Capabilities of Non-line-of-sight Optical Communications with UAV through Water–Atmosphere Interface

А non-line-of-sight optical (NLOS) communication line with an unmanned aerial vehicle (UAV) through the water–atmosphere interface allows removing the limitation to the UAV position within the transmitter divergence angle during a communication session. However, the capabilities of such communication lines were not previously considered. We experimentally and theoretically estimate the capabilities of an NLOS optical communication line between an underwater transmission system and a flying UAV. Field experiments show a possibility of organizing stable NLOS optical communication at baseline distances of 90 m and UAV flight altitude of 30 m. The simulation of a communication line shows a useful signal to be mainly formed by radiation scattered in the atmosphere at short baseline distances and by radiation scattered in water at long baseline distances. Therefore, the useful signal is maximal at high water turbidity (Secchi depth is 3 m). The results make it possible to analyze the main patterns of generation of a useful signal in communication lines under study and can serve a basis for the creation of such systems.