# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 2 Materials and Methods
### Water Quality Monitoring System with Amphiious Drones  
Amphibious drones used for developing the water quality monitoring system were the SplashDroneTM 3+ and SplashDroneTM 4 (Swellpro, China, localized and sold by GSM Co., Ltd., Tokyo, Japan). Each drone has been rated as an Ingress Protection (IP) 67 product (dust- tight and waterproof to one meter of depth for 30 min) in China. The maximum payload capacity for the SplashDroneTM 3+ was 1 kg, whereas that for the SplashDroneTM 4 was 2 kg [18; 26]. The maximum controllable distance between the flight body of the SplashDroneTM 4 (ISM 2.4 GHz, compliant with Japanese regulations) and the controller was 500 m on water, whereas that on the free (air) space could be as long as 5 km [18]. A system schematic is shown in Figure 2. Water quality sensors (pH, EC, DO, and temperature) and their outputs are presented in Figure 3. These water quality sensors were laboratory-grade low-cost models for application with protecting mounters on drones. For a SplashDroneTM 3+ system, the water quality sensors were the DFRobotTM-brand analog output sensors with signal isolators (pH, SEN0161; EC, DFR0300; DO, SEN0237-A; isolator, DFR0504; Zhiwei Robotics Corp., Shanghai, China). For the SplashDroneTM 4 system, water quality sensors were mounted on the landing gear because of the advanced wireless flight control systems of the SplashDroneTM 4 with two large antennas. The pH sensor was a gel-filled immersive electrode sensor (pH Kit; Atlas Scientific, Pasadena, CA, USA). The EC sensor was also replaced with a wide-range (10 to 200,000 \(\mu\)s cm\({}^{-1}\)) sensor (Conductivity K 1.0 Kit; Atlas Scientific, United States) for freshwater monitoring. The DO sensor with an Ag-Zn electrode (Dissolved Oxygen Kit; Atlas Scientific, United States) was also used for unified control of water quality sensors in the I\({}^{2}\)C communication mode. These sensors  
Figure 1: A conceptual illustration of wireless data relay for water quality monitoring using multiple amphibious drones. Upper: no data relay, middle: data relay by a flying drone, lower: data relay with alternated roles of drones for large-scale applications with variable receiver locations. Drones A and B are identical but situated in different locations. were calibrated with the standard solutions with a 9 V source to the sensor controller (a 5 V source was insufficient for the sensors). The water temperature sensor was an SEN-18367 (SparkFun Electronics Inc., Boulder, CO, USA) with a 4.7 k\(\Omega\) pull-up resistor or a KIT0021 (Zhiwei Robotics Corp., China) configured to read the temperature at 9-bit resolution over the 1-Wire interface (a one-wire device communication bus system with a ground wire). The temperature compensation coefficient for EC data was 2.0% K\({}^{-1}\). The coefficient for DO data was obtained to minimize the standard deviation of the data of oxygen-saturated water. These sensors were controlled by an ArduinoTM (open-source microcomputer brand) nano-compatible board (Shenzhen Keyes Diy Robot Co., Ltd., Shenzhen, China), a GroveTM (connecter) shield for the ArduinoTM Nano (Seeed Technology Co., Ltd., Shenzhen, China) ('shield' means stackable electronics board on ArduinoTM series computers), and a six-port GroveTM hub (Seeed Technology Co., Ltd., China) for multiplying the I\({}^{2}\)C port. The resolution of the analog-digital converter for the analog-output sensor was 10-bit. An XBeeTM add-on for ArduinoTM Nano (Gravitech LLC, Minden, NV, USA) was also stacked for XBeeTM wireless communication in the industrial, scientific, and medical (ISM, 2.4 GHz) band. An atmospheric sensor (air temperature/humidity/air pressure sensor; SEN-15440, SparkFun Electronics Inc., United States) was also used for evaluating the dissolved oxygen concentration.