2
Undersea Vehicle Capabilities and Technologies

This chapter describes the types and capabilities of undersea vehicles that will provide the functions necessary to serve as undersea tools for science and industry and respond to vital national needs in the oceans. When describing its capability, it is essential to conceive of an undersea vehicle as a system that encompasses: (1) a vehicle platform plus vessel and shore-based support; (2) a payload of task-performing devices and sensors; (3) a human in control, located either remotely or on site; and (4) the technology and hardware necessary to support, launch, and retrieve the vehicle. This system level vehicle concept must take explicit account of potential missions and objectives.

This chapter points out the importance of system integration—the "glue" binding the human-vehicle system into a single operational unit. The contribution of each subsystem to the overall technical performance capability of the vehicle assembly is assessed in the system context. Advances in the technology and capabilities of some subsystems may provide more overall system performance benefits than would improvements in other subsystems. Finally, and very importantly, each key vehicle subsystem is assessed in regard to the current state of practice and where significant improvements can be made.

In this systems context, an assessment of the state of practice is the starting point for assessing vehicle technology and capability for responding to the nation's needs. An understanding of development trends and influences within, and external to, the undersea vehicle industry is the next important step in determining research and development strategies and priorities. Indeed, some of the valuable aspects of vehicle development are driven largely by advances in other industries, both domestic and foreign. This relationship is a natural process induced by the intense competition among a few small vehicle engineering and manufacturing organizations that must focus their limited development capital on adapting technologies to the special requirements of the undersea environment and missions. The committee identified those technologies that are vital to advances in undersea vehicle capability and are not already being pursued in other arenas.

VEHICLE SYSTEMS

Vehicle systems designs are driven by mission requirements, available technology, and cost. A system must be an effective combination of human operators and vehicle subsystems and components that are integrated to achieve optimal performance (see system elements schematic shown in Figure 2-1). Consequently, the task for builders and designers of a vehicle system is to integrate this multiplicity of subsystems into a working whole. The attributes of each subsystem will be assessed and established by the committee based on the state of technology, relative cost, and mission requirements.

For a given mission or task, designers and engineers begin by considering its particular requirements. These vary according to mission requirements for maximum depth, endurance in range and in time, and sensors; the optimal human role in command and control; power available; cost; and a host of other factors. Depending on these criteria, a particular type of vehicle (DSV, ROV, or AUV) is selected and its subsystems chosen to best fulfill mission requirements. Subsystem capabilities improve with time as technology develops, so the optimum solution in the near future may be significantly better than what is optimal using today's technology.

Table 2-1 summarizes typical capabilities and examples of DSVs, ROVs, and AUVs. The characteristics listed are generic, and there are exceptions; but the table outlines the characteristics and relative strengths and limitations inherent to each type of system.

DSVs place the human in the environment and benefit from the human's high-resolution, three-dimensional observation capability and full visual depth of field that is still superior to the observational capabilities provided by remote sensors. This capability enhances the performance of inspection and imaging tasks as well as manipulation. In general, today's DSVs have relatively large payload capacities and good manipulation capabilities but do not provide real-time feedback to the surface. Moreover, they require costly



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2 Undersea Vehicle Capabilities and Technologies This chapter describes the types and capabilities of undersea vehicles that will provide the functions necessary to serve as undersea tools for science and industry and respond to vital national needs in the oceans. When describing its capability, it is essential to conceive of an undersea vehicle as a system that encompasses: (1) a vehicle platform plus vessel and shore-based support; (2) a payload of task-performing devices and sensors; (3) a human in control, located either remotely or on site; and (4) the technology and hardware necessary to support, launch, and retrieve the vehicle. This system level vehicle concept must take explicit account of potential missions and objectives. This chapter points out the importance of system integration—the "glue" binding the human-vehicle system into a single operational unit. The contribution of each subsystem to the overall technical performance capability of the vehicle assembly is assessed in the system context. Advances in the technology and capabilities of some subsystems may provide more overall system performance benefits than would improvements in other subsystems. Finally, and very importantly, each key vehicle subsystem is assessed in regard to the current state of practice and where significant improvements can be made. In this systems context, an assessment of the state of practice is the starting point for assessing vehicle technology and capability for responding to the nation's needs. An understanding of development trends and influences within, and external to, the undersea vehicle industry is the next important step in determining research and development strategies and priorities. Indeed, some of the valuable aspects of vehicle development are driven largely by advances in other industries, both domestic and foreign. This relationship is a natural process induced by the intense competition among a few small vehicle engineering and manufacturing organizations that must focus their limited development capital on adapting technologies to the special requirements of the undersea environment and missions. The committee identified those technologies that are vital to advances in undersea vehicle capability and are not already being pursued in other arenas. VEHICLE SYSTEMS Vehicle systems designs are driven by mission requirements, available technology, and cost. A system must be an effective combination of human operators and vehicle subsystems and components that are integrated to achieve optimal performance (see system elements schematic shown in Figure 2-1). Consequently, the task for builders and designers of a vehicle system is to integrate this multiplicity of subsystems into a working whole. The attributes of each subsystem will be assessed and established by the committee based on the state of technology, relative cost, and mission requirements. For a given mission or task, designers and engineers begin by considering its particular requirements. These vary according to mission requirements for maximum depth, endurance in range and in time, and sensors; the optimal human role in command and control; power available; cost; and a host of other factors. Depending on these criteria, a particular type of vehicle (DSV, ROV, or AUV) is selected and its subsystems chosen to best fulfill mission requirements. Subsystem capabilities improve with time as technology develops, so the optimum solution in the near future may be significantly better than what is optimal using today's technology. Table 2-1 summarizes typical capabilities and examples of DSVs, ROVs, and AUVs. The characteristics listed are generic, and there are exceptions; but the table outlines the characteristics and relative strengths and limitations inherent to each type of system. DSVs place the human in the environment and benefit from the human's high-resolution, three-dimensional observation capability and full visual depth of field that is still superior to the observational capabilities provided by remote sensors. This capability enhances the performance of inspection and imaging tasks as well as manipulation. In general, today's DSVs have relatively large payload capacities and good manipulation capabilities but do not provide real-time feedback to the surface. Moreover, they require costly

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FIGURE 2-1 Schematic diagram of vehicle systems. pressure housings and life support and safety systems for the human operators. DSV endurance at the work site is limited by prolonged surface periods for crew change and vehicle replenishment. The cost per operating hour is usually much greater than for other vehicle types because of the extra cost attributable to crew accommodation and life support, the larger surface support ship needed to handle the heavier human-occupied vehicle in launch and retrieval, and the more limited availability—therefore, the normally higher day rate—of support ships. In addition, the capital cost component of the day rate for DSVs is potentially higher than for unoccupied vehicles because they can be operated fewer hours due to human limitations. In comparison to DSVs, ROVs provide greater endurance and greater range, including maneuverability of surface support vessels, at lower cost. Because life-safety support is not required, ROVs can operate in hazardous environments and provide simultaneous real-time observation and control to multiple remotely situated observers (Robison et al., 1992; Bowen and Walden, 1993). AUVs are free from the tether restraint common to ROVs and can perform tasks with little or no operator input. However, AUVs must operate on a limited energy budget and can provide little real-time feedback to the operator, who is limited by the bandwidth available when using acoustic communication. However, new advances in task-level control architecture will enable the human to command tasks at high-level in real-time, and acoustical advances are continually expanding the available acoustic communication bandwidth. DEEP SUBMERSIBLE VEHICLES In the 1960s and 1970s many DSVs (with a human in situ) were in operation in a variety of different applications worldwide. Estimates indicate that more than 100 DSVs of all types were built during this time. DSVs have dominated ocean exploration, and some systems, such as Alvin, have been operational for the last three decades. DSVs continue to be used in support of certain military tasks and marine scientific research. However, few have been built recently (only four since 1990), and the number operating today is significantly less than it was 20 years ago. Since the 1970s, ROVs have replaced DSVs for most commercial work tasks, and the committee anticipates that ROVs will provide increasing support for marine science projects. However, there will continue to be certain vital exploration tasks that can be performed only by humans in situ. Most contemporary DSVs require the on-site presence of a mother ship to provide logistical support for the vehicle and its personnel. The submersible's crew usually consists of a pilot and one or more observers. The observers are usually scientists, researchers, or technologists with an active part to play in conducting the mission. Due to limitations on human endurance and on-board power, mission times rarely exceed 8 hours, although some have extended to 16 hours or more. Emergency life support systems must be capable of operating for 72 hours beyond the maximum mission time. The DSV places the crew directly at the site of interest. Visual observations are augmented for close-up inspections (less than 0.5-meter range) by video cameras, an important tool for direct observation (Robinson et al., 1992). Most DSVs are significantly larger than ROVs used in comparable missions. Its size makes the DSV a stable platform to support viewing and manipulative tasks, including biological and geological sampling. However, because it is essential for the DSV to be large enough to accommodate several persons, it is more difficult to handle at sea and more difficult to position when performing tasks in restricted work areas. Essentially, DSVs are vertical probes with limited horizontal range; therefore, they are less suitable for large-scale

