ALTEX Arctic Cruise
October 7 - November 7, 2001
Tromso, Norway to the Arctic Circle
Equipment
Ship & Vehicle : (R/V = Research Vessel, AUV= Autonomous Underwater Vehicle)
United States Coast Guard Healy
USCGC HEALY (WAGB 20) was constructed by Avondale Industries in New Orleans, Louisiana. Her keel was laid on September 16, 1996. A spectacular launch followed on November 15, 1997. Delivered to the U.S. Coast Guard and placed "In Commission, Special" on November 10, 1999, HEALY joined the icebreakers POLAR STAR (WAGB 10) and POLAR SEA (WAGB 11) in their homeport of Seattle, Washington. The ship departed New Orleans on January 26th, 2000, arrived in Seattle on August 9th, 2000 and was placed "In Commission, Active" on August 21st, 2000.
HEALY is designed to conduct a wide range of research activities, providing more than 390 square meters of scientific laboratory space, numerous electronic sensor systems, oceanographic winches, and accommodations for up to 50 scientists. HEALY is designed to break 1.4 meters of ice continuously at three knots and can operate in temperatures as low as -50 degrees F. The science community provided invaluable input on lab lay-outs and science capabilities during design and construction of the ship. At a time when scientific interest in the Arctic Ocean basin is intensifying, HEALY substantially enhances the United States Arctic research capability
As a Coast Guard cutter, HEALY is also a capable platform for supporting other potential missions in the polar regions, including logistics, search and rescue, ship escort, environmental protection, and enforcement of laws and treaties.
The ALTEX AUV
The design of the ALTEX AUV was guided by a set of needs distilled from both the specifics of the Atlantic Layer Tracking Experiment, and from more general observations of Arctic science. The primary requirement is for an AUV with a range that enables basin-wide observations. The vehicle must have a depth rating of at least 1,500 meters for Arctic hydrography, but ideally should be capable of operating through the entire range of depths of the Arctic basin. Furthermore, the vehicle must be small enough to be logistically manageable, and must be cost effective. Finally, the system must be reconfigurable to satisfy a wide range of science objectives
The complete ALTEX vehicle is 5.5 meters long and 0.53 meters in diameter. The vehicle has been constructed and tested to a 4,500-meter depth requirement, except for the communication buoys, which are rated to 1,500 meters. A full ALTEX vehicle with a minimal payload has a design range in excess of 1,000 km.
A modular vehicle has been designed, separating components into four functional sections: the forward payload section, the buoy section, the semi-fuel-cell section, and the tail section that contains propulsion, guidance, and control systems. The aft section constitutes a complete, self-contained control and propulsion module. All necessary control, navigation, and communications hardware reside in a single 43.18 cm spherical pressure vessel. This includes the main vehicle computer, acoustic and radio frequency modems, and an Inertial Navigation System (INS). Distribution of power and data take place through a backbone of cables in oil-filled tubes and junction boxes. This solution was chosen for its high reliability and flexibility.
The propulsion and control module is one integral unit, where the ducted propeller-and-thruster motor moves in unison by means of two linear actuators. This compact and robust tailcone is designed to withstand the rigors of extended arctic deployment, as well as provide outstanding maneuverability and stability. The articulated tailcone design is capable of +/- 20 degree motions and is constructed primarily of plastic and aluminum. The tailcone electronics are housed in an oil-filled enclosure, and operate at ambient pressure. Many of the electronic boards and test protocols have been adapted from systems developed for MBARI's Tiburon Remotely Operated Vehicle (ROV).
The main vehicle software architecture is an upgraded version of the code originally developed for the MIT Odyssey AUVs, proven on nearly two dozen field experiments over half a decade. The vehicle’s core software is object-oriented, running under the real-time operating system QNX, on industry-standard PC 104 hardware. Critical design requirement for the new code were that the software be robust, expandable, and maintainable for years to come.
The mechanical infrastructure consists of an exoskeletal stressed skin connected by interlocking rings and axially oriented members. This concept results in high structural, packaging, and hydrodynamic efficiency. The skin is made from impact-resistant ABS plastic, which makes the vehicle extremely robust in launch, operation, and recovery. The interlocking joining rings are designed to allow rapid assembly and disassembly of the vehicle into its individual modular sections, making the logistics of development, maintenance, and shipping manageable.
