## 4 Discussion
Detecting a Mavic Air drone using auditory signals is more difficult than that detecting a Phantom 4 drone. At the distance of 155 m, the probability that our participants could hear the drone indistinctly was only 8.3% when the protector was not used. This probability was even lower (4.2%) when the protector was used. The previous study indicated that auditory signals provided partial cues in drone researching and detection. The results of the current study, however, indicated that auditory detection of the drone was insignificant in finding the LOS distance for a Mavic Air drone because none of the participants could hear the drone before it reached the visual LOS location. The CAAC [22] has adopted the LOS distance of approximately 514 m. Figures 4 and 5 have shown that the probability of visually catching the Mavic Air drone at a distance of 502 m was zero. The estimated probabilities of detecting a Mavic Air without a protector at a distance of 514 m, based on Equations (4) and (5), were only 2.4% and 5.2% when the DY and DPY criteria were used, respectively. Our participants could not catch the Mavic Air drone at the LOS distance of the CAAC [22]. These are consistent with the findings in the literature [28]. There were limitations of this study. The first one is the atmosphere environment. It is not possible to have a controlled atmosphere environment in a field experiment. Our experiment was conducted in an open space where the sky was cloudy with light breeze and the visibility was good. These weather conditions were measured before the experiments started and some of the weather conditions, such as the gust speed, could be changed after the experiment began. Changes of the atmosphere conditions during testing could affect human responses in detecting a drone. Even the weather conditions of the current study were different than the previous study [28], the consistency between the major findings of the two studies indicate the reliability of the LOS measures of our study. A second limitation of this study was about the participants. It was difficult to recruit experienced drone operators as human participants. The participants in this study were all novices. The literature [33; 34; 35] has pointed out that experience could have influences on the performance of visual search and detection tasks. Future research is recommended to investigate the effects of drone operating experience on the LOS distance and visual angle. In addition, our participants have normal visual acuity and color vision. Our results of visual capture could, then, be higher than those of ordinary observers who have less than normal visions. A third limitation of our study is that the drone was navigated to the predetermined hovering location without the witness of the participants. This was because we liked to focus on the effects of distance while removing the influence of moving cues of the drone. In practice, a ground observer watches the drone flying more often than observing a stationed one. Drone movement could provide cues for drone search and detection. A moving object catches human attention and is, therefore, easier to be detected than a fixed one [36]. Our LOS distance and visual angle data might need to be revised if the participants have additional movement cues of the drone before responding. Future studies are required to incorporate the effects of drone movements on drone catching. Finally, only one drone model was tested. Future research may be conducted to test the LOS distance and visual angle of many other models.