# MViDO: A High Performance Monocular Vision-Based System for Docking A Hovering AUV
## 1 Introduction
### Related Works
#### 1.1.1 Vision-Based Approaches to Autonomous Dock An Auv  
In recent years, the use of artificial vision and identifiable markers for pose estimation in an underwater environment, has become a subject of interest for the scientific community working in subaquatic robotics. Recent examples are the work of [4] where it is proposed a solution using active light-marker localization for docking an AUV. For that, the authors use a module based on a monocular system for detect a set of active light markers. Since the light-markers are placed in known positions of the docking station the authors in [4] can estimate the pose between the docking station and the camera. In order to detect the light-markers, the authors in [4] use a blinking pattern to allow the markers to be discerned. Visual markers are one of the most used systems to help to guide the AUV to the dock, and other approaches have been proposed by [5]. In [5] the authors propose a docking method for a hovering AUV at a seafloor station. The proposed method is based on visual positioning and makes use of several light sources located on the docking station to visual guide the AUV. The proposed method is divided into two stages by distance, a first stage in which the vehicle is visual guided based on the light sources and a final stage in which the AUV just keeps its attitude and moves forward into a corn-shaped station. This method is not applicable to our case because the entrance of our docking station is not corn-shaped. Note that in the method proposed in [5], the authors suspend vision-based guidance when the AUV is at the dock entrance since the light sources are too close for the AUV to see. The solution we propose in our work does not need to suspend guidance based on vision, allowing a more aware docking of the docking maneuver. The use of a single camera on-board of an AUV and multiple light markers mounted around the entrance of a docking station in order to guide visually an AUV is also a solution presented in [6]. In this type of approach, the estimation of the relative pose required for autonomous docking is done using the markers geometry. As in the case of the work mentioned above ([5]), also in the work presented in [6] there was a position where the lights on the dock were outside of the camera viewing range when the AUV was close to the dock, and the vision-guidance is invalid in this area. In the literature, we also find works that are based on solutions that use passive targets to signal the dock. In the case of [7] the passive targets are docking poles. Passive markers in an underwater environment limit the process both in terms of minimum distance to the target and visibility conditions. In the solution we propose, we tried to circumvent these limitationsby building a hybrid target (active and passive). In other works, such as [8], the authors proposed a method to estimate the relative pose of a docking station using a single camera. They assume that the dock is circular so the proposed method is for a 5-DOF pose estimation neglecting the roll effects because all the markers are physically identical, and there is no way to distinguish them. Furthermore not considering the roll effect, the proposed method in [8] presents ambiguities in angles estimation.