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Rising importance of Non- Acoustic detection technologies of Stealthy submarines in Anti Submarine Warfare

Ship detection for offshore production operations, military activities, transportation and other marine applications is very important. Submarine is one of the most important weapon in modern wars. strategic submarines seem to be key to strategic stability, providing what is generally believed to be the most survivable nuclear second-strike force.


Anti-submarine warfare (ASW) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, and/or destroy enemy submarines. Such operations are typically carried out to protect friendly shipping and coastal facilities from submarine attacks and to overcome blockades.  A number of  submarine threats  have advanced stealth and weapon delivery ability. ASW developments are being stressed by the improvements required to detect these threats. Anti-submarine warfare (ASW) has always been a game of hide and seek, with adversarial states looking to adopt and deploy emerging technologies in submarine stealth or detection to give them the strategic edge. The advantage has shifted back and forth, but, on the whole, it has proved easier to hide a submarine than find one: the oceans are wide, deep, dark, noisy, irregular and cluttered.


Sonar detection is one of the most important techniques for the detection of overwater and underwater vessels. Acoustic techniques comprise active and passive sonar which requires the insertion of sensors into the water either to detect sound waves produced by the vessel’s propulsion systems (passive) or detect reflected sound waves emitted by the sensor system itself (active). But with the development of noise reduction and stealth material, the overall noise of some advanced warships is close to ocean background noise level, resulting in traditional sonar detection ability of this kind of “quiet” type ship have been on the verge of the limit.


ASW traditionally relies on a limited number of costly manned platforms such as attack submarines (SSNs and SSKs), frigates and maritime patrol aircraft fitted with a variety of sensors. Today, there’s evidence of a move away from this model towards unmanned aerial vehicles (UAVs), unmanned surface vehicles (USVs), and unmanned underwater vehicles (UUVs) fitted with equivalent sensors, which are more expendable and are becoming cheaper to develop, produce, modify and deploy at scale. Detected information gathered by passive and active sonars  is transmitted back to an air or seaborne platform where processing is carried out. Elevated sensors on a Low Earth Orbit (LEO) satellite, or aboard an Uninhabited Aerial Vehicle (UAV), provide improved capabilities for conducting wide area surveillance (WAS) within the spatial and temporal requirements.


Similarly Modern nuclear and diesel electric submarines have become very stealthy and quiet. Work done over decades has resulted in some remarkable advances in radiated noise management to make detection by passive sonar more difficult. Against active sonars, submarine stealth is achieved by anechoic tiles and clever design to reduce the strength of reflected signals. Today’s submarines are many decibels quieter than their predecessors and present lower target strengths to active sonars. But ultimately, large submarines are large, and there’s a limit to what can be done to reduce propeller and flow noise, and target strengths can’t be reduced below some physical limits—especially at low frequencies.


Rising Importance of Non-Acoustic detection

Therefore there has been rising importance of non-acoustic detection techniques. Besides visual detection, the primary non-acoustic method is Magnetic Anomaly Detection (MAD). This technology is mature and is used by the RAN and the RAAF’s long-range maritime patrol aircraft.


There are several emerging techniques in oceanographic remote sensing for detecting submerged vessels. There are two methods by which a submerged vessel could be located: direct and indirect detection. Direct detection involves locating the vessel structure itself; the indirect method involves the detection of environmental anomalies caused by the presence of a submerged vessel.


Advances in technology, such as detector sensitivity, are now making the operational use of these techniques more
feasible. For decades, non-acoustic methods were considered inferior for being limited in range and reliability compared to sonar. However ever rising increase in computing power and signal processing and AI technologies may be bringing this field to prominence again.


Direct Detection

Submerged vessels may be directly detectable by observing how a hull absorbs or reflects blue-green laser light (450-550 nm). This response could be used to create an image with the vessel appearing as either a bright spot in the normal background scattering of the ocean, or as a hole.


LIDAR Sensors

LIDAR (laser radar) technology has been used successfully in depth sounding systems for seabed mapping, and has also been very effectively
employed for the detection of seabed and tethered mines in the US Navy Airborne Laser Mine Detection System (ALMDS). It has also been raised at various times since the 1970s as a potential ASW sensor.


LIDAR works by emitting laser (or LED) pulses and measuring the return time and strength of the reflected light. When deployed on space, aeronautic, or naval platforms, LIDAR can track a submarine’s disturbance to the ocean surface or directly image a vehicle. LIDAR is presently limited to sensing depths up to 200 m—projected by some to reach 500 m.


