# MViDO: A High Performance Monocular Vision-Based System for Docking A Hovering AUV
## 8 Conclusions  
In our conceptualization and development of a system to aid autonomous docking of a hovering AUV, It was intended to realize how far we can get by using only a single camera and a dedicated processing unit small enough to be accommodated onboard the MARES. In this sense the results showed that this minimal system is a very capable system. We chose to explore the maximum capabilities of data provided by the image sensor in an environment where visibility is very low. The solutions found to work around this problem yielded very interesting results. Since no target was found in the literature that would meet our needs, namely that it met the following requirements:  
* versatility (active in certain situations and passive in others)
* be easily identifiable even in low visibility situations
* allow to be seen from different points of view. A 3D target was designed and built that meets the proposed requirements. The 3D hybrid target maximizes the observability of the markers from the point of view of the AUV camera and proved to be a successful solution. Since an algorithm for estimating the relative attitude that fit our needs was not found in the literature, namely, a low computational cost algorithm, an algorithm was developed from the ground up. Together with the marker detection algorithm, the developed algorithm occupies 83 ms of processing time, which satisfies our requirements. Not having been found in the literature a solution for situations of partial occlusion of the target and the presence of outliers (which affect the performance of the pose estimator), an innovative solution was developed that presented very satisfactory results. One of the challenges of this work was the persistence of tracking the target even in the case of partial or momentary occlusions of the markers, this challenge was met with good results as a consequence of the implementation of the filters that were designed. The particle filter-based solution also allowed a significant reduction in false detections, which improved pose estimator performance. Regarding the need for a guidance law for docking the AUV, none of the works found in the literature takes into account our concern to never lose sight of the target until the AUV is fully anchored in the cradle. For that reason, it was necessary to find a solution to this problem. With the characterization of the system, it was possible to gain knowledge of the system as a whole, which allowed us to advance in choosing an approach to the problem of how to ensure that during docking maneuvers the vehicle is guided to the dock station without losing sight of the target markers. For this purpose, a guidance law was defined. Such law can ensure a much faster convergence in the horizontal mode to zero that of vertical to zero as intended. Our approach is pretty general and can be applied to any hovering vehicle. Conceptualization, A.B.F. and A.C.M. ; methodology, A.B.F. and A.C.M. ; software, A.B.F. ; validation, A.C.M. ; formal analysis, A.B.F. and A.C.M. ; investigation, A.B.F. ; resources, A.C.M. ; data curation, A.B.F. ; writing-original draft preparation, A.B.F. ; writing-review and editing, A.B.F. and A.C.M. ; funding acquisition, A.C.M. All authors have read and agreed to the published version of the manuscript. [PERSON] acknowledges the support of the Portuguese Foundation for Science and Technology (FCT) through grant SFRH/BD/81724/2011, supported by POPH/ESF funding. This work is financed by National Funds through the Portuguese funding agency, FCT--Fundacao para a Ciencia e a Tecnologia, within project UIDB/50014/2020. The authors also acknowledged the support of the INTENDU project (ref. MARTETA/0001/2017) funded by FCT within the scope of MarTERA ERA-NET (EU grant agreement 728053) for providing technical conditions to perform the work. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: