Tactics 101: Anti-Submarine Warfare (ASW)

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ACTICS 101: ANTI-SUBMARINE WARFARE (ASW): PART 1 - THE FUNDAMENTALS

In Part 1 of this discussion, we'll address the fundamentals of naval oceanography, i.e. the nature of the subsurface environment and how it impacts on anti-submarine warfare (ASW).








A. FEAR GOD, DREADNOUGHT, AND THE SUBMARINE!

The watchword of subsurface warfare, and by extension, anti-submarine warfare (ASW), is stealth. Submarines are absolutely dependent on stealth for both mission effectiveness and self-defense.

The introduction of submarines brought about a new and revolutionary dimension in naval warfare, one that was measurable in fathoms. Compared to the heavily armored ironclads, dreadnoughts, battleships, and battlecruisers that ruled the surface, directly confronting each other in hails of gunfire, a submerged submarine instantly became invisible, allowing it closely approach the most powerful surface ship. It could then deliver a torpedo, a weapon which could avoid the armor and strike below the waterline, where a ship was most vulnerable, thus allowing the smallest submarine to sink the largest ship.

This unique combination of stealth and offensive punch has therefore been the defining characteristic of the submarine from the very beginning.

British Admiral Sir John Fisher, the father of the the first big gun battleship, HMS Dreadnought, had the prescience to know that the submarine would seriously constrain surface ship operations, so much so that during WWI, the Grand Fleet did not operate in the southern part of the North Sea because of the threat posed by U-boats. Fisher had hoped to develop a “blue water” force projection capability that would combine the surface strike firepower of the battlecruiser with the submarine, making both the open ocean and the narrows untenable for the enemy. He envisioned that that the future of naval warfare lay in the ability to move quickly and strike at long range, as well as to go below the surface. Unfortunately for the Allies, Fisher's vision proved to be ahead of its time.

The goal of modern ASW has remained unchanged from the time that the effectiveness of submarine warfare was first demonstrated during WWI. That goal is to deny an adversary submarine the ability to exercise its considerable influence on military objectives throughout an area of interest.

ASW no longer involves destroyers and corvettes racing around, pounding the ocean with active sonar and dumping seemingly endless numbers of depth charges on the heads of hapless and terrified submariners. (Though, admittedly, popping the movie Das Boot into the DVD player is still a favorite pastime).

Image credit: Das Boot, 1981, Columbia Pictures et al.

Rather, in the modern era, passive sonar is the instrument of choice (on both sides of the equation) when it comes to trying to detect and locate an enemy, and active sonar is usually relegated to the final attack and other special circumstances. Therefore, since passive sonar is so critically important, both surface ships and submarines take every effort to reduce their acoustic signatures.

Despite their stealth, submarines are inherently quite vulnerable, and they are easiest to detect when they poke their masts (or their hulls) through the surface. In an encounter between a surface ship and a submerged submarine, however, the advantage most definitely shifts to the submarine, as it tends to have a quieter acoustic signature.

This advantage diminishes significantly, however, when the surface ship can act in concert with other friendly assets, especially aircraft. In fact, aircraft are perhaps the most effective way of countering submarines.

More on that later, but firstly, let's examine the environment in which submarines operate.

B. NAVAL OCEANOGRAPHY

1. The basics of SONAR

When the first sonar (sound navigation and ranging) systems were developed (the ASDIC system) between the world wars, it was believed that submarines would henceforth be stripped of their stealth. As was made obvious during the Battle of the Atlantic in World War II, this was hardly the case.

Sonar exploits underwater sound propagation to navigate, communicate, or as a means of acoustic location. There are two kinds of sonar: active and passive.

Active Sonar

Active sonar requires a transmitter to create a sound pulse (the famous "ping") and a receiver, to "listen" for the reflection (the echo) of that sound pulse.

"Give me a ping, Vasili. One ping only, please."
For snarf.

With active sonar, the source of the sound is the sonar system itself, the transducer (which converts electrical energy into acoustic energy). You also need a receiver to catch the signal (echo) reflected back from the target. That signal is degraded with increasing distance to the target in exponential fashion, so that four times the power of the active sonar produces only about twice the range.

