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

翻译：精确的水下定位仍是水下机器人领域的一项根本性挑战，因为全球导航卫星系统（GNSS）信号无法穿透海面。本文提出Raspi$^2$USBL，这是一种基于树莓派的被动倒置超短基线（piUSBL）定位系统，为水下机器人研究提供了低成本、易获取且可复现的平台。该系统由被动声学接收器与主动信标组成。接收器集成了水听器阵列、多通道前置放大器、恒温晶振（OCXO）、树莓派5以及MCC系列数据采集（DAQ）板。信标集成了匹配网络、功率放大器和发射换能器。一个开源的C++框架支持单程旅行时间（OWTT）消息的时钟同步与触发，同时执行匹配滤波、阵列波束形成和自适应增益控制，以估计飞行时间（TOF）和到达方向（DOA）。该系统在消声水池、淡水湖和开放海域试验中完成了验证。结果表明，其斜距精度优于0.1%，方位角精度在0.1°以内，且在长达1.3公里的距离内性能稳定。这些发现表明，低成本、系统级可复现的硬件能够达到研究级的水下定位精度。通过发布软件框架并提供可复现的硬件架构，Raspi$^2$USBL提供了一个参考平台，降低了水下机器人实验室的入门门槛，并促进了水声导航与群体机器人领域中的可复现研究。