# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 2 Materials and Methods
### Field Tests for the Wireless Data Relay  
To examine the capabilities for wireless data transfer of the water quality monitoring system with the XBeeTM chips, two amphibious drones were prepared with the water quality monitoring system and the data receiver system (Figures 8 and 9). The demonstration experiment was conducted on a large marsh pond with shoreline vegetation (2 m maximum depth, 0.3 km\({}^{2}\); Ogata Marsh Pond in Akita Prefectural Koizumigata Park) in Akita, Japan, where most of the water surface is ideally able to be covered using a single amphibious drone (a 500 m radius semicircle). The transmitter drone (SplashDroneTM 3+ system) was placed on the nearshore water with the minimum reinforcing float (Figure 6d); then, the relayer drone (SplashDroneTM 4 system) was brought to the offshore site for data relay to the receiver. The transmitted EC data were zero-filled for the high-concentration range sensor of the SplashDroneTM 3+ system (DFRobotTM EC sensor) because of the low electrical conductivity of the pond water. The receiver was set on a portable tripod (SLIK GX-m 7500; Kenko Tokina Corp., Tokyo, Japan) of 1.8 m height (60% of the Fresnel radius at 2.4 GHz for communication through 300 m of presumed line of sight to a drone). The receiver was located on an elevated (approximately +3.5 m by the Digital Elevation Model 5B (DEM5B), Geospatial Information Authority of Japan) site to increase the allowable distance in wireless communication, but the antenna height of the drone on the water was approximately 0.25 m. The drone and receiver locations were determined using the basic two-ray multipath model applied for earlier studies [19; 20; 21]. Data relay tests were conducted for (1) extending relay (19 July 2023, cloudy with occasional rain) and (2) bypassing relay for shoreline vegetation (27 September 2022, cloudy with occasional sun). The data relay was attempted for both floating and hovering states (<10 m above the drone controller) of the relayer drone. The use of other wireless communication tools (e.g., smartphone, 5W transceiver) was avoided during the drone flight. A waterproof (IP68) flashlight (Wuben L50; Shenzhen Shengqi Lighting Technology Co., Ltd., Shenzhen, China) was used to confirm the drone direction visually. The interval for water quality measurements was set as 7.0 s for the bypassing relay test, but the interval was extended to 10 s for stable communication in the extending relay test. The exact location of the transmitter in the extending relay test was adjusted by approaching the shoreline vegetation (approximately 2 m adjustment to disable the direct wireless communication). After a gentle splashdown of the amphibious drone system on water, stable readings for pH, EC, and water temperature were awaited. The first complete set of readings after splashdown was used for DO because of the internal oxygen consumption by the sensor electrode. Manual sampling measurement of nearshore water was also conducted with portable water quality meters (WM-32 EP; DKK-TOA Corp., Tokyo, Japan and HQ30d; Hach Co., Milwaukee, WI, USA). We presumed that the operational flight time of these drones with a fully charged battery was less than 10 min in the field tests because of the water quality sensor system payload and the repeated splashdown and flying from the water. Figure 8: Scheme of the wireless data relay tests. The transmitter drone measured surface water quality data and transmitted the data. The relayer drone relayed the data to a receiver. The receiver received the data. The communicable distance over the flat freshwater surface in low-altitude connections (reflection coefficient \(\approx-1.0\), total external reflection) was estimated simply based on Equations (2) and (3) [19; 20] (Figure 10).