# Unmanned Aerial Vehicle Based Wireless Sensor Network for Marine-Coastal Environment Monitoring
## 1 Introduction
This type of UAV assisted WSN system architecture has become increasingly used in the past few years, given recent advances in both areas. In the agricultural sector, these type of implementations have been used to monitor crops remotely, by using a UAV as a data collector [21; 22] and, in some cases, as a trigger for countermeasures against pests [22]. Similarly, the use of UAVs has been proposed as a means of managing disaster situations, in conjunction with both WSN [23; 24] and cellular networks [24; 25], which presents several improvements over current implementations. In other areas, such as the entertainment sector, UAV assisted WSN have been proposed to provide enhanced user experiences in various types of events, such as music concerts or sports [26]. In aquatic environments, similar approaches as the one proposed in this article can be found in the literature, such as the work of [PERSON] et al. [27] and [PERSON] et al. [28]. [PERSON] et al. propose the use of a UAV to collect data from stationary low-cost and low-power consuming buoys, which provide data regarding various underwater sensors. To transmit data to the UAV, they used an 868 MHz radio module which implements the Tiny Mesh protocol, with a power consumption of 24 mA in reception and 560 mA in transmission, both at 3.3 V. In their experimentation, they were able to transmit with a maximum data rate of 4399.97 bps and a data link range of 485 m, whilst the UAV was hovering at approximately 9 m above sea surface. [PERSON] et al. use a similar system architecture with a UAV as a data mule and custom designed modular stationary buoy devices, which house a temperature sensor, an Inertial Measurement Unit (IMU), GPS module, micro SD storage, and a wireless transmitter, all managed by a 16 bit microcontroller. The wireless transmitter used in their implementation is a Radiocrafts RF1280 module, which transmits at 868 MHz frequency with a FSK modulation with an estimated data rate of 4.8 kbps. According to their study, this provides an approximate range of communications of 264 m with the UAV, with a low current consumption of 21 mA in reception and 28 mA in transmission, and a 3.3 V voltage supply. The system proposed in this article aims to use a swarm of drifting buoys, contrary to static ones presented in the previously mentioned work, as primary environmental sensors, covering wide areas of seashore, and a UAV as a data sink and dynamic network router. The use of drifting buoys makes the use of short-range communications with the UAV completely unviable, thus, to be able to locate and extract the data with a UAV, it is desirable to have long-range communications to simplify their interaction. The selection of a proper wireless communication protocol to fully integrate these devices is paramount. There are several possibilities for these type of transmissions, yet the recent development of the Low Power Wide Area Network (LPWAN) protocols present new opportunities with the benefits of a reduced energy consumption and moderate data rate [29; 30]. One of the most promising technologies in LPWANs is the LoRa spread spectrum modulation, which has been proven to be able to communicate data across several kilometers with a low power consumption [31; 30; 32; 33]. The unique modulation of this technology not only offers the possibility of transmitting below the noise floor, but has also a high resilience to external interference [34]. As a slight drawback, similar to most LPWANs, the LoRa technology presents a limited transmission bitrate, which is sufficient for most sensing applications although not ideal for high data streaming. Having stated the capabilities of the LoRa modulation, as well as the requirements of the application, it can be seen that it seems to properly suit the necessities of this implementation, avoiding the use of satellite or GSM communications, as well as additional cumbersome infrastructure. In the following sections of this paper a full description of the proposed system and its functionalities are encompassed.