## 4 Discussion
However, the effects of both the LOS distance and visual angle on the likelihood of visual detecting a drone in the air are very complicated. For example, a certain LOS distance or visual angle may be composed of different horizontal distance and altitude combinations. The sky background of the drone could be quite different for different horizontal distance and altitude combinations. Variations of the sky background will surely affect the capability of ground observers in drone detection. It was unfortunate that combinations of horizontal distance and altitude comprising of the same distance were not tested in this study. Orientation of the drone to the observer with respect to the sun also affects the visual detection. An observer may have difficulty in detecting a drone when there is glare even when the drone is within the LOS distance. Orientation of the drone and the sun also affect the reflection of the drone and thus the drone's visibility. Larger drones are supposed to have longer LOS distances than the smaller ones. Our LOS distances of the Mavic Air were, however, significantly higher than those of the Phantom 4 in [PERSON] et al. [28]. This contradicts to what we have anticipated because the former has a smaller diagonal size (34.7 cm) than the latter (59 cm). The reason for this contradiction may be attributed to drone color and background contrast of the two studies. In [PERSON] et al. [28], the Phantom 4 drone was white and was hovering in a clear sky with hardly any cloud. The Mavic Air drone, in the current study, is a black one (observing from the bottom) and was hovering in a cloudy sky. The color contrast between a black drone in the cloudy sky and a white drone in the clear sky could be quite different. In addition, illumination could also play a role affecting the drone detecting capability of the participants. The illumination at the takeoff spot of the previous [28] and current one were 60,435 lx and 293 lx, respectively. Such a huge difference could result in different drone detection responses. The effects of color contrast and illumination at the test site on drone detection behaviors should be explored in the future. It is a common phenomenon that a spinning propeller looks partially transparent. This leads to the blurring of the boundary of the propeller in vision. It was one of our suspects that the actual visual angle might be smaller than that measured using the diagonal size of the propeller because the image of the two ends of the propellers might not be present in vision simultaneously. This was rejected by our results comparing the probabilities of visual detection between the drone with and without a protector. When a protector was used, there was a solid boundary outside the propellers, which clearly delineated the size of the drone. The insignificance of the difference of the probabilities of visual detection between the drone with and without the protector indicated the validity of visual angle calculation using the diagonal size of the propellers. This applies to other drone models in determining the visual angle. Cue utilization is believed to affect the performance of visual target searching detection [32]. The observers with more visual target information, in general, have more cues and hence may have higher cue utilization than those with less target information. We could not quantify the target information of the drone hovering in the air especially when it was at a distant location. The view of a drone in the air is mainly composed of the spinning propellers with a drone body in the middle. When the drone is approaching the LOS location, it appears as a dot as a whole with a blurry boundary in our vision. The difference of the cue utilization of observing the Mavic Air and the Phantom 4 drones is unknown. Quantification of the cue or target visual information for drones will provide challenging research topics in the future. The sound level generated by the Mavic Air is lower than that of a Phantom 4.