  Although the use of multiple Unmanned Aerial Vehicles (UAVs) has great
potential for fast autonomous exploration, it has received far too little
attention. In this paper, we present RACER, a RApid Collaborative ExploRation
approach using a fleet of decentralized UAVs. To effectively dispatch the UAVs,
a pairwise interaction based on an online hgrid space decomposition is used. It
ensures that all UAVs simultaneously explore distinct regions, using only
asynchronous and limited communication. Further, we optimize the coverage paths
of unknown space and balance the workloads partitioned to each UAV with a
Capacitated Vehicle Routing Problem(CVRP) formulation. Given the task
allocation, each UAV constantly updates the coverage path and incrementally
extracts crucial information to support the exploration planning. A
hierarchical planner finds exploration paths, refines local viewpoints and
generates minimum-time trajectories in sequence to explore the unknown space
agilely and safely. The proposed approach is evaluated extensively, showing
high exploration efficiency, scalability and robustness to limited
communication. Furthermore, for the first time, we achieve fully decentralized
collaborative exploration with multiple UAVs in real world. We will release our
implementation as an open-source package.
