Evolution of Anti-submarine warfare

[Lord ZeN]

While mining and mine clearing have existed almost as long as ships, undersea warfare first emerged as a significant area of offensive and defensive military operations in World War I (WWI). Several countries in that conflict began to use submarines on a large scale to attack civilian shipping and, occasionally, enemy warships. This created the need for antisubmarine warfare (ASW) and began a “hider-finder” competition between submarines and ASW forces. In the century following the war, this competition evolved through several distinct phases, each characterized by the predominant ASW detection method.

LIFETIME OF ADVANCEMENTS IN THE WWII ASW COMPETITION


In WWI and World War II (WWII), the hider-finder competition between submarines and ASW forces largely played out above the water, through radio and radar transmissions in the electromagnetic(EM) spectrum. Submarines were relatively slow and limited to short-range visual dtection of targets.2 They needed to be “cued” or directed toward convoys by radio communications from shore or other submarines. These communications could be intercepted by ASWforces, which decrypted submarine orders and reports or geo-located transmitting submarines using high-frequency direction finding (HFDF) equipment. Further, submarines in both wars were vulnerable to visual and (in WWII) radar detection because they were more like submersible ships than true submarines. They could only operate submerged for 1–2 days and spent most of their time on the surface in order to use their diesel engines for faster propulsion, to refresh their atmosphere, and to recharge their batteries.

The WWII hider-finder competition led to a cycle of moves and countermoves; as ASW forces developed new ways to detect submarines, submarines attempted to counter by employing new methods to evade detection. For example, submarine forces deployed radar-warning receivers (RWR) once they realized radar was being employed successfully against them. ASW forces responded by fielding higher-frequency radars that were more effective and not detectable with the existing RWRs. Once submarine forces realized they were being tracked by new radar frequencies, they developed a new RWR to compensate

Similarly, when one side determined its communication codes were likely broken, new codes would be introduced to restore the abilityto securely coordinate operations. In turn, these new codes would eventually be broken. Thesecycles repeated with increasing speed until the war ended, as reflected in Figure 1.

Although ASW forces in both World Wars periodically gained an advantage in the EM spectrum based hider-finder competition, they were unable to sink a significant number of enemy submarines until late in each conflict. Shipping losses to submarine attack, however, decreased shortly after dedicated ASW efforts began, as illustrated in Figure 2.3 This suggests that, instead of eliminating submarines, ASW efforts reduced submarine effectiveness by slowing their deployment to patrol areas, preventing them from getting into firing position, and disrupting their coordination of attacks. This ASW approach exploited the inherent disadvantages of submarines in that they are relatively slow, lack self-defense systems, and cannot rapidly assess the effectiveness of an incoming weapon. As a result, even unsuccessful ASW attacks often compelled a submarine to evade and lose the initiative or made it more detectable for ASW re-attacks.

The first major disruption in the hider-finder competition came with the introduction of snorkels, improved RWRs, and “burst” communications in the latter part of WWII.4 This combination of capabilities enabled submarines such as the German Type XXI to remain submerged and minimize their vulnerability to radar detection when snorkeling, effectively ending the EM-based submarine-ASW competition. Submarine forces, however, were unable to deploy these advancements in relevant numbers before the end of the war.

Navies pursued several efforts after World War II to use sonar for ASW.5 But submarines proved too quiet to hear with passive sonar when travelling on battery power and disappeared in surface noise or sounded like diesel-powered surface ships when snorkeling. Active sonar was somewhat effective against submarines when they were operating at shallow depths, such as when snorkeling, but the detection range was short due to propagation losses incurred as the sound travelled both to and from the submarine.

GERMAN TYPE XXI SUBMARINE AND THE USS NAUTILUS


This changed with the introduction of the nuclear submarine early in the Cold War. Nuclear submarines did not need to surface or snorkel, making them nearly impossible to find with radar and active sonar. However, during early exercises with nuclear submarines such as USS Nautilus, the U.S. Navy realized the new boats had an unexpected vulnerability—they generated continuous noise from their nuclear and steam plant machinery. This sound could be detected at long range with passive sonars the Navy developed to find diesel submarines. As the Soviets shifted to using mostly nuclear submarines for operations outside their home waters, the U.S. Navy adopted passive sonar as its primary ASW sensor. This began a new hider-finder competition between submarines and ASW forces based on passive sonar. The U.S. Navy exploited its “first mover” advantage in passive sonar by starting a methodical sound-silencing program for its nuclear submarines and establishing the passive Sound Surveillance System (SOSUS) network off the U.S. coast as well as at key chokepoints between the Soviet Union and the open ocean. These efforts enabled an operating concept from the early 1960s to the late 1970s in which SOSUS, patrol aircraft, and submarines would trail—and be prepared to attack—Soviet nuclear submarines throughout their deployments.

