# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 3 Results
### Wireless Data Relay by Introducing Additional (Relayer) Drone
#### 3.2.2 Extending Relay  
Wireless data relay testing was conducted under line-of-sight conditions. Direct communication between the transmitter drone and the receiver was disabled by separating the transmitter by 757 m of distance from the receiver for the relay test (Figure 17). The maximum distance for wireless communication by the XBeeTM S2C chips is 1200 m in free (air) space [17], but the actual communicable distance in this study was decreased by the water surface and the shoreline vegetation. The antenna height of the transmitter on water was approximately 0.25 m (0.1 m antenna vertical length + 0.15 m thickness of the system above the draft line). That of the receiver was approximately 1.95 m (0.1 m antenna vertical length + 1.8 m height of the tripod + 0.05 m thickness of the receiver). The actual difference in antenna heights was approximately 4.9 m (1.7 m + 3.5 m difference in altitude-0.3 m raised water level by a previous heavy rain). The estimated communicable distance by the two-ray ground reflection model was 755-866 m with \(\pm 0.7\) m of altitude error by the Digital Elevation Model 5B (DEM5B), excluding effects of multipath formation by shoreline vegetation. Figure 14: Arrangement of wireless communication devices for quantifying the efficiency of drone hovering in data relaying. The test was conducted on 12 December, 2023 in a flat field at Akita Prefectural University (Akita Campus), Japan. The distance between the transmitter drone and the receiver (connected to a laptop computer) was approximately 100 m. The relayer drone was positioned at the center of the line between the transmitter drone and the receiver. No tall grasses were present in the field on the date of the test. Networking metrics (throughput, RSSI, and PDR) were measured both with and without hovering of the relayer drone. The hovering height was maintained at approximately 3 m.  
Without hovering relay (relayer drone was placed on ground)  
With hovering relay (relayer drone was kept above 3 m from ground)  
Figure 15: Results of throughput tests with hovering relayer drone positioned at the center of a 100 m line of sight. The test for each condition was repeated three times. The test duration (cutoff time) was set as 60 s for the rapid environmental monitoring with drones. The average throughput was 0.28 \(\pm\) 0.07 kbps for the on-ground relayer and 0.77 \(\pm\) 0.05 kbps for the hovering relayer,. Without hovering relay (relayer drone was placed on ground)  
Figure 16: Results of range tests with a hovering relayer drone positioned at the center of a 100 m line of sight. RSSI: Received Signal Strength Indicator. Success (%) on the right-side vertical axis represents the Packet Delivery Ratio (PDR, %). The test for each condition was repeated three times. Each packet was to be sent ideally at 1000 ms intervals, but the receiver timeout was set as 10 s for a measurement of water quality at 10 s intervals. The average PDR was 97 \(\pm\) 2.7% for the on-ground relayer and 100 \(\pm\) 0.3% for the hovering relayer. The sensitivity of the receiver (XBe\({}^{\text{TM}}\) S2C coordinator in the boost mode) was \(-\)102 dBm [17]. The controller of the relayer drone (SplashDroneTM 4) continuously displayed the highest accuracy level of the GNSS (10, Figure 17), which indicates at least \(\pm\)1 m of accuracy for the relayer drone location (the maximum instrumental accuracy of the GNSS was \(\pm\)0.5 m [18]). The relayer drone was able to fly along a line-of-sight direction to the transmitter drone. The relayer drone landed on water four times. The horizontal distance from landed sites to the transmitter drone were L1, 624 m; L2, 500 m; L3, 381 m; and L4, 368 m (Figure 18). During each relayer drone flight, the water quality data were relayed to the receiver, except for the first flight to L1 (Figure 18). No complete dataset (pH, EC, DO, and water temperature) was relayed by the relayer drone on water.