Space-heritage SPAD-based flash lidar for underwater 3D mapping: from system design to field demonstration

We present a novel direct time-of-flight (dToF) flash light detection and ranging (lidar) system, specifically designed for bathymetry and underwater 3D imaging. Initially conceived for space missions, the dToF sensor is integrated with a system on chip (SoC) in a compact (< 6.5E-3 m 3 ) and lightweight (< 6.5 kg) housing, including illumination and imaging optics, as well as a pulsed laser source. This lidar system was designed, manufactured, and tested on both an unmanned aerial vehicle (UAV) and an unmanned surface vehicle (USV) in shallow water (<15 m). We demonstrate a pixel-level depth precision of 1.8 cm at a range of 96 m, achieve a complete three-dimensional mapping of an underwater site in Lake Neuchâtel, and successfully acquire 3D images of stationary targets at optical attenuation lengths (AL) up to 4.7 under smoke conditions. Capable of acquiring 3D images in a single snapshot, flash lidar is a key enabling technology for autonomous vehicles (e.g., obstacle detection), shallow water mapping (e.g., sandbars), and underwater infrastructure inspection (e.g., bridge foundations). Integrating a 2D SPAD array with firmware embedded in an SoC enables real-time underwater lidar imaging, paving the way for fast, cost-effective, and efficient depth measurements that can be used to monitor changes or detect anomalies in underwater manmade and natural structures.