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TABLE 2-1 Comparative Undersea Vehicle Capabilities   DSVs ROVs AUVs DEFINITION Untethered, human-occupied, free-swimming, undersea vehicle Tethered, self-propelled vehicle with direct real-time control Untethered undersea vehicle, may be totally preprogrammed and equipped with decision aids to operate autonomously; or operation may be monitored and revised by control instructions transmitted by a data link. DEPTH Many to 1,000 m Very many to 500 m Several to 1,000 m   Few to 3,000 m Many to 2,000 m Few to 3,000 m   Very few to 6,000 m Few to 3,000 m Very few to 6,000 m   One to 6,500 m Few to 6,000 m       One to 11,000 m   ENDURANCE       Time Normally 8 hours, 24 to 72 hours max Indefinite, depending on reliability and operator endurance 6 to 48 hours of propulsion May sit on bottom for extended periods Range < 50 km Limited in distance from host ship by tether 350 km demonstrated; near-term potential 1,500 km, depending on energy source PAYLOAD 1 to 3 people, 45 to 450 kg (100 to 1,000 lb); adaptable to tools and sensors 45 to 1,590 kg (100 to 2,000 lb); adaptable to tools and sensors 11 to 45 kg (25 to 100 lb); adaptable to measuring equipment, tools, and sensors SUPPORT       Ship Most DSVs require large ship support; ship size varies with DSV size Depends on ROV size and mission requirements Medium—depends on AUV size and mission requirements Handling Systems Depend on DSV size Depends on ROV size Similar to ROVs, depending on AUV size Navigation Systems Relative to seafloor or surface vessel Relative to surface/seafloor Seafloor and inertial navigation STRENGTHS Direct human observation and manipulation Real-time feedback to operator, long endurance capability, low- cost per operating hour Potential for automated operations, ability to operate with or without human command and without tether; minimum surface support   Real-time feedback to controller     LIMITATIONS Large size, weight, and cost due to manned requirements Tether cable potentially limits maneuverability and range Energy supply       Bandwidth of data link   Limited mission time   Capacity of internal recorders   Potential personal hazards   Limited work function complexity mapping and surveying operations. As with other systems, advances in propulsion, energy storage, and manipulators will contribute to DSV utility and may even reduce their cost. Developments incorporating innovative uses of materials have already reduced the size and weight of the next generation of DSVs, and a system designed to bring a single pilot to the deepest part of the ocean is in progress (Hawkes and Ballou, 1990; Broad, 1993). REMOTELY OPERATED VEHICLES ROVs are by far the most common type of undersea vehicle; more than 1,000 ROVs have been built since their introduction in the 1960s. ROVs connect to a surface vessel or platform by a tether that carries power and control signals and feedback data from the vehicle. Originally developed for the military, ROV technology was further developed by the civil sector in the early 1970s, when private firms developed ROVs in response to the needs of the offshore oil industry. ROVs were one factor that enabled the offshore industry to move beyond diver depth range. The results were reliable platforms serving a broad commercial market, with some technology transfer back to the military (McFarlane, 1987). ROVs continue to be used reliably in the offshore industry, and innovations in operational techniques and tool packages are expanding the scope of tasks these vehicles can perform (Langrock et al., 1992; Sucato, 1993). Nevertheless, ROVs are vertically operating systems that require significant surface support with attendant costs. ROV manufacturing has been a highly competitive business. After attempts by several companies to compete in the rapidly growing offshore market of the 1970s, only one company, Perry Tritech, Inc., remains in the United States that builds full-size work platforms for the offshore industry. Worldwide, there probably are no more than five companies that have built more than one large ROV system. In the area of the smaller, low-cost ROV systems, a U.S. company, Deep Ocean Engineering, is the largest supplier among eight companies in the world that are in serial production of these vehicles.

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Much of the commercial success gained by ROVs is due to the activities of service companies that operate vehicles under contract. Many of these companies began as commercial diving services, then gradually introduced ROVs as a lower cost alternative to many underwater tasks previously performed by divers. Other companies began as ROV service organizations only, following the pattern set by former successful commercial diving contractors. In the past two decades, the primary driver for ROV technology advancement has been commercial sector demand. Light and medium-weight ROVs tend to have electrically powered thrusters. Heavy work-class ROV systems carry much larger payloads and tools, weigh up to 3,000 kilograms, and are fitted with hydraulic thrusters. These large ROVs found a niche in the offshore oil and gas industries and in the communication industry for underwater manipulation, cable burial, and inspection tasks. Large work-class ROVs are usually fitted with hydraulically powered manipulators, and some have protective cages that are used for launch and recovery. Nonmarine applications of ROVs, principally in nuclear and hydroelectric power plants and municipal water works, have evolved over the last two decades to provide small platforms for inspection of the radioactive, or potentially radioactive, components of nuclear power plants. The technical development and operational experience derived from the nuclear application has benefited the evolution and application of small vehicle designs for undersea application. The mobility of ROVs is often restricted by tether drag, and their stability can be affected by wave action on the surface vessel, which is transferred down the tether. Despite the constraints to horizontal operations within the sea imposed by the tether, ROVs have provided a platform for conducting in situ observations within the water column with little disturbance of surrounding sea life. ROVs have also extended the horizontal search range for larger, more limited survey vehicles such as the DSV Alvin. AUTONOMOUS UNDERWATER VEHICLES Although AUV research has been under way for several decades, the technological challenges and applications are such that these vehicles have developed more slowly than ROVs. A few systems were in operation as testbeds in the 1970s, but it was not until the 1980s, with the advent of microprocessors and associated software architectures, that these systems began to approach truly autonomous operations (Walsh, 1994; Michel and Le Roux, 1981). AUVs have potential advantages over other vehicles types. Because they lack tethers and carry no human occupants, AUVs permit sensing in areas where humans cannot go, such as under ice, in militarily denied areas, or in missions to retrieve hazardous objects. For the past two decades, more than 75 percent of AUV development has been funded by the military, so continued development of this technology may be vulnerable to reductions in defense research budgets (Walsh, 1994). Further, because most of this work has been experimental or directed at military objectives, experience with AUVs in support of scientific missions has been limited, and there has been virtually no experience in the commercial sector.1 Several different types of AUVs have already been developed, each designed to respond to one of a variety of missions. The AUSS, shown in Box 2-1, addresses large area search and detailed inspection requirements. The lightweight AUV sensor platform (e.g., Odyssey; see Box 2-2) aims at fulfilling needs for medium-area surveys under ice and in deep water. The long-duration, deep sea survey vehicles, for example, ABE (see Box 2-3), perform detailed inspections in deep water over long time periods (Michel et al., 1987; Walton, 1991; Walton et al., 1993; Bellingham et al., 1992; Bradley and Yoerger, 1993). A recent emphasis in AUV development is on very small vehicles that can be deployed in large numbers to perform localized, relatively simple tasks such as shallow water mine clearance. These vehicles could be very simple and inexpensive, using basic sensor suites and commercial off-the-shelf electronics. Working together, a constellation of such devices could accomplish the same work as a single, larger AUV. Because multiple units could be programmed to carry out the same task, redundancy would provide reliability. Although this concept is quite new and unproven, it has shown sufficient progress and promise to warrant continuing development. As AUVs enter nonmilitary applications, it is likely that marine science will become a primary early mission and technology driver. As the technology matures, a number of applications in the offshore oil and gas industry, such as cable laying and inspection, and in a variety of other fields can also be envisioned (Fricke, 1992; Collins, 1993; Asakawa et al., 1993; Walsh, 1994). At present, virtually all AUVs are experimental prototypes or proof-of-concept vehicles. None is in routine operational service. AUVs in existence today are vehicles with limited decision making capabilities and endurance. The missions for AUVs call for simple data-gathering, conducting searches, performing surveys, and laying fiber-optic cable. However, until more advanced capabilities evolve, missions requiring probabilistic decision making and true autonomy will be developed only for high-value objectives. AUVs are still in their infancy, and the lack of operational experience with these vehicles in the open ocean marks them as an immature technology with very important future potential. Even now, 1   Examples of scientific work by AUVs (other than under ice) include the Spurv vehicle, operated by the University of Washington in the 1970s, which collects oceanographic data; the French Epaulard, which performed more than 300 dives between 1970 and 1990 (many to depths of 6,000 meters); the AUVs of the Russian Institute for Marine Problems in Vladivostok, which have conducted surveys at depths to 6,000 meters; and the Odyssey, which made dives in the Antarctic and in the Haro Strait off the state of Washington.

