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Antisubmarine Warfare

ASW: A CRUCIAL UNDERPINNING FOR FUTURE NAVY MISSIONS

Geography dictates that the United States is a maritime nation. It shares land borders with only two other countries; the remaining nations of the world community lie overseas. The United States has vital economic, political, and military interests and commitments around the globe. Recognizing this fact, the National Military Strategy1 states that naval forces "…ensure freedom of the seas and control strategic choke points…provide strategic freedom of maneuver and thus enhance deployment and sustainment of joint forces in theater." "Forward…From the Sea"2 addresses the Navy's enduring contributions to strategic deterrence, sea control, power projection, forward presence, and peacekeeping.

Fundamental to all of the above is the nation's ability to fight and win on, over, and below the seas. Antisubmarine warfare (ASW), which is one aspect of the ability to fight and win, was designated as one of the Navy's core competencies by the report of the Commission on Roles and Missions of the Armed Forces3 on May 24, 1995. Highly capable ASW forces that are enhanced through the infusion and implementation of emerging technology and operational innova

1  

Joint Chiefs of Staff. 1995. National Military Strategy of the United States of America, U.S. Government Printing Office, Washington, D.C.

2  

Department of the Navy. 1994. "Forward…From the Sea," U.S. Government Printing Office, Washington, D.C.

3  

Department of Defense. 1995. Directions for Defense, report of the Commission on Roles and Missions of the Armed Forces, U.S. Government Printing Office, Washington, D.C.



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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare 1 Antisubmarine Warfare ASW: A CRUCIAL UNDERPINNING FOR FUTURE NAVY MISSIONS Geography dictates that the United States is a maritime nation. It shares land borders with only two other countries; the remaining nations of the world community lie overseas. The United States has vital economic, political, and military interests and commitments around the globe. Recognizing this fact, the National Military Strategy1 states that naval forces "…ensure freedom of the seas and control strategic choke points…provide strategic freedom of maneuver and thus enhance deployment and sustainment of joint forces in theater." "Forward…From the Sea"2 addresses the Navy's enduring contributions to strategic deterrence, sea control, power projection, forward presence, and peacekeeping. Fundamental to all of the above is the nation's ability to fight and win on, over, and below the seas. Antisubmarine warfare (ASW), which is one aspect of the ability to fight and win, was designated as one of the Navy's core competencies by the report of the Commission on Roles and Missions of the Armed Forces3 on May 24, 1995. Highly capable ASW forces that are enhanced through the infusion and implementation of emerging technology and operational innova 1   Joint Chiefs of Staff. 1995. National Military Strategy of the United States of America, U.S. Government Printing Office, Washington, D.C. 2   Department of the Navy. 1994. "Forward…From the Sea," U.S. Government Printing Office, Washington, D.C. 3   Department of Defense. 1995. Directions for Defense, report of the Commission on Roles and Missions of the Armed Forces, U.S. Government Printing Office, Washington, D.C.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare tions are required to effect undersea battle space dominance. Whether in peace or war, naval forces must be capable of surveilling and controlling that battle space to the degree necessary to accomplish their mission. ASW forces will be required to operate effectively both in the open ocean and in the littorals. The challenges of each are different and, in some cases, require unique capabilities. In any future conflict with submarine-capable nations, initial engagements will likely involve littoral ASW. This is an enabling mission that must be carried out prior to the transit of heavy-lift forces through straits and choke points or landing forces ashore. Submarines and mines are two practical means that can be deployed by an enemy to interrupt the flow of joint forces. Naval forces of the future must be able to rid the battle space of the threat posed by hostile submarines to allow for follow-on operations. ASW is also a key element in the nation's strategic posture, since much of the world's nuclear striking power will be based at sea. In many situations, time will be of the essence. To be effective, ASW forces will require system capabilities that allow for accurate remote sensing, targeting, and effective employment of weapons. Currently, ASW resources are relatively constrained by the overall pressure on military spending, competing warfare priorities, and a continuing debate over the relative significance of the submarine threat. The continuing drawdown in naval forces and current deemphasis on ASW have seriously eroded the Navy's capabilities in this warfare area at a time when potential future adversaries are rapidly acquiring advanced quieting techniques and other offensive submarine technologies. It is possible that U.S. maritime forces will indeed face a credible submarine challenge within the strategic planning horizon of this study. The who, what, and when of future submarine threats remain uncertain. Suffice it to say, however, that global interest in advanced submarine capabilities continues to provide the clear potential for credible submarine opposition in future conflicts. This opposition can generally be described as sea denial—a capability that is founded in the inherent stealth of the submarine. A small number of unlocated submarines, even with limited capabilities, can pose sufficient threat to disrupt operations of maritime forces. Unlocated submarines can influence events by forcing an advancing battle group to proceed with caution. The primary technical challenge in this warfare area is the requirement to detect increasingly quiet submarines. However, detection is not the only ASW requirement: effective weapons, fire control, and self-defense capabilities are all essential elements of a credible war-fighting capability. There are shortfalls in ASW in all of these areas, but each of these problems generally follows from detection limitations. THE GROWING WORLDWIDE SUBMARINE THREAT The Russian Federation and the People's Republic of China have publicly declared the submarine as the capital ship of their navies. Many potentially

