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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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-

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

acoustics, oceanography, and signal processing, and a strategy for providing this education is required. Such intensive training will no doubt be costly, but it is considered necessary if future high-performance ASW systems are to be deployed effectively. This type of comprehensive and extended training cannot be acquired in short order with the onset of a crisis.

SENSORS FOR MOBILE PLATFORMS

With the diminished effectiveness of fixed systems for basin-scale coverage, lightweight sensor arrays that can be deployed from mobile platforms have acquired added importance. The SURTASS towed array system, particularly the latest twin line version, now has the widest area coverage for regional control. Towed arrays, forward spherical and cylindrical arrays, and conformal arrays from both submarines and surface ships provide tactical ASW. Since these systems are mobile and can be deployed rapidly in areas of potential conflict, they most likely will form the major components of future ASW systems. The experience of the offshore oil exploration industry suggests that arrays of up to several kilometers long with 10 or more multilines, more than ~2×104 sensors, and multiship operations are within the realm of current technology; there is every confidence that the rates of massively parallel digital signal processors soon will be adequate for real-time processing with such systems.

Towed array technology has advanced rapidly with longer, multiline systems that have an increasing number of sensors. For submarine-based ASW, the thinline TB-23 is routinely deployed. The TB-29 is longer and has a sensor location system. It is now available on a few platforms and will be deployed on the Seawolf and the nuclear-powered attack submarine, new version (NSSN). It will be the operational submarine-towed array for the foreseeable future. Experiments with adaptive beam forming on both arrays are demonstrating impressive results especially for cluttered environments. For surface ships, the twin line SURTASS has been deployed, and systems with multiline arrays are in advanced development. Multidimensional array geometries lead to both gain and better target motion analysis because they break the right-left ambiguity of a single line array without the need for ship maneuvering; moreover, these systems have employed state-of-the-art sensor location systems that maintain signal gain and adaptive array processing for superior noise suppression performance. A very important aspect of these arrays is that they can operate in the VLF band where it is most difficult to suppress the threat signature. Although there are significant engineering difficulties, multiline submarine arrays should be examined for the next-generation array beyond the TB-29.

Towed arrays are also an essential component of the Navy's mobile active systems that have evolved from the Critical Sea Test and Low-Frequency Active (LFA) programs. Source levels today are high enough so that the performance of active sonars is largely a measure of reverberation, or clutter, suppression. This

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

is now usually accomplished by resolving targets along a beam in range with wide-bandwidth signals or in Doppler with long-duration signals or with a combination of the two. The same technology of longer, multiline arrays with very accurate sensor positioning that will likely be the foundation of improved passive sonar systems will also serve to enhance the performance of active systems.

Bistatic active systems offer many advantages for reverberation suppression with mobile systems, as has been demonstrated in several tests in the LFA and CST programs. The same issues of increasingly capable arrays that more fully exploit the coherence of acoustic signals in the ocean is just as relevant to bistatic systems. Since active systems reveal the location of the source signal, their use is a liability for submarines. The technology for expendable, leave-behind sources that can mitigate this restriction has recently been developed, so submarines can now operate in bistatic modes. There is still a liability associated with the use of active sonar deployed on an off-board vehicle, however. It is possible under some circumstances for an enemy submarine to acquire the echo of one's own submarine from the active sonar signal generated by the off-board source.

One of the major advances in the last decade has been the recognition that the vertical structure of both signal and noise fields represents an opportunity to improve ASW signal processing. Although this has long been recognized by using arrays at endfire instead of the usual broadside geometries, the introduction of towed vertical apertures has led to impressive experimental results. This technique has been carried out using vertically distributed multiline arrays or the purposeful slanting of an extended SURTASS array.