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BOX 2-1 AUV Example: AUSS AUSS, the Advanced Unmanned Search System locates and inspects objects on the ocean bottom over wide areas. Mission logic allows the vehicle to perform a side-scan sonar search, break off the search when it identifies a target for close-in optical inspection, and then resume the search where it left off after imaging the target. AUSS acoustically transmits its imagery and data to a vehicle supervisor on a surface ship. The vehicle uses Doppler sonar and a gyrocompass to navigate. It employs a cylindrical graphite epoxy pressure hull with titanium ends. It has demonstrated sustained searches at a rate of 1 sq-nm/hr, including evaluating individual targets discovered along the search path. Owner: U.S. Navy Designer / Builder: Naval Command, Control, and Ocean Surveillance Center Operator: Oceaneering Technologies for U.S. Navy Supervisor of Salvage Purpose: Deep ocean search Depth: 6,000 meters (20,000 ft) Size: 5.2 m long (17 ft) x 0.8 m (31 in.) diameter Displacement: 1,230 kg (27,000 lb) Speed: 6 kt maximum Endurance: 10 hours @ 6 kt Maneuvering: Two vertical thrusters, two canted longitudinal thrusters Energy: 20 kWh silver-zinc batteries Payload: Side-scan sonar, forward-looking sonar, electronic still and 35-mm cameras AUVs are capable of performing a number of clearly defined missions, but they have not been used because of their high initial development cost, lack of awareness of present vehicle capabilities, or lack of confidence by the potential user community. Vehicle success in missions attainable with present system technical capability should provide both the experience and the support required for critical technology advancements to enable undertaking more complex missions. There is already evidence of this progress, as demonstrated by the AUV under ice operations in the arctic (Bellingham et al., 1993) and in recent Odyssey II surveys over the Juan de Fuca Ridge (see Chapter 3). For example, high-level (low-bandwidth) human control of tasks requiring use of manipulators in real-time is one important area under development. The human operator can command directly what is to be done with the object of interest within the water column or on the sea bed, and the vehicle manipulator system will plan and carry out that command in near real-time (Wang et al., 1992; 1993). This technology is similar to the approach being developed to deal with the time delays associated with robotic manipulation in space, and there has been some crossover of

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Box 2-2 AUV Example: ODYSSEY The Massachusetts Institute of Technology Sea Grant AUV Laboratory has built six Odyssey class vehicles, of which five are presently operational. The five operational vehicles, designated type Odyssey II, are being used for Autonomous Ocean Sampling Network (AOSN) research and for development of a rapid event-response capability. A variety of field operations have employed the vehicles, including operations of the original Odyssey from the Nathaniel B. Palmer during an Antarctic cruise in January 1993. In March of 1994 Odyssey II was operated from an ice-camp in the Beaufort Sea. Deep water operations of Odyssey II in summer 1995 culminated in dives to 1,400 meters deep and surveys lasting more than three hours. Vehicle development has been funded by the MIT Sea Grant College Program, the Office of Naval Research, the MIT, U.S. Navy Program Management Office 403, the National Undersea Research Program, and the National Science Foundation. The construction of the latest five vehicles has been supported by the Office of Naval Research. Owner: MIT Sea Grant College Program, AUV Laboratory Designer/Builder: Same Operator: Same Purpose: Science survey/AOSN research Depth: 6,000 meters (20,000 ft) Size: 2.2 m long × 0.57 m diameter Dry weight: 115 kg present vehicles (maximum 160 kg) Speed: 3 kt cruising, >4 kt maximum Endurance: 6 hours at 3 kt (24 hours with maximum battery configuration) Maneuvering: One electric thruster aft of 4 control surfaces Energy: 1.1 kWh Ag-Zn batteries (>5 kWh maximum) Payload: Sensors used include: video camera, conductivity, temperature, depth (CTD) instrument, optical backscatter instrument (OBS), mechanically scanned sonar, altimeter sonar, side-scan sonar, and acoustic Doppler current profiler. Navigation: On-board, long-baseline acoustic navigation, dead-reckoning, ultrashort baseline navigation also used for homing in Arctic missions. Special Features: Small and self-contained computer system ensure minimal support requirements. Free-flooded fairing provides large wet volume for addition of oceanographic sensors. Low-cost through use of commercially available parts and selected manufacturing technologies.

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Box 2-3 AUV Example: ABE Woods Hole Oceanographic Institution built ABE, the autonomous benthic explorer for deep, near-bottom seafloor surveys. In 1995, ABE completed a geophysical survey on the Juan de Fuca Ridge at 2,200-meter depth, including magnetometer, conductivity, temperature, depth (CTD) instrument, and video survey. ABE has the ability to dock to a mooring and remain in "sleep" mode to perform preprogrammed, repeatable seafloor measurements over extended periods. Owner: Woods Hole Oceanographic Institution Designer/Builder: Same Operator: Same Purpose: AUV deep ocean bottom surveys Depth: 6,000 meters (20,000 ft) Size: 2.2 m long Displacement: 550 kg Speed: 1 m/s Endurance: 8–120 miles, depending on battery type and terrain Maneuvering: Full hover, terrain-following Energy: Lead-acid, alkaline, or lithium batteries Payload: Stereo video snapshot TV, CTD, magnetometer, altimeter Special Features: Low power sleep mode, docking, closed loop positioning, terrain-following, high and low-frequency acoustic navigation. National Aeronautics and Space Administration (NASA) research with undersea vehicle development (Stoker, 1994). OPERATIONAL ATTRIBUTES OF VEHICLE SYSTEMS Each type of vehicle has inherent attributes that make it more suitable for certain tasks than are other systems. Because of their stability and ability to provide direct viewing for occupants/operators, DSVs are very good for observation and most work tasks requiring manipulators and samples. However, because of their limitations of range and time on bottom or in the water column, which is dictated by the on-board energy source, human endurance, and the cost of support ship and crew, DSVs generally are not well suited for large area search and survey or extended observation—tasks that are most efficiently carried out by towed vehicles at present.2 Furthermore, because of their human occupants, 2   DSVs have been used in some cases. An example was the Challenger search when the Johnson Sea-Link I and II had a large area survey role in water depths of less than 1,000 meters.

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TABLE 2-2 Current Undersea Vehicle Capabilities Vehicle Functions Requisite Characteristics and Capabilities DSVs ROVs AUVsa Reconnaissance Forward observation, search, measurements Good Good Good Local survey Small area observation and measurement, precise navigation Good Good * Broad area survey Large area (up to 300 sq km) observation and measurement, medium geodetic or relative navigation accuracy Limited Limited * Waste site monitoring Specific site observation, sensing, and water sampling Good Good * Mapping Terrain feature survey, tied to accurate geodetic navigation, larger areas Poor Limited Good Search Relatively large area coverage, acoustic and optical sensing, object identification, good navigation Good Limited * Inspection Close-up observation, optical and other sensors, good vehicle positioning and stability Good Good * Observation Similar to inspection, but implies real-time witnessing of dynamic events Good Good Poor Work General tasks involving vision, object manipulation, and use of tools Good Good * Sediment sampling Specific work task involving collection of material, including coring Good Limited Limited Installation/Retrieval Placement or recovery of objects and instruments in/from specific locations Good Good Good*         Limited Accident investigation Observation, local area search, collection of material evidence Good Good Limited Waste disposal Transport and placement of toxic materials in predetermined locations. May be large quantities or a deep site. Poor Good Poor Water quality measurements In situ sampling and analysis in varying depths and locations Good Good * a Asterisks indicate that, while current AUVs are not suited to these tasks, developments are under way that could improve system capabilities to the point that the vehicle system should be suitable for the application. DSVs are not suitable for operating in dangerous areas, such as in tunnels or around explosives. ROVs, which are powered from the surface, have no real energy limitations. They are also generally stable. Viewing facilities for the human operator are good and continue to be improved, including stereo and new "augmented reality" compatibility.3 Hence, ROVs are inherently suited for working for extended periods, performing local surveys, operating in high-risk areas, and passing large quantities of real-time sensor information back to a surface support vessel. However, due to tether drag, ROVs are limited in how far and how fast they can travel from their support craft, and they are less suitable for large area, long-range search or survey and long, under ice transits. AUVs can move rapidly and, subject to limitations of the on-board energy storage, can generally traverse great distances relative to the other two types of vehicles. This makes them well suited for transporting sensors over large areas for surveys of various kinds. Some AUV systems have used a fiber-optic communications link for all or part of their operation. However, this is not a routine mode of operation, and, without a tether, the communication mechanisms for real-time human intervention are limited. Nevertheless, the narrow beam acoustic links have passed at least 50,000 bits per second (bps), and even low-bandwidth links can pass some useful data. To extend the range of AUV applications, increasingly effective autonomous work systems are evolving rapidly along with the general field of robotics. Another present AUV limitation is that data transfer must wait until the AUV vehicle is recovered on-board a mother ship. In response to this problem, acoustic telemetry schemes now emerging offer a hybrid arrangement where the human can intervene on a limited, non-real-time basis. New advances in task-level control architecture will enable the operator to command tasks at a high-level in real-time (within the new bandwidth). AUVs are limited by the current lack of maturity of task-management architecture that can be placed on-board. Overcoming this limitation is the focus of present research at the Monterey Bay Aquarium Research Institute (Wang et al., 1993; Marks et al., 1994a; Wang et al., 1995). Table 2-2 summarizes the foregoing discussion and includes a list of generic tasks that may be performed by vehicle systems. The committee has characterized the relative abilities of the different vehicle types to carry out these tasks; several qualitative descriptors are used, each representing the collective opinions of the committee and each based on considerations such as those discussed above. EVALUATION OF THE STATE OF TECHNOLOGY This section evaluates the state of the art and state of practice for vehicle technologies and assesses the potential for future developments. In the total system context described 3   For "augmented reality," the human can move an icon (i.e., a finger) inside the video scene to identify features (e.g., "that edge of that rock"), which the computer then uses in its task planning.