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare adversarial Third World countries essentially have done the same, including Iran, North Korea, India, and Pakistan. A former Indian Navy submarine flag officer, B.S. Uppal, commented in 1994 that developing nations desire submarine forces because they are a cost-effective platform for the delivery of several types of weapons;4 they counter surface forces effectively; they are flexible, multimission platforms (e.g., antisurface warfare (ASUW), special forces, intelligence and warning, and ASW); they are covert and thus can be deployed with minimum political ramifications; and finally, they can operate without supporting escorts. The quieting of advanced non-U.S. nuclear submarines and advanced conventional submarines operating on battery power is now at parity with U.S. submarines. The United States no longer enjoys a comfortable acoustic advantage against the front-line submarines of some other nations. The Russian Federation, for example, continues to build new classes of highly capable submarines and to operate its latest vessels outside of home waters, including waters contiguous to the United States. Russia has recently given additional emphasis to the importance of its Navy, particularly submarines, by creating a budget line for them that is separate from the rest of the armed forces. The People's Republic of China, which currently has a submarine force that is, for the most part, obsolete, is investing heavily in submarine technology, including designs for nuclear attack submarines, strategic ballistic missile submarines, and advanced conventional submarines expedited by the purchase of KILO-class submarines from Russia. China hopes to leap generations of submarine technology in its ambitious buying and building program. There are currently more than 150 submarines in the navies of potentially unfriendly countries in the rest of the world other than Russia (see Appendix B for further detail). Forty-five of these are modern, nonnuclear types. Forty-five more submarines are on order worldwide, principally from Russian and German shipyards. By 2030, it is projected that 75 percent of the submarines in the rest of the world will exhibit advanced capabilities. Most of them will have air-independent propulsion systems that allow 30 to 50 days of submerged endurance without surfacing or snorkeling. When these submarines are in a defensive mode, that is, when they do not have to travel great distances or operate at high speed, they have a capability nearly equal to that of a modern nuclear submarine. Quieting technology will likely proliferate, which will render these submarines difficult to find even with the latest ASW equipment, and they may be armed with highly capable combat and weapon systems. The readiness and proficiency of submarine crews in the rest of the world are improving, and their performance is generally underestimated. Today, training of crews is offered by the countries that export these submarines. Operated competently, these submarines are particularly difficult to locate, especially since 4   Comments made to American Systems Corporation personnel on December 14, 1994, during a presentation entitled "A Third World Submarine Perspective" by B.S. Uppal.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare they operate mostly in their home waters in a defensive posture, can operate slowly, and can take advantage of acoustic and oceanographic factors to maximize their covertness. In summary, by 2035, the capability of the United States to project power in the world may be seriously and competently challenged by submarines from major powers (Russia and China) or from a number of potentially unfriendly Third World nations. OPEN OCEAN TO THE LITTORALS—A WIDE RANGE OF ENVIRONMENTS Unless a resurgent Russia or an equivalent submarine threat from a country with global reach and ambitions emerges, U.S. ASW operations are expected to be needed principally in the regional waters of adversaries-frequently, littoral waters. This situation will arise because potential adversaries are expected to use their submarines mostly for blockades, mining, and interdiction of surface vessels; these submarines generally will have somewhat limited range and endurance. Air-independent propulsion and, perhaps, the limited proliferation of nuclear propulsion could remove the restrictions to range and endurance. For the foreseeable future, however, identifiable adversaries have neither the need nor the capability for global, far-ranging submarine deployment. Hostile coalitions, however, could conceivably pose simultaneous threats in widely separated regions. It is a misconception that littoral waters are always shallow. Although it is true that shallow waters are always in littoral areas, an examination of the bathymetry of potential regional conflict areas reveals that the littoral regions encompass the full range of depths from deep to shallow. To illustrate by just two examples, consider first a potential conflict with the People's Republic of China (PRC) over the status of the Republic of China (ROC) on Taiwan. PRC submarines that would be of primary concern to U.S. naval forces would likely be engaged in antishipping operations or possibly threatening U.S. carriers, combatants, and logistical supplies. Such operations would likely occur east of Taiwan out to a radius of perhaps several hundred miles from the Chinese mainland. A look at the water depths in that region reveals that more than 80 percent of the water is deeper than 200 fathoms and only a tiny fraction is shallower than 100 fathoms. The second example is a possible conflict with Iran over the closing or blockade of the Strait of Hormuz. Iran's submarines are based at Chah Bahar and can be expected to operate mostly south or southeastward from that base. Water depths of more than 100 fathoms are reached just 15 miles off the Iranian coast. In oceanographers' lexicon, the waters of Earth's littoral regions cover the spectrum from blue to blue-green to green to brown. A characteristic that is commonly assigned to littoral waters, however, is complexity, exemplified by much shorter scales of variability in both space and time compared to the deep waters of the

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare open ocean. Important features of the littoral regions include tides, irregular and tide-produced internal waves, diurnal variations, variable bottom topography, and a cluttered acoustic and visual background-all of which make ASW operations more difficult. Effective naval operations in the littoral regions require detailed knowledge and understanding of the maritime environment there. This is a Navy-unique requirement not only for undersea warfare functions such as submarine warfare, antisubmarine warfare, and mine or countermine warfare, but also for amphibious warfare (surf zone, charting, and other near features). Indeed, all naval warfare areas require inputs on weather, ocean condition, aerosols, refraction, and the like. RECENT TRENDS—REDUCED INVESTMENT IN ASW CAPABILITY The lack of consensus on a perceived submarine threat and competing warfare priorities, combined with mounting pressure on the overall defense budget, has put the Navy's ASW program at historic low levels in recent years. This deemphasis on ASW, especially on new development, has occurred at the same time that global interest in submarine capabilities has continued apace with steady progress and increased investments in submarine technologies on the part of potential adversaries. Detecting and then classifying weak signals from future quieter submarines in highly dynamic, complex environments will be extremely difficult. There are technical options that can be pursued, however. The increased availability of computer processing power when applied in combination with new sensor technologies, including nonacoustics and microelectromechanical systems (MEMS), offers the potential of greatly enhanced detection capability. There are no quick fixes, however; a dedicated, focused research effort will be required to develop and effectively deploy these solutions. For example, advanced signal processing techniques must be coupled with a thorough understanding of target and environmental parameters and their variabilities. Historically, development of enhanced ASW capabilities has arisen out of long-term R&D projects including significant at-sea testing and experimentation. Notable examples include the Critical Sea Test (CST) program, the nuclear-powered ballistic missile submarine (SSBN) security program, the Sound Surveillance System (SOSUS) program, and the Surface Tactical Surveillance System (SURTASS) program. These projects provided the solid foundations for progress in ASW that may be missing in the current restricted funding environment. ASW will likely continue to be a people-intensive art. Although advancements in automated data fusion and machine-based reasoning will no doubt occur, the human is likely to remain the best integrator of multiple inputs, reaching