The forward spherical and cylindrical arrays are useful at mid frequencies where they provide fully directional resolution. Although it is unlikely that the size or number of sensors on these arrays can be increased, their performance can be improved with state-of-the-art signal processing. Full-area, conformal arrays distributed over a large extent of a submarine offer wide apertures and are not encumbered by the tactical constraints associated with long towed arrays. They are subject to structural self-noise, however, as well as flow noise at high speeds. There have been a number of development programs for conformal arrays, for example, advanced conformal submarine acoustic sensors (ACSAS) and conformal acoustic velocity sensors (CAVES). The wide-aperture array (WAA), which has already been deployed, has demonstrated the utility of depth-of-field beam forming in tactical scenarios. Advances in low-noise sensors and signal processing should improve conformal array performance at mid frequencies.

High-gain arrays require accurate sensor positions and well-calibrated response functions to avoid signal gain degradation and large cancellation ratios. This is especially important with highly directional noise interference such as nearby shipping lanes or a nearby friendly battle group and with high-sidelobe beam formers such as those found with vertical apertures. Sensor location systems for towed arrays have advanced significantly using GPS, high-frequency acoustic ranging, heading sensors, and tracking algorithms that are based on the

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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known dynamics of the array. Some of these location systems have been incorporated or are now being retrofitted. Minimizing positional uncertainty is imperative for both passive and active sonars to (1) push array gains to their environmental limits, for high gain; (2) exploit vertical arrays; and (3) enable beam forming while maneuvering. Similarly, sensor response variability caused by structural inhomogeneities leads to the same liabilities in achieving high gains for mounted and conformal arrays. This can be minimized by incorporating accurate test range and structural acoustic models into the array processing algorithms.

The technology for towed arrays has two major components that ultimately limit their size and number: the sensors themselves and the signal telemetry. These components are also important elements of hull-mounted arrays. Sensor technology has made tremendous advances with solid-state electronics. The transduction unit itself is now much smaller, and VLSI digitizers at the sensor eliminate some of the limitations of analog telemetry. More importantly, when used in conjunction with fiber-optic technology, digital telemetry enables an order-of-magnitude increase in array size and sensor number. Telemetry from the sensor to the on-board signal processor typically has used twisted pair cables. When there are a large number of sensors, this type of cabling resulted in large-diameter arrays and reliability problems simply because of the number of wires and connections. This in turn required large winch diameters and awkward handling systems. Fiber-optic telemetry changed all of this. Cable diameters are smaller, and so longer arrays can fit on the same size winch, and bandwidths far exceed twisted pairs, so that more sensors can be employed. This enabling technology has already been demonstrated in the oil exploration industry where it has dramatically improved array performance. It is just making its impact felt on passive and active sonars used in naval applications.

Hull-mounted arrays are particularly prone to self-noise problems. Self-noise is dynamic and dependent on speed, depth, operating conditions, machinery configurations, and so on. It adds to the clutter environment on displays and distracts operators. Modern adaptive algorithms are very useful for both monitoring and canceling self-noise fields.

The real-time acquisition of environmental data is an imperative, but challenging, task for a mobile system that must be capable of operating anywhere in the world. The research community has gone to great lengths to acquire such data, but the requisite oceanographic data are both site and time specific, with many scales of variability, and tend to be under sampled in both space and time. The important issue is that up-to-date site-specific environmental data must be incorporated in the deployment of mobile sonar if high detection gains are to be realized.

Operational beam formers for towed arrays assume the array geometry to be straight and horizontal aft of the tow ship, whereas in reality there is always some deformation from this geometry. This is now measured by heading and depth sensors with varying degrees of success. The passive TB-23 and the active LFA

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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arrays can monitor gross deformations, and recently developed systems, such as the TB-29 and the twin line SURTASS, enable compensation in the beam former. A major limitation in current systems is that beam forming cannot be carried out when the ship is maneuvering, which can result in downtimes as great as 50 percent. If the turn is gradual enough, the increase in self-noise is not dramatic; therefore, there is no fundamental limitation to implementing beam forming during a maneuver if the sensor positions are measured accurately. Increased availability of beam-formed data could have a significant impact on tactics. There are also a number of potentially useful properties of deformed towed arrays that deserve exploration. The right-left ambiguity can be distinguished with horizontal deformations, and the introduction of vertical tilt leads to the possibility of using matched-field processing.