AKULA-CLASS SUBMARINE

This ASW concept depended on a temporary U.S. submarine silencing advantage that began to erode in the mid-1970s after Soviet leaders learned of their submarines’ acoustic vulnerability from the John Walker-led spy ring and subsequently obtained technology for submarine quieting. 6 The resulting silencing program produced Soviet submarines such as the Akula and Sierra classes that approached the sound levels of contemporary U.S. boats.7 Consequently, U.S. ASW forces would not be able to continuously track Soviet submarines, and the operating concept of destroying them at the outset of conflict was no longer executable.


In response, the U.S. Navy adopted a new approach in the 1980s that applied lessons from WWI and WWII. Rather than planning to sink Soviet submarines, U.S. ASW efforts would focus on degrading their operational effectiveness.8 U.S. nuclear-powered attack submarines (SSNs) deployed to waters near Russia (also known as “bastions”) to seek out Soviet ballistic missile submarines (SSBNs). This operating pattern compelled the Soviets to keep their best SSNs in the bastions to protect their SSBNs, rather than deploying them out into the Atlantic and Pacific oceans to attack U.S. naval forces. A small portion of the U.S. Navy’s dozens of front-line SSNs were needed to conduct this operation, but the costs they imposed on the Soviets were disproportionately large since the Soviets had fewer than 10 comparable submarines.

 

[Lord ZeN]

Undersea Game Changers
Technological advancements, many of them driven by rapid increases in computer processing power (or “big data”), will likely spur a new round of dramatic changes in undersea
warfare through:
• New ASW capabilities to find and attack undersea platforms;
• Undersea platform improvements that will enhance their endurance and stealth; and
• New undersea weapon, sensor, and communication systems.

ASW capabilities.
Since the Cold War, submarines—particularly quiet American ones—have been assumed to be largely immune to anti access threats. Yet the ability of submarines to hide through quieting alone will decrease as each successive decibel of noise reduction becomes exponentially more expensive and new detection techniques mature that rely on phenomena other than the sounds emanating from a submarine. While the physics behind most of these alternative techniques has been known for decades, they have not been exploitable until very recently because computer processors were too slow to run the detailed models needed to see small changes in the environment caused by a quiet submarine.

Today, “big data” is providing the capability to run sophisticated oceanographic models in real time so these detection techniques can be used. And as computer processors continue to shrink, some of them will soon be small enough to fit on ships, aircraft, UUVs, and deployable systems placed on the sea floor. These systems have the potential to make coastal areas far more hazardous for manned submarines, likely driving greater reliance on UUVs to conduct tactical operations in enemy littorals. Emerging acoustic techniques will continue to exploit new forms of active sonar and methods of analyzing the ambient noise already present in the ocean. Most active sonars on ships and submarines are “medium frequency” (MF), meaning they transmit sound between 1000 and 10,000 hertz (Hz). “Low frequency” (LF) sonar, at less than 1000 Hz, has greater range than MF sonar because the sound suffers less attenuation, but it also provides less precise bearing and range information. Advancements in modeling and computer processing will enhance this target information similar to how photographic images can be enhanced. This will likely make LF sonar useful as a tactical or operational-level ASW sensor. “Big data” could also enable detection of a submarine by comparing expected ambient noise from marine life, waves, and seismic events to measured noise fields, possibly identifying where sounds are being reflected off a submarineor obscured by its hull.10

Emerging non-acoustic detection techniques also show great promise.11 The theoretical possibilities of detecting minute changes on the ocean’s surface caused by a submarine or the wake it leaves underwater have been widely recognized since the Cold War, but only now have processing power and oceanographic modeling improved to the point where these approaches may be operationally feasible. Methods to detect radiation or chemicals emitted by a submarine also date from the Cold War and may benefit from the improved sensitivity “big data” could provide.
Lasers and light emitting diodes (LED) can support non-acoustic ASW by bouncing light off the submarine hull, similar to active sonar. Due to material and computer control limitations, previous generations of these systems could only operate in frequency ranges in which the light energy was highly susceptible to attenuation (being turned into heat) or absorption by water or other molecules. Emerging lasers and LEDs, however, can be precisely tuned to wavelengths in which the light energy suffers smaller losses, increasing their range of detection to operationally useful distances.