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare a conclusion from imperfect data better than any machine. This skill is maintained only by practice, with feedback and learning on a continuous basis. Therefore, regular periods of training at sea, with realistic scenarios and platforms, should be a part of a balanced ASW effort. The increasing sophistication of ASW technology, including the increased automation of ASW assets, will require more highly trained and educated officers and operators, and the Navy must ensure that its personnel have the requisite technical expertise. Expertise in ocean sciences and geophysics will be particularly valuable. The general issue of education and training to enable sailors and marines to effectively utilize the capabilities afforded by new technology is addressed in Volume 4: Human Resources of this series. OCEAN ACOUSTICS: WHAT ARE THE LIMITS? The challenge to ASW is to increase detection gains faster than the gains in quieting or stealth. The 35 decibels of quieting since 1960 has reduced open-ocean coverage from basin scales to a few kilometers, well within range of weapons. For passive systems the issue now is, Can enough gain be recovered to obtain operationally significant increases in detection ranges? For active systems the issue is, Can the advances in source technology, reverberation reduction, and target identification lead to systems with low false alarms while target strengths continue to decrease? For the next decade, improvements now programmed expect performance gains of 10 to 15 dB for both passive and active improvements. Although less certain, evolutionary gains on the order of 1 dB per year up to an overall gain of an additional 20 dB seem very possible within the 2035 time frame. There are several bases for these projections: Experiments have demonstrated5,6 that the spatial and temporal coherence of acoustic signals in the very low frequency (VLF) band (8100 Hz) is high, and so wide-band, coherent processing using wide-aperture arrays with a very large number of sensors can lead to high-gain systems. Signal processing algorithms based on coherent, range-dependent propagation codes using real-time environmental inputs now can exploit this coherence. The reduction of wide-band threat signatures in the VLF band is difficult because it involves the entire platform, and hull coatings are less effective at damping these long-wavelength acoustic signals. Consequently, it is more likely 5   Duda, T.F., et al. 1992. ''Measured Wave front Fluctuations in 1000 km Pulse Propagation in the Pacific Ocean," Journal of the Acoustic Society of America, 92, pp. 339-355. 6   Munk, W., et al. 1994. "Heard Island Feasibility Test," Journal of the Acoustic Society of America, 96, no. 4, pp. 2330-2342.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare that gains in detection capability will exceed quieting and stealth gains in this part of the spectrum. The coherence of the noise field increases in the VLF band since shipping is a dominant component. Adaptive processing methods using large-aperture arrays have high performance for both resolution and sidelobe control in suppressing these target-like noises. In the past, system performance was constrained by limited computational power. However, relatively high computational power is now available and can be deployed on mobile platforms or with rapidly fielded fixed systems. Similarly, array technology using wide-band, fiber-optic telemetry and miniaturized sensors enables high-performance systems that can exploit ocean coherence. DEVELOPMENT IMPERATIVES The development imperative is to improve the gains of sonar devices to enable the detection of potentially hostile submarines that will be characterized by low source levels and target strengths. Sonars have evolved continuously from a few bulky sensors with analog signal processing to arrays with hundreds of miniaturized sensors and high-speed digital signal processing. The understanding of oceanography has grown from the discovery of propagation channels in the ocean to the development of accurate, range-dependent propagation codes and environmental models for noise and reverberation. Similarly, reliable and smart sensing systems can now be deployed using advances in ocean engineering, a lot of hard-won field experience, and rugged very large scale integration (VLSI) electronics. This evolution has been scientifically and technologically intensive; at this point, the easily attainable performance gains have been achieved and even greater exploitation of the science and technology will be required in order to develop future systems than incorporate significant performance gains. The necessary components of an effective ASW technology development program are as follows: Well-posed science and technology; At-sea experiments with sensors that are both well calibrated and accurately navigated to provide real-time environmental data; Fundamental exploitation of the advances in ocean acoustics, oceanography, and signal processing; Robust ocean engineering for their deployment; Integration of communications, navigation, and high-speed computation; and Highly trained operators. At the end of the Cold War the merging of these components showed prom-

