## 4 Experimental Results
### Hovering  
Figure 8 shows the 60 GHz band transfer characteristics during UAV hovering. The distance between the two UAVs was varied from 10 to 50 m in 10 m increments. For each distance setting, data throughput and the received signal strength indicator (RSSI) were evaluated over a one-minute period. The plots are time-averaged over one minute, and the error bars represent the standard deviation of temporal fluctuations. The RSSI diminishes with increasing distance due to free-space path loss, thereby causing a reduction in the throughput attributed to shifts in modulation scheme and coding rate (as described in Section 3.1.2).  
While UAVs can sustain a specific position through their inherent flight controllability, practical scenarios involve slight positional perturbations owing to external factors such as wind, turbulence, and errors in on-board sensor output. These perturbations can result in beam misalignment and consequent deterioration in communication quality. The error bars at distances of 10 or 20 m indicate this phenomenon, with the average throughput hovering around 3 Gbps accompanied by fluctuations of 10% or even more during hovering. As the inter-UAV distance decreases and the spot size narrows, fluctuations become more significant. This indicates the need for precise beam alignment to perform high-speed communication over short distances. The identical trend holds for spot size reduction as frequency increases, and it is readily foreseeable that these effects will become even more critical when higher frequencies are employed. Managing consistent communication while hovering becomes challenging under such circumstances.  
Figure 8: Data throughput and RSSI of 60 GHz band data transfer with different distances in hovering.  
Figure 7: Illustration defining the pass-by flight. The blue arrow indicates the direction of flight, and the red areas indicate the communication range.  
Figure 9 presents the temporal evolution of 60 GHz band data transfer at distances of 10, 20, and 30 m, which are several extracted datasets derived from the source data of Figure 8. At \(d=10\) m, shown in Figure 9a, although a throughput exceeding 3 Gbps is maintained, fluctuations in the throughput and RSSI are evident. These fluctuations are considered to originate from beam misalignment due to variations in the UAVs' position and attitude. According to RTK positioning data, the inter-UAV distance changed by approximately 5 cm within a 5 s interval (Figure 9d). This positional variability, coupled with angular jitter of a few degrees, prompts antenna misalignment, making it impossible to consistently sustain the maximal achievable throughput (max. 3.5 Gbps observed). The impacts of such misalignment on mmWaves have been numerically analyzed and reported by [PERSON] et al. [29]. Shorter distances increase the average throughput but decrease the spot size, contributing to RSSI and throughput fluctuations due to UAV movement jitter. For distances exceeding 30 m, where a sufficiently large spot size is established, this impact becomes negligible, even in cases of comparable positional or angular misalignment, ensuring sustained and reliable throughput.  
Figure 9: Throughput, amount of transferred data, and RSSI of the experimental inter-UAV 60 GHz data transfer in hovering with (**a**) the distance \(d=10\) m, (**b**) \(d=20\) m, and (**c**) \(d=30\) m. The corresponding distance measured by RTK positioning and relative attitude between the UAVs in hovering are shown in (**d–f**), respectively.