# UAV Charging Station Placement in Opportunistic Networks
## 2 Related Work
### UAV Applications  
UAVs have gained popularity due to their their wide spectrum of applications and services, both in military and civilian domains, with new UAV applications being addressed in literature every day.  
[PERSON] et al. [18] and [PERSON] et al. [19] have presented extensive reviews on UAVs applications, the expected challenges, and open topics for future research. [PERSON] et al. [20] surveyed the use of UAVs in studying marine animal life, their behavior, movement patterns, and health. [PERSON], [PERSON], and [PERSON] [21] provided a theoretical framework for single drone use in animal monitoring application scenarios. The animal monitoring scenario covers the case where there are static sensor locations and random animal movements in the environment with the UAV collecting the information. [PERSON] and [PERSON] [13] studied the UAV-based data collection system from stable sensors with a discrete-time Markov chain approach. Similarly, [PERSON] et al. [21] experimented on the scenario where a UAV uses a hidden Markov model to guess the next location for the cluster head while monitoring the animal movements. ASTRO+, an aerial-ground pollution monitoring platform, has been built by [PERSON] and [PERSON] [22] to collect aerial pollution measurements. The paper showed that the dynamic airflow caused by the movements of the drones may affect the temperature and humidity levels of the ambient air, which, in turn, could affect the quality of the sensor's measurement. They also designed an algorithm that modifies the routes of the drones while taking into consideration the aerial measurement quality. [PERSON] et al. [23] developed Druber, a delivery service system where the service fleet is comprised of drones that belong to multiple private owners. The system allows customers to request parcel deliveries where a drone picks up a parcel and delivers it via multiple coordinated drones that share resources with intermediate battery replacement spots. PENGUIN, a UAV-based solution for identifying and classifying aquatic plastic pollutants according to specifics of the material is proposed in [24]. It is a two-phase sensing process that first uses computer vision techniques to recognize underwater plastic wreckage. Then, optical sensing is used to classify the type of plastic for each recognized object to one of six Resin Identification Codes (RICs) which is a set of symbols written on plastic products that identify the plastic resin out of which the product is made. Another UAV application in package delivery within opportunistic network environment has been introduced by [PERSON] et al. [25] using different machine language techniques. [PERSON] et al. [26] developed an optimization model for belt conveyor inspection used in the mining industry. The goal of this model was to define how many UAVs are suitable for monitoring the system based on mixed-integer linear programming. [PERSON] et al. [27] proposed two opportunistic routing protocols to handle the scenario where a UAV is collecting data from mobile sensors moving with different velocities. Ref. [28] proposed a system in which clusters of small UAVs share their computing resources to help clusters with limited resources finish their