Underwater Robot Tested Beneath the Arctic Ice Sheet 

 

After four years of work and numerous test runs in the Monterey Bay, a team of MBARI engineers took the institute's first autonomous underwater vehicle, or AUV, for a trial cruise in the Arctic Ocean last fall. The group spent a month aboard the Coast Guard icebreaker USCGC Healy, testing the AUV and its components under and along the Arctic ice sheet.

The cruise was in preparation for the AUV's first real Arctic mission: the Atlantic Layer Tracking Experiment, or ALTEX. ALTEX, which is slated to take place sometime in 2004, will study the warm layer of seawater that flows from the Atlantic into the Arctic basin. The volume of this layer has increased by 20 percent in recent years, and some scientists believe the increase might be related to global warming.

Associate Director of Engineering Bill Kirkwood declared the trial cruise enough of a success that the same basic design will be used for the actual ALTEX experiment. "Engineering-wise, we learned a lot from the cruise," said Kirkwood. "We tested the navigation system and the core vehicle at high latitude, which worked well." Kirkwood said the engineering team will continue to fine-tune the AUV, working to improve the vehicle's range and navigation system. 

The science and engineering crew of the Arctic cruise on 
the ice in front of the USCGC Healy.

Although this new class of AUV, called the Dorado, is designed to be useful for a variety of missions, MBARI hopes that the Dorado will fill a need for new technology with which to explore the Arctic. The Arctic is a source of great scientific interest—it is still largely unstudied, and the effects of global warming are expected to appear first in the region—but marine scientists lack the tools to work effectively under ice. Most research to date has been accomplished using nuclear submarines on loan from the military, but these subs are limited (safety precautions require that they stick close to the surface), and they will soon be retired from service. Although scientists have used AUVs in the Arctic before, they have typically been deployed from holes in the ice, and have ventured only a few hundred meters below the surface. MBARI hopes that the Dorado, with a depth rating of 4,500 meters and a range of more than 50 kilometers, will open the region up to scientific exploration.

The version of the Dorado that will be used for the ALTEX mission, called the ALTEX AUV, will be equipped with several special components. One of the most unusual of these is the ice-penetrating buoy. Because working in the Arctic is so difficult, Dorado’s designers decided that the first priority of any Arctic mission would be to recover the data collected, even if the vehicle itself was lost. With this in mind, MBARI asked an engineering company, Scientific Solutions, Inc., to design buoys that could transmit the data gathered by the AUV back to scientists — all while the mission was still underway.

The ALTEX AUV can hold a dozen ice-penetrating buoys, each of which contain software to download data collected by the vehicle's scientific sensors. When the AUV is ready to launch an ice buoy, it slows and searches for a suitably thin patch of ice. The buoy is then released, floating up towards the surface until it comes to rest just underneath the ice cover. Seawater is pumped into the nose cone, mixing with a chemical compound, Pyrosolve-Z, in a heat-producing reaction that melts through ice up to one meter thick. Once the buoy bobs to the surface, it extends its antennae, beaming both the data and a GPS fix of the buoy, via satellite, back to the scientists.

The ALTEX AUV will also carry an ice-profiling sensor, a device that uses sonar to determine the thickness of the ice pack above the vehicle. The data it collects will be useful for scientists studying global warming, since the melting of the polar ice sheets is likely to be one of the early warning signs of rising global temperatures.

One of the biggest challenges of working in the Arctic is navigation, since magnetic compasses lose their effectiveness close to the poles. The ALTEX AUV will use a combination of two systems: an inertial navigation system and a doppler velocity log. The inertial navigation system measures the acceleration of the vehicle and its turning rate, while the doppler velocity log tracks how fast the AUV is moving relative to the ice pack above. Together, the two systems provide fairly accurate navigation. Scientists will also be able to use the GPS fixes taken by released ice buoys to reconstruct a more accurate path of any mission.

Aside from the modifications to equip the vehicle for the Arctic, several traits  characterize the Dorado class as a whole. The first of these is the tailcone, or rear section of the AUV. The tailcone comes from an MBARI design that improves both the durability and efficiency of the vehicle. A duct encircles the propeller, protecting the fins from bumps and scrapes against the ice or against the ship when the vehicle is launched or recovered. The duct also improves efficiency, by reducing the vortices of water that spin off from the fins and slow the vehicle. The entire tailcone section is capable of rotating up to ten degrees, making the vehicle more maneuverable. The tailcone design has been licensed to Bluefin Robotics Corp., and has been put to use in military and commercial AUVs.

The nose of a Dorado with its outer "skin" removed, showing oceanographic sensors housed inside.

The Dorado is also one of the first modular AUVs, and can be modified according to the demands of its mission. Scientists will be able to easily add and remove instruments and sensors, and the core vehicle itself can be assembled in several configurations that range from just over two meters to more than six meters long.

The Dorado has already been put to work in the Monterey Bay. In August of 2000, two early-stage Dorado AUVs joined a fleet of research vessels, buoys, and drifters in a large-scale investigation of the biology and chemistry of the upper water column of Monterey Bay. MBARI scientists took a Dorado to sea this summer to explore the Monterey Bay submarine canyon in a dual study of bioluminescence and ocean fronts. The Dorado’s depth rating and long range make it capable of a variety of other missions, from midwater work to deep-sea hydrothermal vents.

Ultimately, MBARI’s engineers envision an even more sophisticated version of the Dorado that could play a role in MBARI’s planned ocean observing system. These vehicles would be permanently docked at moorings in Monterey Bay. When an event of interest occurred, such as a phytoplankton bloom, scientists on shore could command the AUV to go collect data, giving it a set course. The vehicle would then return to the mooring to recharge its batteries and transmit the gathered data, via satellite, back to the scientists.

Before the ocean observatory plan can be realized, there are many technological hurdles to overcome. But Jim Bellingham, MBARI's director of engineering, notes that AUV technology has, in a very short time, advanced our capability to study the ocean. "Ten or 12 years ago," he says, "it wasn't clear that AUVs could provide scientific data." Today, he says, these vehicles have proved that they can deliver data sets of a much finer resolution than can be made from a ship—and in conditions most scientists and ship crews would prefer to avoid.

Details from the 2001 Arctic cruise—including daily cruise reports— are available at MBARI's Expeditions report on the expedition.

MBARI's collaborators on the Dorado project include MIT's AUV lab, the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory, and Scientific Solutions, Incorporated.

Click on the photo to see a slide show of the Arctic cruise.

—Sierra Senyak
August 2002