# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 4 Discussion
For larger-scale monitoring, reciprocal hovering of the amphibious drones (Figure 1, lower) can be an easily attainable practice for overcoming the limited availability of wireless communication on large water bodies. The maximum communicable distance found during the field tests was influenced by the water level of the pond on the test day (+0.3 m water level for the extending relay test) (Figure 21a). The estimated communicable distance by the two-ray ground reflection model [19; 20] was 835 m, but the distance decreased to 812 m because of the +0.3 m rise in the water level. The actual communicable distance in the field was shorter than the values predicted by the surface reflection model, indicating the influences of other factors including the shoreline vegetation, noises from the surrounding environment, the surface roughness/high dielectric permittivity of water [32], signal loss from the lower part of the dipole antenna of the amphibious drone on water, and inaccuracies of the altitude data. During testing, we were able to keep the amphibious drone systems away from other park users (Figure 21b,c). In earlier studies, the greater surface roughness, dielectric permittivity, and conductivity of the seawater surface in comparison with a flat ground have been found to provide only a 2-3 dBm decrease in the received signal strength at 2.4 GHz [32]. When the antenna height used in the two-ray ground reflection model calculation was lowered to (a) the antenna base height or (b) the phase center of the antenna, the estimated value of the maximum communicable distance was decreased (a, 703 m; b, 757 m). The result obtained with the phase center calculation (b) was comparable to those in the field test (approximately 757 m of direct communication was possible when the transmitter drone was kept away from the shoreline vegetation). The high actual performance which necessitated consideration of the antenna length was attributed to the conformation of the \(\lambda\)/4 monopole antenna, inclusion of the s-waves (horizontally polarized electric waves), incidental coherent signal enhancement by multipath transmission, and inaccuracy of the altitude data (approximately 0.99 m of synthetic error for the altitude difference between the transmitter and the receiver). The altitude error might be an important factor affecting wireless communication. Based on the synthetic error for the altitude difference, the predicted communicable distance ranged from 731 m to 887 m (including the +0.3 m of water rise independently from the error on the mapping survey). Among all factors recognized, the shoreline vegetation demonstrated a strong effect, which is consistent with earlier studies of the numerous complicated paths of wireless communication generated by vegetation [21]. Although the line of sight in the bypassing relay test was concealed only by thin (approximately 70 m) shoreline vegetation (Figure 19), direct data reception was unattainable. The vegetation included some tall, foliated tree species (e.g., oak, pine, pseudoacacia) grown to over 20 m in height. The path loss through vegetated paths has most frequently been estimated using the International Telecommunication Union-Radiocommunication sector (ITU-R) model [21]. The model predicts a path loss of 26.4 dB for the vegetated path of 70 m. The path loss for the remaining path (373 m \(-\) 70 m = 303 m), estimated by the two-ray ground reflection model, is 84.9 dB. The estimated signal intensity through the entire direct path is -103.3 dBm, a value below the sensitivity of the receiver (\(-102\) dBm [17]). Experimental evidence of a sharp signal drop in 2.4 GHz Bluetoothand Zigbee through a 25 m path of thick foliage has also been reported by an earlier study [33]. The first landing site in Figure 19 was only approximately 36 m away from the shoreline vegetation, but the wireless data relay network was established during flight.