The ability of Laser radar, or LIDAR (LIght Detection And Ranging), to penetrate the water’s surface, reflect from an object and then be detected remotely, makes this sensor a potential candidate for the detection of submarines. However, this technique is limited by the inability of LIDAR to penetrate clouds and other high attenuation effects caused by fog, haze and atmospheric pollutants.

Despite these limitations, it is reported that the Swedish have used this technique to detect submarines in national waters from an airborne platform, although the  effectiveness of this system is not known. In addition, the US Defense Advanced Research Projects Agency (DARPA) has developed and tested an airborne LIDAR system for the purpose of detecting mines at sea. The system, known as the Airborne Laser Radar Mine Sensor (ALARMS), uses a pulsed blue-green laser operating at 510 nm. Trials of the system showed that the laser shadow cast by the object under inspection produced the best results at depths of up to 200m. An Australian application of LIDAR, the Laser Airborne Depth Sounder (LADS), uses the time difference between surface (blue) and the sub-surface (green) reflections of a 532 nm (yellow-green) laser, for sub-surface laser ranging.


To improve the accuracy of depth soundings using lasers, an understanding of effects such as turbidity (absorption and scattering) of the water is required. A number of techniques using low frequency electromagnetic pulses (Electromagnetic Bathymetry), used for airborne prospecting for mineral deposits, has the potential to measure subsurface targets with no restriction due to turbidity. However, with all such measurements, some false targets are likely to be observed due to returns from other submerged objects such as whales.


Indirect Detection

Submerged vessels also generate a diverse range of indirect effects on the surrounding marine environment; these are categorised as physical, optical and thermal effects. While such effects are highly variable, being dependent on submarine type and operational parameters, they do provide potential means of detecting submerged vessels using both airborne and elevated sensors.


Physical Surface Effects

Physical effects range from the production of a wake which may be detectable on the surface, to the generation of internal waves which manifest themselves through subtle surface effects.


The submarine will form a wake on the sea surface when it passes by, and the wakes will vary with the depth and the speed of submarine. So we can detect submarines by the SAR image of sea surface. The major physical surface characteristic is the wake developed by a vessel when it is mobile. The characteristics of the wake will be a function of the speed, depth and size of the vessel. Three separate hydrodynamic phenomena are either directly or indirectly caused by the wake: the Benoulli hump, Kelvin waves, and the surface effect of internal waves.


The Benoulli Hump. If a submarine travels at high speed near the surface of the ocean it produces a characteristic hump of water which is sometimes referred to as the Benoulli hump. The size of the Benoulli hump decreases rapidly with submarine speed and depth. For example, the height of the hump reduces from about six centimetres to one millimetre when a given submarine reduces it’s speed and increases it’s depth from 20 knots and 50 metres to five knots and 100 metres, respectively.


Kelvin Waves. Kelvin waves are produced by both ships and submarines and are responsible for the characteristic “V” shaped wake that can be seen to linger behind a moving vessel. They have an angle of approximately 39º which is independent of the size of the vessel or the speed at which it is travelling. Kelvin waves, like the Benoulli hump, reduce rapidly in size with submarine speed and depth. Using the above example, the wave size reduces from about two centimetres to immeasurably small.


The physical surface effects caused by a submerged vessel may be detectable either by accurate measurement of the ocean surface height or
by imaging the ocean’s surface. LIDAR is well suited for precision measurement and may be suitable for the detection of the Benoulli hump or Kelvin waves. However, the effectiveness of this technique will be severely limited by the depth and speed of a submarine.


Internal Waves. Internal waves are periodic variations in the temperature and density of water at depths near a thermocline, an ocean layer in which the temperature drops and the density rises sharply with increasing depth. The period of internal waves, known as the Brunt-Vaisala period, varies with time and location but is typically between 10 and 100 minutes.


The displacement of water associated with internal waves is influenced by many factors, including atmospheric pressure variations, ocean
currents, and the presence of a submarine. In addition, internal waves are often formed in areas where the ocean bottom is irregular and the tidal range is large. Internal waves are rendered visible on the surface because the internal currents generated modulate the small scale surface waves overlying the internal waves, which leads to periodic variations in surface roughness.


