A graph localization approach to assist a diver-in-distress

In this paper, we focus on the problem of locating a diver-in-distress using graph localization tools. Without GPS reception while being submerged, a diver cannot report on his location and upon distress can only rely on help from fellow divers by sending periodic SOS messages using underwater acoustic communication. In areas where many obstacles exist like harbors or shallow reefs, these SOS messages may not reach all the divers in place, and thus localization through triangulation is not guaranteed. Instead, we gather information of the propagation delay between the divers and use graph localization tools to allow a distributed evaluation of the location of the SOS source by an assisting diver. Considering the need to fastly propagate the SOS information, we flood the SOS packet while piggybacking propagation delay information across the divers' network. To combat the expected packet collisions, we evaluate the propagation delays by identifying the times collisions occur. When the assisting diver forms the graph of the propagation delays between the divers, he is able to estimate the location of the SOS source relative to his own using only a single anchor position. Numerical results as well as a proof-of-concept test in a water tank show a relative localization accuracy on the order of a few tens of meters, which is beneficial for quickly locate the diver-in-distress.