This paper describes field experiments with an X8 Unmanned Aerial Vehicle (UAV) operating as a wireless communication relay while loitering over a REMUS 100 Autonomous Underwater Vehicle (AUV) being at the ocean surface. The paper describes the design of the communication relay payload, network configuration, optimal flight conditions and UAV antenna mounting, and experimental results. Experiments were conducted under less than ideal conditions with rain and turbulent winds leading to unfavorable roll and pitch motions of the UAV, and small waves surrounding the AUV. The results with data download from the AUV through the UAV communication relay to a ground station shows that at the tested (typical) flight conditions the distance and attitude between the AUV and UAV are not the bottlenecks in the communication network. The main bottleneck was identified as the capacity of the proprietary wireless system on the REMUS 100 AUV which was not specified as a high capacity data link, and seems to be set up by the AUV system manufacturer to provide a relatively low capacity, but very robust, wireless data link regardless of signal strength and quality.
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