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March 2002 Excerpt

Signatures and Silencing—An Overview


By Brian Bowers and James King, Reprinted from Tech Digest

NWEST BETHESDA—A primary NSWCCD mission since WWII has been to promote, through technology development and performance evaluation, achievement of U.S. naval surface ship and submarine signature control that will ensure operational superiority. The Division collects signatures on all pertinent Fleet assets and conducts research, development, test, and evaluation (RDT&E) in all aspects of ship signatures and signature control. Historically, NSWCCD supports NAVSEA in the successful development and implementation of signature control and silencing technology in all major combatants and support ships.

Operational stealth can be considered a measure of the ability of a ship (surface ship, submarine, or other naval vehicle) to operate undetected against specific threats in designated mission areas. It is highly desirable for ships to embark on assigned missions with a degree of stealth that provides a necessarily low level of vulnerability to detection, classification, and localization by threat sensors.

A ship’s stealth is controlled not only by its own signatures, but equally by the capabilities of threats that take advantage of and exploit the ship’s signature characteristics. Consequently, while advances in silencing and signature reduction improve a ship’s stealth, advances in threat capabilities reduce the ability of a ship to operate undetected (i.e., reduce the ship’s level of stealth). It has been the nature of ship design, accelerated in the late 20th century, to require progressive signature improvement to maintain an acceptable level of stealth.

Generally, signatures can originate passively (and be detected by passive threat systems) from sources onboard, and actively as a result of scattered directed sound (sonar ping) or radar energy, against moving and stationary ships. All of a ship’s individual signatures are related to its operational stealth. As the weakest links of a chain control the chain’s utility, so a ship’s operational stealth is controlled by the progression of most observable/detectable signatures. The threat migrates to the stealth vulnerability.

Signatures can be acoustic (propagation by mechanical vibration of physical particles), electromagnetic (EM) (propagation by periodic variations in electric and magnetic fields), or other observable entities that result from ship-design or ship-system components, singly and in combination, dynamic and static. Purposeful operational transmissions such as electrical/ mechanical signal transmissions of communication, navigation, weapons launch, and active emissions of radar and sonar are signatures that are detectable by threat sensors, but are not the focus here.

A variety of pertinent ship signatures are listed in Table 1. The table summarizes the signature characteristics, related sources and mechanisms, and the nominal upper limit of the distance (range) at which the signature may be detected by modern sensor technologies. Radiated noise (passively detected) and the sonar target echo (active detection) are both acoustic, and therefore can propagate great distances underwater. Much submarine silencing has been accomplished over the last several decades, so that current U.S. submarines are only marginally detectable under the best of circumstances. Many of the sources of radiated noise also negatively influence performance of own-ship sonar systems, in active and/or passive mode. Such sources often simultaneously contribute to the self-noise of hull-mounted and towed sonar arrays, thereby reducing the range at which a threat can be detected.

The EM signatures (radar target echo, infrared, and electro-optical) shown in Table 1 are important because surface ships and surfaced submarines also can be detected by relatively remote threat sensors. Although the electrical and magnetic signatures generally require threat sensors to be in close proximity to the ship, these signatures are crucial because coastal regions offer relatively easy deployment of those sensors. As an example, the littorals provide an ideal environment for acoustic, magnetic, pressure, and electrical influence-activated weapons (mines).

Figure 1 illustrates and relates acoustic, radar, IR, and electro-optical signatures of submarines and surface ships to regions and design aspects of the ship that are most prominent in controlling those signatures. Various sources associated with these regions and design features have been demonstrated to control signature temporal, spectral, and spatial characteristics. Some acoustic quieting technology developed for submarines over the years has been applied successfully to surface ships. Figure 1 also presents general equations related to the detectability of radar, IR, and acoustic signatures by threat sensors. Properties of the ship, local sea environment, and threat target must be accounted for in the passive and active sonar equations in Figure 1. The determination of the variable properties is usually very intricate, as shown by that for target strength. U.S. Navy ship characteristics such as general and local geometry, material, external and internal structure, and quieting devices impact some sonar equation parameters (such as target strength where geometry, materials, and hull vibration control Ps2 (t)), providing a range at which the ship is detectable to a threat sensor. Similarly, these types of ship characteristics are strongly represented in the equations governing signal-to-noise ratio of a radar echo (top), and infrared radiance, N. Range, R, may be determined directly from the radar equation once the other parameters are defined.

