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SYNOPSIS
Autonomous underwater gliders, and in particular autonomous underwater gliders, represent a rapidly maturing technology with a large cost-saving potential over current ocean sampling technologies for sustained(month at a time) real-time measurements.
This report gives us an overview of the main building blocks of an underwater glider system for propulsion, control, communication and sensing. A typical glider operation, consisting of deployment, planning, monitoring and recovery will be described using the 2003 AOSN-II field experiment in Monterey Bay, California.
We briefly describe recent developments at NRC_IOT, in particular the development of a laboratory-scale glider for dynamics and control research and the concept of a regional ocean observation system using underwater gliders.
INTRODUCTION
Sampling the oceans has traditionally been conducted from ships, with the first Global Oceanographic Research Cruise by Sir Wyville Thomson on the HMS Challenger From 1872-1876. It led to numerous discoveries such as Mid-Atlantic ridge and the Challenger Deep in the Mariana Trench to name only a few. Surprisingly it took over23 years to compile the results.
Today with increasing use of remote sensing technologies from satellites and airplanes more and more data becomes available and needs to be processed. Current remote sensing technologies, airborne or from space, do not penetrate far below the Oceans surface. In order to gain more insight into the temporal and spatial processes below the surface we were until recently depending on ship based measurements and moorings. Over the last few decades alternative technologies such as sub-surface floats, remotely operated vehicles (ROVâ„¢S) and Autonomous underwater vehicles (AUVâ„¢S) have emerged to complement the existing sensing techniques. Visions of autonomous platforms roaming the oceans have not come true yet, but technological advances pushed by these visions brought us a long way from the CHALLENGER cruise. These small, smart, inexpensive instrument platforms offer the promise of describing the ocean interior with much higher resolution in space and time than is possible with techniques reliant on ships and moorings. Autonomous floats have demonstrated the power of a distributed network to describe circulation at comparatively modest cost. Profiling versions of these floats are poised to monitor the large scale hydrographic structure of the ocean interior.
BACKGROUND
The motivation for developing autonomous underwater gliders is essentially economy. Oceanographers would know more about the ocean if the access price were lower. Ships have served the ocean community well in describing the basic phenomenology of physical, chemical, temporal and spatial scales too short to be resolved, and extents too long to be covered, by ship based measurements without prohibitive cost.
The need for technology to collect oceanic sections by remote control without the cost of ships was recognized by Henry Stommel in the early days of WOCE, the World Ocean Circulation Experiment.
Dougg Webb had the idea for a thermally powered glider and approached Stommel with it in 1988.Stommel suggested naming it after Joshua Slocum, the New Englander who made the first single handed global circumnavigation in his sail sailboat SPRAY. They secured a contract from the office of naval technology in 1990 to develop a battery powered prototype with which they made 29 dives to as much as 20m depth in Wakulla Springs FL in January 1991 and 14 more dives in Seneca Lake NY in November that year. Stommel passed away two months later. The basic features of their prototype survive in the current generation of gliders: buoyancy engines coupled with mass shifters to control vehicle attitude. The idea is simple enough that an enterprising engineering student recently built a glider from LEGO parts of her senior thesis at Princeton. Making the transition to a functioning open-ocean glider has been the challenge over the last decade.
HMS CHALLENGER
The office of the Naval Research awarded about 30%of its support for an autonomous Oceanographic Sampling Network (AOSN) program to develop seaworthy gliders from 1995 to 2000. Three gliders development groups emerged from the AOSN program, which featured demonstration experiments showing that gliders could navigate at sea under remote control and return data in near real-time. Webb Research Corporation developed Slocum, a Scripps Institution of Oceanography/Woods Hole Oceanographic Institution consortium developed Spray, and a University of Washington group developed Sea glider.
The design philosophy of gliders follows that for profiling floats: a distributed network of modestly priced platforms is appropriate to studying systems where no single element is vastly more important than any other so that integration over the
system is requisite. The fluid ocean is such a system and a distributed approach
provides the fault tolerance to understand it. This approach is the opposite of that
taken by cabled observatories, for example, focused on systems where all the interesting signals are highly localized in space and time (Volcanic eruptions). It also is counter to the temptation to overly complicate platforms so each becomes invaluable, a common tendency on oceanographic instrumentation.