In combination, new sensors and related improvements to torpedo seekers could enable completely new approaches to finding and attacking submarines. Most significantly, ASW forces could shift away from today’s skill- and labor-intensive tactics that result from the short detection range of sensors that are precise enough to support ASW engagements. This limitation requires ASW ships and aircraft to methodically search a wide area for a submarine, then track it until they can get within weapons range for an attack. New sensor and seeker capabilities could instead enable a “fire and forget” approach in which ASW forces detect a submarine at long range and apply computer processing to obtain enough precision for an attack using longrange missiles with torpedo warheads. This kind of attack may not sink the submarine, but would likely compel it to at least evade, breaking its initiative and making it more detectable. Platform enhancements. New technology will also address the limited endurance of nonnuclear undersea platforms and the growing vulnerability of manned submarines.
Advances in battery and fuel cell technology are expected to enable non-nuclear submarines, UUVs, and other undersea systems to conduct long-duration military operations far from friendly waters.12 For example, the newest Japanese Soryu-class submarines will use lithium-ion batteries instead of AIP for power when submerged.13 And large UUVs are expected to achieve one to two months of endurance within the next two years using a combination of fuel cells,batteries, and traditional propulsion sources.14 These vehicles could carry sensors for coastal surveillance missions and/or large weapons such as torpedoes and mines, making them able to take on some missions conducted today by manned submarines.

ECHO RANGER LARGE UUV

The same improvements that are making submarine detection easier may also enable a new generation of sophisticated counter-detection technologies and tactics. Against passive sonar, a submarine or UUV could emit sound to drown out its own radiated noise, similar to the method used in noise canceling headphones, or deploy decoys to create false targets. Against active sonars, undersea platforms could—by themselves or in concert with UUVs and other emitters—conduct acoustic jamming similar to that employed by airborne electronic warfare systems against radar.
Both active and passive counter-detection systems will benefit from continued improvements in computer processing and oceanographic modeling that will enable them to control and adapt their operations in real time as part of an overall undersea deception operation.15 One implication of new stealth-enhancing capabilities may be that manned submarines will need to be largerto host additional on-board and deployable systems.

T-AGOS COMPACT LOW FREQUENCY ACTIVE SONAR SHIP

VIRGINIA-CLASS SUBMARINE


Undersea systems.
The ability of large UUVs and submarines to conduct and coordinate operations will improve with the introduction of new weapon, sensor, and communication systems.
For example, the U.S. Navy is fielding the Common Very Lightweight Torpedo (CVLWT), which is less than a third the size of the smallest torpedo currently operated by the fleet.16 Although the CVLWT has a short range, large UUVs could carry substantial numbers of them as offensive weapons and exploit the UUV’s quietness to position the torpedoes close to a target. CVLWTs could also be employed as active defense weapons by submarines. Similarly, small, unmanned air vehicles (UAVs) such as the Navy’s Experimental Fuel Cell (XFC) UAV have relatively short endurance but can be launched by submarines or UUVs close to an adversary’s coast. They can exploit the ongoing miniaturization in electro-optical, infrared, and radar sensors to conduct surveillance or electronic warfare missions, providing targeting information directly via line-of-sight to a submarine or strike aircraft in the vicinity.17 Such systems could even carry warheads and be used as loitering, anti-radiation homing weapons to attack enemy air defense radars.

COMMON VERY LIGHT WEIGHT TORPEDO


New technology will also address the longstanding vulnerability of undersea platforms with regard to communications. In previous competitions, submarines transmitting over operationally relevant distances often did so at the risk of jeopardizing their greatest strength: their stealth. With new ASW technologies, undersea platforms will risk being detected even when passively receiving communications near the surface.
These risks could be reduced in the future with new or improved undersea communication methods that will enable undersea platforms to communicate directly with one another, with systems on the ocean floor, and with the above-water
joint force while remaining deeply submerged. In general, undersea communications benefit from the same technological advancements as ASW detection methods. In parallel with improvements to active sonar, acoustic communications are increasing their range and bandwidth to the point where they can support undersea operations over relevant distances in real time.18 In addition to their use in undersea sensing, tunable lasers and LEDs could provide high-bandwidth underwater communications, albeit at shorter ranges than acoustics. And drifting or seabed-mounted cables and floating radio transceivers will enable submerged platforms to communicate with forces above the surface without risking detection.19 Increasing computing power will also enable undersea systems to do more on board processing of sensor
data to reduce the amount of communication bandwidth needed to pass their information to undersea platforms or battle networks

 

[Lord ZeN]

