Approaches to reducing a surface ship's signatures range from evolutionary alterations to ships' structures to revolutionary designs that could change completely our concept of the warship. To achieve the latter, stealth must be part of the design process from the start.
Survivability is one of the most important attributes of surface ships, and as the public becomes more sensitive to casualties and our ships become more expensive, it has become more critical than ever. Total ship survivability (TSS), a systems-engineering approach to survivability, ensures a more efficient incorporation of ruggedness into new ship designs and acquisitions. Stealth is a major element of survivability because it helps prevent damage and reduces demands on defensive systems, but in each case, stealth must be directed at a specific set of ship objectives. Because stealth is relatively new to surface ships and fundamental to ship design and operation, a new organizational approach to stealth is needed.
A ship can survive combat either by avoiding damage or by continuing to function after she is hit. Understanding this ship survivability timeline offers opportunities to identify sturdiness and stealth capabilities that can be introduced into the designs of new ships.
Ship Survivability Timeline
Figure 1 illustrates the ship survivability timeline. This commonly is divided into two phases: the kill chain, (which emphasizes hit avoidance) and damage tolerance (which provides for limiting damage and recovery). Obviously, it is best to break the chain as early as possible. It is far better, for example, to avoid attack than to recover from one, and it is even better to prevent an attack from being made at all.
First, an enemy must detect the ship and identify it as a target worthy of attack. The ship then must be tracked accurately and precisely. The enemy must be able to engage with its own sensors, which also requires detection, identification, and tracking. Finally, the enemy's weapon must engage the target through impact. It also has to detect, identify, and track the ship throughout its attack.
In response, we can try to reduce the likelihood of these events taking place. The system that directs the attack, the attacking platform, and the weapon all can be attacked. Jamming can be used to prevent the attacking platform from functioning properly, or countermeasures can be used to distract a weapon from its target.
If an attack is successful, we can limit the extent and severity of damage. Built into the ship before attack, compartmentalization, redundancy, distribution of systems, selection of materials, and armor contribute to this. After an attack, the crew goes to work to prevent further damage (such as from fire and flooding), save the ship, and recover functions—including maneuverability, crew support, and even combat capability. As crew sizes are reduced, the ship must be able to do more on her own because human-intensive methods may be limited.
The various means to prevent or survive an attack can work together. For example, if the range at which an attacking platform can attack a ship is reduced, that platform may have to come into range for attack by that ship's weapons. If the time for the attacking platform to identify a ship is increased, that platform becomes more vulnerable to attack. In addition, inaccuracies imposed on an attacking weapon may help limit damage.
It is important to recognize that not all attacks are made using the classical approach. Some small boat or terrorist attacks, for example, have compressed timelines and reduced ranges. Mines lie in wait for an unsuspecting ship and follow an abbreviated kill chain. Submarines may be directed from another source, and so may follow the pattern described or may perform the entire kill chain independently. In all these cases, however, each of the same steps in the kill chain are followed.
Some of the tools that affect survivability are technological. We must never discount, however, the importance of tactics and training. Of course, material tools can affect these. For example, tactics for a low-observable ship must account for the ship's signature. The crew must be trained to take greatest advantage of features provided for recoverability. Navy Rear Admiral John Morgan pointed out that three elements are needed to ensure recoverability: a well-designed and -constructed ship; a captain who trains and leads his crew after damage; and a well-trained, courageous crew that fights to save its ship.
The total ship survivability process is intended to reduce the likelihood that the components within the survivability domain compete with one another. We must consider, however, that resources are limited and all survivability features have an impact on the ship. Tradeoffs must be made, and we must know exactly what is being gained or lost in each case.
Stealth and the Ship Survivability Timeline
A typical attack against a warship begins with detection. This may be by a land-based surveillance site, a maritime patrol aircraft, a ship, an unmanned vehicle, or a satellite. Using these same systems, the observers identify the target. Then, the detecting platform, or another platform, localizes the target. An attacking platform is directed to attack the ship. It has to target the ship separately. This involves detection, identification, and localization. When it has sufficient quality information, it launches a weapon—which engages the target.
The portion of the timeline before the hit is the kill chain. Consider how each link of the chain can be broken and how stealth can contribute.
Detection: This occurs when the signal from the target is sufficiently greater than that of the background clutter to allow the observer to confidently declare a detection. To avoid detection, the target does not have to be invisible—it merely has to have signatures small enough to hide in the background. Historically (and even today), this was done by hiding in fog, beyond the horizon, or behind topographic features.
Identification: The observer now must determine that the target is a ship he wants to attack. He should be certain the target is not a neutral or friendly ship, a decoy, a commercial or fishing vessel, or an unimportant naval ship. Identification can be delayed or foiled by modifying a signature to appear to be that of a much smaller or less-- threatening ship. Ships have used paint schemes, false structures, or false flags to conceal their identities.
