Investigation of underwater acoustic positioning using a Ritchey-Chrétien reflector

To reduce labor in the development, maintenance, and operation of port areas, underwater drones and unmanned underwater construction machines are being utilized. Accurate positioning of these systems is essential, making underwater acoustic positioning technology critically important. The authors have previously studied positioning using underwater acoustic lenses and demonstrated that acoustic paths can be separated in multipath environments. However, in port areas, underwater sound speed varies significantly due to weather conditions, making the application of lenses difficult. To address this, we conducted a basic simulation study on underwater positioning using a reflective mirror. A Ritchey–Chrétien reflector was adopted, in which acoustic waves are reflected twice—by a primary and a secondary mirror—and focused through a central hole in the primary mirror. By adjusting the hole size, the focal acoustic pressure for incident angles of 6° or more was reduced to 10% of that at 0°. Additionally, comparing side lobe pressure levels allowed us to detect incident angle changes as small as 0.1°. These findings suggest that this method can suppress unwanted reflections in multipath environments and enable high-precision direction estimation.