Underwater acoustic positioning using a ray-tracing-based ToA–wavefront database in very shallow water with a non-uniform sound speed profile

Abstract This study proposes an underwater acoustic positioning method designed for very shallow water, where severe multipath propagation and non-uniform sound speed profiles (SSPs) often obscure the direct path and degrade ranging accuracy. The method combines the spatial convergence of backward-propagated synthetic wavefronts from imaginary sources with time-of-arrival (ToA)-wavefront shape database based on ray tracing. The proposed method actively exploits reflections and reconstructs the corresponding backward-propagated wavefronts from multiple ToAs, enabling ranging even in environments with complex reflection and refraction. Simulations using a measured SSP demonstrated a ranging accuracy of 0.06 m over a maximum distance of approximately 340 m, outperforming an existing ToA-based approach. In field experiments conducted in a very shallow water environment, the proposed method demonstrated higher positioning accuracy than existing methods, even under conditions in which the required reflections were difficult to obtain, by extracting wavefronts with no or few reflections from the database.