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earlier, vehicle technologies are discussed here in relation to the various subsystems that are common to all subsea vehicles. The technologies described are typically applicable to several if not all types of vehicles covered in this report. This section groups the subsystems into two categories: those that directly support vehicle operations, that is, energy, propulsion, and control; and those that are related to payloads to support various missions, that is, sensing, survey, and manipulation. These two categories may overlap in some cases, but the distinction is useful for analysis. New developments with near-term usefulness are cited for each area, and the status of synergistic technology developments from other industries is discussed. Vehicle Subsystems Each subsystem and its driving technologies play a role in overall vehicle performance and contribute to the vehicle's capability to accomplish specific mission objectives. Lack of development in certain technology areas inhibits progress and further applications because they determine or facilitate vehicle capabilities. The technologies in other subsystems are highly developed, and further advancement will not appreciably improve the overall performance of the system. Accordingly, during the committee's evaluation, each subsystem was given an importance rating of "critical," "incremental," or "mature," depending on our evaluation of its impact on further vehicle development. These ratings are characterized as follows: Critical—Improvement in the subsystem will enable or create important new vehicle capabilities. Incremental—Vehicle progress can benefit from development of subsystems technologies in an evolutionary manner. Mature—Development has been successful and further improvement may occur, but development will contribute only marginally to improved vehicle performance, and improvements will be used only if they are cost-effective compared to current techniques. Energy Existing energy sources pose limitations for systems without cable connections (i.e., DSVs and AUVs), affecting system size, payload, and endurance. Energy limitations on AUVs are critical, and they are becoming more critical for DSVs because of the growing power demands of sensors, lights, computers, and manipulators. High-energy density batteries could lengthen missions and generally improve performance. Energy sources are rated in terms of both energy and power. The most frequently used ratings are "specific energy" (watt-hours per kilogram) or "energy density" (watt-hours per liter). Batteries are usually used in underwater vehicles; numerous other energy technologies are also available, but they are more costly. The performance characteristics of available energy sources are compared in Table 2-3. The table is divided into four types of energy systems: secondary batteries, primary batteries, fuel cells, and heat engines. Secondary batteries are electrically rechargeable, while primary batteries are used for only a single cycle. (Ag-Zn may be included in either category; in its primary configuration it may be recharged as many as five cycles, which hardly counts as rechargeable.) Fuel cells are electrochemical devices that passively (without heat) react a fuel and an oxidizer to produce electricity; power levels are controlled by the amount of reactant injected into the cell. Many fuel cells are rechargeable, but not in the same way as secondary batteries; instead, reactant tanks are filled, and in some cases sacrificial metallic elements are replaced. Heat engines are generally closed-cycle, air-dependent systems that react fuel and oxidant in a mechanical cycle to drive an engine, which in turn directly drives the propulsion system or a generator to support electronic equipment. Table 2-3 is not intended to be all-inclusive. Instead it provides an overview of available energy technologies that can be considered for undersea vehicles. Battery and fuel cell technologies developed for applications in space, automobile, and communications industries have not been adapted for use in undersea vehicles because of their cost, safety, immaturity in development, or incompatibility with marine missions. Cost is a primary factor and, as shown in Table 2-3, varies by orders of magnitude for different systems. Cost roughly increases as the energy density increases. For commercial AUVs and DSVs, the most advanced, high-energy systems are presently out of economical reach. These types of energy sources are found mostly in military systems where mission and endurance are primary factors that outweigh cost. Factors important in selecting a battery include power density (the ability to deliver stored energy at the rate needed), outgassing properties, failure modes, reliability, ease and speed of recharge, and ability to operate over broad temperature and pressure ranges. Considerations of safety in handling energy sources, both aboard ship and aboard the vehicle, are critical and have limited the use of some chemistries, such as lithium, despite their high energies. (Some systems give off explosive gases during operation, and others may start fires if they fail.) Current battery types used in undersea vehicles include standard lead-acid and nickel-cadmium batteries as well as silver-zinc and lithium-thionyl-chloride batteries. Figure 2-2 provides a helpful way to visualize the power and energy capabilities of various battery chemistries. For military applications, silver-zinc has been the de facto standard for over 20 years. Recently lithium-thionyl-chloride (see primary lithium in Figure 2-2) has seen some use because of its significantly higher energy rating. However, both silver-zinc and lithium-thionyl-chloride batteries are quite expensive ($300 to $1,000/kWhr) and have high life-cycle costs. Silver-zinc batteries are typically usable for 20 to 30 cycles,

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TABLE 2-3 Performance Characteristics of Available Energy Sources   ASSESSMENT OF ENERGY TECHNOLOGIES FOR USVs Technology Specific Energy Wh/kg Energy Density Wh/Liter Cycle Life Cost $/kWh Maturity for Undersea Vehicles Safety Concerns SECONDARY BATTERIESa             Lead Acid (Pb/Pb0) 35 90 800 50 Proven H generation Nickel Cadmium (NiCd) 55 130 1,000 1,500 Proven Cd toxicity Nickel Hydride (NiH2) 60 150 10,000 2,000 Proven High pressure H Nickel Metal Hydride (NiMH) 70 175 300 50 Proven High pressure venting Silver Zinc (Ag-Zn) 140 380 20 1,000 Proven H generation Silver Iron (Ag-Fe) 150 200 200+ 500–800 Demo H generation Li-Solid Polymer Electrolyte (Li-SPE) 150 360 200 100–1,000 Lab Lithium fire Lithium Ion Solid State (Li-Ion-SPE) 150 360 1,000 100–1,000 Lab None Lithium Ion (Li-Ion) 200 200 2,000 500–1,000 Proven Venting Lithium Cobalt Dioxide (LiCoO2) 220 300 50 1,000 Lab Pressure venting, Li fire PRIMARY BATTERIES*             Lithium Sulfur Oxide (LiSO2) 140 500 1 400 Demo Li fire Silver Zinc (Ag-Zn) 220 400 5 3,000 Demo H generation Lithium Manganese Dioxide (LiMnO2) 400 450 1 200 Proven Li fire Aluminum-Seawater 450 400 1 100 Demo N/A Lithium Thionyl Chloride (LiSoCl2) 480 500 1 300 Demo Thermal runaway Lithium Carbon Monofluoride (Li(CF)x) 800 1,200 1 1,700 Proven Li fire FUEL CELLS             Alkaline 100 90 400 5,000 Demo Gaseous H and O fires Proton Exchange Membrane (PEM/GOX/GH) 225 200 50 10,000 Demo Gas H and O fire Proton Exchange Membrane (PEM/LOX/LH) 450 400 50 15,000 Lab H and O fires Proton Exchange Membrane (PEM/SOX/SH) 1,000 883 50 5,000 Lab N/A Aluminum-Water Semi-cell (A1/H2O/LOX) 1,200 800 1 10,000 Demo H and O fire HEAT ENGINES (Closed-Cycle Air Independent Propulsion Systems)             Internal Combustion Engine 75 170 2,000 50–100 Demo Fuel fire Diesel Engine 125 75 1,000 100–200 Demo Fuel fire Brayton-Lithium Sulfur Hexafluoride (LiSF6) 400 700 1 15 Demo Fuel fire Stirling 200 250 2,000 50–100 Proven Fuel fire a Battery parameters are based upon single cells; non-battery performance parameters are system level. and lithium-thionyl-chloride batteries are primary batteries (not rechargeable). The higher energy batteries hold the potential for violent release of energy under certain circumstances and are not generally used in commercial and scientific applications (Harma, 1988; Moore, 1988). Although higher energy systems are critically important to high-endurance AUVs, cost and safety must be primary objectives as new sources are developed. Newer secondary lithium chemistries hold the promise of reasonable production cost, high numbers of recharge cycles (approaching the lifetimes of lead-acid automobile batteries), no outgassing, benign failure modes, and energy densities better than offered by silver-zinc batteries. Development of these batteries is largely being driven by laptop computer and portable electronics applications, but they are presently being adapted to larger-scale batteries for automotive use and should be available for military and commercial undersea vehicles within three to five years.4 Recharging techniques for secondary batteries are being developed for a variety of user applications in the telecommunications, automotive, and undersea vehicle industries. The intent is to reduce the recharge time, extend the ratio of operating to charging time, improve battery cycle life, and promote personnel safety. Charging techniques such as pulse charging can be used to manage the recharge process and decrease recharge time, to reduce heat generation, and to minimize cell degradation. For undersea vehicles, some new 4   This information is based on current Lockheed Martin Corporation programs and plans and on independent research and development related to energy sources as described in internally published documents (Gentry, 1995).