In the case of Benoulli humps and Kelvin waves it is clear that reasonable precautions could be taken to avoid detection by limiting speed and remaining at sufficient depth. However, this is not the case with internal waves which seems to be the most promising physical effect to exploit for the detection of submerged vehicles over a wide area.


Of the physical effects, detection of internal waves is probably the most realistic approach; detection of Kelvin waves and the Benoulli hump is severely limited by submarine depth and speed. Indeed the detection of internal waves using SAR is an area where the majority of research seems to have focussed.


Recent developments in Laser Doppler Velocimetry, a mature technology, may now permit the remote measurement of fluid parcel velocities in the ocean using the Doppler shift of a laser beam. However, a number of artefacts such as the effects of waves, and turbulence in the ocean, will also cause a Doppler shift of the laser beam.  The military application of this technology includes the detection of the propeller wakes, and possibly internal waves.


In the scope of underwater warfare, strong signature of ship wakes is attractive for its exploitation in ship detection, tracking and classification. It is known that nations are working on the development of wake-homing torpedoes and countermeasures against wake-homing torpedoes.


Ship wake detection through SAR images

Synthetic Aperture Radar (SAR) is well known for its ability to monitor wave patterns and determine sea surface roughness, and has been
shown to successfully detect internal waves. The Seasat satellite launched in 1978 effectively imaged ocean surface features such as internal waves and ship wakes using SAR. Subsequent reports claim that Russian scientists have demonstrated a way of detecting submerged submarines using microwaves reflected from internal wave generated surface effects.


Although the ship itself can often be seen as a bright region close to the wake pattern, there are at least two reasons why it would be more advantageous to detect the wake pattern rather than the ship itself: the wake pattern is larger and more distinct than the ship signature, and it can yield a better estimate of the ship’s true location. The latter is true because the motion of the ship in the ocean causes it to appear in the SAR image displaced in azimuth from its actual location, since conventional SAR image reconstruction techniques assume no motion of objects in the image scene.

Image result for ship wake


The common assumption of the existing approaches is related to wake structure, which is basically composed by the turbulent wake, two narrow-V wakes, and the Kelvin pattern. The turbulent wake is imaged as a dark line, surrounded by two bright lines, representative of the narrow-V wakes. The Kelvin pattern shows three features: the transverse, divergent, and cusp wakes, all imaged within an angle of 19.5◦ with respect to the ship track. Since the cusps are the most discernible features in SAR images, only the detection of the cusp line  is implemented in the proposed algorithms.


In a SAR image, a ship wake always has a linear feature pattern. The existing approaches to detect wake components exploit the Radon Transform, which emphasizes linear features. Radon transform converts straight lines in the image domain in dots in a transformed domain. Hence, dark lines, e.g., turbulent wake, appear as dark dots in the Radon domain and, similarly, bright lines are imaged as bright dots. For sake of clarity, since in the real images the wake components are not lines but streets of bright/dark pixels, they are converted into clusters of bright/dark dots in the Radon domain, thus rendering the detection process more difficult.


Image result for ship wake


Moving ship generates a set of multiscale surface gravity wave patterns. In general, this produces a wedge-shaped pattern in Synthetic Aperture Radar (SAR) images, which is related to the ship’s size and speed As in SAR images the ship always tum out just as some bright points, it is hard to see about the size of the ship.

Image result for submarine wake detection

SAR (Synthetic Aperture Radar), with the characteristics of all day, all weather, high resolution and wide coverage, provides a perfect approach for ship monitoring. The wake visibility in SAR images depends on several parameters, related both to radar characteristics and ship motion, as well as to local conditions, such as wind speed and direction.


Ship parameters such as ship position, ship size and ship orientation can be estimated after the ship target is detected. Ship target’s moving relative to the SAR platform may cause azimuth displacement in SAR image and the ship speed can be estimated by this Doppler shift. Moreover, ship wake usually extends 5 to 15 kilometers in SAR image and much larger than ship target. Ship orientation estimated by the ship wake will be more accurate than the value estimated by the ship target. So ship wake detection plays an important role in ship detection.


Other technologies of wake detection

The strong signature of ship wakes is due to bubbles found inside them. Ships generate bubbles by propeller cavitation, by breaking of ship generated waves, and by air entrapment in the turbulent boundary layer under the ship hull. Ship propeller and hull design, ship speed and maneuver affect the bubble distribution and hence wake signature. In various experiments, it has been observed that ship wakes could last for about 15 minutes


The bubbles are nonlinear oscillators, acting as acoustic sources giving out characteristic acoustic emissions. Also they affect acoustic propagation such that acoustic waves through the bubble cloud have frequency dependent average sound speed and attenuation. Backscattering of the acoustic waves is observed due to the scattering cross sections of the bubbles. These properties of bubbles create the acoustic signature of ship wakes.