The Next Chapter in Undersea Competition
Undersea research and development has been a distinct U.S. military advantage since the end of WWII, but commercial and scientific interest in offshore resources is prompting a rapid expansion and diffusion of undersea study and expertise. American undersea forces will likely become more vulnerable to inadvertent detection by civilian and foreign entities, while rival military and non-state forces could more easily access and incorporate new technologies in their undersea sensors, unmanned vehicles, and weapons.20
Today, many American leaders assume quiet U.S. submarines can access almost any ocean
area, even those defended by enemy A2/AD systems. New ASW technologies and improvements of non-nuclear undersea platforms, however, will likely enable adversaries to complement their surface and air A2/AD networks with undersea surveillance and attack systems.
These may not have the reach of anti-ship ballistic missiles or modern surface-to-air missiles,
but they have the potential to make the undersea littorals of a potential adversary an increasingly denied zone. Consequently, unless U.S. forces adapt to and lead the new competition, the era of unrivalled U.S. undersea dominance could draw to a surprisingly abrupt close.
U.S. forces will need to develop a novel approach for the next chapter in undersea warfare that addresses the use and exploitation of emerging technologies such as those discussed above, while developing new concepts of operation that harness and integrate both new and legacy systems. In particular, the new approach to undersea warfare should consider the following:

Technological Considerations

• A new basis for the submarine-ASW competition. The effectiveness of traditional passive sonar will continue to erode as submarines become quieter, their stealth is enhanced with countermeasures, and rivals deploy more unmanned systems that radiate less noise. New detection methods may need to leverage something other than noise emitted from an undersea vessel and function at far greater ranges to enable engagements from beyond the envelope of submarine-launched weapons. Whereas the EM spectrum wasthe basis for the WWI and WWII undersea competition, and the Cold War competition centered on passive sonar, the detection method of choice in the first half of the 21st century in ASW may be low-frequency active sonar, non-acoustic detection, or some other previously unexploited technique enabled by ongoing advances in computer processing and material science.

• The advent of undersea “battle networks.” New long-range sensors, such as LF active sonar or wake detection, and emerging undersea communication capabilities will enable the development of new undersea fire control networks analogous to those using radio signals in above-the-surface warfare. For example, long-range ASW weapons such as a missile with a CVLWT warhead could be networked with long-range sensors to create an effective standoff ASW capability that delays or drives off submarines by exploiting their inherent limitations in speed, situational awareness, and self-defense. Undersea networks could also enable coordinated surveillance or attack operations with swarms of UUVs operating autonomously or controlled from a manned submarine or other platform.

• Disruptive technological shifts. With computer processing power continuing to rapidly increase and become more portable, dramatic breakthroughs are imminent in undersea sensing, communications, and networking. Advancements are also underway in power generation and storage that could yield significant increases in the endurance, speed,and capability of unmanned vehicles and systems. These improvements would compel a comprehensive reevaluation of long-held assumptions about the operational and tactical employment of undersea capabilities, as well as the future design of undersea systems. Of course, they would also have a broad effect on naval and joint force architecture writ large.

Operational Considerations

• A new (old) ASW approach. During both of the world wars and the Cold War, U.S. forces eventually adopted ASW operating concepts that emphasized preventing enemy submarines from being effective instead of sinking them—approaches that exploited the inherent disadvantages of undersea platforms. U.S. forces will likely have to take a similar tack to counter growing adversary undersea forces in the future.

• A new approach to offensive undersea operations. Manned submarines will likely need to shift from being front-line tactical platforms like aircraft to being host and coordination platforms like aircraft carriers. New ASW sensors will increasingly rely on phenomena other than radiated noise, so acoustic silencing will not be enough to maintain a submarine’s stealth. As a result, manned submarines will have incentives to reduce their exposure to risks in hostile littorals while maximizing their use of a growing array of deployable acoustic and non-acoustic decoys and jammers to prevent detection. Large UUVs and other deployed systems will increasingly be relied upon as substitutes for manned submarines in conducting tactical operations with a greater probability of detection such as coastal intelligence gathering, land attack, or anti-ship missions in hostile littorals. In addition to being less detectable than a manned submarine, UUVs should be cheaper than manned submarines, leading commanders to be bolder in using them for extremely high-risk operations. Meanwhile, the next generation of manned submarines may need to be considerably larger than today’s Virginia-class submarines to accommodate new counter-detection, communication, and command and control systems, as well as to host an array of unmanned vehicles and weapons.

• Expansion of undersea infrastructure. The seabed increasingly supports a burgeoning array of commercial oil and mineral extraction equipment and pipelines, communication transmission cables, power generation equipment, and acoustic and non-acoustic sensors. Civilian systems are becoming more common as scientists and governments increase monitoring and management of undersea resources such as fish stocks and hydrocarbons. An undersea warfare approach will have to take account of this new form of undersea “encroachment,” especially in terms of inadvertent detection by non-military sensors, protection of friendly infrastructure, and opportunities to inflict damage on enemy undersea infrastructure during a conflict.