## 1 Introduction  
Unmanned aerial vehicles (UAVs), or commonly known as drones, have been adopted for military missions for decades. With advancements of miniaturizing hardware, better affordability, and an easy-to-use design, UAVs are widely used in entertainment and applications in industry such as aerial photogrammetric survey [1], wildlife and coastal habitat monitoring [2], transportation engineering [3], parcel shipping [4], inspection of construction sites [5], disaster management [6], and movie filming [7]. Drones have even been preprogrammed to provide air shows in the night and many other entertainment purposes. Privately owned UAVs are mostly operated by an external pilot or operator on the ground using a remote-control device. The operator reads the flight information from a monitor and moves the joysticks to send a flight command [8]. UAVs with multiple propellers, especially the four-propeller ones, are widely used for commercial and personal purposes because of the performance in stability and flight control. The emergence and popularity of the UAVs has created safety problems [9, 10, 11, 12, 13]. The literature indicates that the accident rate of the UAV is, in general, much higher than that of manned aircraft [14]. UAV-related ground people injuries and even fatalities have been reported [15, 16]. In addition to safety problems, UAVs taking photos or filming videos of ground targets on private properties may also raise the concerns of privacy for the public [17]. People may feel that the UAV is taking a photo of them, which violates their privacy even if the camera on board is actually not activated. Drone invasion into a prohibited airspace has, then, become an emergent issue because of safety and privacy concerns of the public [18]. For the safety of the public, the aviation authorities in many countries employ different approaches to avoid drone accidents. These approaches include implementing UAV ownership requirements, drone operator training, and implementation of drone operation administration. Codes and regulations concerning these requirements have been announced. The drone registration requirement in the USA was implemented in 2015. The Federal Aviation Administration has received 300,000 registrations within only one month after announcing this requirement [19; 20]. The drone registration requirements in China and in Taiwan were implemented in 2017 and 2018, respectively. At the end of 2018, the Civil Aviation Administration of China (CAAC) has received 287,000 drone registrations [21]. Drone operator training is achieved by pilot licensing regulations. These regulations have been implemented via a community outreach program. For example, a UAV operator license is required to operate a UAV with a takeoff weight of 7 kg or higher in China [22; 23]. In Taiwan, a common license is required to operate a UAV of 2-15 kg, and a professional license is required to navigate a UAV 15 kg or higher [24]. The drone codes, in general, prohibit a flight in a no-fly zone (NFZ) and a flight above a certain altitude in a restricted-fly zone (RFZ). A low altitude flight above crowds of people without prior approval from local authority is also not allowed [22; 24; 25]. Alcoholic consumption is prohibited for drone operations. In Taiwan, the alcoholic offence for pilots of civil manned aircraft also applies to drone operators [24]. The aviation authorities generally require that privately owned UAV may fly in unrestricted open space not higher than 120 m (or 400 ft) above ground level (AGL) and within the line-of-sight (LOS) during daytime [22; 25; 26]. Defining the LOS is one of the fundamental tasks in the administration of small drone operations in open spaces. The LOS may be defined using the distance between the drone and the operator and the corresponding visual angle. The visual angle is defined as \"the angle a viewed object subtends at the eye\" [27].