A Heterogeneous Multirobot System for Autonomous Object Retrieval in Challenging GNSS-Denied Maritime Environment

Marine intervention tasks for retrieving large and heavy objects from vessels at sea are significantly important for maritime security. However, it remains a challenging task due to the lack of effective retrieval methods. Harsh open-sea conditions, unreliable Global Navigation Satellite System (GNSS) due to either interruptions or deliberate spoofing by other vessels, and the limitations of single robots in localizing and manipulating these objects pose significant challenges for the operations. To address these challenges, this article proposes an autonomous heterogeneous multirobot system comprising two commercial Unmanned Aerial Vehicles (UAVs) and a six-degrees-of-freedom robot arm mounted on a Unmanned Surface Vehicle (USV) to perform this complex task in a GNSS-denied marine environment. GNSS limitations for UAV localization are addressed using ultrawideband beacons specially installed on the USV. Our approach involves the robot arm initially estimating the object's position. Then the UAVs fly over the target vessel and search for the object using this information. Once the object is located, a custom-designed tethered-bar mechanism attached to the UAVs drags the object towards the workspace of the robot arm mounted on the USV. This study also introduces a decentralized framework to facilitate effective information sharing and collaboration among the robots. Field trials in maritime conditions demonstrate that by combining these elements, our system possesses significant potential for real-world maritime security and rescue applications. Videos of the field trials can be found at http://youtu.be/a_JzZf7QqWM.