## 5 Conclusions  
Tests in real conditions are always the best way to verify the correctness and effectiveness of designed systems, especially flying systems such as UAVs. Assumptions made during the design process are usually reformulated, and valuable knowledge about the tested object is collected. In the case of the tested tethered hexacopter, the flights at sea during a standard \"Wolin\" ferry voyage made it possible to identify some exciting issues. The most important issue concerned the magnetometer readings on the ferry's landing pad. Varying drift from the real magnetic north, which decreased with a flight altitude increment, was observed, especially on the plots of the magnetometer z-axis readings just after take-off and just before landing (Figures 15 and 20, respectively). Moreover, the z-axis readings of the magnetometer were very noisy in comparison to the x- and y-axis readings. Of course, the problem's source lay in the material from which the ferry's structure was made. Steel is a ferromagnetic material, which interferes with the weak magnetic field of the Earth. The magnetometer was the most crucial device among the hexacopter onboard equipment, because it was the only source of heading that was required to track the landing pad's position while the ferry was cruising. At the low cruise speed of the ferry, the GNSS heading readings were unreliable. During the tests, a solution to this problem was found, which included magnetometer recalibration before each take-off. Thus, our conclusion was that the automated process of flight management, especially if it has to be carried out by  
Figure 19: Tethered hexacopter linear velocities and ground speed during the altitude following test. Figure 20: Tethered hexacopter magnetometer readings and course angle during the altitude following test. the computer of an autonomous vessel, must include a flight preparation phase responsible for magnetometer tests and recalibration. Another important issue arose from air vortices around the landing pad, at an altitude of up to 5 m. These were generated by the wind's air mass reflected from the vessel's side and other equipment located around the landing pad and by the airflow related to the vessel's movement. In the case of the ferry, its landing pad was almost at the top of the ferry's body, just behind the captain's bridge, and these were ideal conditions for such vortices. Because of these vortices, the take-off phase was extremely unstable up to 5 m above the surface of the landing pad. There was no possibility of mounting wind deflectors around the landing pad, so the only way to reduce the crash risk was to perform take-off with the maximum vertical speed to jump through the vortices' area. During flights performed in real maritime conditions, no significant impact of the power cord's aerodynamic drag was observed. The unwinder always provided the proper tension and required length of the cord. The only issue was related to the winding speed while the hexacopter was in freefall due to a motor fault. The winding speed was too low because it was tuned to the normal landing phase. Therefore, an emergency procedure enabling the maximum speed of the unwinder motor in the case of a fault in one of the hexacopter's motors should be provided and launched by the flight management computer. Thus, during the tests in real maritime conditions, the issues that could be critical in the implementation phase of the hexacopter were identified. The gathered experience and knowledge will be utilized during future projects, aiming to design UAV systems for unmanned vessels. Preparing and carrying out the tests on the \"Wolin\" ferry's landing pad was highly risky, because the landing pad and the ferry's deck were not prepared for flying UAVs. Any freefall of the tethered hexacopter was dangerous to the researchers, ferry crew, and navigation devices on the ferry deck. Thus, successful tests without any crashes must be considered a great achievement. Conceptualization, L.A., M.C., A.W., S.R., D.O., A.B. and C.K.