting of a station which vehicles navigate to recharge their batteries and extend their data gathering capabilities. In [15] the authors propose a three-phase wireless charging system that could be used in a field-deployable charging station capable of rapid, efficient, and convenient AUV recharging during or in between missions, while taking care not to affect the instrumentation in the AUV itself. In the case of the subCULTron swarm, the \"docking station\" is an ASV--the aPad--bearing four docking mechanisms, and it navigates to and docks the AUV--the aMussel--since the agent requiring charge has limited movement capabilities beyond the vertical. A funnel-like shape is used on the aMussel top cap, while the aPad dock consists of two prongs and a servo-powered latch. The subCULTron swarm uses non-contact power transmission via inductive coil coupling, while bespoke mechanical design ensures coil alignment for maximizing efficiency. Although the main purpose of docking stations is battery recharge, also relevant is data and mission configuration download and upload without a need for vehicle recovery [16]. In the system described in [17], two modes of communication are employed: acoustic communication for long ranges and optical for short ranges. Both in this monitoring system and the one given in [18], fixed underwater sensors collect data about the environment while AUVs act as communication relays periodically delivering the data to the surface. In contrast to these approaches, the subCULTron swarm is fully reconfigurable, and the aPads, taking on the role of communication relays, remain on the surface ready to transfer information at all times. While underwater, aMussels communicate with them using acoustic modems, and when surfaced and docked, the aMussels upload collected data logs to the aPads using WiFi. As opposed to more traditional docking/charging stations and more similar to approaches used in the subCULTron swarm, Ref. [19] proposes an autonomous mobile charger robot which acts within the swarm, monitors the other agents' location and battery level, and seeks out low-powered agents to respond to their charging requests. Robots in need of charging approach the charging robot and dock to one of its several charging stations. It also contains algorithms developed for cooperation between robots in need of charging and the robot carrying the charging stations. In [20] specialized agents serve as the starting points of energy supply chains within a robotic swarm. The algorithm proposed and implemented here is an image-based visual servoing variant (IBVS) with end-point closed-loop control, since the visual sensor moves together with the effector towards the target, while itself integrated into the control loop of the system. The information about various offsets within the image acquired from the image processing segments of the algorithm is used directly in the control algorithm to calculate reference signals, as opposed to being used as a basis for estimating the real-world 3D space position or pose of the target. This is a classical, well-established and well-documented approach, as seen in [21; 22; 23].