# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 4 Discussion
The link waiting time was expected to be sufficiently short (<10 s) to conduct a 10 s interval measurement because the 30 s hovering relay provided two complete datasets at 10 s intervals (Figure 18). The connected line-of-sight relay used for this study can be expected by classical theories, under the condition of (1) a line of sight formed with mutually observable pair(s) of drones and (2) a sufficient node discovery backoff time. The validation of the system in the field tests confirmed the feasibility of drone swarm applications in amphibious settings. Such applications include water quality monitoring with immersive sensors or direct water sampling by landing on water, providing crucial conditions for water-sensitive hardware. By utilizing amphibious drones and wireless communication tools with mesh-networking abilities, water quality monitoring with drones can be conducted safely and is available for environmental scientists and engineers with limited knowledge in other fields of study. By considering the data rate and the network structure of wireless communication in this study, the possible size of the amphibious drone swarm, accounting for latencies caused by hardware settings [34] such as baud rate, USB-UART conversion, and data display processes, is estimated at 15 drones with 10-second measurement intervals. In the estimation, the serial connection of all nodes is assumed as the worst case of the mesh-type network structure. The overall traffic in the serial connection is calculated as \((n+1)/2\)-folds (\(n\): number of transmitters) of that in the star-type network (direct connection of each transmitter to a receiver). The throughput obtained in this study's implementation and test conditions was 1.7 kbps for 16-byte payloads (2-byte data for four water quality indicators and four sensor numbers). Therefore, the estimated traffic with 15 drones is 15 \(\times\) (15 + 1)/2 [transmission packets] \(\times\) 16 \(\times\) 8 [bits per transmission packet]/10 s = 1.536 kbps (<1.7 kbps). Pilot contamination by the 2.4 GHz WiFi drone controller could be avoided by fixing the channel for each drone and using the XBeeTM network for the least-populated water body, but the number of channels for the drone controller was limited to 10 [26]. For the actual settings of this study, water quality data for each drone (pH, EC, DO, temperature, and the sensor numbers) were sent separately to secure each data point. In this case, the practical size is up to five drones. The traffic in this scenario is calculated as 5 \(\times\) (5 + 1)/2 \(\times\) 8 [transmission packets] \(\times\) 2 \(\times\) 8 [bits per transmission packet]/10 s = 0.192 kbps (<0.25 kbps actual throughput). The theoretical maximum speed (250 kbps for 2.4 GHz XBeeTM S2C [17]) without rerouting (optimized route is maintained reactively) and latencies, except for wireless communication, is estimated by multiplying the ratio of payload (16 bytes for a drone)  
Figure 21: Photographs of the field test site. (**a**) Path to the pond shoreline with raised water (+0.3 m). (**b**) Overview from the shore in the extending relay test. (**c**) Overview from the shore in the bypassing relay test. to frames (37 bytes of transmission frame and transmission status frame). The given theoretical throughput is 108 kbps. With the optimized throughput of 108 kbps, the theoretical maximum of the drone swarm is estimated to be 129. In this case, the traffic is calculated as 129 \(\times\) (129 + 1)/2 [transmission packets] \(\times\) 16 \(\times\) 8 [bits per transmission packet]/10 s = 107.328 kbps (<108 kbps). Even if the network structure can be approximated to a star-type network, there are still other constraints. To prevent collisions of numerous packets at the coordinator, the backoff time in Carrier Sense Multiple-Access-Collision Avoidance (CSMA/CA) [17] becomes crucial. The waiting time for sending a packet from a node under challenging conditions was assumed as approximately 400 ms.