It has been recognized for some time that wide apertures can provide instantaneous passive ranging at mid frequencies. Several systems have been tested, and the beam forming for the WAA implements range-dependent focusing. Several towed array experiments have demonstrated this as well. Passive ranging information is very useful for tactical ASW and should be a feature available in all future beam forming systems.

Submarine detections on mobile platforms are still made by human operators despite extensive research on automated detection, pattern recognition, and neural networks and this is likely to remain the situation for some time. More capable sonars will provide both more resolution and more varied means of analyzing the data. This will significantly expand the search space dimensionality, but there is a real danger of overloading the operators with information.

Advances in database management and display enhancements can improve operator performance so that the full potential of the sonar can be used. Some aspects of the detection process could possibly be automated enough to lead to manning reductions.

SENSORS FOR FIXED SYSTEMS

The bottom-mounted SOSUS arrays were one of the major elements of the ASW effort during the Cold War. They provided enough gain and directionality for basin-wide surveillance to be maintained at ocean basin scales in the theaters of interest. Now that the threats are quieter, basin-scale coverage of low-speed targets is no longer possible. The High Gain Initiative was the last effort to regain basin-scale coverage, but it was cut short with the end of the Cold War. Currently, there is seldom a need for ASW throughout the ocean basins, and the Navy is on a track of abandoning SOSUS and its associated support infrastructure. Nevertheless, with the submarine being the capital ship of choice for many countries, one must seriously consider the need for a SOSUS capability within the 2035 time frame.

Now and for the near-term future, the potential operational theaters are re-

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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gional, where it is difficult to deploy permanent array systems. ASW control in critical sea lanes, for carrier battle groups and forward troop introduction, requires the ability to achieve regional acoustic superiority. This indicates a need for rapidly and covertly deployable array systems with lifetimes on the scale of a year and regional coverage of hundreds of square kilometers, containing both bottom-mounted and moored vertical arrays. Miniaturized sensors with numbers on the scale of 104, digitized in situ and connected with wideband, fiber-optic telemetry, are needed. Accurate sensor positional calibration for the horizontal array sensors and mooring location of the vertical arrays using either navigation surveys or sources of opportunity are necessary. The vertical arrays would also require tilt and compass sensors for dynamic positioning in response to ocean currents. The advanced deployable system (ADS) and the fixed distributed system (FDS) are bottom systems that have these general features, but the addition of the vertical apertures is important.

There is need for a short-lifetime, very rapid, air-deployable array system for situations in which there is not enough time to field an ADS or FDS system. A high-gain sonobuoy-type array system is needed since the approach of a grid of single sonobuoys does not have enough sensitivity for current and projected threat levels. Each sonobuoy would contain a vertical array, and the entire network would be navigated by a real-time positioning system. At one time the Navy developed the star tracking rocket altitude positioning (STRAP) and vertical line array difar10 (VLAD) systems, which had this type of construction. The STRAP had single sonobuoy sensors instead of vertical arrays and far fewer sensors, but it addressed many of the important technical issues, including sensor positioning. The VLAD sonobuoy had a small vertical array for improving the signal-to-noise ratio (SNR). The sensor technology has now advanced significantly.

The coverage that a fixed system can provide against modern quiet threats will not have basin scales, but the performance can be maximized with imperatives described earlier. Wide apertures with sensor numbers far exceeding those of SOSUS, the use of fully coherent processing with accurately navigated arrays, adaptive beam forming, and the exploitation of Doppler all can be used with the same measures of effectiveness. Fixed receivers with active sources also can be used advantageously. ASW detection depends exponentially on signal-to-noise ratio; even 5-dB increases are important and 15- to 20-dB gains are dramatic. Ocean acoustic propagation is ducted, so recovering a significant fraction of basin-scale coverage is certainly feasible.

