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A well-structured and reliable communication system is key to the successful operations of multi-robot systems. In this letter, we present our design and implementation of a multi-robot communication architecture CHORD (Collaborative High-bandwidth Operations with Radio Droppables) based on two popular robotics middleware, ROS 1 and ROS 2. We discuss the benefit and best practices of combining two different frameworks that share the same spirit and show its performance from large-scale real-world experiments. The proposed system is developed as part of Team CoSTAR's effort for the DARPA Subterranean (SubT) Challenge. The system has been field-proved and demonstrated in the Urban Circuit event, where team CoSTAR won first place. To our knowledge, this work is the first real-world demonstration of a ROS 2-based multi-robot system in such large-scale extreme environments. From the significant improvement of the communication performance and the ease of transition from existing ROS 1 systems, this work encourages wider adoption of ROS 2 in field robotics applications.

The importance of autonomy in robotics is magnified when the robots need to be deployed and operated in areas that are too dangerous or not accessible for humans, ranging from disaster areas (to assist in emergency situations) to Mars exploration (to uncover the mystery of our neighboring planet). The DARPA Subterranean (SubT) Challenge presents a great opportunity and a formidable robotics challenge to foster such technological advancement for operations in extreme and underground environments. Robot teams are expected to rapidly map, navigate, and search underground environments including natural cave networks, tunnel systems, and urban underground infrastructure. Subterranean environments pose significant challenges for manned and unmanned operations due to limited situational awareness. In the first phase of the DARPA Subterranean Challenge (held in August 2019; targeting underground tunnels and mines), Team CoSTAR, led by NASA JPL, placed second among 11 teams across the world, accurately mapping several kilometers of two mine systems and localizing 17 target objects in the course of four one-hour missions. While the main goal of Team CoSTAR at the end of this three-year challenge (August 2021) is a fully autonomous robotic solution, this paper describes Team CoSTAR's results in the first phase of the challenge (August 2019), focusing on supervised autonomy of a multi-robot team under severe communication constraints. This paper also presents the design and initial results obtained from field test campaigns conducted in various tunnel-like environments, leading to the competition.