AUTONOMOUS UNDERWATER GLIDERS
Autonomous underwater gliders represent a rapidly maturing technology with a large cost saving potential over currently available ocean sampling techniques, especially for sustained month at a time, real time oceanographic measurements.
Underwater gliders move efficiently through water column by exploiting their ability to change their weight in water. As a result there is upward/downward force acting on the glider. Successive weight changes combined with a change in attitude result in a concatenation of up/down glide cycles. The combination of the upward/downward force with the change in attitude (i.e. pitch) allow the wings and the body to generate the hydrodynamic lift and drag forces which propel the gliders horizontally and vertically through the water. The mechanism to achieve this change in weight is referred to as a buoyancy engine. Currently operational gliders, such as Sea glider, Spray and the electric Slocum gliders use a mechanical displacement actuator, pump or piston to change their weight. A prototype glider using an alternative thermally driven buoyancy engine is currently under development. The closed loop control of attitude and depth is performed by an onboard computer that also executes a pre-programmed mission while submerged. At the surface the gliders acquire their location using a receiver and compare that position to that desired position from the mission plan. The position error is used to compute and estimate the current flow encountered between the two surfacings. The current estimate is then used to correct the dive parameters (i.e. heading) for the next dive cycle. At the surface the gliders are able to communicate globally using an IRIDIUM satellite connection or, for local line of sight communication, some gliders are equipped with a high bandwidth RF modem. An ARGOS transmitted is implemented as a fall back solution. The antenna is integrated into the gliders such that while the glider is at the surface, the antennae are at a maximum height above the water surface for reliable communications. In the case of SLOCUM gliders, the antennae for communication and GPS are embedded within the Rudder Assembly and by means of an inflatable bladder in the tail cone, can be brought out of water. Once the communication to a control centre has been successfully established, the current glider mission can be updated and/or data recorded during the previous missions can be downloaded form the vehicle.
Besides the vehicle position, attitude and other internal states, the gliders collect data from the scientific sensors. Typically the gliders carry a conductivity, depth and temperature sensor, but more recently additional instrumentation such as Photo synthetically Active Radiation (PAR) sensors and Fluorometers have successfully been operated. The drawback due to additional sensors as well as a frequent communications and shallow dives, which imply frequent changes in buoyancy, is an increase in power consumption and therefore a reduction in mission length. Currently the operational endurance of the gliders varies from 3-4 weeks for the shallow SLOCUM gliders to several months for the deeper diving gliders. All three gliders are comparable in size and
handling requirements. Their weight in air is approximately 50kg and their total volume
change capacity is in between 0.5 and 1% of their total displacement. The horizontal speed relative to the surrounding water is typically around 35cm/s.
APPLICATIONS
A. AUTONOMOUS OCEAN SAMPLING NETWORK-II MONTEREY BAY 2003 (AOSN-II)
The autonomous ocean sampling network-II field experiment was conducted during the summer of 2003 in Monterey bay, California. This bay was chosen for its accessibility, resident research institutions with on site hardware (ships, airplanes, AUV s) and its interesting bathymetry. Since the region is well studied there is a large amount of historic data available for inter comparisons. The objective of the experiment was to demonstrate the feasibility of an integrated ocean observation, modeling and prediction system. This experiment differs from previous efforts because of its high degree of system integration, allowing for re-adoption based on ocean model
Predictions. The sampling patterns of mobile observational assets such as ships, airplanes and underwater gliders that is SPRAY, SLOCUM and propeller driven AUV s (REMUS, DORADO) were planned and in some cases, adapted using the numerical modeling and prediction capabilities of two independently running numerical modeling codes developed by two groups. Those models were in turn supplied with data coming from the mobile assets, as well as other sources such as CODAR (Continental radar), satellites, fixed moorings and surface drifters.