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FIGURE 2-2 Battery cell comparisons. Source: Lockheed Martin Corporation. lithium chemistries that emit little or no gas during charge and discharge are attractive because they can be charged in place (without recovery or vehicle disassembly). Other charging schemes include charging with solar cells, either by surfacing or by connecting to a subsurface charging station powered from the surface. Other energy system developments include seawater batteries, which react metals with the oxygen dissolved in seawater. These batteries have been difficult to use because they produce very low- voltage and power due to the limited quantity of dissolved oxygen in seawater. However, efficient dc-to-dc conversion can overcome some of these limitations. Since seawater batteries depend on dissolved oxygen, these batteries also may not be suitable in some parts of the ocean, such as in hypoxic areas (Blase and Bis, 1990). Military research and development efforts have explored using seawater batteries, but little work is ongoing in this area in the United States. The Norwegian Defense Research Establishment has built and successfully tested a long-range (approaching 1,800 km) AUV that employs magnesium seawater batteries with a specific energy of over 540 Wh/kg (Apel, 1993; Zorpette, 1994). Metals can also be reacted with oxygen carried on-board a vehicle. An aluminum-oxygen semi-fuel cell has been built and tested at sea with some success (Collins et al., 1993; Walsh, 1994). ARPA is developing a higher power density version (Gibbons et al., 1991) of this type of cell that contains a chemically stable anode (aluminum), works at low temperature, and has environmentally benign byproducts, mainly Al2O3. The key concerns in this technology are oxygen storage and manufacturability. Canadian companies have also developed an aluminum semi-fuel cell, which uses pumped alkaline electrolyte and oxygen sources. This energy subsystem has been tested in an AUV (Stannard et al., 1995). NASA has used alkaline fuel cells widely in spacecraft for over 20 years. One of these fuel cells was successfully demonstrated in a DSV in the late 1970s; however, cost and logistical problems limited further development for undersea use. ARPA is now evaluating a proton exchange membrane fuel cell (Meyer, 1993; Pappas et al., 1993). In this concept, stored hydrogen and oxygen are reacted in a fuel cell with the potential of rechargeability and specific energies of over 600 Wh/kg. The newer proton exchange membrane fuel cells offer many advantages over alkaline fuel cells, including lower cost, higher power capacities, improved tolerance to impurities in the reactant gases, and better long-term cycle performance. Energy capabilities of fuel cells are high, but they are limited by the difficulty of storing hydrogen and oxygen at high densities. To achieve truly high specific energies in fuel cells (>1,000 Wh/kg) practical cryogenic (liquid oxygen) storage and solid hydride fuel are required. The logistics of reactant handling and storage continues to make cost reductions and practical usage of fuel cells elusive. Isotope-based systems and small nuclear reactors have been proposed as ultrahigh energy sources (10,000 to 50,000 Wh/kg) for AUVs; however, these have not been implemented due to regulatory and cost restrictions. Closed-cycle heat engines are another potential high-energy source for AUVs and DSVs. These can use conventional or hydrogen fuels in combustion cycles (e.g., Diesel,

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Significantly higher data rate transmissions can be achieved using pulsed lasers. An ARPA-sponsored project demonstrated 100 megabytes per second data transmission from an autonomous vehicle to a submerged submarine in 1992.10 The attenuation of light in the ocean, however, limits the pulsed laser range to approximately 100 meters or less. An important area of communications for undersea vehicles is the use of satellite or radio frequency links. Both the Woods Hole Oceanographic Institution and the Monterey Bay Aquarium Research Institute have used this method to display science results from ROVs working offshore in real-time at the laboratories on shore. AUVs may also benefit from this technique by docking at subsea data transfer stations periodically to recharge batteries and transfer data for transmission ashore via surface buoys. This is a very active field, drawing on advances in the electronics and telecommunications industries. Significant advances are also being made in sonars and acoustic sensors and in understanding acoustic oceanography. The impact on DSVs and ROVs will be incremental. However, as improvements are made in communication systems, especially acoustic links, and as the new systems become cost-effective, they will have a major impact on AUV performance capability. Payload Subsystems Payload systems are those carried by the vehicle to perform mission tasks. Payload requirements influence the design of the basic vehicle platform for specific missions. The challenge for the payload designer is to achieve the maximum system performance within size and energy consumption limits. Payloads systems include: work systems (e.g., manipulators, end-effector tools, tool racks, and bins for storing samples) sensor systems (acoustic, optical chemical radiation, gravity, and magnetic field sensors; fluourometers and transmissometers; and conductivity, depth, temperature sensors) current meters payload power systems (often separate from the main power system, so that a failure of payload power will not shut down the full system) The number, size, and weight of payload systems affect vehicle size, mass, and propulsive power requirements. The number of energy-consuming payload systems helps determine the total energy system requirements; energy efficiency is important. Work Systems Manipulator arms are used on both DSVs and ROVs to accomplish common scientific and industrial tasks; manipulator development is likely to result in incremental increases in performance capability for these vehicles. Manipulator use an AUVs is still embryonic, but improvements could significantly enhance AUV capability for performing a greater breadth of operations. Current practice involves rate or master-slave manipulators, where the operator (located inside a DSV or on a surface vessel controlling an ROV) operates the arm by throwing switches or by moving a miniature version (the "master") of the manipulator on the vehicle (the "slave"). Typically, modern hydraulic arms on large ROVs can lift hundreds of kilograms, even when fully extended. New control techniques drawn from space developments will allow the human operator to command directly at the task-level what is to be done with the object of interest, and the vehicle manipulator system will respond by carrying out that command. The operator needs no special "crane operator" skills, and a scientist or the field engineer can play the operator role. The operator can then focus completely, in real-time, on the task itself and the objects to be manipulated, whether they are science samples, cores, or equipment (Wang et al., 1993). Manipulators need a device, called an "end-effector," to perform the actual task. End-effectors are often general-purpose hands or grippers, but they can also be special purpose power tools, such as drills, cutters, or wrenches, especially in offshore oil applications. These tools are often grouped into "tool packages" that allow the vehicle to use several different tools during one dive. Whereas formerly it was considered an accomplishment for an ROV to open and close valve handles on offshore platforms, new tool development now allows undersea vehicles to perform increasingly complex tasks in increasingly deeper water, including lubricating, pipe-cutting, making and breaking hydraulic connections, rigging support, and maintaining communications cable under water (Bannon, 1992; Gray et al., 1992). Important tasks performed at mid-ocean depth, where capturing an object involves moving and controlling the vehicle and its manipulators as a single system, require more advanced capability. The new capability, "object-based task-level control," enables the human to command the task to be done; the control system then plans and executes the task, using the on-board vehicle manipulator control system. This capability will allow near real-time control of AUVs in mid-water tasks (Wang et al., 1993). Scientific applications also require manipulators and tools, especially for selecting and gathering samples. New and diverse tools being developed for these tasks will continue to increase the scope of scientific work that undersea vehicles can perform. Nonetheless, general-purpose manipulators will always be important in undersea vehicle science. Current low-cost ROVs usually come with some basic manipulation capabilities, and their dexterity will improve to approach that of the larger systems, although their load capability will likely remain limited (Schloerb, 1992; Sprunk 10   Tests were successful, but no subsequent test reports have been publicly released (Pappas, 1995).

