# Improvement of Hexacopter UAVs Attitude Parameters Employing Control and Decision Support Systems
## 4 Laboratory and In Situ Measurement Results and Discussion
### Hover Flight Tests
#### 4.3.2 Hexacopter Flight Parameters Extracted from Sensors during Hover Flight
), GPS position errors can cause the hexacopter to \"feel\" that it is in a different location than the correct one, which can lead to the drone flying aggressively to correct its perceived erroneous location information. These \"errors\" appear in both tlogs and dataflash logs as a decrease in the number of visible satellites and an increase in the horizontal HDop accuracy value. Hdop values less than 1.5 are very good, and values above 2 could indicate that the GPS positions are not correct. Decreasing the number of satellites below 12 leads to erroneous measurements of the hexacopter position and speed relative to the ground. A significant change in these two values often accompanies a change in GPS position. Figure 32 proves that the number of received satellites is 15 and the horizontal position accuracy is 0.65-0.71 m, so both values correspond to a parameterized operation of the GPS signal receiving equipment from satellites. Figure 33 shows a graph recording the relative speed of the hexacopter to the ground, based on the information received from the GPS. Given that the UAV performs the ascent and hover manoeuvre at a fixed point with small position adjustments, it can be seen that the value of this velocity is generally close to 0 m/s. Mission planner, via the IMU batch sampler menu, has the option to record high-frequency data from IMU sensors to the flash data log on the flight controller. These data can be analysed after the flight to diagnose vibration-related problems using graphs created employing fast Fourier transforms (FFTs). A common feature of these plots is a peak at  
Figure 31: Gyro rotational speeds and accuracy of data received from GPS satellites. (**a**) Gyro rotational speeds in rad/s for IMU (0) and (1); (**b**) Accuracy of data received from GPS satellites. Figure 32: Accuracy of HDop positioning data received from GPS satellites. the \"propeller blade crossing frequency\", that is, the frequency at which the blade passes over the arms and causes an acceleration in the frame. In the graphs presented in Figure 34 with data collected from accelerometers and gyroscopes, there are, however, certain noises corresponding to the eigenfrequencies of the motors. The accelerometer and gyroscope data show on the vertical axis the amplitude and on the horizontal axis the natural rotational frequency of the motors. The amplitude is not scaled to a useful value, so we cannot tell whether the levels of these values are high or low, which means that the graph is only useful for determining the frequency of vibrations. Vibrations at frequencies higher than 300 Hz can lead to attitude or position control problems. In this case, peak frequencies are observed at 40 Hz/2400 rpm, 47 Hz/2820 rpm, 95 Hz/5700 rpm, 130 Hz/7800 rpm and 153 Hz/9180 rpm. It is possible to filter out these noises in order to increase the hexacopter performance and to allow a better parameter tuning by activating the harmonic notch filter(s). The harmonic notch filter is designed to match the frequency of the noise introduced by the engine rotation. Its value changes as the motor rotates by means of interpreting the value of the engine acceleration. The frequency is scaled up from the hover frequency and will never drop below this frequency. However, during a dynamic flight, it is quite common to reach low motor operating frequencies during propeller rotation. To address this, it is possible to modify the reference value in order to scale the filter to a lower frequency. For the operation of the hexacopter beyond visual line of sight (BVLOS), the necessary components for implementation on the drone have been acquired. Ground and flight tests can be carried out to demonstrate their ability to control the hexacopter via 3G/4G LTE mobile networks. These include the Raspberry Pi 3B board, IR camera + EO camera and 4G LTE mod  
Figure 34: Vibration frequencies induced by motors rotation. Figure 25:0 Relative speed of the drone to the ground. and autonomous, can be carried out to test the drone limits.