I. Introduction
Recently, the deployment of autonomous underwater vehicles (AUVs) has been receiving increased attention in distinct research activities [1], for instance in monitoring of deep-water rivers for environmental reasons, interaction with underwater sensor networks, ocean floor mapping for exploration of minerals, oil, gas, etc. Nevertheless, to achieve long-term and truly autonomous underwater operations, the AUV should be able to successfully recharge its batteries at an underwater docking infrastructure by means of wireless powering techniques, while avoiding any human intervention. As such, besides the demanding operational requirements for autonomous navigation, also a powerful transmitter system must be present at the docking station, whereas the AUV must comprise an efficient rectifier system and circuitry to regulate the charge rate of the batteries. Each side of the powering system should have a coupling coil, preferably the closest possible to maximize the magnetic coupling during the charging process [2], [3]. The coils need to be in a sealed housing to avoid changes in their electrical parameters along time, however this poses some limits in the minimum distance possible, which depends on the physical structures of the AUV and docking station [4]. Besides the relative distance, another inherent challenge is the proper coil alignment, which influences the impedance seen from the transmitter [5]. For that reason the system resonance can be affected, thereby having an impact on the power delivery as well as on the overall efficiency of the powering system [6].