Underwater acoustic positioning using orbital angular momentum vortex beams

Underwater acoustic positioning has been commonly used in a wide variety of underwater work, including deep underwater construction, oil and gas extraction, ocean sciences and underwater robot navigation. To date, various methods have been developed to achieve accurate underwater coordinate localization, such as ultrashort baseline (USBL), short baseline (SBL) and long baseline (LBL) acoustic positioning system. The surveys of these classic hydroacoustic localizations are typically executed by employing the time-of-flight method to measure the distance and the orientation (vertical and horizontal angles) of the transducer to the transponder. USBL and SBL provide accurate positioning estimation in the short distance, but the error depends on the additional depth sensors and accumulates as the distance increases. Deep measurement is not essential for LBL, but the requirements for the positioning accuracy are strict. Acoustic vortex beam, an acoustic wave with a helical phase front pattern, possesses an additional spatial degree of freedom called orbital angular momentum (OAM), offering potential solutions to overcome the above limitations. In this work, we propose an approach using the acoustic vortex beam carrying OAM to investigate the underwater positioning. Theoretical model reveals that the received signal has the hybrid form of inverse Fourier transform and Bessel function. The two-dimensional Fourier transform is then employed to calculate distance and azimuth angle. Simulation results show that the OAM-based positioning method can achieve more accurate coordinates than the conventional hydrolocation method. The application of OAM provides exciting opportunities to break the limits of current underwater positioning methods.