Topo-bathy LiDAR sensor for characterizing proud underwater objects in shallow waters from a UAS platform

Abstract

A breakthrough LiDAR technology in mapping, detecting, and identifying proud underwater objects in shallow waters (< 10 m) from an above-water, unmanned aircraft system (UAS) is presented. This new and novel capability addresses a strong need in the Munitions Response area to find unexploded and discarded munitions underwater in depths less than 10 meters. This shallow-water domain includes an assortment of unexploded ordnance (from proud, partially buried, to buried) that are the most likely to be encountered by the public and are expected to experience the most mobility. Shallow water of less than 10 meters constitutes a wide variety of aquatic environments, such as ponds, lakes, rivers, estuaries, and coastal areas. Many sensor technologies designed to detect, classify, and remediate munitions are challenged by this unique environment and suffer in performance, access, navigation, deployment, viewing, sensor standoff distance, and possible damage by changing bottom topography or obstructions. This new technology has demonstrated unprecedented centimeter-level range resolution in estimates of water depth, river cross-sectional area, seamless land-water transitions, detailed bottom topologies, and detection of objects submerged just below the water surface. The Lidar can be deployed from a variety of platforms including UAS, fixed-wing aircraft, boats and even bridges or piers. Such an above-water capability enables munitions identification by target morphometry (the ability to accurately determine the dimensions - length, width, and height - of targets identified compared to the known dimensions of these same targets) and target positioning (the ability to accurately determine the specific location of targets) for reacquisition and remediation. Two particular challenges in accurately mapping and detecting underwater objects in this shallow-water environment are surface waves and turbidity. Surface waves vary the angle of incidence of the transmitted laser beam and vary the optical path length through the water. These effects produce uncertainty in 3-D mapping of underwater objects. Water column turbidity causes laser light to scatter out of the beam and reduces the ability to detect submerged objects, or equivalently reduces the depth penetration of the lidar. We have investigated the influence of surface waves and turbidity on the above-water topographic/bathymetric LiDAR signals and their impact to applying target morphometry and positioning for mapping underwater objects. Indoor controlled experiments were performed using a wave tank and known suspended particulates to assess laser beam propagation and signal detection schemes through different levels of turbidity and surface wave conditions. Outdoor experiments under different surface wave and turbidity conditions were also performed in a variety of environments including river beds, coral beds, sandy beaches, vegetation, and underwater breakwater construction. The results of these indoor and outdoor experiments will be presented and the shallow-water capabilities of the above-water topographic/bathymetric demonstrated for a variety of environmental conditions. The outcomes will illustrate the promising application of this approach for providing Munitions Response services in shallow waters.