Raspi$^2$USBL: An open-source Raspberry Pi-Based Passive Inverted Ultra-Short Baseline Positioning System for Underwater Robotics

TL;DR

Raspi$^2$USBL addresses underwater positioning where GNSS is unavailable by delivering an open-source, Raspberry Pi–based piUSBL system with a six-channel hydrophone receiver and a surface beacon. The authors integrate modular hardware (LNA, AGC, DAC/DAQ, OCXO) with a unified C++ software framework that performs real-time DSP to estimate TOF and DOA via a matched filter and conventional beamforming, aided by adaptive gain control. Experimental validation across an anechoic tank, lake, and sea demonstrates slant-range accuracy better than 0.1% and bearing accuracy around 0.1°, with long-range tracking up to 1.3 km. By releasing both hardware designs and software publicly, the work fosters reproducibility, collaboration, and rapid advancement in open underwater acoustic navigation and swarm robotics.

Abstract

Precise underwater positioning remains a fundamental challenge for underwater robotics since global navigation satellite system (GNSS) signals cannot penetrate the sea surface. This paper presents Raspi$^2$USBL, an open-source, Raspberry Pi-based passive inverted ultra-short baseline (piUSBL) positioning system designed to provide a low-cost and accessible solution for underwater robotic research. The system comprises a passive acoustic receiver and an active beacon. The receiver adopts a modular hardware architecture that integrates a hydrophone array, a multichannel preamplifier, an oven-controlled crystal oscillator (OCXO), a Raspberry Pi 5, and an MCC-series data acquisition (DAQ) board. Apart from the Pi 5, OCXO, and MCC board, the beacon comprises an impedance-matching network, a power amplifier, and a transmitting transducer. An open-source C++ software framework provides high-precision clock synchronization and triggering for one-way travel-time (OWTT) messaging, while performing real-time signal processing, including matched filtering, array beamforming, and adaptive gain control, to estimate the time of flight (TOF) and direction of arrival (DOA) of received signals. The Raspi$^2$USBL system was experimentally validated in an anechoic tank, freshwater lake, and open-sea trials. Results demonstrate a slant-range accuracy better than 0.1%, a bearing accuracy within 0.1$^\circ$, and stable performance over operational distances up to 1.3 km. These findings confirm that low-cost, reproducible hardware can deliver research-grade underwater positioning accuracy. By releasing both the hardware and software as open-source, Raspi$^2$USBL provides a unified reference platform that lowers the entry barrier for underwater robotics laboratories, fosters reproducibility, and promotes collaborative innovation in underwater acoustic navigation and swarm robotics.

Figures (11)

• Figure 1: Hardware architecture of the acoustic receiver and beacon cabins in the Raspi$^2$USBL system.
• Figure 2: The system diagram of the Raspi$^2$USBL system.
• Figure 3: Software workflow of the Raspi$^2$USBL system.
• Figure 4: Signal processing workflow of the Raspi$^2$USBL system. Top Left: The reference 10-12 kHz LFM signal used for acoustic transmission and reception. Top Middle: The time-frequency spectrum of the received signal from one hydrophone channel. Top Right: The beam pattern obtained from conventional beamforming for DOA estimation using the raw received signal. Bottom Right: The matched filter output for TOF estimation and the extracted target signal segment.
• Figure 5: Anechoic tank experiment setup. Left: The acoustic beacon cabin and the acoustic receiver cabin are both mounted on overhead traveling crane's guide columns. Right Top: The depth control panel of the overhead traveling crane. Right Bottom: The rotation control panel of the overhead traveling crane.