Improvements in material are also important for applications to AUVs, where lightweight materials can be translated into energy and payload for additional range and endurance and for work or sensing capabilities that require energy.

Currently, nonmetallic materials, including filamentwound epoxies, Kevlar, and graphite composites and ceramics, are used for military applications in both primary structures (pressure hulls) and secondary structures (fairings). However, the cost of some of these materials discourages commercial use. The Navy's AUSS uses graphite reinforced plastic/epoxy for its main pressure vessel, but the future of such materials in pressure vessel applications remains unclear, primarily because of difficulties in manufacturing processes and high cost (Stachiw and Frame, 1988; Stachiw, 1993). Ceramic alumina cylinders have been tested for pressure housings and hulls, with potential weight reductions of 85 percent compared with titanium; however, economical manufacturing techniques are still under development (Stachiw, 1992; Kurkchubasche, 1992, DeRoos et al., 1993). Other ceramics being considered include silicon nitride, silicon carbide, and boron carbide materials (Ashley, 1993).

Advanced materials for fairings include graphite epoxy layup or fiberglass constructions using a fiber-impregnated, high-density polyethylene that is also acoustically transparent (Sloan and Nguyen, 1992). AUSS uses this polyethylene material for its fairings. Advances in high quality acrylic and quartz glass will provide greater visibility to pilots of DSVs. Developments in important materials technology for vehicles aim to provide low-cost, lightweight, high-buoyancy materials for flotation. Sandwiched composite and syntactic buoyancy materials are being used to provide lightweight, high-displacement secondary structures. Although strength, density, and buoyancy are key design factors, longevity, corrosion resistance, and reliability also affect materials selection.

Design innovations have been demonstrated during development of new structures despite the relative maturity of conventional technology in this field. The two Deep Flight vehicle prototypes for a single-occupant, free-flying, full-ocean depth DSV represent an innovation in alternatives for supporting human activity in the deep sea (Hawkes and Ballou, 1990; Ashley, 1993). Deep Flight's pressure hulls are wound glass filament and epoxy matrix. Such a vehicle depends on advanced materials for structures to support its performance goals.

Russian and Ukrainian undersea vehicle programs have developed advanced techniques for fabricating structures of titanium, ceramic, and composite materials, according to two teams of experts who recently surveyed the undersea vehicle programs of western Europe and the former Soviet Union, under the auspices of the World Technology Evaluation Center (Mooney et al., 1996; Seymour et al., 1994).⁵ (Appendix B reviews the status of foreign undersea vehicle programs throughout the world.)

Another materials technology area of importance to system improvement is using coatings and other methods to resist biofouling or degradation of the vehicle's outer skin. Biofouling can create dynamic drag and interfere with the performance of skin-mounted sensors. This can be an especially critical problem for long-duration missions. Conventional coating systems that are used on surface ships may not be desirable for vehicles because the toxic compounds they use to kill organisms might cause chemical contamination of the vehicle's scientific sensors.

The success of most undersea vehicle applications depends on accurate navigation and positioning. Navigation is the function that continuously locates the vehicle within geodetic or relative coordinates and is critical to vehicle safety, operational productivity in real-time, and post-mission scientific and information processing. Positioning refers to the localized and more precise measurements often used to determine specific distances relative to some fixed point. For example, vehicle work in the offshore oil and gas industry frequently involves precision measuring and positioning of equipment relative to installations on the seafloor. When operating a vehicle in a localized area, most contemporary navigation and positioning systems make use of acoustic transponders such as the long-baseline networks widely used in many types of deep water work. Systems of this type use bottom-placed transducers in array fields with typical transponder separations of up to 4 km and can offer accuracies of 1 meter at frequencies of 26 to 36 kHz. Recent developments in acoustic positioning include a high-frequency, high-accuracy system that determines the position of a vehicle with an accuracy of a few centimeters in a bottom-placed transponder field. Other systems, which utilize transducers mounted on the surface ship and a transponder on the vehicle (short baseline), do not require transponders on the seafloor. These systems are widely used for navigating vehicles relative to a support ship. Combinations of these acoustic systems are used to maximize the advantages of each for best navigational accuracy for specific environmental conditions.

Numerous other acoustic and nonacoustic sensor technologies are used on the vehicle to enhance navigation and positioning. Simple video cameras are useful, especially for ROVs and DSVs, when operating near the bottom of a structure and can provide the operator a reference for motion. Computerized image processing techniques have been developed that can use information from video cameras to navigate vehicles automatically (Wang et al., 1992; Marks et al., 1994a, 1994b). Further developments of this type will enhance the value of video as a navigation aide, especially for AUVs, where precise autonomous, near-field navigation is