A modern hull mounted active sonar can produce hundreds of thousands of watts of sound energy, reaching up to around 250 decibels (dB). Compare that to the sound level of a jet engine at 30 meters distance, which is just 150 dB!

Passive Sonar

With passive sonar, the sound source is the target itself. A transducer that can only receive acoustic energy is called a hydrophone.

Passive sources fall into two main categories: broadband and narrowband.

Broadband sources produce acoustic energy over a wide range of frequencies. When it comes to ships and submarines, typical broadband sources are noises that emanate from the propeller or shaft, flow noise, and some kinds of propulsion systems (e.g. steam boilers).

Narrowband sources, on the other hand, produce acoustic energy within a small band or class of frequencies. Typical ship and submarine sources are the various pieces of machinery found in every ship. For example, pumps, motors, electrical generation equipment and propulsion systems. When specifying narrowband sources, it is important to also specify the frequency at which it occurs.

Each type of sonar is dependent on a different characteristic of the sonar target that in turn influence sonar performance. For active sonar, that characteristic is the target's sound reflection characteristics, or "target strength". For passive sonar, the target's own radiated noise is critical.

Frequency

Lower frequency (longer wavelength) sounds - generally in the 1-5 kHz range - tend to propagate the furthest distance, and require a large sonar dome to maintain gain (directionality). Medium frequency sonars fall generally into the 5-15 kHz band, and high frequency in the 20-30 kHz range.

High frequency (shorter wavelength) sonar can be broadly associated with short, direct path ranges in the surface layer (again, more to follow). Medium frequency sonars can be used to detect targets below the surface layer, and can sometimes exploit the bottom bounce technique. Low frequency sonars, meanwhile, have the potential to achieve convergence zone (CZ) detections. More to come on the terms direct path, surface layer, and bottom bounce.

It is also very important to note, however, that in recent years, the distinctions between frequency classifications and capabilities (as well as the boundaries of the frequencies being achieved) have blurred somewhat as technology has pushed the boundaries of what sonar can do. It is also important to realize that the ocean environment (including such variables as temperature, salinity, depth, and even geographic location) plays a huge role in affecting how sonar behaves, and notably, what it can and cannot do.

2. The Characteristics of Sound in the Ocean Environment

While light is highly absorbed by water, sound waves are not. Sound waves can thus be effectively used to probe the ocean's depths, communicate underwater, and most importantly for the purposes of this discussion, locate submerged objects.

It is notable that although sound travels vastly farther through water than does light, it is lost in reverse order, i.e. highest frequency sounds are absorbed first, and lowest frequency sounds last. This frequency loss is also one reason why it can take quite awhile to determine the nature of, or develop, a sonar contact. It requires analysis of many sounds of many different frequencies, coming from different sources on the target, to determine its identity.

The speed that sound travels underwater varies from about 4,750 feet per second (fps) to about 5,150 fps. Its speed increases with:

1. Temperature, at a rate of about 4.3 fps per degree Fahrenheit.
2. Salinity, at a rate of about 4.3 fps per thousandth part increase in salinity.
3. Depth, at a rate of about 1 fps per 60 feet of depth.

The speed of sound underwater (in fps) can be determined as follows: 4388 + (11.25 × temperature (in degrees Fahrenheit)) + (0.0182 × depth (in feet)) + salinity (in parts per thousand)

(A special instrument, called a sound velocimeter, can be used to measure the sound velocity profile of a certain area of the ocean).

The average salinity of the world's oceans is 35 parts per thousand (ppt), that is to say, the same as 35 grams of salt dissolved in each kilogram of water. Salinity in the oceans varies from about 32-37 ppt, except in the polar regions and near the shore (where fresh water from ice and the flow from rivers and streams act to dilute the salinity), where it may be less than 30 ppt.