Various acoustic methods are used for probing underwater bubble clouds to estimate parameters like average sound speed, average attenuation, target strength, bubble size distribution and void fraction. Mostly used acoustic methods for probing bubble clouds are pulsed and continuous wave forward propagation measurements using separate projector and hydrophones, backscatter measurements using multi-beam sonars, and measurements using acoustic resonators. While each method has different advantages making it suitable for bubble clouds with specific structures, best approach seems to be using a combination of these methods


Submarine detection

Submerged submarines, no differently than surface vessels, produce a wake of disturbed water when running, produced by vortices in the water excited by the motion of the submarine hull and screw. Submarines produce many different types of wake. As well as the familiar V-shaped wake they leave underwater disturbances known as “internal waves”, flat swirls called “pancake eddies” and miniature vortices which spin off from fins and control surfaces. These all depend not only on speed and depth but also on the submarine’s hydrodynamics (the underwater version of aerodynamics).


This wake expands in a roughly conical shape behind the submarine, as it dissipates in intensity with distance and time. When the front of the wake impinges on the surface of the water above and behind the submarine it produces a surface disturbance in the shape of a non-linear paraboloid curve. The Internal waves modify the roughness and steepness of the surface waves which can be detected from space.


A wake detection system (WKS) are special sensors that analyze the state of water near a submarine. And sometimes from this analysis it can be determined that another submarine has passed here recently – nuclear submarines leave a small footprint from the reactor, emit warm water used to cool the reactor, diesel submarines leave exhaust from a diesel engine, or a trail of bubbles, etc. That is, with a competent analysis, can not only determine that another submarine passed in this place, but can even determine the direction of its movement, speed and distance. Well, in much the same way as on modern fighters, it is determined by special thermal radars from the thermal and gas trail left by the engines of the enemy aircraft.


Militaries employ Wake Detection

In the late 1980s, the Soviet Union claimed a feat many military experts thought impossible. K-147, a Victor-class nuclear-powered attack submarine, secretly followed the trail of a U.S. boomer (most likely the USS Simon Bolivar) in an underwater game of chase that continued for six days. The CIA’s Directorate of Science & Technology produced the report on Soviet Antisubmarine Warfare Capability in 1972, but it was declassified only this summer. A lengthy portion about Soviet technology under development gives details never previously revealed about devices with no Western equivalents.  created something else entirely. As it became known from a recently declassified CIA report, Soviet submarines tracked American submarines without the use of sonar.While NATO were concentrating almost all their efforts on sonar,  The USSR developed other tricky means of detecting submarines.


One of the methods described in the report is the Soviet mysterious SOX – wake-track detection system. This device, installed on Russian assault submarines, tracks the wake that the submarine reserves. SOX were first installed on the K-14 in 1969.  Since then, subsequent versions under code names were installed on each new generation of Soviet and Russian torpedo submarines, including those installed on modern submarines of the Shark and Ash class. The Soviets developed not one device, but several. One of these devices could capture “activation radionuclides” – a faint trace that remains from radiation emitted by an onboard nuclear power plant. Another instrument was a gamma spectrometer, which detects traces of radioactive elements in seawater.


The British fleet had shown continued interest in wake detection devices and guidance systems. Reports of the presence of non-acoustic detection devices on British nuclear submarines appeared in the 1980s and 1990s. In 2008, the allegedly tested SOX device was observed on the retractable guards of the British atomic multipurpose submarine S 107 Trafalgar (decommissioned in 2009).


The CIA report adescribes how submarines leave behind a range of chemicals. Their proportions with respect to the ocean are negligible, but sophisticated equipment can detect them. And, unsurprisingly, a nuclear reactor leaves a lot of heat. According to the report, a large nuclear submarine requires “over 10,000 liters of refrigerant per minute.” This water, used to remove heat from the reactor, can be 10 degrees warmer than the surrounding seawater, which changes the refractive index of the water — a change detected by the optical interference system.