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et al., 1993). Vehicle manipulators can be improved and more sophisticated tasks commanded at the task-level with the development of new underwater sensors for proximity, force, touch, and audio—to give the operator feedback on the performance of manipulators and other mechanical systems—which will most likely be based on devices created for terrestrial and space applications. The payload-carrying capabilities of vehicles are important for selecting or designing tool packages. More capacity means heavier tools capable of performing greater work; and, especially in geology, carrying capacity affects the ability to transport sample rocks to the surface. More research and development needs to be done before AUVs will be able to perform more than simple tasks. However, there are several applications that could use even primitive autonomous recognition and manipulation. This is particularly important when the surrounding geometry can be controlled and anticipated, such as maintaining structures whose physical characteristics are well known or structures specially built for robotic maintenance. Work in outer space has had to deal with similar time delays and task structure problems to those that are faced in some AUV designs. Therefore, it is expected that useful techniques for accomplishing tasks will derive from space-related work as well as from subsea contexts (JPL, 1995). Projects to develop robotics applicable to undersea vehicles have been completed or are under way at NASA's AMES facility, the Stanford University and MBARI joint program (Wang et al., 1993), and the Pennsylvania State University and Woods Hole Oceanographic Institution joint program. Sensors An important undersea vehicle mission is collecting data from various types of sensors. Thus, sensors tend to be a technology limiter or driver for vehicle applications. Sensors in the context of "payloads" refer to those sensors that are not directly involved in the functioning of the undersea vehicle but are used to collect data from various external sources, such as the environment. Such sensors can be carried by all classes of vehicles; typically, the sensors are matched to the type of host vehicle that is transporting and supporting them. For example, sensors that are applicable to large area searches would not normally be installed on DSVs, which may have poor range and endurance capabilities. Furthermore, the characteristics of specific vehicle types can have a significant influence on the design of sensors. A case in point is AUVs, where sensors are critical to overall capability, particularly because of limitations to direct interaction by humans in system control. From a handling and cost point of view, a common desire is to make AUVs as small as possible, thus imposing payload-carrying capacity and resident-energy limitations. This, in turn, imposes similar restrictions on allowable sensors, which must be smaller and more energy efficient. If AUVs make possible longer missions than those of DSVs and ROVs, they will need sensors that are more resistant to fouling and with longer-lasting calibration characteristics. DSVs and ROVs will derive benefits from improvements in these same sensor characteristics. Foreign work on sensors is exemplified by the European Community Cooperative Research Program, Marine, Science and Technology, which funds research and development in sampling and measuring instrumentation, including optical plankton analysis systems, electrochemical instrumentation for in situ determination of trace metals, in situ acoustic characterization of suspended sediment, and anti-fouling coatings for submarine sensors. Acoustic and optical sensors are the most broadly applicable and deserve the highest research and development priority. Acoustic Sensors. Acoustic payload sensors include side-scan sonars and special scanning sonars, subbottom profilers, and imaging sonars. Side-scan sonars represent a well developed engineering practice, with advances primarily in the areas of processing and display, not the on-board transceiver. However, in the case of AUVs, significant efforts have been directed toward digitizing and processing side-scan sonar signals on-board the vehicle, either to conserve data storage space or to form the basis for autonomous action. Various types of scanning sonars can require significant on-board processing and may even contain their own processors. Synthetic aperture sonars, which promise to increase range and resolution by an order of magnitude, require very accurate navigation for short periods. If such sonars can be reduced in size and power consumption to be readily installed on AUVs, while at the same time meeting their navigation requirements, then all potential host vehicles will benefit. Acoustic imaging sensors have been under sporadic development for over 20 years. These sensors, which are useful even in murky water, have relatively high frequencies—on the order of 0.3-2.0 MHz—and can achieve image-level resolutions. Consequently, useful ranges are commonly limited to 100 meters or less. Scanning sensors have frequently been used on ROV systems for approximately 15 years. With a size reduction, these sensors can be installed and operated on some AUVs. The trend toward processing acoustic data on-board as a director for vehicle action, without human intervention, has several direct benefits. One is to reduce time required to implement such action; another may be to assist a human operator. Algorithms imbedded in resident software can assist with specific object recognition for tracking animals or locating objects. This can be done by programming the resident acoustic sensor processor with the unique characteristics of objects to be recognized. When these characteristics are matched with the incoming sensor data, a specific action, such as a vehicle maneuver or an alarm, results. A variety of acoustic sensors are available to study the distribution and abundance of animals in the water column.

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Currently, there are three principal architectures used to count targets, estimate target strength, and estimate volume backscattering. Multifrequency systems use inverse techniques to estimate biological properties; dual-beam and split-beam echo sounders measure the properties directly. Recent research has demonstrated wide variation in scattering properties of zooplankton and micronekton (Greene et al., 1989, 1991, 1994; Wiebe et al., 1990). Developments in these systems will be vital to making accurate estimates of biological properties. Optical Sensors. Video cameras are the most commonly used optical sensors. A number of camera technologies, including charge coupled devices (CCD) with one to three chips, silicon intensified target, low-light-level imaging tubes, stereo pair imaging, and CCD based electronic still cameras, have been adapted from their parent commercial applications for use on undersea vehicles. Sources for these technologies include the television industry and military systems. Because of the severe limitations on available energy in AUVs, low-light imaging sensors (which do not require power-consuming lights) are an important optical payload component. Video cameras and high-speed strobes can be configured to provide high-resolution images of plankton at frame rates of up to 60 per second. Individual targets as small as 60 microns can be identified. Automated image analysis techniques are being developed, but considerable work remains to be done in this area. Sample volumes at the highest resolution are extremely small, and much higher capacity CCDs are required. Increased image sizes, however, will require new data compression schemes that reduce the data loading and significantly enhance low power data storage media. As with acoustic perception capability, the real power of an artificial vision system is the perception capability that carries far beyond optical sensing per se. Providing a degree of scene analysis on-board the AUV makes it possible to send key information over the acoustic data link to the human task director in near real-time, which in turn enables the human to direct tasks in near real-time. For example, using newly developed bottom-mosaicing techniques, the AUV can accomplish local real-time vehicle guidance by scene matching. The scientist looking at the transmitted scene image can instruct the AUV to "hover over that starfish there," and the AUV will obey. Other optical sensors that are emerging in development with direct applicability to undersea vehicles are the laser line scanner (LLS) images and range-gated laser imagers. LLS sensors work on the principle of a single laser, directed to scan a sector of the ocean bottom while the host vehicle moves forward at a uniform rate. The result is a good resolution waterfall (continuously scrolling) image display of much larger areas than conventional camera systems can cover in water of equivalent clarity. Recent enabling developments in LLS technology include the incorporation of solid-state lasers that require only 100 watts power and folded optics, that can reduce overall size by 50 percent. High sensor cost has restricted widespread use of the LLS to date. Laser range-gating uses a traditional CCD image sensor, but "gates" the light reflected from a laser pulse to eliminate unwanted backscatter (Swartz, 1993). Unlike the LLS, which requires the vehicle to be moving to provide the second display axis, range-gated systems can be stationary, that permits precise positioning in the vicinity of an area of interest. Range-gated optical sensors have recently become available, and they have already undergone a tenfold reduction in required power and a significant size reduction; the cost remains high, however. Chemical Sensors. Chemical sensors allow undersea vehicles to perform tasks that would otherwise require collecting water samples for laboratory analysis. It is necessary to distinguish between chemical sensors (devices in which passive diffusion transports the substance to be analyzed to the detector) and chemical analyzers (devices that actively transport the substance through an analytic process). Both types of devices can work on undersea vehicles. Sensors are used to monitor dissolved chemicals. However, few chemical sensors with adequate sensitivity and selectivity are available today for the in situ determination of dissolved chemicals in seawater. Electrochemical sensors for oxygen and pH are the only devices that are in widespread use for in situ measurements. Changes in oxygen and pH reflect the rates of primary production (or respiration) through production of oxygen and consumption of carbon dioxide during photosynthesis. They do not provide direct information on the nutrient elements that limit these rates. Today, the only method for remotely monitoring dissolved nutrient or trace element (e.g., NO3, NH4, PO4, SiO2, Fe, Co, Mn, Zn) concentrations is to use fully automated chemical analyzers (devices in which mass transport moves the chemical through the instrument) that are adapted to operate in situ. Work on such devices is in progress at several laboratories, and it is now possible to monitor concentrations of nutrient species for extended periods of time (up to several months) with sensors appropriate for mounting on a vehicle. Such analyzers are based on the principle of flow injection and are inherently more accurate and complex than sensor systems. Recent advances have made it possible to produce systems with only a few moving parts (Johnson et al., 1986b; Jannasch, 1992). Analyzers also have the advantage of providing a direct, chemical calibration while operating in situ. Chemical analyzers with sufficiently rapid response rates (< 30 seconds) for use on ROVs have been used in situ to map distributions of nitrate, silicate, sulfide, manganese, and iron (Johnson et al., 1986a; Johnson et al., 1990; Coale et al., 1991). Currently, these analyzers have a lifetime of about one day when operated continuously, but their lifetime may be extended if operation is intermittent. They also are susceptible