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare ise for substantial gains in ASW capability, but the momentum has been lost as a result of budget cuts and program cancellations. Arrays have been essential for almost all sonars. They provide signal gain, noise suppression, and target bearings. Many of the expected future performance gains will likely come from more capable arrays that exploit both horizontal and vertical properties of the signal field. One can look to the oil exploration industry to contemplate what is now possible. There, up to 18 multiline, towed arrays, each with 8-km apertures leading to a total of 2×104 sensors, are now routinely used. Two-ship operations with cross-registered arrays are also routine. Finally, mechanisms need to be in place to ensure that competition keeps costs low. In ASW, although specific array designs certainly depend upon the specific application, the imperative for larger, more capable arrays with multidimensional apertures and a very large number of sensors is clear. This is the only means by which additional performance gains can be achieved. More sensors are needed for noise suppression, and larger apertures for better resolution. The acoustic field has a three-dimensional structure that can be resolved only with multidimensional arrays. The spatial and temporal coherence of both submarine signals and ambient and reverberation noises is the fundamental acoustic attribute that will enable the development of high-performance ASW signal processing systems. Experiments have demonstrated that the ocean supports much greater acoustic coherence than is now exploited by current operational ASW arrays if one compensates for source-receiver motion effects and makes use of accurate propagation models. This is especially so in the VLF bands where acoustic signals propagate most efficiently and where submarine quieting is most difficult to achieve. High coherence implies the potential for high processing gains. Several experiments have achieved signal gains greater than 40 dB, and a few greater than 50 dB, in contrast to the 20- to 30-dB gains realized in present systems. Similar gains can be expected against coherent directional noise fields such as those found in high-clutter or battle group environments. Table 1.1 shows the apertures scales that may be required to fully exploit the spatial coherence of the ocean.7-9 Similarly, the temporal coherence of the ocean can be exploited further. Oceanographic processes such as internal waves and sea surface motion introduce time fading, as well as multipath and boundary interactions, leading to time spreading which limits coherence; nevertheless, careful compensation for motion can push the processing closer to the limits of temporal coherence. Experiments 7   Baggeroer, A., W. Kuperman, and P. Michalevsky. 1993. "An Overview of Matched-Field Methods in Ocean Acoustics," IEEE Transactions, Journal of Ocean Engineering, 18, pp. 401-424. 8   Flatte, S., et al. 1991. "Impulse-Response Analysis of Ocean Acoustic Propagation," pp. 161-172 in Ocean Variability and Acoustic Propagation, J. Potter and A. Warn-Varns, eds., Kluwer Academic Publications, Norwell, Mass. 9   Carey, W.M., J.W. Reese, C.E. Stuart. 1997. "Mid-frequency Measurements of Signal/Noise Characteristics," IEEE Transactions, Journal of Ocean Engineering, 22, pp. 548-565.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare TABLE 1.1 Aperture Scales Required for Sonar Arrays Low frequency, deep water Low to mild frequency, shallow water Horizontal Vertical Horizontal Vertical 1,000 wavelengths 100 wavelengths 200 wavelengths 20 wavelengths here suggest coherent integration intervals on a scale of 105 kiloperiods (>1/2 hour at 50 Hz) for deep water and 104 kiloperiods (>3 min at 500 Hz) for shallow water are attainable. Source-receiver and target motion leads to Doppler and differential Doppler effects that are robust phenomena in many ASW scenarios, yet sonar systems do not exploit them to nearly the extent used in space-time processing and synthetic aperture radar systems. An acoustic version of synthetic aperture radar has been a long-sought goal for sonar systems. Three principal obstacles have frustrated achievement of this goal: (1) For a ship to be able to steer itself at sea, it must travel fast enough for its control surfaces to be effective. The ratio of the average cruising speed of a ship to the speed of sound in water is relatively high for sonar compared to the relevant ratio for ship radar (10−3 for sonar versus 10−6 for radar), and this limits the maximum range. (2) Reverberation persists for a long time, which leads to low pulse repetition frequencies. (3) Navigating the sensors to the small fraction of a wavelength needed for coherent beam forming is very difficult. Advances in precision navigation using the global positioning system (GPS), miniaturized sensor positioning systems, and coherently navigated receiver arrays, either towed or hull mounted, can mitigate these difficulties for both passive and active sonars, especially at very low frequencies. More generally, fully navigated beam formers (i.e., those that track targets in position and velocity) can lead to longer coherent processing intervals, and hence, higher gains. In radar this is termed space-time beam forming and leads to impressive noise suppression for both clutter and jamming, yet it has not been explored in sonar. Coherent sonar signal processing can be considered a generalized form of matched filtering, so that spatial and temporal replicas, or matching signals, are required for implementation. The processing for operational arrays, whether mobile or fixed, is based on narrow-band, plane wave fronts for passive systems plus direct replica correlation for active systems. The ocean waveguide introduces vertical inhomogeneity and multipath and modal dispersion, as well as differential Doppler effects in acoustic signals. These effects are very significant for long-range, deep-water VLF and for shallow-water propagation. Consequently, plane wave beam forming and replica correlation processing do not lead to the gains of a fully coherent processor. Acousticians have developed numerical codes for this waveguide propagation. They enable fully coherent processing in these environments with a generalized form of beam forming, usually termed

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare matched-field processing. With this technique, the complexities of the acoustic waveguide can actually be used to improve array performance. In fact, these methods are most effective when the arrays have a vertical extent. This fully coherent processing technique incorporates environmental data for wide-band prediction of the signal and noise fields. It also uses coherence limits of the propagation to establish array lengths and coherent processing intervals, as well as the division between the coherent and incoherent sections of the sonar signal processor. Target detection is a function of both the signal and the noise fields. The acoustic noise field for both passive and active systems has considerable structure that can be exploited by modem adaptive algorithms. The noise field for passive systems can be very directional because of commercial shipping and nearby friendly ships, and this is amenable to cancellation techniques. Similarly, the reverberation in an active field is governed by water column multiple reflections and bathymetric features that can also be mitigated. Doppler phenomena are important features of a noise field as well, and they have not been exploited in sonar systems to the extent that they have in radar systems. Oceanographic models are now used to predict quantities such as transmission loss and noise levels, which are entries in the sonar equation. Although there has been progress toward greater integration into the sonar system, such as the real-time implementation of the parabolic equation and on-line ambient noise models, the environmental data now available from real-time sensors, databases, and satellite observations are not well utilized for optimizing sonar performance. More importantly, oceanographic data generally are not brought to bear on sonar signal processing problems. For example, virtually all of the algorithms now in use are based on a plane wave signal model, such as that used in radar. Acoustic propagation in the ocean is significantly more complicated than the propagation of microwave radar signals. Nevertheless, there has been substantial progress in the development of codes for predicting acoustic propagation when accurate environmental models are available. Although random ocean dynamics and complicated boundaries will always preclude exact models, the limits on environmental data accuracy must be pressed to exploit the coherence of the signals. Oceanography coupled with accurate propagation models can permit the extraction of useful signals from what would otherwise appear to be noise. Advances in the technology of sensors deployed from either moored or remote vehicles, tomographic systems, and satellites will continue to make these data even more useful for improving sonar performance. Adaptive beam forming has long been pursued in the sonar research community; yet with a few exceptions, it has not been employed routinely in operational systems. Until quite recently, improvements in adaptive beam forming techniques have not justified the added complexity and computational resources required, and performance could actually be degraded if adaptive beam forming was implemented incorrectly. The utility of adaptive algorithms depends on the