GLIDER OPERATIONS
The core observational assets of AOSN-II were autonomous underwater vehicles and in particular a fleet of underwater gliders. Two types of gliders were available, five SPRAY gliders operated at the SCRIPPS institution of Oceanography and 10 SLOCUM gliders operated at the Woods Hole Oceanographic Institution. Prior to the experiment all gliders were shipped to the MBARI (Monterey Bay Aquarium Research In Moss Landing, California). On site the gliders were assembled, ballasted and tested in MBARI s test tank. Since the gliders were deployed for long periods of time, special attention was given to Sensor Calibration; this sensor data were closely monitored during the course of the experiment. After initial shakedowns close to the shore, the gliders were directed towards their operational area. To take full advantage of the different depth capabilities of the gliders, the five SPRAY gliders were deployed in the deep water further outside the bay while the SLOCUM gliders were flown closer to the bay. Figure below shows a snapshot of the glider tracking display from 25 august 2003, with a three day position history plotted behind each glider. The two large dots represent fixed moorings in the bay (M1, M2). The SPRAY gliders were flying on straight lines almost perpendicular to the shore line, while the SLOCUM gliders were either flown on a fixed racetrack (a, b, c, d) or operated in an adaptive sampling mode while the trajectories of several gliders were coordinated and adjusted in a real time experiment communication to and from the gliders during regular operations using the IRIDIUM satellite system. Due to their more frequent inflections and the higher sensor load, the SLOCUM gliders had to be recovered during the course of the experiment. The gliders were either directed to a designated recovery area close to a surface vessel for recovery. When in range of the surface vessel, the gliders were able to directly communicate with the vessel and were controlled using the high bandwidth RF-link. After recovery the gliders battery packs were replaced, the systems re-ballasted, checked out and readied for deployment.
In order to manage the number of different assets in the water, as well as to provide a quick overview of the last available portions, a real time operational display was designed and made available in the control center at MBARI. The display was developed in the beginning of the experiment and was continuously improved during the course of the experiment. The display was automated and ran continuously during the experiment which enabled the control room staff to closely monitor the progress of the gliders and if necessary intervene. On several occasions the operators noted that the gliders were advancing only marginally over the course of several hours. This behaviour was associated with strong currents close to the southern end of the bay (MONTEREY); those currents were on the order of the gliders horizontal velocity. On other occasions the gliders progress was far above the theoretical limits and continued slightly on shore. This behaviour was observed 3-4 times and was attributed to fishermen recovering the equipment. The vehicles were retrieved from the recovery teams, checked out and redeployed if necessary.
The AOSN-II experiment successfully integrated all the above mentioned components and collected a valuable data sat for evaluation of various sampling strategies and modeling efforts. The performance of several multi-vehicle experiments during the course of AOSN-II show the potential for added value by using coordinated control strategies. New tools are under development that allow for improved planning & monitoring of the observational assets which will provide a higher degree of autonomy during future deployment.
FUTURE DEVELOPMENTS AND APPLICATIONS AT THE INSTITUTE FOR OCEAN TECHNOLOGY:
A. Newfoundland Ocean Observation; Modeling and Prediction Facility (NOOMPF)
A team of researchers from NRC-IOT, Memorial University Of Newfoundland & Labrador (MUN) AND Department of Fisheries and Oceans is currently developing a plan to implement a regional coupled Ocean Observation and Modeling system in Newfoundland (NOOMPF). Possible sites of implementation are Conception Bay, Trinity Bay, and Placentia Bay as shown in the figure below. The goal is to develop a capability for automated coupled ocean observation and model predictions on a regional scale. NOOMPF will integrate novel approaches to ocean sampling, modeling & prediction. The potential improvements in the modeling and prediction capabilities of the ocean will significantly enhance our ability to predict and manage the ocean as a resource of food production, transportation and exploration. This facility will provide a unique testing ground for future developments in sampling strategies and technologies as well as a possibility to benchmark future improvements in the modeling and prediction of the ocean environment.
Observations will be based on a suite of different sampling platforms. We will perform conventional observations based on time series for moorings, weather stations and ship.