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to clogging and to failures of valves, pumps, and other hardware. Commercial versions of these instruments are beginning to become available and should be accessible for deployment on operational vehicles in the near future. Expected development includes stop-flow systems, which can be activated at any time from a dormant state (Jannasch, 1992; NRC, 1993). Significant development also is likely to come from fiber-optic sensors ("optrodes"), light transmitted to a target through an optical fiber where it interacts with an indicator designed to respond to the presence, absence, or concentration of the substance to be analyzed. These sensors may be smaller, may require lower power, and may retain more stable calibrations than other types of sensors (Walt, 1992). Conductivity, Temperature, Depth Sensors. Conductivity, temperature, and depth are among the most common sensors in oceanography and have undergone a great deal of technical development. The state of the art in CTDs has advanced to the point that commercial, off-the-shelf CTD sensors are acceptable for most situations encountered by undersea vehicles, including AUV requirements. Fouling of electrodes can be a problem for long-term deployments in shallow water (Bales and Levine, 1994), and new CTD developments focus on ultra low power specialized electronic to eliminate fouling and on dynamic calibration (Brown, 1991). Fluorometers and Transmissometers. Fluorometers measure chlorophyll a small volume of water (Bartz et al., 1988). Photons that enter the chloroplasts in phytoplankton cells cause the chlorophyll to emit light at a different wavelength (about 670 nm). This fluorescence concentration in situ by strobing blue light (typically 480 nm) through is measured and, with proper calibration, can be related to the amount of chlorophyll in the volume. Typically, fluorometers consume 3 to 10 watts of power, weigh under 10 kg in air, and provide resolution of 0.01 mg/l for concentrations up to 100 mg/l. Transmissometers measure particle density by projecting light of a particular wavelength (usually around 670 nautical miles) through a volume of water and measuring the amount of light received at the end of the path (Bartz et al., 1978). The decrease in light received relative to that transmitted is due to absorption of light by the water, dissolved organic matter and particulate matter, and scattering by particles in the water. With appropriate calibration, particle concentrations can be estimated (Bishop, 1986). Transmissometers tend to be lightweight and low power (0.1 to 0.3 w) but are rather large, because of the length of the light path (25 to 100 cm) through the sample volume. Replacing transmissometers with smaller backscatter detectors has been studied. This would provide a similar measurement, but in a size suitable for an AUV (Bales and Levine, 1994). The backscatter detector projects light into a sample volume using two modulated 880-nm light emitting diodes, and the scattered light from particulate matter is detected by a "solar-blind" sensor. The device weighs 0.26 kg in air and requires about 28 milliamperes at 9 to 28 volts. Calibration must be exact if more than a qualitative index of suspended particle concentration is needed. It is important however, that sensors on AUVs be equivalent to those currently used by scientists in other oceanographic applications. Newer optical instrumentation has been developed that is designed to measure spectral absorption and attenuation simultaneously (Moore, 1994). The device requires a pumping system to continuously move water through two flow tubes: one to measure scattering and the other to measure both scattering and absorption. With data on up to nine wave-lengths, it is possible to derive estimates of dissolved organic matter concentration, phytoplankton pigment type, and total chlorophyll content. The device weighs 7 kg in air, is about 70 cm long, and requires 8 w at 12 volts. Other upwelling and downwelling light sensors are also available, some of which would be useful in AUV applications. Magnetic Field Sensors. Small flux-gate compasses are available at low-cost for magnetic surveys. However, scientists also need total field measurements, which cannot be performed with these devices. Proton precession magnetometers are also available. These have been used since the mid-1960s for surface-based surveys (Larson and Spiess, 1969) and could be adapted to AUV applications (Tivey, 1992). Gravity Sensors. The principal development work on compact gravimeters systems was done to support military objectives. Early models were used on DSVs in the 1960s. Smaller gravimeters are being developed that could be used on both ROVs and AUVs; however, accurate compact systems are still expensive and complex. Gravimeters are evolving rapidly. Current Meters. Several sophisticated current meters are available, most of which could be adapted for undersea vehicle use. Vector current meters have been used in moorings to measure two or three components of current at a point as a function of time. Acoustic Doppler meters have also been used on ships and moorings to measure three components of current close to a platform using Doppler shift on backscattered sound pulses. With some modifications, these are being applied to vehicle navigation systems. Several new oceanographic current meters operate on principles that could be incorporated into miniaturized sensors suitable for installation on undersea vehicles. Physical Samplers In addition to data obtained by the various types of sensors and from on-board analytical devices, in situ physical samples need to be collected. As noted earlier, manipulators will remain the primary means for recovering samples from the seafloor. This includes direct manipulator intervention as well as the use of specialized tools, such as sediment corers, grab samplers, scoops, and sieving apparatuses. The

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ability to drill and recover rock cores from DSVs was demonstrated during Alvin dives in 1991 in the Juan de Fuca Ridge (Stakes et al., 1992). A core drilling system was added to the payload package of the ROV Ventana in 1992 (Stakes, 1996). Sampling devices used in the water column include pump-driven suction samplers, closing chambers, and "water catchers" (Youngbluth, 1984; Robison, 1993). For many biological, geological, and chemical samples, maintaining ambient temperature and pressure during transport to the surface is an important requirement, and several such systems are under development. Efficient, large-capacity external stowage systems for physical samples are also an important part of the submersible vehicle's external payload. Generally these consist of bins, with or without lids, or hydraulically activated drawers that withdraw into the vehicle's framework. Elevator systems are being developed that can raise very large or heavy samples that exceed the vehicle's lifting payload to the surface independently. Surface and Shoreline Support A significant component of vehicle system technology and management is the surface support and the shore-side support, as shown in the schematic drawing in Figure 2-1. Surface support requirements vary widely among vehicle systems according to their size, characteristics, and the nature of their applications. Cranes and A-frames are the most common lifting elements, with the largest most complex vehicles requiring dedicated support vessels. Many modern systems are readily deployed from ships of opportunity. Both DSVs and AUVs require battery-charging and support facilities, and DSVs require recharging life support systems. ROVs need tether management and handling systems. In some ROV applications requiring precise placement, the surface support ships must have dynamic positioning capabilities. Shore-side support is equally important to mission planning and development; storage; overhaul, and maintenance; and timely, accurate resupply. Training, while noted as a key element in shore-side support, is also at the heart of safe and successful vehicle system operations. Training is a key consideration when selecting the type of vehicle to perform a task. Launch and Recovery At sea vehicle operations are generally limited by the sea state in which they can be launched and recovered. If the launch and recovery system can be designed to work safely in higher sea states, then the on-station time of the support vessel increases, and the vehicle is made more productive, that is, more diving days are available without shutting down operations because of weather and sea state. Undersea vehicles are usually positioned navigationally and launched, tracked, and recovered by surface ships, semi-submersibles, or platforms. A-frames, elevators, and ramps are typically used to accomplish the recovery. Some vehicles use dedicated vessels and others can work from ships of opportunity. The size and the specialized handling equipment required for a particular vehicle, combined with the size of the crew required to support the operation, determine the size of the ship required for a particular mission and significantly contribute to overall mission cost. Techniques for launching DSVs and ROVs are adequate for most vehicle applications; improvements in launching and retrieval are likely to have an incremental influence on overall system performance. An exception is research related to mitigating snap loads in long ROV umbilical lines when the vehicle is being deployed from a small vessel. Most large ROV launch and recovery systems have a motion-compensation and tether-compliance component to lessen the influence of sea surface motion on the vehicle. On the other hand, launch and recovery techniques for AUVs are still evolving. To take advantage of an AUV's lower cost and minimize surface support, new and unique launch and recovery techniques will be required. SYSTEMS INTEGRATION The systems engineering and integration process applies a formal, disciplined approach to focusing, with quantitative specifications (metrics), on the mission the system is to perform and on using the technologies, hardware, and software that will produce a harmonious union of subsystems to achieve the best combination of reliability, economy, and operational effectiveness. For the development of undersea vehicles this means the systematic integration of all the components of a vehicle system such that an optimum whole is achieved with respect to cost and effectiveness in performing task or mission objectives. As the engineering of submersible systems becomes more complex, the process necessarily becomes more formal and rigorous. Systems engineering and integration (SE&I) is particularly relevant to undersea vehicles because of the diversity of the components and technologies involved. Some of the very elements that appear to be holding up wider usage of these systems are elements that lend themselves to SE&I. Examples are the integration of new sensors and other payloads with existing vehicles and the innovative merging of payload subsystems with new vehicles, both of which require careful attention to ensure reasonable cost, flexibility, and operational effectiveness. In reviewing the preceding sections, it can be argued that a great deal of the current effort is ongoing with respect to components and operational techniques, but relatively little attention is being directed toward bringing all this technology together to form specific payloads and integrate them with new or existing vehicles in a cost-effective manner. In other words, new technology per se is only one enabling