10  

Difar is a type of directional sonar.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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SCIENCE AND TECHNOLOGY ISSUES

Several science and technology issues must be addressed to ensure the continuous evolution of improved performance for both mobile and fixed sonar systems. Although signal processing algorithms and computational capabilities are necessary for a high-performance sonar, the acoustic-oceanographic coherence is what ultimately sets the limits. The science and technology issues for coherence have several environmental contexts.

Littoral waters, where these sonars must operate, can be quite shallow (10 to 200 m) or deep (kilometers) and include a range-dependent shelf break. In shallow water and on the shelf, strong horizontally anisotropic internal waves driven by tidal and topographic forcing, usually with a diurnal period, can modulate sound speed profiles dramatically. In upslope-downslope geometries these can precipitously interrupt surface duct propagation and impact coherences through mode coupling and/or ray path fluctuations. When bottom refraction sound speed profiles are present, bottom interaction can significantly impact coherence. Even well-lineated, constant-depth, shallow water introduces problems because of differential absorption; the complexities of rapidly range-dependent slope with high geologic roughness are even more challenging.

Coherence in deep water is greater than in the littoral, especially when the signals are not bottom interacting. This presents an opportunity to significantly increase detection ranges by pushing coherence to the limits. VLF deep-water experiments have demonstrated significant frequency dependence on coherences, with those in the lower edge of the band demonstrating remarkable ray-mode coherence, whereas those in the upper section have different coherences for the high-angle paths and the ducted paths. The cumulative effects of internal waves appear to be the problem, but there is a great deal of controversy about this issue. Bottom interaction has received less consideration; but it is an unavoidable issue, especially for active systems.

It is useful to examine what has been learned from some recent programs to suggest some of the needed R&D. The programs discussed below are conce rned primarily with deep-water acoustic phenomena, as U.S. ASW efforts have, in the past, been focused on deep water operations. Some of the knowledge gained from these programs is applicable to operations in shallow water, but, in general, additional R&D will be required to extend ASW capability to shallow waters.

  • The High Gain Initiative (HGI) was a response to the appearance of quiet threats and the loss of basin-scale coverage by SOSUS and SURTASS. It was a fixed system that several vertical array geometries spanning a large fraction of the water columns and applied matched-field processing. Environmental monitoring was extensive, and there were ancillary tomographic experiments. Matched field methods are a generalized form of beam-forming when the vertical multipath-multimode features of the signal field are significant. The first successful experi-

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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ments with matched-field processing were conducted in the Arctic at ranges of 250 km. The HGI demonstrated that in the VLF realm, the ocean coherence supported matched-field processing. Augmented sources were resolved in depth to within 50 m and in range to within 2 km at detection ranges of 1,000 km at 25 Hz. Since the HGI, there have been several experiments with vertical arrays that have matched-field processing in both deep and shallow water.

  • The Defense Advanced Research Projects Agency (DARPA) 3X experiment used three concatenated SURTASS arrays to create a very long aperture. The ocean supported fully coherent processing over this extended horizontal aperture, which was approximately 600 wavelengths in extent. Wave front curvature and range-dependent beam forming were successful in near-field and far-field targeting. The array was purposely allowed to deform so that it had a vertical extent, and matched-field processing was applied. Sources were resolved in range and depth at long ranges, again suggesting the gains obtained with fully coherent processing.

  • The Heard Island Acoustic-Tomography-Climate (ATOC) experiments for acoustic monitoring of ocean climate have demonstrated coherence at several-thousand-kilometer ranges. During the Heard Island Feasibility Test, matched filtering was successfully carried out for 3-minute signal durations at 5,000-km ranges. Phase shifts induced by source motion were tracked to within a fraction of a wavelength at ranges of up to 9,000 km. The ATOC experiments demonstrated coherent matched filtering for more than 20-minute signal durations, and vertical beam forming was coherent for the deep reflection and refraction paths at 5,000-km ranges. The surprising result was the modal scattering of the very energetic axial signals. Overall, although the source level was high compared to current threat signatures, these experiments demonstrated that the ocean supports coherent propagation and passive localization at very long ranges.