Weather will also influence salinity, particularly where precipitation exceeds evaporation (such as in the rainy North Pacific) or where evaporation exceeds precipitation (such as in the Indian Ocean). An isolated body of water also tends to have higher salinity, such as in the Mediterranean, where excess evaporation works to increase salinity.

The polar seas (the Arctic and Antarctic) are the least saline (at 30-33 ppt), followed by the Indian Ocean (32-35 ppt), the Pacific (32-36 ppt), and lastly, the most saline ocean is the Atlantic (at 34-37 ppt).

Water pressure, which increases with depth, also has an effect on the propagation of sound. As stated, the speed of sound underwater increases with greater depth (i.e. increased pressure), which causes sound waves to bend (or refract) away from the area of higher sound speed. More on the effect of refraction in just a bit.

Let's look more closely at one of the most significant factors affecting underwater acoustics: temperature.

3. The Thermocline or the Layer

The world's oceans are stratified vertically with respect to temperature. Considered globally, seawater has a relatively large temperature range that depends upon location and time of year. Water temperature in the open ocean varies from a low of 28.4 degrees Fahrenheit (-2 degrees Celsius) to about 86 degrees F (30 degrees C), and can reach nearly 100 degrees F (37.8 degrees C) in shallow coastal waters around the Equator.

The thermal structure of the ocean is divided into three zones:

First is the surface layer or the mixed zone, where temperatures are almost uniform with depth. This is where waters are mixed together by wave action, solar driven circulation cells, tides, and so on. The depth of the surface layer varies with location and season. During winter months, it becomes more defined at all latitudes, and may extend to depths of 1,000 feet or more in mid latitudes during stormy weather. In the polar regions, water can become thoroughly mixed in winter, so that it is very nearly the same temperature from the surface to the bottom. Under normal conditions, the daily temperature of the surface layer varies as little as 5 degrees.

The bottom of the surface layer or mixed zone is marked by the next zone, the thermocline, where the temperature begins to decrease rapidly with depth.

The third zone is the deep layer, where temperature decreases very slowly with depth.

Ocean layers

Image credit: ES310, Introduction to Naval Weapons Engineering.

Here we are most concerned with the characteristics (i.e. acoustic properties) of the thermocline.

There may be a number of seasonal thermoclines present, which vary in depth and numberin accordance with the season (being most numerous and extending to the the deepest depths during the summertime), and also, standing or permanent thermoclines which appear year around and usually occupy deeper waters than do the seasonal kind. (Thermoclines may be identified on a bathythermograph trace, abbreviated BT or XBT, which is a graph that measures temperature as a function of depth, with temperature normally on the horizontal axis, and depth on the vertical axis).

Distinct density boundaries appear at the various thermoclines that change the water's acoustic properties. Within the surface layer, sound travels generally in straight lines (particularly at angles of less than 15 and more than 45 degrees relative to the horizontal from the source).

The "layer"

Image credit: ES310, Introduction to Naval Weapons Engineering.

However, sound tends to bend (i.e. refraction) as it passes through the thermoclines and tends to produce "shadow zones" above and below the angle of the sound. The boundaries of these shadow zones are referred to as limiting rays. As a result, much of the sound generated by surface sources is trapped in the mixed layer and can travel over substantial distances (sometimes called the "surface duct").

Surface duct

Image credit: ES310, Introduction to Naval Weapons Engineering.

There are exceptions to this effect, a good example being in the Red Sea, where hot water seeping from thermal vents pool and accumulate at the bottom, and another example, in the polar seas, where extremely cold surface water is present. These produce reverse thermoclines with temperatures that increase with depth (rather than the usual behavior of decreasing with depth).

Unsurprisingly, submariners pay close attention to keeping an eye on where the thermoclines are located, and may pass across these boundaries periodically to listen for targets above and below the layer.

Submarines enjoy an additional beneficial effect, in that sound generated below the surface duct may often only be detected in the mixed layer within a 45 degree cone from the source. Surface vessels, and sonobouys dropped by aircraft, get around this obstacle by using variable depth sonars (VDS) and hydrophones. The detection gear is suspended below the layer to detect sounds under it.

