UAVs Beneath the Surface: Cooperative Autonomy for Subterranean Search and Rescue in DARPA SubT

This article presents a novel approach for autonomous cooperating unmanned aerial vehicles (UAVs) in search and rescue (S&R) operations in subterranean domains with complex topology. The proposed system was ranked second in the Virtual Track of the Defense Advanced Research Projects Agency (DARPA) Subterranean Challenge (SubT) Finals as part of the team CTU-CRAS-NORLAB. In contrast to the winning solution that was developed specifically for the Virtual Track, the proposed solution also proved to be a robust system for deployment onboard physical UAVs flying in the extremely harsh and confined environment of the real-world competition. The proposed approach enables fully autonomous and decentralized deployment of a UAV team with seamless simulation-to-world transfer and proves its advantage over less mobile unmanned ground vehicle (UGV) teams in the flyable space of diverse environments. The main contributions of this article are present in the mapping and navigation pipelines. The mapping approach employs novel map representations—SphereMap for efficient risk-aware long-distance planning, FacetMap for surface coverage, and the compressed lightweight topological-volumetric map (LTVMap) for allowing multirobot cooperation under low-bandwidth communication. These representations are used in navigation together with novel methods for visibility-constrained informed search in a general 3-D environment with no assumptions about the environment structure while balancing deep exploration with sensor-coverage exploitation. The proposed solution also includes a visual-perception pipeline for onboard detection and localization of objects of interest in four RGB streams at 5 Hz each without a dedicated graphical processing unit (GPU). Apart from participation in the DARPA SubT, the performance of the UAV system is supported by extensive experimental verification in diverse environments with both qualitative and quantitative evaluations.