  • The CST and LFA programs have been the focus for active sonars over for the last decade. High-powered, vertically transmitting arrays and towed horizontal receiving arrays have been used to resolve the spatial structure of the reverberation and submarine target strengths. The Acoustic Reverberation Special Research Program (ARSRP) investigated fundamental properties of acoustic scattering. The arrays of the CST-LFA system demonstrated the importance of high-resolution bathymetric maps of the bottom geology and oceanographic models for the insonification. These have led to environmentally driven models for the coherence of the reverberation and the limits on target detection by range gating. Similarly, models for the signal modulation due to surface waves and entrained bubbles led to environmental models for Doppler coherence and limits on target detection using narrowband signals.

  • The twin line SURTASS experiment used two parallel towed arrays with a sensor positioning system. It implemented a simple form of adaptive beam forming, with the array shape compensation from the positioning system. The gains with even this simple form of adaptive beam forming were impressive when operating in a high-clutter, shipping lane environment. Ranges exceeded

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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those of all other systems operating simultaneously. The right-left ambiguity was broken, which led to much less clutter on the displays and better operator performance.

These and other experiments suggest some of the critical issues in acoustics, oceanography, and signal processing for future ASW systems.

Acoustics

  • Coherence scales. Although there have been many theories and experiments addressing the issues of spatial and temporal coherence, the limits have yet to be established for the environments and spectral bands where ASW operates. Careful experiments with accurate environmental data and modeling have frequently revealed coherence scales larger than those predicted by theories and simulations. Data for horizontal and vertical apertures, low and high frequencies, and shallow and deep water are all necessary.

  • Noisefields. The ambient noise field structure is a key issue in ASW. If the noise is directional or has spectral features, these can be exploited to improve ASW detection performance. The coherence of the noise field is just as important as that of the signal in signal-to-noise measures. Important aspects of the noise include its dependence on environmental parameters, excitation mechanisms such as shipping and natural processes, frequency dependencies, and coherence. It can also be used to make environmental assessments much like ocean weather.

  • Reverberation processes. Most active systems are limited by reverberation noise, which is caused primarily by bottom and surface scattering and sometimes by sea life or other objects in the water column. Acoustic models for wideband range resolving and very narrowband, Doppler resolving scattering for systems operating in monostatic and bistatic geometries are necessary if one is to take advantage of large-aperture arrays.

  • Doppler processes. The temporal structure for moving source-receiver is important for synthetic aperture arrays, fully coherent matched-field processing, forward-scattering systems, and detection by Doppler gating for active systems. It has a complicated dependence on surface and internal ocean waves and source-receiver motion in a multipath/multimodal, range-dependent medium; nevertheless, experiments have demonstrated a remarkable robustness for Doppler phenomena over very long ranges.

  • Range-dependent and three-dimensional propagation. Acoustic propagation is strongly dependent on the temporal and spatial variability of the medium even at modest ranges. Although there has been significant progress starting with the parabolic equation, a need remains for range-dependent propagation codes that can accommodate rough and elastic seabeds, surface wave modulations, and internal wave scattering. There are many ASW sites in the littorals, but

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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relatively little work has been done on the shelf regions where range dependence is very significant. Three-dimensional effects such as slope refraction and horizontal refraction are also just beginning to be appreciated. Propagation modeling often separates into two approaches—deterministic and stochastic; since both are inevitably necessary, better coupling between the two is sorely needed.

  • Wide-band signal models. Almost all acoustic modeling has been based on narrow-band representations with Fourier synthesis used for wide-band signals. Wide-band representations are needed for all active systems that use travel times and for passive ranging systems such as matched field. Wide-band representations in all bands (VLF, low frequency [LF], and medium frequency [MF]) now press the limits of computational capabilities when range dependence, time variability, and scattering are present.

Oceanography

  • Environmental data and models. Oceanographic data are undersampled in space and time, and so a variety of strategies must be used to provide environmental inputs to the acoustics. These include high resolution; seasonally dependent atlases; on-board, off-board, and cooperative sampling; satellite data; and tomographic networks. All must be coupled with oceanographic models for data assimilation.

