# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 2 Materials and Methods
### System Stability Evaluations  
#### 2.5.1 Flight Stability Test  
To evaluate the drone flight stability for use with water quality measurement systems, flight stability testing was carried out (Figure 7). We measured the time to deviate from the center of a 3 m line during hovering stability tests (Figure 7). We did not control the drones with the aileron until time measurements were completed. Therefore, we excluded effects of individual flight skills on lateral flight stability evaluation.  
To simplify the flight stability evaluation procedures, the throttle and the elevator of the controller were used for manual adjustment of the hovering location. The initial location of the drone at each test was the center of the 3 m line at 1.5-meter height above the ground mat (four sheets of loosely filled air mat with 22 cm thickness and a landing pad at the center) (Figure 7). The test line was demarcated with a soft rubber string tightened on two flagpoles supported by the flagpole bases (8 kg) (Figure 7). Each of the two vertical lines indicating 1.5 m of deviation from the center was made by suspending a soft string with a square water bottle as a plumb. The vertical soft strings were used to detect deviation of the drone from the test line. The calibration steps provided by the manufacturer's manual (i.e., compass calibration, gyro sensor calibration, accelerometer calibration or inertial measurement unit calibration) were followed before starting the test. After those calibrations, >15 min of waiting time was added to confirm stable GNSS data reception. Hovering time without the water quality sensor systems was measured before starting the test. If the measured time was reduced by >10% (reduced hovering stability), then the drone was recalibrated before testing. We used the GPS (actually used both GPS and GLONASS) mode of the drones. The leg height of the upper mounter was optimized by referring to the GPS quality indicator of the drone system to maintain less than 1 m accuracy of the GPS positioning. A wind-cup-type anemometer (CHE-WD2; Sanwa Supply Inc., Okayama City, Japan) was used for measuring the average wind speed during hovering stability tests. The measured hovering time values were classified under five grades of average wind speed based on the Beaufort scale (<0.5, 0.5-1.5, 1.6-3.3, 3.4-5.5, and 5.6-7.9 m s\({}^{-1}\)). The test was canceled when the average wind speed exceeded 8.0 m s\({}^{-1}\) (the maximum limit specified  
Figure 6: Simplified top view (horizontal view) of the amphibious drone system with additional floating materials: (**a**) symmetric L-shaped polystyrene-foam bars (the concave of each L-shaped bar is fitted to each of the drone arms, approximately 60 g total weight) made by cutting a polystyrene foam box, (**b**) empty PET bottles (approximately 940 mL volume, approximately 136 g total weight), (**c**) coiled tubular polyethylene sponge foam discs (approximately 400 g total weight), and (**d**) the least amount of sponge-form arm-covers (approximately 85 g total weight) to balance the drones on still water.  
in the SplashDroneTM 4 manufacturer's manual [18]). We compared the hovering stability with additional floating materials for stable floating on water.