The density and temperature profile of ocean changes with depth. When a submarine moves it causes mixing of layers of water having different density and temperature. The mixing results a turbulent wake downstream of submarine. The wake grows uniformly in all directions. After a critical time and distance, the wake collapse vertically and spread horizontally. The collapse generates Internal waves which lasts more than hour


Russian  SOKS wake detection system

On such method highlighted in the report is the Soviet’s mysterious SOKS, which stands for “System Obnarujenia Kilvaternovo Sleda” or “wake object detection system.” This device, fitted to Russian attack submarines, tracks the wake a submarine leaves behind. SOKS is actually visible in photos of Russian subs as a series of spikes and cups mounted on external fins.


Rumors out of Russia about SOKS have been inconsistent and often contradictory, with some saying SOKS measured changes in water density, or detected radiation, or even used a laser sensor. According to these newly declassified documents, the old rumors were accurate in one way – the Soviets did not develop just one device, but several. One instrument picked up “activation radionuclides,” a faint trail left by the radiation from the sub’s onboard nuclear power plant. Another tool was a “gamma ray spectrometer” that detects trace amounts of radioactive elements in seawater.


The report also describes how submarines leave behind a cocktail of chemicals in their wake. Sacrificial anodes that prevent corrosion leave a trail of zinc in the water. Minute particles of nickel flake off the pipes circulating seawater to cool the reactor. The system that makes oxygen for the crew leaves behind hydrogen that’s still detectable when dissolved in seawater. Together these chemical traces may measure only a few tenths of a part per billion, but sophisticated equipment can find them.


And as you’d expect, a nuclear reactor also leaves behind tons of heat. According to the report, a large nuclear submarine requires “several thousand gallons of coolant a minute”. This water, used to take heat from the reactor, may be 10 degrees Celsius warmer than the surrounding seawater, creating a change in the water’s refractive index—a change that’s detectable with an optical interference system.


The technology of search and detection of submarines on the wake trail left by a submarine is being developed by the Krylov State Scientific Center. This was announced in July 2019 by the scientific director of the center Valery Polovinkin. According to Polovinkin, the center was engaged in the development of technology for detecting submarines in their wake trail, a disturbed strip of water left by the ship’s propellers. The reason for this interest was that the wake trail remains in the water longer than other physical fields of the ship, since the acoustic noise and electromagnetic signals produced by the submarine move with it, and the wake trail still remains for some time. With the help of its detection, it is possible to find out the direction of movement of the submarine or the point where it was.


Polovinkin, of course, did not disclose the details of the ongoing research in this direction, but said that work was being done to detect the “fields of evidence” of the submarine, but not acoustic. It is known that the wake stream is an acoustic unmasking factor by which a submarine can be detected, but such noises move with the submarine and quickly fade away from the detection point.


U.S. Navy is testing a new radar pod that  can detect submarines through wake detection

The U.S. Navy, in a break with traditional submarine detection, is working to replace sonar and magnetic detection with radar. The AN/APS-154 Advanced Airborne Sensor (AAS) will spot the invisible wakes left by submarines underwater, telltale clues that something large is lurking beneath the waves. The AAS will be carried by the P-8 Poseidon aircraft, which can then engage submarines with air-dropped anti-submarine torpedoes. The AAS is solid-state, wide-aperture, active electronically scanned array radar housed in a long pod under the fuselage. The sensor is designed to provide standoff detection and tracking of moving targets and high-resolution ground mapping. Flight tests on the first P-8A test aircraft began in April 2014.


Subs create wakes as they displace water in their path, which are barely visible on the surface. A radar like the AAS can pick out these wakes from the pattern of regular ocean waves, betraying a submarine’s location. The AN/APS-154 Advanced Airborne Sensor (AAS) will spot the invisible wakes left by submarines underwater, telltale clues that something large is lurking beneath the waves. The AAS will be carried by the P-8 Poseidon aircraft, which can then engage submarines with air-dropped anti-submarine torpedoes.


One such object is the wake created by a submerged submarine on the surface of the ocean. Subs create wakes as they displace water in their path, which are barely visible on the surface. A radar like the AAS can pick out these wakes from the pattern of regular ocean waves, betraying a submarine’s location.


According to Forbes, the downward-mounted pod features an advanced electronically scanning array (AESA) radar. Unlike traditional dish radars that use one large, powerful radar module, AESA radars use many smaller modules. These modules can collectively operate over multiple frequencies, which means they can overcome jamming or broaden or focus their field of detection, especially against small objects and those invisible to the human eye.