  • Ocean coherence. Acoustic coherence is driven by the spatial and temporal variability of the ocean. Models exist for wave processes at all scales of variability—spatially from basin scales to microstructure and temporally from years to seconds; however, they are usually not specific enough to use for predicting acoustic coherences.

  • Volume and scattering physics. Acoustic wavelengths for ASW are usually between 1 and 100 m, which sets the scale needed for the scattering physics. Ocean environments are seldom measured to such scales in any deterministic way, so random models are necessary to make robust acoustic predictions. Oceanographic models for the sea surface, internal waves, and bathymetry exist; however, they require further development to extrapolate their use down to acoustic wavelength scales and environmental data to constrain the parameters in the models.

Signal Processing

  • Algorithms for wide-aperture, high-density arrays. Large-aperture arrays are capable now of resolving the complexity of multipath, multimodal acoustic propagation. The use of multidimensional geometries such as multiline towed arrays, large networks of bottom arrays, and curtains of vertical arrays further advances this capability. This leads to many opportunities to improve the signal and array processing for ASW from the relatively straightforward, such as range-

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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dependent (depth-of-field) beam forming, to the complex, such as fully coherent, wide-band, matched-field processing. The paradigm of linear arrays with plane wave signals has been the basis for virtually all Navy array processors and has led to well-established design criteria for beam formers. With multidimensional arrays operating in a complex, inhomogeneous acoustic medium, these design criteria are no longer appropriate. Algorithms for side-lobe control, wide-band signals, space-time and Doppler processing, synthetic apertures, and robustness are all needed for fully coherent processing with these arrays.

  • Real-time, environmentally adaptive algorithms. Adaptive array processing offers the potential for higher signal and noise gain, clutter reduction, and detection at lower signal-to-noise ratios. It is needed for multidimensional arrays and matched-field processing for sidelobe control. For passive sonars, larger arrays resolve more directional sources and adaptive processing provides cleaner displays and easier track identification, whereas for active sonars they can mitigate reverberation. Adaptive algorithms have superior resolution, which can improve the accuracy of solutions for target motion analysis, but they are computationally intensive and not applicable in all environments. Incorrect implementations can degrade performance. They can be sensitive to sensor calibration, positioning errors, and environmental modeling errors, as well. For dynamic noise environments, the time required for the adaptation with arrays having a large number of sensors can be problematic. Promising approaches include dynamic dimensionality reduction, calibrating the medium with probes such as self-cohering signals, and conjugate field methods as well as alternative signal representations such as wavelets. Research to exploit fully the capabilities of both the arrays now being deployed and the even larger ones to follow must accompany development of the arrays themselves.

  • Postprocessor algorithms. The emphasis in signal processing is usually on the coherent front-end beam forming and matched filtering, yet the postprocessor that performs an incoherent combination of these outputs across frequency for a threat spectrum and over time for a track hypothesis provides a substantial fraction of the overall processing gain. In addition, the postprocessor provides the track parameters for target motion analysis solutions. The computational resources for large-dimensional search spaces, which include the spectral bands and up to five spatial parameters—azimuth, range, and the velocity vector—are now available; thus, algorithms such as "track before detect" for lower detection thresholds, dynamic tracking for clutter management, and classification for target identification can now be carried out in real time. Tracks are usually established by identifying a directional signal that emerges above a local noise level. The template design for this is one of the subtle, but very important, aspects of low SNR detections, and its effectiveness depends on the ambient noise environment. Both components of the postprocessor will have to respond to the demands of higher-resolution, large-aperture multidimensional arrays.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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The report of the Panel on Technology, Volume 2 of the full nine-volume series Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a -Century Force, describes the supporting computational and sensor component technologies that are expected to have a positive impact on technology development for ASW systems. The following technologies are unique to the Navy ASW mission and may require direct Navy support for their further development:

  • Array technology

    • Low-cost horizontal, vertical, and multidimensional arrays with dense sensor spacing

    • Reliable array deployment and handling

    • Modular, fiber-optic data telemetry

  • Environmental data acquisition

    • High-resolution environmental databases

    • Assimilation of real-time oceanography

    • Real-time satellite data

  • Processing hardware and software

    • Modular, commercial off-the-shelf (COTS) systems with open architectures for rapid insertion of upgrades

    • Teraflop massively parallel processors and beam formers

    • Postprocessors for track and environmental management

    • Data management for networking processing and displays.

