  This paper presents a novel approach for autonomous cooperating UAVs in
search and rescue operations in subterranean domains with complex topology. The
proposed system was ranked second in the Virtual Track of the DARPA 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 UGV teams in the flyable space of diverse environments. The
main contributions of the paper 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 topological-volumetric LTVMap for allowing multi-robot
cooperation under low-bandwidth communication. These representations are used
in navigation together with novel methods for visibility-constrained informed
search in a general 3D 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 on-board
detection and localization of objects of interest in four RGB stream at 5 Hz
each without a dedicated 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
evaluation.
