MetaBlue: A Metasurface-Assisted Acoustic Underwater Localization System

Abstract

Underwater localization is essential for marine exploration and autonomous underwater operations, yet existing radio frequency and optical approaches are limited by rapid attenuation or limited visibility. Acoustic sensing remains the most practical choice, but conventional acoustic systems typically rely on large arrays or multiple synchronized anchors, resulting in high hardware costs and complex deployment. This paper introduces a novel low-cost passive acoustic metasurface, MetaBlue , explicitly designed for underwater localization, which, when attached to an ordinary ultrasonic transmitter, transforms it into a directional "super-transmitter." The metasurface embeds direction-dependent spectral patterns into the transmitted waveform, enabling accurate angle-of-arrival (AoA) estimation using only a single hydrophone. For ranging, we present a new EM-acoustic mixed time-of-arrival (ToA) method that leverages the acoustic transducer's inherent low-frequency EM leakage as a timing reference, enabling precise ranging without shared clocks. This allows complete 3D localization with a single low-cost anchor. We evaluate the system across diverse real-world underwater settings, including pools, tanks, and outdoor environments. Experiments show that our design achieves an average AoA error of 8.7 degree and 3D localization error of 0.37 m at distances over 10 m. Even with a single anchor, the system maintains 0.73 m precision.

Figures (21)

• Figure 1: MetaBlue Overview. The passive acoustic metasurface converts the transmitter into a directional encoder. A single low-cost hydrophone can measure direction by decoding the direction-dependent spectral pattern (for AoA), and achieve ranging by measuring the time difference between the electromagnetic leakage and the acoustic signal. By combining these two measurements, a single anchor achieves full 3D sub-meter localization.
• Figure 2: Past AMS. (a) The structure of a unit cell. The $\lambda$ is the wavelength and the phase shift is determined by $d_1$ and $d_2$. (b)–(c) Instantaneous sound pressure fields of a single AMS unit cell and the AMS array in air and water.
• Figure 3: Waterborne AMS Design. (a) The top shows the structure of the waterborne unit cell, and the bottom shows instantaneous sound pressure with an input of a plane wave. (b) shows the circular AMS array. (c) shows the equivalent far-field wave of the circular array. (d) shows the simulated directional sound pressure of a 20-cell AMS driven by a 196 kHz source.
• Figure 4: 3D Model of AMS Structure.
• Figure 5: Directional Feature. (a) shows raw spectral features at different directions, including cases with and without multipath. (b) shows signal waveforms with and without multipath after applying the proposed suppression.