Countermeasures and Sonar

The emphasis in ASW has been on increasing detection gains faster than gains in quieting and stealth. In confrontational situations, ASW system performance can be affected significantly by aggressive techniques such as jamming and spoofing, which are more generally part of countermeasure technology. Anticipated gains in submarine quieting and increased use of off-board sensors, together with the cost and difficulty of increasing detection range, makes it prudent to increase the emphasis on ASW acoustic countermeasures and counter-countermeasures. ASW has not developed countermeasures and, subsequently, counter-countermeasures in any meaningful degree compared to those now routinely used in radar. ASW could certainly benefit by expropriating some of the radar countermeasure work. Countermeasures can be used to frustrate all aspects of ASW from initial acquisition to localization and, finally, targeting. High-gain processing exploiting coherence to its limits is very susceptible to jamming. Sensor networks have the same vulnerability to jamming of critical links or nodes. The acoustic environment is quite different from radar, and so the applicability of countermeasure technology has to be determined.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Sound Surveillance System

The SOSUS arrays have been the principal fixed arrays deployed over long periods. Although considerable research was done within the classified community, the data were not routinely accessible to the acoustics research community. Yet, addressing several of the imperatives for passive ASW that exploit fully coherent processing with large-aperture arrays requires a facility where these research issues can be addressed. Experiments with fixed arrays are necessary because towed arrays introduce both motion effects and positional variability, which complicate measurements and analysis. Over the years, several large arrays have been deployed for short durations for research purposes; a horizontal array buoyed up into the water column was deployed in the 1970s to examine coherence issues. The most recent notable example was a system of multiple, bottom-moored vertical arrays where matched-field processing was a focal point. Largely because of the expense and the complex ocean engineering required, there has never been a research facility with long-term observations addressing the ultimate ASW capabilities of passive sonars with fixed arrays.

This draws attention to the future of the existing SOSUS system. Although it is not an ideal facility and certainly not the one that is really needed to address the acoustic and signal processing issues outlined above, it does exist and thus constitutes an available source of at-sea data. The use of SOSUS for research has two aspects—one for ASW and a second for oceanography. Important ASW research issues for fixed systems can be addressed in the short term by augmenting SOSUS facilities, and possibly FDS and ADS, with vertical arrays having their own data telemetry; moreover, critical experiments can be done that can help to specify the configurations of future systems. The maintenance of some components of SOSUS should be addressed from a Navy perspective.

The scientific use of SOSUS has been an issue for the last several years because SOSUS represents an acoustic observatory system long sought by the research community. Acoustic observation of earthquakes and marine mammal activity and acoustic tomography are examples of the type of data that SOSUS can provide. The scientific use of SOSUS is compatible with the Navy's ASW needs because it can provide fundamental data on ambient noise sources and acoustic propagation. For example, sea organisms and micro-earthquakes are important VLF noise sources, and much of what has been learned recently about deep-ocean coherence came from SOSUS-acquired data. Although there have been financial issues that are important in today's tight budget climate, security has been the fundamental problem for general use of SOSUS by the scientific community. The acoustics community has long sought an acoustic observatory, and maintaining some parts of SOSUS represents the closest possibility. This issue warrants careful consideration in light of the potential long-term benefits to the Navy's ASW program.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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FIGURE 1.1 Trends and projections for submarine quieting and acoustic detection limits.