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare ambient noise field and array geometries. Adaptive algorithms are most useful in directional fields such as those with high clutter. Although this technique was initially advocated as a means to enhance the resolution of short arrays, its use for sidelobe control in the presence of strong interference and for matched-field processing is very important. ASW against quiet, stealthy targets is always a weak signal detection problem, so the suppression of sidelobes, even low-level ones, is important. Although there are still limitations for wide-band arrays with a large number of sensors, the computational capability to implement adaptive systems has grown dramatically and should continue to increase. Adaptive beam forming has been a topic of extensive research and development, including the development of arrays with a large number of sensors. Now, there is greater understanding of the algorithms and their limitations, and new algorithms are being developed, so that more robust and stable implementations of adaptive beam forming are expected to be available soon. It is important to emphasize that fully coherent processing and adaptive processing are different issues, although they can reinforce each other. The rapid pace of development in acoustics, oceanography, signal processing, and ocean engineering relevant to sonar applications should enable a shortening of the development time between the demonstration of the engineering feasibility of advanced sonar techniques and their implementation. The long development times for new sonar systems, which have in the past often spanned a decade or more, are no longer acceptable—long development times do not allow the Navy to respond rapidly to changing missions and needs. There should be continuous process of system development with a build-test cycle included as a fundamental part of the process. It is anticipated that the signal processing capability will continue to grow rapidly since it is often coupled to commercially driven products. New system architectures should be flexible and well documented so that new hardware and software developments can be implemented quickly and easily. There will likely be fewer versions of sonar sensor hardware because the design, testing, and implementation are unique to ASW and thus more costly; consequently, such hardware should be designed to be modular and flexible, making use, for example, of widely compatible interfaces. The entire development cycle should incorporate a continuous evolution of building and at-sea testing, with extensive support by science and engineering ship riders to complete the design feedback process. Prototype systems will likely be too complex to permit effective evaluation by operators trained on legacy systems. The training of sonar operators and officers is currently not adequate to meet the near-term future threats anticipated by the panel, and the complexity of future systems will only exacerbate this shortcoming. The current practice of training new enlisted personnel in oceanography for only two weeks is certainly not enough for understanding its impact on ASW performance across the range of potential operating environments. Investment in the experience of sonar chiefs will continue to be important. Officers will need much more extensive training in

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare FIGURE 1.2 Basic transmitter and receiver configurations for four different active acoustic concepts: (A) monostatic, (B) bistatic, (C) multistatic, and (D) forward scatter. Varieties of waveforms, including wavetrains, wide bandwidths, and impulsive, combined range and Doppler sensitivities; and Advanced processing algorithms and techniques for clutter reduction. In addition, receiver and processing advances in passive acoustics will directly benefit the active side as well. Typical active acoustics system concepts are shown in Figure 1.2. Concept A represents the more traditional monostatic configuration common to all tactical sonars as well as the SURTASS-LFA system using the research ship Cory Chouest, now assigned to the Pacific fleet. Concept B has proven to be quite valuable in certain environments where surface ships, submarines, buoys, and fixed systems with bistatic receivers can provide detection capability at greater distances from the transmitters. Concept C extends the bistatic concept to include multiple transmitters and, thus, can achieve significant area coverage by distributed fields of multiple transmitters and multiple receivers. The use of distributed sources and receivers greatly complicates the target submarine's ability to hide, evade, or attack and further ensures the safety of manned platforms when the sources (and perhaps receivers as well) are autonomous and separated from the manned platforms. Multiple sources prevent the target from avoiding Doppler or specular orientations while receiver locations can remain covert. Such configurations have been

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare tested fairly extensively by the Navy and are also routinely used in offshore seismic oil and gas exploration. Concept D represents an alternative detection concept to the monostatic, bistatic, or multistatic approaches, all of which are based on the backscatter of acoustic energy from targets. Forward scatter achieves detection only when the target crosses the line between source and receiver but has the advantages of significantly lower transmit power and increased difficulty for countermeasures by target strength reduction. Thus, fields of many low-power sources and receivers could provide a surveillance web tracking a submarine as it cuts across individual source-receiver lines. Continuing advances in active acoustics, in both transmitter and receiver technologies, combined with advances in unmanned systems, computational power, and C4ISR (command, control, communications, intelligence, surveillance, and reconnaissance), could lead to active system concepts that cover large areas at reasonable cost. Unmanned underwater vehicles (UUVs), unmanned aerial vehicles (UAVs), or unmanned surface vehicles (USVs) could deliver or act as transmitters or receivers. UAVs could also act as communications relays or weapon deliverers. Overall, active acoustics could confound an adversary without putting manned platforms at risk. Transmitter technology should continue to advance to provide lighter-weight sources with higher output power and more bandwidth. Autonomous sources and/or receivers will permit the continued development of concepts using fields of distributed sources and receivers to very large scales. Exact system configurations will depend strongly on the environment because it is critical to get acoustic energy on target and the reflected energy to a receiver. A significant issue will continue to be clutter removal by advanced processing techniques. With numerous distributed receivers and accurate navigational capabilities (e.g., GPS) multisensor data processing concepts, either incoherent or perhaps coherent, could provide significant gains in target detection and classification. A major concern for the testing and use of lower-frequency active acoustic concepts, particularly those involving backscatter, is the fear of physical harm to humans (divers) and marine animal life exposed to high acoustic signal levels (see Appendix D). Efforts are under way to establish safe levels for humans, particularly, but longer-term efforts to address the issue, including marine animal life, could lead to limitations on transmitter power, areas of operations, frequency, and so on. The conclusions of such research could affect the types of systems developed in the future. Because of the lower power requirements, multistatic backscatter and forward-scatter concepts may be preferred. The entire subject of what constitutes safe levels of active acoustic emissions in various situations should be addressed explicitly by the Navy. The Navy should become the recognized leader in establishing the knowledge base from which regulatory limits will be determined.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare Overall, active acoustics has significant potential to provide a robust search capability in reasonably large areas, and although it is difficult to avoid counter-detection by an adversary submarine, the bistatic, multistatic, and forward-scatter concepts can mitigate most adversary countermeasure approaches. As with passive acoustics, none of these developments will occur without the dedicated, focused, long-term R&D projects based on extensive at-sea testing that have proven necessary to make real advances in detectability. Although COTS can provide the processing hardware, the submarine detection applications and systems remain unique to ASW, without counterparts in the commercial or, for that matter, other military endeavors. Because of advances in submarine quieting and the use of off-board sensors, detection ranges in some scenarios are beginning to fall into those commonly associated with modern mine-hunting sonars. As detection ranges decrease, the role of high-frequency sonar for both passive and active systems may become more prominent. NONACOUSTIC ASW: A NEEDED COMPLEMENT TO ACOUSTICS Based on the current technical understanding of nonacoustic submarine signatures and their detectability under various operational and environmental conditions, nonacoustic ASW concepts will, in general, complement acoustics in at least the following ways: Exploit shallow submarine operations, particularly when acoustic detection might be degraded, thus inhibiting an adversary from using an important part of his operating envelope and denying him a safe haven from acoustics. Exploit a submarine's hydrodynamic signature, which is unavoidable under many conditions when the submarine must move to conduct most missions. Contribute independently derived glimpses or moderate-quality data to the overall data fusion process. Nonacoustics must be considered in the context of four target operational regimes: Periscope depth with exposed masts or scopes, Periscope depth with all masts or scopes retracted, Nominal safe operating depth to avoid surface ships (roughly 150 feet), and Lower depths down to design depth or on the bottom. Current or developmental ASW capabilities exploit such regimes to varying degrees. For regime 1, sensors such as visual, low-light-level television (LLTV),

