HTML5 Webook

49/84

camera drone passed a certain area (around the vehicles seen at the center in Fig. 4) and then reproduced at faster speed without drop-frames. We supposed that such delays in the transmission of the video data occurred due to Interference Fading*5 caused by the reection of commu-nication radio waves due to iron plates of the vehicle. So, we moved all vehicles to outside the experiment area, but the same phenomenon occurred. Hence, we concluded that some reason other than the eect of the vehicle might have caused the delay in communication at that point (Actually, the PING response time always increased slightly when video reproduction paused. Fig. 5).ere were three base stations for mobile phones near the test eld. We will investigate the cause of such phenom-ena including the eect by the base stations.6Indoor experimentWe conducted an indoor experiment on Sunday, February 26, 2016, at the auditorium (20 m × 17 m × 7 m height) in the No.4 building of NICT in Koganei city, Tokyo. We simulated the investigation mission in an im-portant facility in danger of collapse due to a disaster. We simulated the condition where image data taken by the camera drone that is visually isolated from the ground station set outside of the auditorium was transmitted to the ground station via the relay drone (Fig. 6).Several experiments were conducted, each of them was about ten minutes. e distances between drones and be-tween the relay drone and the ground station were kept at about 10 meters, according to the scenario where the camera drone investigated inside the auditorium. In addi-tion, a partition (7.8 m width × 1.8 m height) was set between the camera drone and the ground station so that the radio waves emitted from the camera drone would not reach to the ground station directly. Communications of encrypted video data and the procedure to reproduce the video data were veried in the same way as conducted by the open-air elds experiments. However, the experiment of disruption was skipped because it was outside the scope of verifying the indoor communication condition.As for the condition of communication between the ground station and the camera drone, we veried only the security of data transmission inside the building and re-production of the video data. For this purpose, we observed it by taking videos of reproduced video data at the ground station. In addition to the reproduction of the video data, FiF5PING response time between the ground station and the camera drone (horizontal axis represents the serial number assigned to the PING signals)02,0004,0006,0008,00010,00012,000147101316192225283134374043464952555861646770737679828588919497100103106109112115118121124127130133136139142PING response time (Ground station－Camera）* A PING signal is sent every one second.[msec][msec][msec]Video pauseVideo restartVideo pauseVideo restartVideo pauseVideo restartStart of descentingLanding 02468101214169193959799101103105107109111113115117119121123125127129131133135137139141143145PING response time (Backward)Taking-offVideo pauseVideo restartVideo pauseVideo restartMove of droneRelay disruption0246810121418202224262830323436384042444648505254565860626466687072PING response time(forward)Start of rotation of propeller of droneTaking-off of droneMove of relay droneRelay disruptionMoveofRelay drone→RelayrecoveryLanding of droneRelay Disruption*5Interference fading is a phenomenon that the received signal level at the radio station varies due to constructive and destructive interference, because the phases of the radio waves from the sender to the receiver shift at the receiver due to the difference of the path caused by reflection by objects therein. Sometimes radio waves cannot be received.453-4 Secure Transmission of Video Data Relayed by UAV