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In the ground station, robotic navigation software called “simultaneous localization and mapping” (SLAM) — which both maps an unknown area and keeps track of an agent inside the area — uses the LIDAR input to localize and capture the position of the drones. This helps it fuse the maps accurately.
The end result is a map with 3-D terrain features. Trees appear as blocks of colored shades of blue to green, depending on height. Unexplored areas are dark but turn gray as they’re mapped by a drone. On-board path-planning software tells a drone to always explore these dark unexplored areas as it flies around. Producing a 3-D map is more reliable than simply attaching a camera to a drone and monitoring the video feed, Tian says. Transmitting video to a central station, for instance, requires a lot of bandwidth that may not be available in forested areas.
More Efficient Searching
A key innovation is a novel search strategy that let the drones more efficiently explore an area. According to a more traditional approach, a drone would always search the closest possible unknown area. However, that could be in any number of directions from the drone’s current position. The drone usually flies a short distance, and then stops to select a new direction.
“That doesn’t respect dynamics of drone [movement],” Tian says. “It has to stop and turn, so that means it’s very inefficient in terms of time and energy, and you can’t really pick up speed.”
Instead, the researchers’ drones explore the closest possible area, while considering their current direction. They believe this can help the drones maintain a more consistent velocity. This strategy — where the drone tends to travel in a spiral pattern — covers a search area much faster. “In search and rescue missions, time is very important,” Tian says.
In the paper, the researchers compared their new search strategy with a traditional method. Compared to that baseline, the researchers’ strategy helped the drones cover significantly more area, several minutes faster and with higher average speeds.
One limitation for practical use is that the drones still must communicate with an off-board ground station for map merging. In their outdoor experiment, the researchers had to set up a wireless router that connected each drone and the ground station. In the future, they hope to design the drones to communicate wirelessly when approaching one another, fuse their maps, and then cut communication when they separate. The ground station, in that case, would only be used to monitor the updated global map.
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