## 1 Introduction
Footnote 1: saipem.com/en/media/news/2019-07-31/sonsub-hydrone-r-launched-water-first-dive-saipems-underwater-intervention  
Given the aforementioned trends, it is clear that there is a need to integrate into AUVs the capability to home into and be able to dock into a stationary or moving docking station, be it underwater such as submarines, a station on the seafloor, or on the surface such as a surface vehicle. The docking maneuver is usually performed in two phases: The first phase is a long-range homing that involves the identification of the homing signal from the docking system by the AUV to proceed in the general direction of the homing signal. As the AUV approaches closer to the docking station, the terminal docking phase is performed where the AUV proceeds further towards the docking system and attains the dock position. Recent trends denote that this homing and approach maneuver is performed largely by ad hoc controllers. In [23], the authors discuss a docking system designed for REMUS AUV that allows acoustic homing to the docking station. The work in [24] presents a docking assessment algorithm for assessing the feasibility of the desired path for docking of a torpedo-shaped AUV using depth tracking via neural network-generated paths, docking feasibility region analysis to provide a possibility region of successful docking, and docking success probability evaluation based on the probability of sensor data. A vision-based docking system [25] was developed for Starbug AUV where a target is detected using a fast pole detection algorithm based on Haar rectangular features and image rotations. In [26], the authors discuss an optical quadrant tracker method for terminal guidance of underwater vehicles to control the horizontal and vertical movements of the vehicle by appropriate proportional fin commands, thereby steering the vehicle. A vision-based localization for resident underwater vehicles, consisting of beacon lights attached to a tether management system, creating a unique light marker fixed at the target location for pose estimation, is discussed in [27]. Docking of intervention AUVs using sonar and video image processing algorithms accompanied with navigational data obtained from conventional sensors is explained in [28]. The work in [29] discusses the concept and testing of a bio-inspired electromagnetic homing guidance system developed for AUVs at close range to the dock, using a differential magnetometer system fitted with magnetic field sensors at the forward and aft of the AUV for detecting the dipole magnetic field of fixed frequency and power generated from the dock. Furthermore, just like ASVs, AUVs needs to be docked to surface docking stations such as piers, mother ships, etc. for system inspection or safe retrieval. Autonomous docking of an AUV to surface docking stations can be assumed as a special case of autonomous docking of ASV. In [30], the authors discuss docking of an ASV using linear- and circular-based vector field guidance methods to overcome the motion constraints in performing docking to certain regions where it is difficult to dock without collision with structures such as pier walls. A combination of circular and linear vector fields are suggested to drive the vehicle out from the collision region. However, if the vehicle starts too close to the structures, there is a chance of collision as the vehicle might perform some tight maneuvers. Coming to moving docking stations, the authors of [31] integrate no-fly zones with virtual obstacles into the navigation function for avoiding collisions while trying to dock from the stern direction into a docking station hoisted on top of a submarine. A method of recovery for AUVs using a visual docking approach is discussed in [32] where the terminal guidance is performed via two cameras that help in estimating the position of a moving docking station. The position and heading of the AUV is corrected before controlling its vertical movement to finally dock to the docking platform.