Once a submarine is detected, a P-8 can drop a Mk. 54 lightweight anti-submarine torpedo to give chase. The Mk. 54, delivered by parachute, will enter the water, turn on its onboard sonar system, and start searching for the enemy sub. When the torpedo finds the sub, it moves to intercept, detonating a 100-pound warhead against the submarine’s hull.


The AAS appears likely to replace older submarine detection systems. Aircraft like the Navy’s P-3C Orion would often drop sonobuoys in waters suspected of harboring enemy submarines. The sonobuoys, pinging away with sonar, transmit their data to the circling P-3C. Another system, Magnetic Anomaly Detection, detects the disturbance in Earth’s magnetic field created by a large, steel-hulled submarine.




Reliable and repeatable submerged submarine wake detection is a challenging task, primarily due to the enormous variability of ocean surface conditions. With increasingly high sea states, a radar attempting to image from a shallow grazing angle will have to confront wave troughs and peaks, which will disrupt the wake pattern and shadow, on average, half of the pattern.


Not only must the radar do a good job of capturing the surface image but the post-processing demands of finding the specific shape of a wake
pattern in a very noisy radar image of the sea surface are quite difficult, typically using a Hough transform or Radon transform algorithm, both of which require a lot of computing power.


While wake detection using SAR even under optimal conditions may result in a highly accurate track of the wake, the delay between the production of the wake and its contact with the surface adds considerable uncertainty as to the immediate position of the submarine being tracked, unless it is running very shallow. While the shape of the wake front could be used to infer the distance and the depth of the submarine, for submarines running deep wake detection radars may well become primarily a ‘tripwire’ sensor, not unlike MAD used for initial detection and tracking rather than prosecution of an attack.


Ultimately, once SAR wake detection technology matures, it will provide a potent capability to detect submarines from orbital and high flying airborne platforms. This will drive submarines to greater depths and lower transit speeds, and result in further design changes in hull shaping to produce the least detectable wake patterns.


Optical Effects

Optical effects range from the stimulation of marine micro-organisms to emit light (bioluminescence), to the scattering of light by the
movement of organisms in an internal wave.


The oceans are populated with organisms that either emit light when they are disturbed (known as bioluminescence), or scatter light under all conditions. Such effects may be detectable above the ocean surface and may be used to reveal the location of subsurface vessels.

Bioluminescence. The turbulent wake of a moving submarine will naturally cause a local disturbance of the surrounding bioluminescent organism population inducing them to emit light. The blue-green component of this light will propagate the greatest distance and may well be detectable beyond the ocean’s surface. The intensity of blue-green light is attenuated by a maximum factor of approximately
two for every seven meters it travels through water.  At this rate, light passing upward through 50m of water will be attenuated by about 21dB; through 200m of water the attenuation would be around 86dB.


It may be possible that a turbulent wake could rise to the surface bringing the bioluminescence with it, however it is more likely that the wake would collapse behind the submarine due to suppression by distinct ocean layers. Another possibility is that the emission of light by excited organisms at one depth may induce other organisms closer to the surface to emit light. While it has been reported that relays of such empathic responses have been observed, what is not known is whether such a mechanism could reveal the location of a submarine. Furthermore, there appears to be very little knowledge at this time regarding the geographic, seasonal and depth distributions of such organisms. One major limitation for the detection of bioluminescence is the overpowering background noise contributed by the sun and the moon which would render a detection system useless during the day-time and possibly also under certain night-time conditions.


Light Scatterers. There are layers of organisms in the ocean which scatter light. The motion of these layers caused by vessel generated internal waves may be detectable. However, again the geographic, seasonal and depth distributions of such organisms is not well known.


The scattering of light from the surface effects of internal waves is perhaps the most promising detection phenomena of the optical effects. While this technique has been experimentally verified, it is probably only feasible during the day or in the presence of sufficient moonlight. The potential for detection of a submerged vessel via its bioluminescent wake requires more research to determine if this is even feasible. This technique will require highly sensitive electro-optical sensors to detect the relatively low levels of light produced. Furthermore, its restriction to night-time use makes its use impractical as a singular surveillance sensing technique.


Electro-Optical Sensors

Thermal imagers and stabilised high definition television telescopes have been widely integrated on LRMP aircraft and ASW helicopters but have mostly been used for the identification and tracking of surface vessels. The technology has been proposed for use in tracking submarines by bioluminescence in surface wakes, or to track minute temperature increases in a surface wake. Both applications would be more suitable for an infrared hyperspectral imaging sensor.


