## 4 Results  
During the ferry cruise described above, a number of hexarotor flight tests were carried out to check the most important components of the UAV system (local control and stabilization system, positioning system, navigation system, landing-pad-following system, etc.). The results of two sample tests are shown in Figures 11-20. The first test presented was conducted to verify if the tethered hexacopter followed the landing pad and ferry over a long distance and in difficult weather conditions. The second trial was designed to test the quality of hexacopter altitude tracking in a simulated UAV observation altitude change scenario. Figure 11 shows the 3D hexarotor trajectory realized during the first test. The take-off and landing points as well as the climb and descent curves are presented and can easily be identified. The total distance covered by the hexarotor during this test was 2.26 km. The average speed over this distance was 23.5 km/h. The hexacopter's trajectory, in the east-north-up (ENU) local coordinate system, is also shown in Figure 12. The UAV's estimated position is compared with the desired trajectory. The hexarotor, despite the difficult conditions, correctly and accurately followed the desired trajectory. Additionally, the take-off and landing setpoints with current values are also marked in Figure 12. The hexarotor's attitude, expressed by roll, pitch, and yaw angles, is shown in Figure 13. The roll angle during the entire cruise was kept at about 8 degrees, while the pitch angle was about \(-6\) degrees. The yaw angle was about 150 degrees, except for the take-off and landing phases, when the hexarotor was close to the ship's metal deck. The ship's large metal superstructure distorted the readings of the UAV's onboard magnetometer. The linear velocities of the propeller are shown in Figure 14. These velocities were used as control signals in the landing pad tracking system. In addition, the ground speed is shown for comparison. The hexarotor followed the desired speed values very precisely, except for the critical take-off phase due to disturbances of the magnetometer and disturbances from gusts of wind, which were intensified at relatively low altitudes close to the deck (air streams were reflected from the deck surface and hit the hexarotor--the ground effect). The magnetometer readings during the test are shown in Figure 15. It is easy to see the effect of the proximity of the vessel's metal deck on the operation of the magnetometer and the determination of the yaw angle. Figure 14: Tethered hexacopter linear velocities and ground speed during the long-term following test. Figure 12: Tethered hexacopter trajectory (local ENU frame) during the long-term following test. Figure 13: Tethered hexacopter attitude during the long-term following test. The second test involved checking the hexacopter and its navigation system in an observation scenario at various altitudes under difficult sea conditions. Changing the flight altitude (ascending, descending) while following the vessel in windy conditions and operating from the vessel's helipad was a challenge. The vessel's structure intensified wind disturbances and gusts, and numerous air chimneys were noted in the vicinity of the vessel's landing pad. Similarly to the results of the first long-distance test, the UAV's trajectory and the most important flight parameters are shown in Figures 16-20. In Figure 16, the hexacopter's trajectory during the second test is presented. It is easy to see the changes in altitude intentionally triggered in the control system. During the second test, the hexacopter covered a distance of almost 900 m, with an average speed of 25 km/h. The hexacopter's trajectory in the ENU local coordinate system is also shown in Figure 17. Here, the set values of the trajectory and the estimated position in the local frame in the ENU reference system can easily be compared. It can be seen that the quality of tracking the desired trajectory was satisfactory, despite the difficult weather and environmental conditions.