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare periscope-detecting radar, and electronic support measures (ESMs) work when exposure occurs. Since exposure can be controlled to a few seconds, the glimpse-only scenario is viable for those sensors. However, the submarine may stay at periscope depth conducting periodic exposures, thus making regime 2 also a viable scenario. Detections by magnetic anomaly detectors (MADs), infrared (IR) detectors, passive optics, and laser detection and ranging (LIDAR) are now possible for regimes 1 and 2. Under certain conditions, MAD and LIDAR may detect submarines in regime 3 and MAD could conceivably extend to regime 4. Most current nonacoustic systems are deployed on fixed-wing aircraft or helicopters. ESM is on all platforms, while surface ships may have limited visual, IR, and radar capabilities. The current U.S. technical knowledge base in nonacoustic ASW is fairly robust, with significant investment by the Navy in many programs and continuity residing currently in the SSBN Security Program. In addition, DARPA, the Central Intelligence Agency (CIA), and currently the Office of the Secretary of Defense (OSD) have projects that contribute as well to the knowledge base. For the long term, this knowledge base suggests areas of significant payoff: Evolutionary improvements in current capabilities that exploit shallow submarine operations (regimes 1 and 2) although some signatures such as periscope cross section, magnetic, and perhaps optic might be successfully reduced. Concepts that exploit the hydrodynamic signatures; hydrodynamic effects cannot be controlled to below detectability thresholds under many conditions. Such concepts can be categorized into three types: Remote detection of surface effects by airborne or space-based sensors (e.g., synthetic aperture radar (SAR) IR, and optical); Remote, but direct, detection of the submerged wakes or internal wave fields by airborne sensors (e.g., optics and electromagnetics); and In situ detection of the turbulent wake, contaminants contained in the turbulent wake, or the internal wave field using sensors mounted on or towed from surface ships or submarines. In addition, extension of the above concepts to space, unmanned air, and undersea vehicles could act as force multipliers for all sensor concepts. In general, nonacoustics is not a robust ASW solution but an opportunity-exploitable approach that inhibits or exploits important portions of an adversary submarine's operating envelope. However, there may well be situations, particularly in the littoral, in which search areas are not large and acoustic conditions are poor; therefore, nonacoustic sensors could become a significant ASW contributor.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare UNMANNED UNDERWATER VEHICLE NETWORKS:A ROLE TO PLAY One development currently in its infancy is the use of networked unmanned underwater vehicles11 for synoptic (fully three-dimensional) environmental sensing. Such environmental information, obtained through local sampling or tomography, could be of significant operational usefulness in improving sonar capabilities. In addition, the imagination is stimulated by the vision that many small, dispersed platforms could be connected with a few manned platforms for networked war-fighting. Such a vision is currently premature but might become practical over the time horizon of this study. The panel believes that the environmental sensing applications of UUV networks should be pursued first. In this way, the Navy will develop the enabling technologies for such networks, as well as operational experience in their deployment. The two key enabling technologies for UUV networks are their power sources and reliable underwater communications. There has been much activity in the area of air-independent long-duration power sources, but there does not seem to be any consensus regarding which of many possible avenues—batteries, fuel cells, and air-independent combustion—should be pursued. This is an area in which considerable interest also exists in the commercial sector, and new long-duration power sources may arise from activities such as the Partnership for Next-Generation Vehicles. Thus, a top-down research approach may be effective in guiding R&D in this area into the most promising paths. Currently, the leading candidate for underwater communication is acoustic communications. Data rates of 2 to 20 kilobits per second (kbps) are currently achievable, although distances are limited to a few kilometers in shallow water. It is even possible to give UUVs Internet addresses and to communicate with them using standard Transmission Control Protocol/Internet Protocol (TCP/IP) protocols. An improved understanding of ocean acoustic coherence, discussed elsewhere, could also improve our ability to communicate underwater. Little work, however, has been done on the vulnerability of acoustic communications networks to acoustic jamming or on covert underwater communication. UNDERSEA WEAPONS—A FUTURE PERSPECTIVE The continuing evolution of the potential submarine threats facing U.S. forces and the expanding set of roles and missions for Navy platforms, particularly submarines, require advancements in weapon capabilities for the future in order to counter the threat and to ensure success in the complex scenarios envisioned. At present, the Navy maintains an inventory of submarine-launched Mk-48 and 11   National Research Council. 1996. Undersea Vehicles and National Needs, National Academy Press, Washington, D.C.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare Mk-48 ADCAP torpedoes and surface- or air-launched Mk-46 and Mk-50 torpedoes. Although important programs are under way to improve weapon capabilities such as the Mk-48 ADCAP modification and the lightweight hybrid (utilizing elements of the Mk-46 and Mk-50), these efforts can be viewed as minimally sustaining the R&D capabilities for undersea weapons. Concurrently, a drawdown of weapon inventories is under way. In the near- to mid-term time frame, the focus will be on improving the performance of undersea weapons in complex, littoral environments and scenarios against an increasingly stealthy submarine target equipped with sophisticated countermeasure devices for thwarting a weapon attack. Insertion of new technology in signal processing, detection and classification, sensors, and guidance algorithms is planned. Design changes will be undertaken to achieve reduction of costs for weapon test exercises and life-cycle support. In the case of submarine-launched weapons, in particular, design changes that reduce the radiated noise signature of the weapon will reduce the counter-detection range by the target and the effectiveness of the target's counteraction. The current inventory of undersea weapons will be upgraded to utilize advances in electronics and computer technology to exploit very wide-band signal processing in order to enhance detection and homing against the low-signature threats employing multiple, complex countermeasure devices. In the next 10 to 20 years, the current inventory of weapons will have to be replaced by weapons with significantly advanced capabilities. The advancements required will be driven by new approaches and scenarios for engaging the target submarine and by consideration of the platform design flexibility that new weapons can provide. One such scenario is a rapid attack situation wherein a sudden detection of a threat submarine is made, perhaps at relatively short range, requiring an immediate response to achieve weapon on target and, for of our own submarine, to ensure survival. Advances in hydrodynamics such as supercavitating flow coupled with new fuels that utilize seawater as an oxidizer will provide an option for very high speed weapons that may be employed to greatly shorten the time from weapon launch to target engagement or, conversely, from weapon detection to response. For engagements at attack ranges similar to those of today, advances in power and energy density of undersea propulsion systems will provide the capability for reducing the size of weapons or increasing performance in terms of speed and range. Reduced weapon size, such as submarine-launched torpedoes that are one-half the current length with equivalent or better performance and payload effectiveness, would significantly increase the options available to submarine designers. Although individual surface, air, and submarine ASW platforms must retain the capability to cope with the advancing threat, future ASW operations will likely evolve in a cooperative engagement sense that simultaneously utilizes the capabilities of multiple assets. In such a situation, the platform delivering the weapon may not be the same as that maintaining track on the target and generat-