4. Convergence Zones (CZ)

Convergence zones (CZ) are another effect caused by the refraction of underwater sound and are chiefly a result of changes in sound speed velocity as depth increases.

Sound waves traveling down into the depths (past the critical depth, where velocity matches that at the source) tends to bend or refract back toward the surface due to the increased pressure (and hence, increased sound velocity). Under the right conditions, the refraction creates parabolically arcing paths through the water, and when it reaches the surface, in a donut shaped area called an annulus, the sound is reflected back down again. Each focus at the surface is called a convergence zone (CZ). The process repeats itself until the sound waves have been attenuated or obstructed.

Convergence zone

mage: ES310, Introduction to Naval Weapons Engineering.

CZ require a minimum depth of about 400 meters in order to form, at which depth they have about a 50 percent chance of developing. The likelihood of CZ forming tends to increase with water depth, so a depth of 600 meters yields an 80 percent probability of CZ, with near certainty in even deeper water. In shallow water, the sound waves echo off the bottom rather than refract upwards. Notably, underwater topography, such as sea mounts and mid oceanic ridges, tend to disrupt CZ paths, as do ocean currents that have significant differences in temperature (e.g. the Gulf Stream).

CZ typically develop at about 30-33 nautical mile (nm) intervals, with the width of the annulus increasing with distance. The first CZ at 30-33 nm typically has a surface annulus some 3-5 nm wide; the second, at 60-66 nm, is about 6 nm wide; and the third, at 90-99 nm, has a width of about 9 nm. Fourth and fifth CZ are possible but rare. Notably, CZ ranges in the Mediterranean are reduced due to layers of warm water near the bottom.

Annulus

Image credit: ES310, Introduction to Naval Weapons Engineering.

 

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CZ do not form perfect concentric cylinders, of course, but rather a series of curved 3-D shapes, so it is quite possible that a submarine within CZ range but below the CZ path could remain undetected. The same is true for the areas between the annuli, where shadow zones will develop.

Both active sonar and passive sonar may make use of CZ. This is especially true of low frequency sound, the kind generated by a high powered active sonar such as that found aboard the American Spruance (DD-963) class and the Soviet (Russian) Udaloy (Project 1155) class ASW destroyers.

In the same way that radar warning receivers (RWRs) and electronic surveillance measures (ESM) gear can detect actively emitting radars well beyond their own effective range, actively emitting sonar can also be detected well beyond the range at which it can effectively receive an echo. As a rough rule of thumb, active sonar can be heard at three times the distance at which it can detect targets.

5. Direct Path and Bottom Bounce

Sounds in the ocean are rarely confined to following a single path, and each of those potential paths must be considered when using acoustic devices to try and detect or locate ocean objects. In addition to the CZ phenomenon, sound may travel by direct path or bottom bounce.

Direct Path

The direct path is, just like the term suggests, the simplest acoustic propagation path. It occurs in the surface layer, and acts much like radar, being essentially a straight line path between the sonar source and the target, with no reflection and only a single change in direction due to refraction. It is the principle upon which fathometers are based.

The maximum achievable range obtained by exploiting the direct path is tied directly to the point at which the surface duct limiting ray is reflected from the surface. For most hull mounted medium frequency sonars, maximum direct path range is typically less than 10 nautical miles (nm), and most often, much shorter than that. (Range is of course largely dependent upon the power and frequency of the sonar in question).

Bottom Bounce

Bottom bounce is an acoustic propagation path that involves reflecting sound off the bottom, so that it "bounces" up to the surface, and then is reflected downward again. It is chiefly an effect of medium (MF) and high frequency (HF) sonars, since low frequency sound tends to be absorbed into the bottom.

Bottom bounce normally occurs in areas where the bottom is fairly smooth and the water depth is slightly greater than critical depth (see our earlier discussion on that term), typically in depths greater than 2,000 meters but less than 5,000 meters. It is useful in shallow seas, such as the Mediterranean, or along the deeper portions of the Continental Shelf.