SIGNIFICANT GAINS POSSIBLE IN PASSIVE ACOUSTICS

The acoustic energy emitted by the world's first-line submarines, both nuclear and diesel and both narrow-band and wide-band, has decreased at a remarkably constant rate over the past 35 years—about 1 dB per year. This came about because of extensive and sophisticated quieting R&D programs involving submarine designers, component producers, and structural acoustics technologists. Despite the impressive 35 dB of quieting, even the best submarines operated properly at low speeds can still be detected—advances in passive acoustic detectability via improvements in sensors, arrays, and processing techniques have almost, but not quite, kept apace of the quieting. Nevertheless, passive detection ranges for these low-speed modern submarines have shrunk from hundreds of kilometers to only a few kilometers.

These trends, illustrated in Figure 1.1, if continued into the next 35 years will lead to essentially undetectable submarines and will reduce ASW capabilities to close-proximity detections and transient or higher-speed situations. However, the technologies are ripe for a sharp change in the slope of the detectability history curve—improvements averaging several decibels per year are on the horizon and could come to fruition with vigorous pursuit of the requisite R&D.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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These new opportunities stem mainly from the advent of massive, affordable computational capabilities synergistic with new and affordable sensor developments and an improved understanding of what the ocean will permit by way of coherent and matched-field processing. It is time for the fulcrum of the lever to shift-to put detectability on the long end. Whereas future quieting will be difficult and will involve continued excruciating attention to detail and extensive testing, advances in detection capability, although not simple, can occur and be inserted on a much shorter time scale than has existed in the past.

The enabler is the now-occurring introduction of open-architecture, COTS-based systems in ASW platforms and acoustic processing chains. A sea change is taking place: special-purpose, MIL-spec hardware and software with decades-long life and replacement cycle times are being replaced with open-architecture, COTS component systems that allow hardware-software refresh times of a year or two. This approach is currently being implemented successfully in submarine combat systems and can, by logical extension, be applied to other ASW systems.

At the same time, relatively cheap but high-performance sensor and telemetry or connection concepts are maturing, based on fiber optics for both sensors and telemetry and MEMS or other miniaturized sensor concepts. These developments enable not only the processing of more signals with higher bandwidths from more sensor elements with ever more sophisticated algorithms, but also exploitation of the details of the local ocean environment through temporally and spatially coherent processing as well as spatial signal replica/adaptive beam forming—the so-called matched-field processing.

As arrays, both mobile and fixed, become larger in both number and length, engineering issues associated with handling and placement or control may become apparent. The oil industry has made major advances in deploying multiple towed arrays. The Navy should maximize the benefits of the experience and lessons learned in the oil exploration industry.

SIGNIFICANT POTENTIAL FOR ACTIVE ACOUSTICS

Active acoustics, although fielded as a Navy tactical capability for many years, has experienced significant advances over the past decade or so with the promise of much more improvement to come. Advances include the following:

  • Exploitation of frequencies well below 1 kHz where reduced attenuation allows longer detection range, perhaps much longer ranges in deep water, compared to the higher-frequency (> 1 kHz) tactical sonars;

  • Development of low-frequency sources with increasing power and efficiency, increasing bandwidths, and smaller sizes, all of which will allow an increasing variety of sources for future applications;

  • Separation of transmitter(s) and receiver(s) into various distributed system configurations;

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×

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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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:

  1. Periscope depth with exposed masts or scopes,

  2. Periscope depth with all masts or scopes retracted,

  3. Nominal safe operating depth to avoid surface ships (roughly 150 feet), and

  4. 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),

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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:

  1. Remote detection of surface effects by airborne or space-based sensors (e.g., synthetic aperture radar (SAR) IR, and optical);

  2. 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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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-

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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  • 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.

Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
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Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 26
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 27
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 28
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 29
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 30
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 31
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 32
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 33
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 34
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 35
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 36
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 37
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 38
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 39
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 40
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 41
Suggested Citation:"1 Antisubmarine Warfare." National Research Council. 1997. Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare. Washington, DC: The National Academies Press. doi: 10.17226/5867.
×
Page 42
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