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare ing a targeting solution. The inherent stealth of the threat submarine, projected to have even further signature reductions in the future, implies that a detection and ASW weapon targeting system will rely increasingly on distributed short-range sensors, glimpses of signals embedded in clutter, and extensive data fusion to function effectively. During hostilities, these occasional valid contacts would have to be exploited rapidly and accurately by a cooperative engagement, rapid attack weapon system. Such a weapon system would consist of a delivery capability to distances up to 10 to 20 nautical miles from the launch platform with a total response time of minutes, a reacquisition capability to locate and track the target within a relatively small area (a few miles radius), and a terminal attack weapon. The enabling technologies for such a concept might include the following: A high-bandwidth sensor data network and fusion capability (i.e., an undersea cooperative engagement capability [CEC]); A rapid-response, high-speed airborne delivery vehicle; A UAV with long loiter time carrying a shorter-range, high-speed delivery vehicle; Rapidly deployed distributed sensor field on datum with fused processing; or Off-board guidance and control of the weapon to the close-in vicinity of the target, potentially using a high-bandwidth data communication to the weapon that permits wide-band, intersensor processing between a weapon and off-board sensors. Further, the possibility of weapon attacks from much longer standoff ranges is envisioned. To this end, weapons capable of long-endurance, stealthy closure of the target before the attack occurs would be needed. One such concept involves use of a UUV-like vehicle a weapon or a weapon delivery platform. Such a weapon would be capable of operating in concert with a distributed sensor field providing long-range target detection and vectoring to a position for launching an attack. The proliferation of sophisticated undersea weapon systems available to potential adversaries will drive a concerted effort to achieve assured self-defense against incoming torpedoes for both surface ships and submarines. A major focus of this effort will be on counter-weapons (i.e., hard-kill antitorpedo torpedo, capable of intercepting an incoming torpedo and destroying it). The technology of the future could permit a small counter-weapon to autonomously detect an attacking torpedo, close on it at high speed, maneuver at high rates, achieve a relatively close point of approach, and fuse a lethal warhead to kill its target.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare EXPANDING NEED FOR COMPREHENSIVE ASW CONCEPT OF OPERATIONS ASW is often perceived as a submarine-versus-submarine problem. This has never been the only scenario and will be less so in the future. ASW has always been the integration of information from mobile and fixed assets; in the future, successful execution of undersea warfare will involve the fusion of data from multiple sensors—below the surface, above the surface, in the atmosphere, and ultimately, in space. These platforms will be fixed as well as moving, surfaced and submerged, flying, and orbiting. The potential of space-based sensors has not been realized. Sensing the various hydrodynamic effects created by a submerged hull moving through the ocean may be possible with key enabling technologies yet to be fully exploited. Future space-based sensors, whether they be developed for environmental information or for military applications, will provide additional information to be fused into a complete picture of the ASW situation. Modern maritime helicopters have proven to be extremely effective ASW vehicles. Their capabilities and strengths derive from the ability to operate from small platforms at sea and use deployed and on-board sensors, including dipping sonars, to prosecute submarine contacts. Speed, range, and endurance are the most limiting factors of current ASW helicopters. New vertical takeoff and landing (VTOL) aircraft, with capabilities at least equal to the tilt rotor V-22, hold promise of overcoming these limitations. There is no question about the desirability of a VTOL aircraft that could proceed at 250+ knots to a distant (200+ nautical miles) contact area and prosecute a submarine target with time on-station and effectiveness equal to or greater than those of the SH-60R at its normally shorter operating ranges. Technology advances in unmanned systems will allow the proliferation of sensors over the undersea battle space without exposing manned platforms to unacceptable risks. Unmanned air, surface, subsurface, drifting, and fixed platforms will act as force multipliers to provide a highly integrated network to address the ASW problem. These same technology advances will also provide a threat multiplier for potential adversaries. The increasing complexity of future ASW concepts of operations and the need to test them in an even more complex joint arena require that modeling and simulation become a significant facet of ASW systems development, training, and decision aids. The difficulty will remain the existence of validated models that faithfully characterize the physical processes associated with the generation, propagation, and detection of acoustic or nonacoustic signals in a wide range of environments, some of which are quite complex and harsh. Although the best possible models of appropriate fidelity should be employed in large-scale and joint simulations, it is important to maintain a strong focus on the R&D necessary