The angles at which the sound enter the water during bottom bounce are generally too steep to produce the sort of refraction that results in CZ (that is, provided the depth is less than 5,000 m), and are typically between 15 and 45 degrees from the horizontal. Maximum effective range for the bottom bounce technique is less than 20 nautical miles.

The chief advantage of the bottom bounce technique is that it can fill the gap between relatively short direct path range and long distance CZ detections, particularly where conditions limit CZ propagation. It isn't as popular in the modern era, largely due to higher than expected absorption rates across much of the ocean floor.

Bottom bounce

Image credit: ES310, Introduction to Naval Weapons Engineering.

C. TACTICAL OCEANOGRAPHY IN ANTI-SUBMARINE WARFARE

What does all of this mean in anti-submarine warfare (ASW)?

Simply put, it means that ocean conditions have a huge influence on how ASW sensors and weapons behave. Because of this influence, it is critically important to be aware of the conditions in which your submarine (or surface ship) is operating.

1. Ocean Depth and Topography

As discussed, water depth, bottom contour, and bottom composition will affect sound propagation, and in turn, sonar performance.

If the sound source is deep and the conditions are right, sound propagation may occur in what is referred to as the "deep sound channel" or DSC (previously called the SOFAR channel, or Sound Fixing And Ranging Channel). Sound is "trapped" in the DSC, with no losses at its boundaries, in turn providing extremely low loss to the receiver. (This is similar in principle to the transmission of light in a fiber optic cable). Low frequency sounds can travel for literally thousands of miles in the DSC.

The famous US Navy SOSUS (Sound Surveillance System) - the chain of bottom laid hydrophones devices strung across the north Atlantic - exploited the deep sound channel phenomenon to listen for Soviet submarines.

The DSC tends to occur deeper near the Equator, and shallower at the Poles. You can even achieve similar propagation in the surface duct under ideal conditions, but there are always some reflection loss at the surface.

The deep sound channel, here at around 750 meters

Image credit: World Ocean Atlas, US National Oceanographic Data Center.

Sound propagation is also affected by absorption in the water itself as well as at the surface and bottom. Sonars tend to perform best where the ocean bottom is hard and flat, and worst where it is muddy (or rocky, in which case, you get more scattering than absorption). This absorption is frequency dependent, as we discussed earlier, with highest frequency sounds being absorbed first, and lowest frequency sounds last. This is why sonars required to operate over long ranges tend to utilise low frequencies to minimise absorption effects.

In shallow water, sound propagation can suffer considerable loss. Certain sonars, for example, such as low frequency types, tend to perform poorly in shallow waters due to reverberation (scattering) and the harmonic noise produced by wave action.

Since underwater topography also influences sound propagation, submariners try to remain aware of features such as ocean ridges, sea mounts, guyotes (which are flat topped features that look like underwater mesas), and islands. The composition of the ocean bottom can likewise influence the performance of bottom bounce sonar or affect the placement of mines. Another obvious, and ominous, reason to remain aware of undersea topography is the hazard they can pose to subsurface navigation. Witness what happened to the USS San Francisco (SSN-711) in April 2005:

Photo credit: US Navy.

2. Ocean Temperature and Salinity

Another consideration is the freshwater outflow from rivers and the outflow from marginal seas, which can significantly affect sound propagation characteristics.

This is because, as we learned earlier in this discussion, both temperature and salinity will affect the speed of sound underwater. In turn, the behavior or performance of active and passive sonar is affected.

The temperature and salinity levels of the ocean vary widely between geographical regions. In the Arctic, low water temperatures can make for nearly uniform conditions from the surface to the ocean floor, which ordinarily might make for consistent sonar performance. However, the presence of ice also means obstacles to sound propagation (increased potential for scattering and reverberation) and high amounts of ambient noise (due to the constant grinding of the ice pack).

Varying salinity levels can also affect sonar performance. For example, the Mediterranean salinity "tongue", the name given to the warm outflow of water passing through the Strait of Gibraltar into the Atlantic, forms a mass of water that stretches nearly all the way to Bermuda and possesses acoustical properties that are distinct from other ocean regions.