In addition to these measurements we will utilize available data products from satellites including AVHRR & Radarsat. Besides these assets several autonomous mobile platforms such as autonomous underwater gliders and propeller driven AUV s will be deployed for extended periods of time. NRC-IOT s role in the development of such a system is to develop the control and communication infrastructure necessary to direct and monitor the observational assets. Modular design of ASMT will allow us to sequentially develop and improve individual components of the system. In the baseline version of ASMT will provide a basic display of asset locations in an area of interest. Together with the first display module (Asset Location) a data interface will be developed that will allow us to assess position information and collect data from a selection of platforms (gliders, AUV s, ships, buoys) as soon as they become available. Other parts to be developed include a planning module, a vehicle health monitor and a general asset status module. Some of these components, such as the planning module, access to metrological and oceanographic databases and models. The vehicle health monitor will analyze data coming from the vehicles to provide an automated early fault detection mechanism to warn the operators of possible feature failures. The ASMT can also be used as a simulation and practice environment using real time, recorded to or generated data as inputs into the system.
LABORATORY SCALE GLIDER
In order to complement the efforts, NRC-IOT is considering developing a laboratory scale glider. The purpose of the laboratory glider is to conduct experiments for hydrodynamic testing and control and to provide a test bed for new actuation and flow sensing technologies. The gliders mathematical model will be used to develop various parts of the ASMT, such as Health minor monitor and a planning module. The performance of those modules can then be evaluated using the data coming from the glider
.
As a first step, currently investigation is on design alternatives and constraints of a buoyancy engine. Buoyancy Engines of operations up to 20m of water depth and a size such that it fits into a cylindrical housing of 10cm diameter. The particular characteristic of the engine, such as volumetric rate &absolute displaced volume are to be designed such that the glider reaches a steady state within 2/3 of the depth of IOT s tank.
After the completion of the design of buoyancy engine we are moving towards the hydrodynamic design of the glider. The design philosophy is to be able to build several glider hulls with significantly different hydrodynamic characteristic and reuse the electromechanical internals of the glider. The approach allows for experiments with uncommon designs such as flying wings or hybrid gliders at a reasonable rate.
CONCLUSION
The outstanding question for gliders are what problems will emerge from a few years of field use. A couple of the open ocean missions to date have ended with juvenile barnacles attached, with attendant increase in hydrodynamic drag evident in atleast one. Issues of corrosion or other potential fouling issues have yet to be discovered. Since gliders are reusable, questions of wear arise. The necessary levels of maintenance have yet to be established for glider components.
The availability of low-power satellite data communications is crucial, yet all commercial providers of such service used by gliders to date have gone through bankruptcy, begging the question of stability. In 1995, prospects for lower power satellite telecommunications looked very promising. Today it appears but a niche market, surviving largely due to military demand.
Transfer of glider technology is in its infancy. Webb Research has sold Electric Slocum gliders to four customers. The Seaglider and Spray user groups have expanded by entraining investigators from within their respective institutions. Demand for gliders is persistent, both domestically and internationally. How to best meet the demand is an unanswered question. Presumably, operation by a broad number of users will result in wide application of glider technology to oceanographic problems. Ideally, an investigator and a technician can operate a few gliders backed by producers who provide services inaccessible to all groups.
If the development experience of profiling floats can be used as a guide, by the end of the present decade, glider technology will have made a clear scientific mark on the escape. In the intervening years, gliders will mature ad developers and groups of users exercise its capabilities.
REFERENCES
BOOKS:
T.Curtin, J.G.Bellingham, J.Catipovi and D.Webb-Autonomous
ocean Sampling networks.Oceanography, 6(3); 86-94, 1993.
.A.R.Robinson- Forecasting and simulating coastal prediction system
In C.N.K.Mooers, editor, Coastal Ocean prediction. AGU Coastal
and Estuarine Studied series pages 77-100. American Geophysical
Union, 1999.
JOURNALS:
IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 26, NO. 4,
OCTOBER 2001.
