## 1 Introduction  
Wireless Sensor Networks (WSNs) are composed of a large number of sensor nodes deployed to monitor physical phenomena. Coupled with the expansion of the Internet of Things, WSNs enable new applications such as intelligent transportation, water distribution and collection networks and the smart grid. WSN are also used in home and industry automation, plant monitoring, scientific experiments, forest monitoring, border patrolling, environmental observation, machine and structure health monitoring, security enhancement, underwater world observation, water quality monitoring, air pollution examination, landslide detection, and natural disaster prevention [1]. In a military context, WSNs can be used to monitor a terrain for ground troop or vehicle movements. In these applications, sensor nodes are low-power devices often using the IEEE 802.11 af and 802.15 protocols and capable of communicating over a short range [2]. In typical WSNs, sinks or gateways are installed to collect the information from the wireless sensor nodes and to forward the data to the internet. However, in the case of a remote WSNs such as in military theater or in emergency situations where the WSNs are rapidly deployed in the environment, there is usually no infrastructure in place to accept the gateways. In such situation, UAVs have been proposed as an alternative to static gateways [3, 4, 5]. The UAVs act as mobile sinks and visit each sensor node, download their data and relay it back to a base station. Compared to unmanned ground vehicles, UAVs can cover more terrain in a shorter period of time. They can also be used in a rugged environment that would be difficult or impossible to reach by land. This is especially the case for landslide and avalanche prediction. With UAVs, it is also possible to ensure line of sight (LOS) with the wireless sensors by controlling the position of the UAVs. To accelerate the data collection, a team of collaborating UAVs can also be used. The use of UAVs to assist terrestrial communication networks is quite recent, but has seen a rapid increase of interest by researchers in recent years. In their survey paper, [PERSON] et al. [6] provide the state-of-the-art in applications and challenges for UAV assisted networks. They highlight the benefits of the UAV assistance paradigm, such as providing a cost-effective alternative for ground networks' limitations, a dependable coordination with ground network infrastructure, an efficient eye in the sky of the area covered, a robust deployable backup network for damaged terrestrial network infrastructures, and a communication bridge between existing networks and isolated users. However, their study also mentions the numerous challenges and technical issues related to the deployment of UAVs for the assistance of terrestrial networks that remains to be investigated and solved. Some of the challenges identified by the authors of [6] are UAV coordination, routing of messages, data gathering or data collection, UAV monitoring, communication with cellular networks, communication with Internet of Things networks, use of UAVs for disaster management, distributed computing, and secure communication. Another noteworthy survey paper on UAV assistance to terrestrial networks was published by [PERSON] et al. [7]. This paper focuses on the networking scheme used by the UAVs and identifies Software-Defined Network and Network Function Virtualization as two promising technologies to support UAV assistance to terrestrial networks. They discuss several use cases and challenges related to UAV-assisted network and identify future research directions with a focus on the softwarization of UAV networks. This present paper addresses the problem of UAV assistance to terrestrial communication networks by focusing on the challenge of data collection using UAVs. In the absence of a reliable communication network to relay the data from the wireless sensors to a monitoring station, UAVs are used to visit the wireless sensors, download their data, and relay the data to the base station.