# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 1 Introduction  
Amphibious drones with capabilities of both aerial and aquatic operations have the potential to provide valuable drone applications in aquatic environments. Typical amphibious multirotor drones are classified as amphibious unmanned aerial vehicles [1], whereas other unmanned vehicles dedicated to aquatic environments are also conventionally designated as drones. For water quality monitoring, some specialized systems using amphibious drones have already been proposed [2; 3; 4; 5; 6; 7; 8]. Earlier studies of advanced water quality monitoring have demonstrated the necessity for real-time wireless data acquisition [9; 10; 11; 12; 13; 14]. For drone applications, without the validation of data from continuously monitored trends, some water quality sensors might record unstable data within rapid measurements or emergency surveys with drones. However, continuous data reception from amphibious drones is not always expected when target sites include obstacles to wireless communication (e.g., vegetation, buildings) [15]. In actuality, the water body itself is an obstacle. Therefore, some valuable modules have been developed for wireless communication in deep water [16]. As a cost-effective approach to surfacewater monitoring, conventional communication devices (e.g., LoRaTM, XBeeTM, LTETM) are also available if the amphibious drone can be used with an antenna protruding above the amphibious drone's draft line. An earlier study used an amphibious drone system with a LoRaTM wireless communication module on the upper side of the amphibious drone [5]. The XBeeTM is an alternative for cheaper and more convenient applications using a higher data transmission rate (<250 kb s\({}^{-1}\)[17]). The maximum distance of wireless communication for water quality data in an amphibious drone system might be as short as the maximum controllable distance of the drone on water (e.g., 500 m [18]) for safe flight. However, an extremely short communicable distance achieved by convenient low-power tools (e.g., Zigbee products) on water could pose an important limitation for real-time monitoring applications (e.g., 45 m for vertically floating half-wave dipole antennas with the XBee S2 CTM [17], as estimated using the two-ray ground reflection model [19; 20]). Even in small-scale applications, wireless signals can be shielded by surrounding objects (e.g., vegetation [21]) at the monitoring site. If a real-time monitoring system is necessary for complex target sites with obstacles, then we can choose from several methods including (a) high-power wireless communication systems after obtaining legal permission/licenses and anti-crosstalk validation, (b) the LoRaTM communication systems by compromising the data rate, and (c) the XBeeTM mesh network by introducing a rescue drone or the parallel use of multiple drones. We specifically examined (c) for this study because legal permissions for the use of high-power wireless communication systems and anti-crosstalk validation can be important issues in some areas and countries, even though a drone-based water quality survey can be completed within a short time. A rescue drone could be made available by repurposing old drones or by purchasing a small drone (e.g., DJI MiniTM) capable of carrying a lightweight (<100 g) data relay system (e.g., ArduinoTM Nano, XBeeTM, XBeeTM shield, antenna, a 9V-006P battery and cables). For a greater-scale drone survey, the concept of a wireless data relay among a swarm of drones has been proposed [22; 23]. It has been examined with multi-hop wireless communication systems for aerial operations [24; 25]. In [25], multi-hop wireless telemetry with a low-frequency (169 MHz, reallocated for telemetry and UAV control from analog TV broadcasting in Japan) band was examined, and it succeeded in Beyond-Line-Of-Sight (BLOS) flight by relaying the telemetry data. The result also provides a hint for the extended conceptualization for an amphibious drone system (Figure 1).