Signature Control Development and Application

Until the mid-1960s, responsibility for ship signature control was distributed across the individual Navy activities engaged in ship and system component design. Operational performance was impacted severely as a result of unintended design conflicts because of the sensitivity of signatures to multiple ship design component interactions. The Chief of Naval Operations (CNO) established strict performance and management requirements in recognition of the need for multi-disciplinary acoustic silencing technology development and ship design management. This led to the establishment of a very successful central ship acoustic signature silencing office within NAVSEA, which functions to this day (currently Ship Signatures Group, NAVSEA 05T) and has been expanded to cover all ship signatures.

Signature control begins with quantitative measurement of signature characteristics, and development of a physics-based understanding of signature sources and mechanisms. Systematic signature measurements have been, and are, obtained during a variety of full-scale trials on all classes of submarines and surface ships while under controlled conditions, and with supporting signature diagnostics. The Office of Naval Research (ONR) has been funding applied research and advanced development for technologies and tools that are used in next generation ships. NSWCCD has developed and adopted effective computational models covering surface ship and submarines, and maintains and cultivates the world’s largest ship signature database. As a consequence, the effectiveness of new-ship design features can be evaluated effectively in model-scale with great savings in time and cost.

Figure 2 illustrates the relative radiated-noise reduction achieved in submarines and surface ships since 1960. Monumental improvements in own-ship sonar performance have been achieved simultaneously because, in addition to reduced radiated-noise components, other less-radiating shipboard sonar self-noise sources have been quieted concurrently. Aircraft carrier silencing is shown to be essentially unchanged over time only because available Navy silencing technology generally has not been applied; however, it may well be applied to next-generation carriers.

Figure 3 illustrates the relative nominal reduction in surface ship RCS and IR signatures for most classes of surface ships since 1960. While important advances are continuing to take place for RCS reduction, electromagnetic signature control is still relatively immature.

Future Technology

Maintaining stealth in the presence of improving threat capabilities will require significant signature advancements. In addition to improved threat capabilities, changes in operational areas, such as operating in littoral regions in closer proximity to a multitude of threats and threat sensors, effectively reduce the level of ship stealth.

Technology developments are required and must respond to current and emerging threat capabilities to achieve adequate stealth performance. The temporal, spectral, and spatial characteristics of ship signatures that can be exploited by threat sensors must be quantified; their sources and mechanisms identified and understood; and effective approaches developed to mitigate sources that compromise stealth.

The technical challenges to achieve the signature characteristics and levels necessary in the future for ship stealth goals will be much more formidable than in the past. The easy technical problems have been solved. The major challenge will be development of sufficient scientific understanding of how sources and materials interact within complex ship structures to produce a signature that can be exploited. This will place a premium on development of procedures and equipment to quantitatively measure signatures. The measurement challenge involves achieving greater fidelity in current signature spectra, and measurement capability extension to new/expanded acoustic and EM signature spectra.

In pursuing technology development, the relative priority of individual signatures must be defined to support both Science and Technology (S&T) and the Research and Development (R&D) investment decisions. Expansion of operational missions to littoral regions has vastly increased the type and number of threats, especially to U.S. submarines. While focusing on littoral regions and their unique stealth demands, the need for stealth in the presence of continuing threats along the transit routes to littoral area operations cannot be ignored. Consequently, a computational capability adequate to determine relative signature priority on a mission basis is a major need. Such a capability must permit anticipating own-ship detection and detectability against threats in all candidate mission environments.

Maintenance of a core capability within the Navy for ship signature scientific advancement is a priority challenge. Significant investment has been made in Navy technical intellect and facilities that has resulted in evolutionary ship signature achievements and permitted operational mission successes. Reduced research and ship program budgets have slowed progress in signature technology advancements and are beginning to degrade the signature core intellect and affect other equities and facilities. The challenge is to maintain a core scientific capability sufficient to advance signature technology and satisfy future threat challenges and operational missions.

As we begin the 21st Century, it is anticipated that the number of Fleet units will decrease while our requirement to operate in harm’s way will increase. Clearly, specific measures are needed to ensure that adequate ship signature technology is available and applied in the future to achieve required stealth levels. The following are offered:

• Continue centralization of ship signature oversight responsibility at the NAVSEA level.

• Maintain a ship signature database, core intellect, and facilities to support Navy and shipyard RDT&E.

• Develop improved measurement capabilities for new or arising signature spectra.

• Continue to assess evolving technical threats and potential operating environments to identify changing ship stealth needs.

• Continue development of basic stealth technology that requires many years to mature.

• Continue development of physical and computational tools necessary to effectively predict and economically support ship signature improvements.

• Aggressively demonstrate ship stealth technology.

• Apply stealth technology on a systematic, mission-driven, balanced basis to counter evolving and arising priority threats.

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