The successful detection of bioluminescence will require visible spectrum radiometers with sufficient spatial and radiometric resolution to enable the low  level of emitted light penetrating the surface to be detected. One major limitation of this technique, however, is that it is limited, at the best, to use at night. Further investigation is needed to determine resolution requirements. The scattering of sunlight (or
moonlight) from the movement of marine organisms or surface effects caused by internal waves has been observed using ship-borne optical sensors and may  be possible using elevated sensors.


Thermal Effects

All active submerged vessels generate heat which is dissipated through the seawater (conventional and nuclear submarines), as well as through the atmosphere (conventional submarines).


Thermal Transfer to Surrounding Seawater. Conventional and nuclear submarines draw in substantial quantities of seawater specifically for the purpose of cooling. In the case of a nuclear submarine producing about 190 MW of useful power, about 188 MW of heat energy is released into the ocean. While this appears to be massive, heat transfer calculations reveal that at a speed of about five knots, the
temperature immediately behind the submarine only rises by about 0.2 degrees Celsius. This temperature differential will diminish rapidly as the submarine moves further away. In addition, this slightly warmer water, as it rises to the surface could, depending on the depth it was generated, eventually encounter water of the same density at which point it will rise no further and therefore not be detectable on the surface.


Thermal Transfer to the Atmosphere. Unlike nuclear submarines, diesel powered conventional submarines need to surface periodically in order to recharge their batteries. This process, known as snorting, requires two pipes to be raised near or above the surface. The first pipe, raised above the surface, is used to draw in fresh air to run the diesels. The second pipe is usually kept just below the surface and allows exhaust gases (and, therefore, heat) to escape. The heat emitted through the exhaust gases of a conventional submarine may be detectable above the normal sea temperature. The major limitation with this method of detection is that a conventional submarine in normal operation only needs to snort for about two hours in every 24.


Passive. Methods for remotely detecting localised increases in water temperature include the measurement of thermal infra-red and microwave radiation. The localised intensity of this radiation is highly dependent on submarine depth and speed. As previously discussed, the temperature increase due to the presence of even a large nuclear submarine is very small and would only provide a weak surface
signature. However, the snorting of conventional submarines may be detectable as a localised point source on the ocean surface. Landsat 5 carries IR sensors with an instantaneous field of view of 120m x 120m and some sensors have a 0.1ºC thermal precision capability. Even at such resolutions it is unlikely that the average temperature rise over a 120m x 120m area would be detectable.

Active. Water, when irradiated with a laser beam, exhibits strong Raman scattering; the ratio of energies in the two strongest scattered lines is temperature dependent. Even though passive detection may be minimal, preliminary estimates indicate that a system based on LADS should have the capability to measure temperatures to an accuracy of at least 0.1ºC to a depth of 50m in moderately clear water, with a
depth resolution of about 1m.



Wake homing is a technique used to guide torpedoes to their target.

The torpedo is fired to cross behind the stern of the target ship through the wake, as it does so it uses sonar to look for changes in the water caused by the passage of the ship, such as the small air bubbles. When these are detected the torpedo turns toward the ship then follows a zig-zag course, turning when it detects the outer edge of the wake, to keep itself in the wake. This will eventually bring it to the rear of the ship, where its warhead can do the most damage, to propulsion and steering.


The system is difficult to jam, though can be distracted by other ships crossing the wake. In 2013 the US Navy tested prototypes of the Countermeasure Anti-Torpedo (CAT) designed to intercept and destroy the incoming torpedo. Deployment of TWS/CAT has not proceeded as planned due to performance issues.


The main disadvantage of wake homing is that the course taken to the target is non-optimal and the target is always sailing away from the weapon, requiring a fast weapon with a longer range than for direct homing.


The ability to detect quiet submarines may allow a substantial enhancement to situational awareness in anti-submarine warfare. The detection of conventional attack submarines may allow a naval force to avoid torpedo attack or destroy adversary firepower. The tracking and targeting of SSBNs may allow for the destruction of submarine-launched ballistic missiles (SLBMs) carrying thermonuclear warheads. Quiet, nuclear-powered SSBNs are essential components of the nuclear strike capabilities of the United States, Russia, United Kingdom, France, India, and China.


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