A Brief Summary of the Autonomous Benthic Explorer (ABE)

ABE has seven thrusters which allow it to move forward or back, up or down, left or right. It can hover and turn in place. ABE is most efficient traveling forward, using about 200 Watts while running level at 0.65 m/sec, but requires up to 340 Watts when it has to go up or down over rough terrain.

 For each ABE dive, we plan a survey path along the bottom, then write a program. This program can be composed using MATLAB scripts, which automatically generate the mission code. After building the survey plan and reviewing the tracklines, the mission code is downloaded and simulated on the vehicle.
 
 

click on this figure to see tracklines from two of our ABE dives in 1996

A typical ABE dive has three parts, a precise descent to the starting point, running the survey, then return to the surface. To save power, ABE uses weights for the trips to and from the bottom. ABE takes about 1 hour to sink or rise 1000 meters, and runs for between three and fours while doing a survey. The entire deployment therefore takes around eight hours from launch to recovery. The figure at right shows the tracklines of ABE's survey 19 and 20, done on the FLOW region in September 1996. You can find other more detailed plots from the 1996 ABE cruise also on-line

 ABE relies on long baseline acoustics to localize itself. The transponders are the same as used Alvin and Jason, and we usually run in the same net as those vehicles (but not at the same time). ABE deals with the transponder returns totally autonomously, filtering out noise, rejecting surface bounces, and switching baselines. Whenever possible, ABE computes least-squares fixes using all available returns (up to 4), but if necessary it can navigate on as few as two transponders. If ABE doesn't get two acceptable ranges, it updates its position by deadreckoning.

ABE uses its long baseline navigation to descend to a specified starting position while consuming limited power. As ABE descends, it tends to glide forward. By periodically pointing itself at the desired spot, ABE executes a spiral pattern, and usually lands within 10 meters of the target. ABE also uses the long baseline acoustic navigation in real-time to fly preprogrammed tracklines

ABE uses an acoustic altimeter to measure its distance off the bottom. We use this both to record the bathymetry and also to drive ABE's bottom-following algorithm. We have been successful in running ABE in some very rugged terrain using a bottom following algorithm based on potential fields. You can see an example plot for a long mission, and also a closeup of a long hill climb

We designed ABE to be extremely stable in pitch and roll and to be reasonably efficient in forward travel. Any resemblance to the Starship Enterprise is coincidental. All the buoyancy is built into the two upper pods, while the majority of the weight (the batteries and the main pressure housing) is in the central lower section. The three-hull structure also allows us to put the vertical and lateral thrusters in between the hulls where they will be protected.

 In 1995, we had our first successful science dives with ABE. We mapped the magnetic field above the New Flow and Floc sites on the Coaxial Segment of the Juan de Fuca Ridge over 6 dives. In a seventh dive, we mapped the temperature field of a hydrothermal plume at the Floc Site. On this cruise, ABE was lauched at night between Alvin dives. While the long baseline navigation worked well enough to guide ABE to the proper starting position, after reaching the bottom ABE relied on dead reckoning (constant heading and forward thrust) to fly tracklines.

In 1996, we operated ABE in conjunction with the ROV Jason. We mapped the magnetic field above the New Flow, above Cage Seamount, and at the Gorda New Eruption site. In 1996, our long baseline navigation was significantly improved, and ABE flew closed-loop tracklines using its in-hull navigation in real-time. This made the surveys much more efficient and more under the direct control of the science party.

In 1999, we participated in an expedition on the Southern East Pacific Rise. On that cruise, we used ABE in conjunction with the DSL-120 sidescan sonar and the submersible Alvin to make detailed maps of the seafloor at a fast-spreading porition of the Mid-Ocean Ridge.

ABE carries two monochrome video cameras in its upper pods, and a strobe light in its tail. These are used to take stereo snapshot video images. We have developed techniques to calibrate the cameras at depth, and we have shown that they can be used to produce good stereo measurements with precision of a few centimeters.

ABE's standard data products include the processed long baseline navigation (recorded every cycle, typically at 10 second intervals), vehicle attitude (heading, pitch, and roll recorded once per second), as well as all the data from the science sensors: CTD, Optical Backscatter, 3 axis magnetometer. We usually distribute these as MATLAB .mat files, but we can make them in a variety of formats.

ABE is a new instrument and still being improved. For example we have added an Imagenix scanning sonar to do microbathymetric surveys. We had a major success making coregistered magnetics and bathymetric measurements on the 1999 cruise.. We will be upgrading ABE's batteries in the future, and we will also increase the speed and range by reducing the drag. In the long term, we hope to use ABE for long-term monitoring of Mid Ocean Ridge sites.

The development and field testing of the Autonomous Benthic Explorer was supported by National Science Foundation Grants OCE-8820227 and OCE-9216775. The principal investigators are Albert Bradley, Dana Yoerger, and Barrie Walden

You can find some pictures and some brief facts about the ABE Team by clicking here. Engineers contributing to ABE include Stephen Liberatore (electronics and acoustics), Rodney Catanach (mechanical systems), Alan Duester (electronics and snapshot video system), Wayne Spencer (mechanical systems), Kenneth Prada (imaging system software), Dermot Dobson (image cature electronics), and Ed Verry (mechanical design). We are grateful to the Massachusetts Maritime Academy for the use of their Sailing Center for ABE's pond tests. We thank Dr. Paul Johnson, Dr. Maurice Tivey,  Dr. Ken Smith, Dr. John Sinton, Dr. Marie-Helene Cormier and Dr. William Ryan for inviting us on their cruises and for their scientific guidance. Our special thanks go to the officers and crew of the Research Vessels Atlantis II, Thomas G. Thompson and Atlantis and to the WHOI's Deep Submergence Operations Group for their skill, dedication, and encouragement.

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