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare to understand and maximize the detection processes, which in turn will result in improved, if not validated, models for use in simulations. The successful Navy of the future will be the one that assimilates disparate data into a meaningful integrated whole. Technologies such as GPS enable sensors at widely spaced locations to be combined as though they were close or fixed on the same platform. Communications capability using high-bandwidth fiber-optic cable and satellites is exploding, driven by the information and communications industry. In addition, special communications based on acoustic and blue-green laser technologies are possible to enhance the connectivity of underwater systems. Leveraging these capabilities puts ASW on the threshold of a major transition. The potential to process raw sensor data, either incoherently or coherently, from a field of sensors of various types is now conceivable, given emerging technologies. Thus, a concept analogous to the CEC of the air defense warfare domain can now be envisioned for ASW. For ASW CEC to be carried out effectively, it must incorporate assets of multiple platforms, technologies, and information sources. This level of cross-platform, cross-disciplinary cooperative engagement will require high-level, authoritative coordination. RESEARCH OPPORTUNITIES Emerging enabling technologies will make currently unachievable system concepts realizable in the coming decades. Examples of these concepts with their requisite enabling technologies are presented in Table 1.2. It is evident that the TABLE 1.2 Possible Future ASW Concepts Concept Enabling Technologies Submarine detection, instant localization from space or air vehicles Hull Wake Surface effects Picobuoys (highly distributed floating sensors) Portable phase conjugate systems for self-adaptive, autofocusing active sonar High-power, efficient, small, high-bandwidth acoustic sources Coherent sensor fusion: acoustics, electromagnetic, optics Undersea acoustic systems for localization, navigation, and adaptive focusing Many decibels of clutter rejection in SAR, optics High-resolution, multipixel focal arrays (visible, IR) New blue-green laser concepts Small, cheap sensors with navigation, communications, processing Lightweight transmitters; computational capability New transmitter concepts, materials Ultrahigh digitization rate technology Network and source technologies

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare research technologies associated with information processing and sensors, much of which are being developed independent of Navy requirements, are the key to realizing these system concepts. These technologies represent a research opportunity for radically enhanced ASW capabilities. RECOMMENDATIONS While ASW remains a technically challenging warfare area, there are a number of steps the Navy can take to remain in control of the undersea battle space into the next decades. Highest-level Recommendations Establish and maintain a dedicated long-term program, centered on at-sea measurements and tests, to provide the science and technology bases for pushing active and passive acoustic array gain to the limits imposed by the ocean. Decades of experience have shown that advances in ASW come about only as a result of such programs. Focus passive and active ASW sonar development on exploitation of the ocean's intrinsic coherence and on the use of large volumetric arrays, as enabled by massive computational power, miniaturized sensors, and high-bandwidth transmission links, with a goal of 20-dB or greater detectability gains beyond near-term programmed improvements. Develop networked, distributed sensor fields, including unmanned platforms (e.g., UUVs, UAVs, and satellites), for both submarine detection and local environmental characterization. Develop weapon concepts and technologies that will exploit distributed sensor networks, permit rapid response, and provide more capability against countermeasure-equipped quiet submarines and torpedoes. Recommendations for Follow-on Action Elevate and maintain the priority for ASW R&D within the Department of the Navy to ensure capabilities to counter the future submarine threat. Determine the limits of acoustic concepts such as coherent and matched-field processing with volumetric (both horizontal and vertical) arrays through comprehensive environmental measurements, accompanied by modeling and testing. Use SOSUS data to explore ocean coherence and other acoustic phenomena that will be fundamental to the next generation of sonar technology. Incorporate engineering experience connected with the manufacture and deployment of large towed arrays gained by the offshore oil exploration industry.

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Technology for the United States Navy and Marine Corps, 2000–2035: Becoming a 21st-Century Force, Volume 7 Undersea Warfare Develop power supplies and other key enablers for autonomous deployable low-frequency acoustic sources. Quantify the operational, algorithmic, and communications requirements for distributed active sonar methods including bistatic, multistatic, and forward-scattering configurations. Ensure that the Navy is the leader in understanding the environmental impact of acoustic energy on mammals and other marine life. Establish and implement a road map to exploit miniature sensor developments for undersea applications, including microelectromechanical systems technology, both inside and outside the Navy. Continue to pursue promising nonacoustic ASW detection techniques, including magnetic, electro-optical, and biological. Establish an ASW research program to exploit the effects of submarine hydrodynamic signatures, especially in littoral environments. Improve the capability of ASW weapons against stealthy submarines operating in littoral environments and deploying complex countermeasures, including the exploitation of advanced sensors, expanded processing bandwidths, and environmental adaptability. Develop technologies that will enable a family of new weapon concepts such as rapid attack, long-range response to off-board sensing and targeting; short-range, close-in, quick reaction; and long-endurance, stealthy UUV-like search-track-kill weapons. Pursue robust enabling technology for protecting surface ships and submarines against threat torpedoes, such as antitorpedo weapons and advanced countermeasure devices. Adopt open-architecture and COTS-based systems in all ASW applications to enable hardware or software refresh cycles of approximately two years. With the aid of GPS, build on the capability to network widely spaced platforms, such as UUVs, and large distributed acoustic arrays and satellites, to provide data, including environmental information, that can be fused into a complete ASW picture. Adapt improved technology VTOL aircraft for ASW to provide greater range, speed, and endurance capabilities than current helicopters. Ensure a continuum of robust fleet ASW R&D projects characterized by at-sea operations, testing, measurements, and experimentation as the principal means of advancing the slate of future fleet ASW capabilities and readiness. The panel is fully confident that taking advantage of the opportunities to incorporate available and emerging technology will enable the Navy to maintain undersea superiority well into the next century.