Localization: Controlled signatures, combined with a ship's mobility, make this difficult. The data passed to the attacking platform can be inaccurate, requiring additional search time. In addition, ships with low signatures may appear only intermittently, which increases the inaccuracies.
Targeting: The attacking platform has to detect, identify, and localize the ship, usually with a less-capable system than that used earlier, albeit at shorter range. The attacker can be confused or distracted away from a ship using decoys or various electronic-warfare tools. These all become more effective when the signatures are lower. In addition, the attacker must make the detection at much smaller range if the signature is lower, which subjects it to attack by a ship's defensive weapons.
Engagement: Against a low-signature ship, the weapon is far less likely to acquire or be able to maintain a track. Countermeasures can be used much more effectively to seduce a weapon away from the ship once the weapon has begun to track.
A balance among signatures is required. Essentially, all signatures should be reduced to levels where they are equally useless to the attacker and become force multipliers for the ship's combat systems. Signature reduction can be thought of, in part, as a margin that enables future combat system upgrades. It is not necessary that the exploitation ranges for all signatures (e.g., acoustic, infrared, radar, electro-optic, electromagnetic) be reduced to the same value. They should each be reduced to ranges that make their operational use impractical or risky. Signature control also contributes by reducing the demands on defensive systems. Repeated studies have shown, for example, that ships with lower signatures require fewer self-defense rounds. Ships with reduced signatures allow for less costly countermeasures that can perform more effectively.
Implications for Ship Requirements
The relationship of sturdiness to ship size is relatively strong. Nonetheless, smaller ships can be provided with some protection against various classes of weapons. Stealth is not strongly related to ship size, but there are advantages for very small ships. It costs somewhat more to control the signatures on a larger ship and there are size influences that limit practical reductions.
As requirements are developed, we must consider what we hope to accomplish with either sturdiness or stealth on a particular ship design before specifying levels of performance. Some examples for stealth application are:
- Special Warfare Craft: These craft need to avoid detection. They would be designed for low detectability across the spectrum. Primary emphasis would be on nighttime operations.
- Submarine: Submarines operate with periscopes and masts up in many conditions and need to avoid detection. This would be involve emphasizing low radar cross sections and visual detection, during both night and day. Sub marines, however, also may need to surface occasionally. In this case, signature requirements would emphasize night operations for the exposed sails in visual and infrared spectra and low radar cross sections.
- Corvette: These ships might want to emphasize difficult identification and take advantage of mobility along with signatures to make localization difficult. They also would use signatures to improve their countermeasures' effectiveness. For some missions, under limited environmental conditions, such as at night, they may want to avoid detection across the spectrum.
- Cruiser/Destroyer: These ships may take advantage of signatures to complicate identification and achieve some advantage in localization under many conditions. They emphasize the targeting phase because of the opportunity to destroy an attacker drawn into weapons engagement range.
- Aircraft Carrier: Carriers would emphasize countermeasure effectiveness because of their size and operational mode. They might be able to avoid detection or identification under very limited environmental and operating conditions.
Managing Ship Design to Achieve Survivability
The importance of sturdiness and stealth to ship survivability may require that the process of ship design be changed. Both need to be built into designs from the beginning. Although some augmentation can be made through a ship's life, major improvements are difficult and expensive. Both sturdiness and stealth influence every system on a ship and must be developed as whole systems. Both require training: the former emphasizes damage control and recoverability training; the latter emphasizes configuration management and tactics training. Systems also must be maintained, though minimizing maintenance requirements is one of the major goals of the technology.
The submarine community has developed a disciplined, comprehensive organizational approach to stealth that has enabled its fleet to stay ahead of evolving threats. A similar approach in the surface community could yield a similar advantage.
Sturdiness and stealth should be considered complementary attributes, designed into a ship in such a way that each takes the greatest advantage of the other. Neither should be considered a characteristic to be traded off. Because they are integrating functions, they need to be fundamental attributes of the ship. Stealth and sturdiness also should not be considered only at the ship level. The ships in a battle group—and the fleet in general—depend on the survivability of every other ship in the group. This is a level of interoperability beyond what is normally considered. It also implies that decisions regarding survivability requirements need to be made on a fleet-wide basis.
We must strive to achieve better survivability at lower cost. This will require considerable investment in technology. Congress recognized the need for investment in survivability technology when it recommended, in the 2001 Defense Authorization Bill, that special consideration be given to maintaining U.S. superiority in naval ship technologies such as "hydrodynamics; machinery, electrical and propulsion systems; stealth; ship protection and structures; and advanced seaborne materials," and that the administration should "give serious consideration to the relationship between increased and consistent funding in science and technology and the maintenance of U.S. technology superiority."
Survivability authorities must advocate and direct the advancement of these important total ship survivability capabilities for our ships and fleet.
Mr. King is the head of the electromagnetic signature technology department at the Naval Surface Warfare Center’s Carderock Division.