Image credit: Dynamics of the Mediterranean Salinity Tongue, James C. Stephens and David P. Marshall, in Journal of Physical Oceanography, Vol 29, Issue 27, July 1999.

3. Ambient Noise

Even without ships and submarines present, the ocean is far from a quiet place. The ocean's own background noise, or ambient noise, varies widely with source, location, and frequency. Turbulence and geologic or seismic activity (such as underwater volcanoes) are the primary contributors to low frequency ambient noise. Ports and harbors will have increased localized noise levels due to industrial activity and merchant traffic. Distant ship traffic is perhaps the dominant noise source in the range of medium frequencies. Surface noise, such as wind and wave action, or rainfall on the surface, is the primary source of high frequency ambient noise.

Biological activity in the ocean also contributes to ambient noise and can influence your sonar. (Look up the fascinating story of the snapping shrimp). Tropical waters year round, and temperate and polar waters in the summer time, are especially prone. Marine mammals, especially toothed whales and dolphins, naturally use a method of echo location (or biosonar) to communicate and find food. These sounds add to the cumulative background noise of the ocean. (There is considerable controversy, as well, surrounding the use of high powered military sonars in the world's oceans and the negative effect it is believed to have on these same marine mammals).

The most obvious contribution to ambient noise is what is happening on the ocean's surface due to weather. Higher winds drive bigger waves, and consequently cause a greater amount of noise. Consistent wind speed is measured by the Beaufort Wind Scale, ranging from 0 (calm, wind speed less than 1 knot) up to 12 (hurricane force, wind speed over 64 kt). There is a direct relationship, then, between the steady wind speed and the sea state (the height, period and character of the waves). The World Meteorological Organization (WMO) sea state codes set out a scale for assessing the condition of the ocean surface, ranging from 0 (calm, glassy seas) up to 9 (phenomenal waves, over 14 meters high).

The greater the size of those crashing waves, of course, the greater the ambient noise contribution. Weather tends to degrade sonar performance, as increasing sea states increase ambient noise in the surface duct. This can deafen hull mounted surface ship sonars, and likewise, mask the presence of surface ships from submarine sonars. (The frequency of the noise from sea state tends to be greater than 300 Hz).

4. Self-Noise

Self-noise is the noise that your own surface ship or submarine produces, and which may be detected by your own sonar (or, worse, by the enemy's sonar system) and which contributes the overall degradation of sonar performance. Self-generated noise has therefore always been a problem for sonar platforms. Essentially, the faster a ship or submarine travels through the water, the more noise it creates and the less it hears. In both instances, it risks destruction by slower moving, stealthy opponents. The combination of a ship or submarine's self-noise, both narrowband and broadband, form its "acoustic signature" and can be exploited by an enemy's passive sonar to classify the vessel.

The noise from a ship or submarine's machinery, propellers, and even the sound of the hull passing through the water (flow noise), all add to self-noise.

Ships and submarines have plenty of potentially noisy machinery, including their engines, reduction gears, generators, hydraulics, pumps and turbines. In addition to their own noise, machinery produces vibrations in the hull that are in turn passed into the surrounding water. Machinery noise is generally independent of speed, as it is masked at high speed by flow noise, and forms the major component of self-noise at low speed. Even electronics and electrical devices can produce noise (thermal noise) if inadequately shielded.

Flow noise is generated by the turbulence of water flowing over the hull, and is dependent upon speed, hull shape and, somewhat ironically, the placement of the sonar transducer.

Propeller noise (typically very low frequency) is dependent on rotation speed and the geometry of the propeller itself. In fact, cavitation noise is the loudest noise arising from normal ship operations. Cavitation occurs when partial vacuums or bubbles are formed by the motion of rapidly turning propellers (at high speeds), and when these bubbles collapse, they produce broadband noise that can be heard at a considerable distance (and also produce shock waves that can actually damage the propeller's blades ... remember the power of those snapping shrimp?).

