# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 4 Discussion  
Although the averaged stable hovering time showed similar values for the PET-bottle float and the polystyrene-foam bar float (Table 1), the flight stability evaluation was sensitive to anomalies of the wind speed (maximum wind speed) for large payloads. Although the base drone (SplashDroneTM 4) was stable during the test (the cutoff time for a stable system was set as 180 s), the flight controller adjusted for the base drone was unable to respond correctly to the additional forces generated by the gusts on the large floats. Possible improvement might be provided by adjusting the parameters for the inertial measurement unit, the Proportional-Integral-Derivative (PID) control, or by increasing the elevation angle for the GNSS signal reception. The manufacturer provides a set of large floats (approximately 4.6 L) of SplashDroneTM 4 for boat-mode operations. The average wind speed allowed for flight with the genuine floats was limited to 5 m s\({}^{-1}\)[18]. The waiting time for the GNSS cold-start after each battery refill was also not negligible for flight tests, which would affect the efficiency of water quality monitoring with amphibious drones. The water quality data stability test gave a fluctuating trend of EC in Figure 11b, but the raw data (temperature-corrected EC in Figure 11b \(\times\) 1.020\({}^{(t-25)}\), \(t\): water temperature in \({}^{\circ}\)C) before temperature correction were stable during testing. The water temperature was stable during testing (Figure 11e). Therefore, a thermal inhomogeneity between the EC sensor (capped with a standard solution) and the temperature sensor was implied. For this study, we did not use the values of the EC sensor for the transmitter drone (SplashDroneTM 3+) during field testing of pond water, because of the low electrical conductivity of the test site (freshwater pond). However, each water quality sensor used for this study has proven performance at standalone operations in the presumed applications [30]. In the field test, the fluctuations in the EC and the water temperature values by the system for the relayer drone (SplashDroneTM 4) were within the range of \(\pm\)2%, but the pH values increased gradually by the repeated landings on water (Table 2). The manufacturer claimed fluctuations by the thermal non-uniformity of the pH sensor boards for the previous version [31]. Therefore, for high-precision monitoring of water quality, a thermal sink or an air circulator with a waterproof filter would be required for the modules sealed in the upper mounter. The DO values relayed from the transmitter drones (Figure 18) were lower than those obtained using the relayer drone (Figure 13), reflecting oxygen consumption by the electrodes. For  
Figure 20: Water quality data received from the bypassing relay test. Distances between the transmitter drone and the relayer drone at each landing site were the following: L1, 313 m; L2, 270 m; L3, 181 m.  
long-term floating applications of amphibious drones, an optical DO sensor is preferable to reduce errors by electrode consumption. The above-ground relay tests have shown a significant increase (by a _t_-test at a 99% level of significance) in wireless communication throughput by the low-altitude (around 3 m) hovering relay (Figure 15). The PDR did not show any significant difference with the 10 s receiver timeout, but the extended test duration for the on-ground relayer (Figure 16) indicated limited availability of the on-ground relay in faster measurements. According to the two-ray ground reflection model, the local RSSI in the range test from the transmitter drone (Figure 16) was consistent with the predicted value (\(-92\) dBm). The predicted communicable distance between two amphibious drones was 180 m for the drones on water, but the +15 dBm increase by the hovering relay enables a search range of 420 m by another drone. The extended range by the hovering relay is comparable with the controllable distance of the amphibious drones on water [18].