Equipment
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Energy Systems for Long Endurance
One of the outstanding challenges of the project has been the creation of an energy system that will support ranges in excess of 1,000 km. The semi-fuel cell developed under this program is innovative in that it is pressure compensated and is designed to support extended periods of autonomy. This provides a number of advantages over more traditional designs that enclose the semi-fuel cell in a pressure housing. The primary payoff is a substantial reduction in the weight of the vehicle, due to the absence of a large, deep-rated pressure housing. Additionally, the system will be safer for operators, as any leaks of the volatile reactants will be vented directly to the water environment, rather than to the electronic-filled interior of a pressure bottle. Finally, the system should be effectively pressure independent, thus providing a powerful new energy source for deep ocean applications. The fuel cell is being designed for a 4,500 meter depth capability.
The semi-fuel cell vehicle section utilizes the high energy density of aluminum, and an oxygen content of 50 percent peroxide to produce a depth-independent refuelable energy source. Depth independence is achieved through storage of only solid or liquid reactants and wastes. Energy density for the fuel cell alone based on current tests of full size cells is projected as over 300 (Wh/kg dry weight). The complete system for 66 kWh of net energy, packaged as a neutrally buoyant section is designed to fit in a 1.5 m (59") long by 0.533 m (21") diameter hull section. For the vehicle 66 kWh net will provide propulsion power (150W) and hotel power (100W) for 260 hours.
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The vehicle navigation consists of two phases. First, the AUV must perform real-time navigation in order to execute the mission. Second, GPS fixes from the buoys should be used in post-processing to improve the vehicle position estimates. The ALTEX real-time navigation needs do not impose great accuracy requirements in contrast to applications like high-resolution seafloor mapping. The specific track line followed is not crucial, but should be well known after the mission. The scientific survey strategy requires that the vehicle follow a specific bottom contour (1300 m) with periodic tracks normal to the isobath. While high quality heading information is critical to the mission, the navigation system must provide only sufficient positional accuracy to keep track of progress along the isobath, so the mission can be altered as major features are reached along the track.
In real-time, the vehicle will navigate using a combination of an INS, bottom bathymetry, and an open-loop model of the vehicle's thrust/speed characteristics. An accurate, drift-free heading reference is required to follow an isobath and to return to it after a cross leg. The vehicle control will be dominated by the heading loop, which will be commanded to head along (or across) the isobath in the nominal, a priori direction. The commanded heading will be altered with very low bandwidth corrections based on the bathymetry. In particular, a good heading estimate is required to avoid a pathological control condition should a closed isobath be encountered (the vehicle would go in circles forever).
During the mission, we will process the navigation based on information from the buoys (GPS fixes) as well as some information from the INS obtained through the communications links. After the vehicle is recovered, we will reprocess the navigation using the full INS data set from the vehicle data logger.
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The ice-penetrating buoys are carried by the vehicle in the buoy section and provide a means of both transmitting data from the AUV to shore and of obtaining GPS position fixes along the vehicle track. The buoys function as follows:
- On release from the vehicle, a buoy will ascend to the surface and penetrate through the ice and deploy a GPS antenna and an ARGOS antenna.
- The GPS system will provide the location of the buoy, which will be used later to correct the track of the vehicle in post processing of data.
- The ARGOS antenna will be used to transmit data transferred to the buoy prior to its launch from the vehicle.
- Similar to all other sections of the vehicle, the design of the buoy section is modular to allow complete flexibility of vehicle configuration.
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Prior to launching a buoy, the vehicle uploads the collected data into the next buoy in the launch sequence and locates a suitable launch site with an ice cover of up to one meter thick. The vehicle then slows to minimum controllable speed (approximately one knot) at a depth of at least 50 meters. The launcher then releases the designated buoy from the AUV. In its launch configuration, the buoy is slightly buoyant to limit its ascent rate to one meter/second. The launcher also releases a weight simultaneously with each buoy in order to maintain neutral buoyancy of the vehicle. Upon its release, the buoy floats towards the surface where it comes to rest against the bottom surface of the ice. Next, the buoy is extended by means of pressurized nitrogen, which increases the separation between its center of buoyancy and its center of gravity. In this configuration, the buoy becomes stable in an upright orientation. The chemical reaction of seawater with Pyrosolve-Z, contained in the nose cone of the buoy, starts the melting process. This reaction generates approximately 1500 watts of power for 30 minutes. Immediately following the termination of the chemical reaction, the nose cone is ejected and the GPS and ARGOS antennae are deployed. After the antenna deployment, the buoy obtains a GPS fix, and initiates its data telemetry via ARGOS. This process continues until the batteries are expended. It is also important to mention that a full set of data can be stored in permanent memory within the buoy's onboard computer. With the knowledge of its GPS position, the buoy can even be recovered for downloading the data.
Click on the links to the left to find out more about this exciting cruise!