``Distributed Computation Issues in Marine Robotics''
Dr. Knut Streitlien and Dr. James G. Bellingham
Massachusetts Institute of Technology
Sea Grant College Program, Autonomous Underwater Vehicles Laboratory
Cambridge, MA 02139-4307
E-mail: knut@mit.edu, belling@mit.edu
URL: http://web.mit.edu/seagrant/www/auv.htm

The Autonomous Underwater Vehicles (AUV) Laboratory at MIT has for the last 10 years designed, built, and operated robotic vehicles for underwater exploration. The current generation, Odyssey IIb, is 3 years old and have completed 400 dives in the course of 17 field expeditions. There are 5 such vehicles in existence. Unencumbered by umbilical cables and free from the requirements of supporting human pilots, they provide an economical and agile platform for a variety of ocean measurements. Each vehicle weighs about 150kg and has a length of 2m, which is a kind of middle ground in AUV size; large enough to allow a variety of off the shelf payloads, small enough for safe deployment off almost any vessel in sea states up to 5. The batteries, computers and attitude sensors are housed in two 17'' glass spheres set in a flooded polyethylene hull. The spheres permit deployment to 6000m depth, a capability unique to the Odyssey IIb. The current operational envelope is a speed of 1.5m/s and a duration of 3h between recharges. Typical deployments take place from our mission command central, inside a standard 20' cargo container where the operator configures missions via a TCP/IP tether connection. Once he or she starts a mission, the deck crew disconnects the AUV and off it goes after a preset delay. The vehicle's intelligence is based on state configured layered control, which provides good real time performance and a flexible development environment for complex behaviors, with low computational effort. At the end of a mission, the vehicle floats on the surface, and upon retrieval it is connected again for data transfer.

Recent developments in the laboratory have focused on enhanced autonomy, extended prescence in remote areas, and higher endurance, through a program called Autonomous Ocean Sampling Networks. We have designed and built, in cooperation with the Woods Hole Oceanographic Institution, a dock station for the AUV, placed on a mooring 500m below the surface. The vehicle is now able to home in on the dock, attach itself, and transfer data and power autonomously. Through a satellite connection at the mooring surface expression, the operator can access the AUV from anywhere in the world, and have remote control of sampling programs that last several months. Vehicle missions, up to 12h, can run on an automated schedule or be initiated via satellite. Low bandwidth acoustic communication to the dock from the vehicle during missions is now also available. We expect to exercise this system in full by the end of the year.

One objective of the AUV Laboratory is to further develop the operational and physical capabilities of its vehicles through integration with powerful field modeling software. The finite velocity of AUVs results in a coupling of space and time through the survey trajectory, and most energy storage devices places constraints on the extent of their surveys. To get efficient estimates for synoptic oceanic fields, AUVs must therefore adapt their sampling strategies by concentrating measurements in scientifically interesting regions of the survey domain. This is best achieved by modeling software such as the Harvard Ocean Prediction System, which assimilates observations from all available sources by melding these optimally with dynamical models to produce nowcasts and forecasts for all the modeled quantities and their error variances. The AUV will be a node in a heterogeneous network of modeling and observation resources, seeking to minimize the uncertainty in the field estimates through model based adaptive sampling.

The laboratory also has a computer model that can represent the AUV in simulations. This model uses the same algorithms as the vehicle, only the physical properties of the AUV is represented in an idealized way. This simulator will play an important role in Observing System Simulation Experiments, where the entire network described above is exercised in simulation before actual experiments are fielded.

Another trend that is likely to continue is the miniaturization of ocean sensors, which then become potential payloads for the AUV. These can take novel in situ measurements such as optical or biological properties, or in the case of acoustics; integral as well as local samples. Our data holdings are therefore bound to become more complex and multidisciplinary in nature. It is important to the laboratory to make this data readily available to other investigators, both in real time and the long term.

The AUV Laboratory's interest in this workshop follows from these objectives. We want to be able to register our vehicles, simulation tools, and data through metadata or other means, so that they can be made part of larger scale workflows. Our ambition is to make AUVs full nodes in a communication/knowledge/observing network, in a way that deals naturally with the limitations of low bandwidth and long time lags inherent in the communication channels. The AUVs will rely on the distributed computing services of the network for their model based adaptive sampling, which is computationally intensive. We wish to participate in an early stage in the development of standards in this area, both to contribute to them and to guide our future developments.

  1. J. G. Bellingham, C. A. Goudey, T. R. Consi, J. W. Bales, D. K. Atwood, J. J. Leonard, and C. Chryssostomidis, ``A second generation survey AUV,'' IEEE Conference on Autonomous Underwater Vehicles, MIT, 1994.
  2. J. G. Bellingham and J. S. Willcox, ``Optimizing AUV Oceanographic Surveys,'' IEEE Symposium on Autonomous Underwater Vehicle Technology, Monterey, CA, June 1996.
  3. T. B. Curtin, J. G. Bellingham, J. Catipovic, and D. Webb, ``Autonomous Oceanographic Sampling Networks,'' Oceanography, 6(3):86-94, 1993.
  4. P. Malanotte-Rizzoli, ed., Modern Approaches to Data Assimilation in Ocean Modeling, Elsevier Oceanography Series, 1996.
  5. A. R. Robinson, ``Physical Processes, Field Estimation and an Approach to Interdisciplinary Ocean Modeling,'' Earth-Science Reviews, 40:3-54, 1996.