## 4 Discussion  
The human-machine system in operating a drone consisted of two subsystems. The first one involves accessing the flight information of the operator. The second one includes the flight control of the drone via the remote controller. Flight information may be accessed by either observing the drone directly or watching the monitor on the remote controller. When the drone is flying within a short distance, the operator could catch flying information such as the altitude, horizontal distance, and moving direction, by direct observation of the drone movements. On the other hand, when the drone is flying at a distant location with a small visual angle, direct observation of drone movements provides little information to the operator. For example, an operator may have difficulty catching a drone several hundred meters away, not to mention catching the altitude and moving direction of the drone. The operator, then, needs to rely on flight information on the monitor of the remote controller. When the operator is depending on the monitor to access the flight information, he or she may not be aware of whether the drone is within LOS unless the operator has prior LOS information in terms of distance. Providing the LOS distance is therefore important for flight control especially when the drone is at a distant location. Even though an operator may access flight information from the monitor on the remote controller, he or she may, sometimes, need to observe the drone in the sky in order to gain feedback of the flight control. This is very important especially for novice operators. For example, a novice may not be sure whether pushing a joystick forward will lead to a forward or a backward movement because he or she may not be aware of the head orientation of the drone because quadcopters do not have a distinct head and tail in appearance. By watching the drone movement in the air, the operator may confirm whether the drone is moving in the way consistent to his or her control. This, however, can only be done when the drone is within the LOS. The LOS information will be beneficial for drone trainers to help their trainees in achieving movement compatibility in drone operation training. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **DY** & & **DPY** & \\ \hline
**Probability** & **Distance** & **Visual Angle** & **Distance** & **Visual Angle** \\ \hline
0.1 & 397 & 5.0 & 464 & 4.3 \\
0.2 & 335 & 5.9 & 406 & 4.9 \\
0.3 & 293 & 6.8 & 367 & 5.4 \\
0.4 & 259 & 7.7 & 336 & 5.9 \\
0.5 & 228 & 8.7 & 307 & 6.5 \\
0.6 & 197 & 10.1 & 278 &  
The aviation authorities, in general, require that drone operation conducted during the day and within LOS. The LOS is a spatial domain in which the operator can visually catch the drone. It may be defined [28] using the largest distance between the drone and the operator or the minimal visual angle by which the operator has a 50% probability to catch the drone visually. [PERSON] et al. [28] found that the estimated distance and visual angle of the LOS for a DJI Phantom 4 drone were 245 m and 0.065\({}^{\circ}\), respectively, when the DPY criterion was adopted. In the current study, the estimated distance and visual angle of the LOS for the Mavic Air flying without a protector were 307 m and 0.065\({}^{\circ}\), respectively, considering also the DPY criterion. When a DY criterion was adopted, the estimated LOS distance and visual angle were 228 m and 0.087\({}^{\circ}\), respectively. The estimate LOS distances of the Mavic Air drone in Table 2 were significantly (\(p<0.05\)) higher than those in [PERSON] et al. [28]. The differences between the estimate LOS visual angles of the Mavic Air in this study and those of the Phantom 4 in [PERSON] et al. [28] were, however, insignificant. Both LOS distance and visual angle affect how large the image is of a visual target on the retina and thus affect the likelihood of visual detection. The LOS distance is, theoretically, equivalent to the LOS visual angle.