Making maps of the ocean floor is what autonomous underwater vehicle Sentry does best. But once underwater, it can drift, and the area the vehicle thinks it is mapping may not correspond to its actual location.
This isn’t an issue when making maps of unexplored sea floor—one of the main reasons Sentry was designed. But it is a problem when scientists are observing sea floor changes year-to-year to track volcanic activity.
In spring 2022, Sentry acquired some new smarts, in the form of software developed by engineers at MBARI (Monterey Bay Aquarium Research Institute) and adapted by the Sentry Team, which enable the vehicle to course correct as it goes, using a map of the area to recognize where it is.
“Once Sentry submerges it uses dead reckoning,” said Sentry team lead Sean Kelley. “It knows where it came from and its distance, time, and direction. But as it drives a straight line, the vehicle has no idea that it has drifted.” This is where the new software comes in.
Since 2006, the National Science Foundation (NSF) has funded research to do repeat mapping of Axial Seamount. By measuring how much the surface of the summit has risen, scientists are studying the cycle of in this volcano: where the magma is stored before an eruption and how much the chamber beneath the surface fills with magma before it erupts. This seamount is an ideal study site because it erupts every ten to twenty years and is only a day’s journey off the coast of Oregon. Also, it has a 40-year history of being studied, and hosts many real-time sensors as an Ocean Observatories Initiative location.
For the mapping portion of the project, the group needed a vehicle to repeat the same lines in a spider web pattern for 20 kilometers (~12 miles). Before using the MBARI software, Sentry could drift up to 40 meters (~130 feet) on some of the dives, which made comparing changes to the ocean floor difficult.
“Navigation is the hardest part of doing underwater surveys,” said David Caress, a principal engineer at MBARI who led the team that adapted new software for use on Sentry. “As soon as you are not at the sea surface and don’t have GPS, you have to do other kinds of navigation.”
The software Caress and his team developed uses terrain relative navigation, which works by loading a pre-existing map into the vehicle and then using the real time seafloor map collected during the dive to figure out its position and make corrections.