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factor in achieving more widespread use of vehicles; the other is more effective system integration of the technologies that are already available. Implicit within this observation is the recognized value of using common interfaces, particularly mechanical, power, and signal interfaces, between payload components and host vehicles. TECHNOLOGY TRANSFER FROM OTHER INDUSTRIES AND TECHNICAL FIELDS Undersea vehicle development has drawn and will continue to draw on the technology bases from other industries and technical fields. Without this technology infusion, the cost of vehicle development would be prohibitively high, system support would be expensive, and rapid prototyping would be virtually impossible. Key examples of such technology transfer are summarized in Table 2-4. Note that for purposes of clarity, no distinction has been made between any of the undersea vehicle types (i.e., DSVs, ROVs, and AUVs). The table also includes the special requirements of undersea vehicle technology, in some cases calling for significant adaptations of components transferred from other fields. Since so much of the technology for undersea vehicles comes from other fields, the question then becomes how best to take advantage of that transfer. Rarely is the technology automatically transferable and usable without cost. Rather, technology transfer occurs through focused efforts, usually as communications among technical experts. In addition, the movement of skilled technical leaders from one industrial or technical sector to another provides an efficient mechanism for technology transfer. Such efforts tend to be inexpensive and can have a great impact if they improve the total level of transfer by even a small percentage. Still, the amount of effort required for technology transfer is proportional to the advantages it yields and the level at which it occurs. High-level ideas from other disciplines can be the most effective contributors to technology transfer, but they also require the most work to adapt and implement. It is wrong to assume that because technologies are under development in other technical disciplines they require no development within the field of undersea vehicles. The process of adaptation alone imposes significant effort and cost. For a technology assessment, it is worth asking whether one can anticipate developments in undersea vehicles by looking at the future (or even current) directions of these other disciplines. It is probable, for example, that significant progress will be made in energy systems that can be applied to undersea vehicles (particularly AUVs) because of the development effort currently focused on power systems for electric cars. Similarly, as defense-dependent industries, such as aerospace, feel the effects of the current reduction in defense budgets, they will seek new markets for the technologies developed for military applications. The undersea vehicle TABLE 2-4 Technology Transfer Vehicle Subsystem Other Industries and Disciplines Unique Requirements and Adaptations for Undersea Vehicles Energy Auto industry/electric cars, computers, and communications Air independence, shipboard handling Propulsion Hydraulics, pumps, motors, valves, filters, plumbing, brushless dc motors, propellers Hydrodynamics, pressure tolerance, ability to work in oil Materials and Structures Aerospace, boat building, aluminum composites, 316 SS, acrylics, graphite reinforced plastics Pressure tolerance, corrosion resistance Navigation and Positioning Aerospace/compass and gyros, video cameras, lighting, global positioning system/inertial navigation system (GPS/INS) Need to operate in acoustic rather than radio regime; GPS available only occasionally Guidance and Mission Control PC industry, automatic control Unique hydrodynamics, long-term reliability Data Processing PC industry, object-oriented programming, computer-aided software engineering, computer science Packaging for pressure housings, uniqueness of acoustic signal processing, pressure-tolerant electronics Communications Fiber-optics, signal processing, electronics Electromagnetic spectrum not available, acoustic medium only; packaging space restrictions Task-Performance Systems and Tools Construction, robotics, and automation Moving platform and manipulator system, acoustic bandwidth, denser medium, high pressure Sensors Other ocean sciences, instrumentation, micromachinery, medical sensors Seawater medium, long-term stability, biological fouling, corrosion Launch and Recovery Other marine applications/boat handling Ability to work in multiple sea states, tether handling

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engineering community can take advantage of these trends (e.g., laser gyroscopes and inertial navigation systems) by remaining vigilant about how its own needs overlap with those of other industries. FINDINGS Finding. Research and development programs outside the field of undersea vehicles have had major impacts on undersea vehicle technology (Table 2-4). The automobile industry has led the development of batteries; the computer industry, the development of on-board processing; and the communications industry, the development of fiber-optic cable and signal processing techniques (although applying those developments in the ocean environment has required considerable effort). However, the few important subsystems that are unique to undersea vehicles, such as launch and recovery, small acoustic sensors, and undersea vehicle and manipulator control systems that can be commanded acoustically at the task level, do not receive attention from other industries and will progress only from well-supported and sustained efforts of the undersea community itself. Finding. The subsystem technologies that are essential to highly effective undersea vehicle systems are in various states of evolution. These technologies include the following: Energy. A great deal of development effort in energy sources has taken place outside the undersea vehicle field. ARPA has supported work fuel cells, specifically for undersea vehicles, and progress is being made. However, there is a need for high-energy density, low-cost-energy sources that can be commonly used on vehicle systems and that are not under development elsewhere. These energy sources are essential for operation of untethered AUVs, as well as AUVs used in the hybrid mode with telemetry tethers, to accommodate operations requiring long periods of time and long-distance traverses, such as large area surveys. Propulsion. Some interesting developments have taken place, but current technology is generally satisfactory for most foreseeable requirements. Navigation and Positioning. Several acoustic systems are available for relative navigation over small to medium distances (less than 3.5 km, or 2 nm), using local networks of transmitters with accuracies in the range of centimeters. Dead-reckoning capabilities have evolved, fusing information from multiple sensors for increased accuracy. Positioning over longer times and distances, however, will continue to require the use of geodetic coordinates. Locating surface ships and buoys with very high-precision using differential Global Positioning System measurements is commonly accomplished at low-cost. Guidance and Mission Control. Guidance and control of ROVs have been improved through automation. Even greater potential returns are available from automation of AUVs, but unique requirements of undersea vehicles will necessitate major advances in control logic to permit widespread practical use of AUVs beyond the simple tasks that are currently automated. Moreover, new advances in task-level control architecture and acoustic bandwidth will permit human operators to direct AUVs, observe events, and issue high-level commands in real-time—a major advance in the range of tasks that AUVs can perform. Materials and Structures. The current state of practice is generally satisfactory for most applications of undersea vehicles. Major benefit will come from the ceramics work initiated by the Navy for lightweight, deep ocean, pressure-resistant housings and buoyancy structures. Pressure-tolerant electronics may allow significant weight reductions with the elimination of some pressure housings, but the cost of providing these changes may offset the limited weight reductions achievable. Communication. Significant advances in wideband acoustic communications are being made, with parallel advances in computing power. This, and the development of task-level controls, are the pair of technologies that will enable the powerful human near real-time command of AUVs. Work now being supported by ARPA and the Navy is beginning to provide good short-range communications for AUVs. Using optic fibers in ROV tethers has increased the bandwidth and enhanced the supervisory control capability of those systems. Data Processing. Computational power is linked directly to developments in the computer industry. Issues for undersea vehicles are strongly oriented to AUV on-board data logging and manipulation, in situ analyses, and systems management. These will involve low power consumption, high-volume data storage, and innovative manipulation techniques. As with guidance and control, the state of practice is embryonic, vehicles will have a major impact on the practicality of AUV operations. but significant advances in adaptations of data processing to undersea Launch and Recovery. There appear to be no serious technology issues here. This area is constantly evolving, with most emphasis on reducing size so that smaller, lower cost surface support vessels can be used. Task-Performance Control Systems. General-purpose manipulators and tools are reasonably well developed for most anticipated tasks. Improved dexterity can be achieved for special purposes by adapting industrial robot manipulators, which have sensors for proximity, force, and touch. New real-time task planning and task-management architecture with sophisticated interfaces will enable humans to direct operations from the task-level for AUVs equipped with the expanded bandwidth now being developed.

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Sensors. Sensor capabilities impose limits on undersea vehicle missions. A wide variety of ocean-specific sensors is needed—acoustic, optical, chemical, magnetic, and others—to improve the abilities of undersea vehicles to search, identify, and measure pollutants and other vital substances; to help make undersea work more dexterous; and to present human operators with visual and other information on undersea activities. Advances in optical and acoustic sensor systems have been made, but much remains to be done, especially in the area of chemical sensors. The integration or fusion of data from different sensors offers significant potential for advancing this field. Finding. Foreign developments in undersea vehicle technologies can contribute to U.S. needs. By being well aware of technical developments in foreign countries and participating in selected projects, researchers in the United States can incorporate that knowledge into their own work, thus avoiding duplication of research. 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