The DARPA SubT Challenge: A Robot Triathlon

One of the biggest urban legends growing up in New York City were rumors about alligators living in the sewers. This myth even inspired a popular children’s book called “The Great Escape: Or, The Sewer Story,” with illustrations of reptiles crawling out of apartment toilets. To this day, city dwellers anxiously look at manholes wondering what lurks below. This curiosity was shared last month by the US Defense Department with its appeal for access to commercial underground complexes.

The US military’s research arm, DARPA, launched the Subterranean (or SubT) Challenge in 2017 with the expressed goal of developing systems that enhance “situational awareness capabilities” for underground missions. While the prospect of armies utilizing machines to patrol sunken complexes conjures up images of the Matrix, in reality one of the last frontiers to be explored on Earth is below the surface. As SubT moves closer to its culminating event planned for 2021, the agency is beginning the first phase of three planned real-world tests. According to the contest description the initial focus area will be “human-made tunnel systems,” followed by “underground urban environments such as mass transit and municipal infrastructure,” and then concluding with “naturally occurring cave networks.” This summer DARPA issued a Request For Information for subsurface infrastructure in the interest of “global security and disaster-related search and rescue missions.”

Competing technologists will have the chance to win $2 million for hardware inventions and $750,000 for software innovations that “disruptively and positively impact how the underground domain is leveraged.” The types of solutions being considered include platforms “to rapidly map, navigate, and search unknown complex subterranean environments to locate objects of interest.” In further explaining the objectives, Timothy Chung, DARPA program manager, said: “One of the main limitations facing warfighters and emergency responders in subterranean environments is a lack of situational awareness; we often don’t know what lies beneath.” Chung’s boss, Fred Kennedy, Director of the Tactical Technology Office, confirmed, “We’ve reached a crucial point where advances in robotics, autonomy, and even biological systems could permit us to explore and exploit underground environments that are too dangerous for humans. Instead of avoiding caves and tunnels, we can use surrogates to map and assess their suitability for use.” Kennedy even coined a catch phrase for the challenge – “making the inaccessible accessible.” 

In an abandoned Pennsylvania coal mine, on a sweltering August afternoon, eleven teams from across the globe came with 64 terrestrial robots, 20 unmanned aerial vehicles and one autonomous blimp to compete in the first wave of the SubT Challenge. The course included four events each lasting an hour deep inside the mine, which was originally built by the Pittsburgh Coal Company in 1910. Each team’s fleet of machines had to autonomously locate, identify and record 20 items or artifacts. The only team to score in double digits in all four independent runs was Explorer of Carnegie Mellon University. CMU is a DARPA Challenge favorite with a winning record that includes the 2007 Urban Challenge and 2015 Robotics Challenge. This year it had the distinct advantage of being local, scouting out the location beforehand to better plan its tactics for live competition. As Courtney Linder of Popular Mechanics writes, “Explorer regularly practiced at the Tour-Ed Mine in Tarentum, which is normally only frequented by tourists who want to check out a coal mine formerly owned by Allegheny Steel. They periodically flew drones and watched their ground robots exploring the cavernous, maze-like depths.”

The biggest hurdles for teams competing below ground are the lack of Global Position System (GPS) signals and WIFI communications. To safely navigate the cavernous course of these GPS-denied environments, SubT machines had to rely solely on a fusion of on-board sensors, including: LIDAR, cameras and radar. In explaining how his team won to the Pittsburgh Post-Gazette, CMU lead, Sebastian Scherer said they employed up to eight robots that created its own WIFI network to “talk” to each other while simultaneously mapping the environment with its sensors. Deploying a swarm approach, the robots acted as a collective unit working together to fill in data gaps, which meant that even if one went offline it was still able to pilot using its onboard systems and previously downloaded maps. Leading up to the competition the CMU team utilized computer simulations to strategize its approach, but understood the limitations of exclusively planning in the virtual world. As Scherer’s collaborator, Matt Travers, explains, “Our system may work perfectly in simulation and the first time we deploy, we may take the exact same software from the simulation and put it in the robot and it drives right into a wall and you go figure out why.” CMU’s geographic proximity to the test site seemingly played a critical role in their team achieving high scores.

While Explorer walked off with some nominal prize money, all eleven teams are committed to the same goal of full autonomy regardless of the environment and manual input. As Travers exclaims, “We’d like to build a system that’s going to be agnostic to the types of mobility challenges that we’ll face. And this is certainly a difficult thing to do.” Reflecting on the August gathering, the creativity of invention unified a global community towards a single purpose of saving lives. In the words of the program’s organizer, Chung, “We are inspired by the need to conduct search and rescue missions in a variety of underground environments, whether in response to an incident in a highly populated area, a natural disaster, or for mine rescue.” The next round will take place in February quite possibly in the sewers of New York City (alligators and all). As Chung cautions the contestants, “Prepare for a few new surprises. The SubT Challenge could be compared to a triathlon. DARPA is not looking just for the strongest swimmer, runner, or cyclist, but rather integrated solutions that can do all three.”

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