Acoustic stealth can also be degraded by short, transient noises caused by such things as rudder movement, start up and stopping of machinery, a dropped wrench, or famously, the opening of the torpedo tube doors. Transient noises are very characteristic of submarines and can help classify them as such.

Unsurprisingly, navies have sought to reduce self-noise wherever possible in an effort to further enhance their own stealth. Vibration dampening, isolation, and shielding of noisy machinery and components, specially shaped or skewed propeller blade geometry, and pump jet propulsors, are examples of this effort. And, of course, crews exercise a serious noise discipline, particularly aboard submarines.

Considerable amounts of money and effort have been expended to find further ways to reduce the self-noise of submarines (and the surface ships that might hunt them), even since the days of WWII. One of the earliest efforts, one that continues today, was the addition of anechoic tiles or coatings to the hulls of submarines. The Kriegsmarine (German Navy) introduced the technology first in 1944, with a synthetic rubber coating for its U-boats called Alberich. Although the principle was (and is) sound, problems with the adhesive sometimes resulted in the coating coming off, flapping in the U-boat's own wake and causing even more noise.

Modern anechoic tiles or coatings serve two purposes: to absorb active sonar, reducing the strength of the return and thereby reducing its effective range (which also reduces the acquisition range of active homing torpedoes); and to reduce self-noise, thereby reducing the effective range of an enemy's passive sonar.

Clearly visible anechoic tiles on the bow of USS Key West (SSN 722)

Photo credit: Rob Mackie, steelnavy.com, 7 October 2000.

A noise reduction system called "Prairie Masker" was developed by the US Navy for several classes of its warships (including the Spruance, Perry, Arleigh Burke, and Ticonderoga) to reduce or mask their self-noise. The Prairie system is fitted to the ship's propellers, while Masker is fitted to the external hull in the vicinity of the propulsion plant. Compressed air is forced through machined perforations to create air bubbles that form a barrier around the propellers and hull, thereby shielding their radiated noise, and interfering with the ability of enemy passive sonar (particularly submarine sonar) to conduct analysis of the sound to classify or identify its source. It is reported that the acoustic signature of Prairie Masker sounds like rain to a passive sonar.

Prairie Masker

Image credit: Mariana Ruiz.

D. CONCLUSION

Hopefully this discussion has provided a decent foundation for an understanding of the anti-submarine warfare (ASW) environment, the "battlefield" in which submarines operate and their pursuers face off.

In Part 2 we'll move to the tools of ASW - the sensors and the weapons.

Source references:
Jane's Navy International, November/December 1995.
Naval Institute Guide to World Naval Weapons Systems, 1997-98.
Jane's Information Group: August 1999; June 2003; September 2005.
Journal of Electronic Defense, March 2001.
Proceedings, June 2002.
National Defense, January 2003.
World of Defence, UDT, Issue No.2, 2004.
Navy Times, August 2005.
Harpoon 3 Sonar Model, AGSI, 2007.
Proceedings, June 2007.
ES310, Introduction to Naval Weapons Engineering.
Ocean Talk, Naval Meteorology and Oceanography Command.
uboataces.com

 

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Part II

TACTICS 101: ANTI-SUBMARINE WARFARE (ASW): PART 2 - THE TOOLS OF ASW

In Part 1, we learned the basics of naval oceanography, and in large part, how sound behaves in the subsurface ocean environment.

Now, in Part 2, we move to the tools of anti-submarine warfare (ASW), the sensors, platforms, and weapons. Much of the material contained here is probably "old hat" to seasoned Harpoon players, and certainly generally available to anyone who is willing to take the time and effort to dig up the information.

The purpose of this "Tactics 101" discussion, however, is to provide a workable foundation for those who may be completely new to naval warfare and its concepts. Part 2 tries to touch on many of those concepts.

A. ANTI-SUBMARINE WARFARE (ASW): A VERY SHORT HISTORY

War never changes (there's my Fallout 3 quote of the day ), but in the case of anti-submarine warfare (ASW), it has perhaps changed just a little.

The emergence of unrestricted submarine warfare in World War I and the early days of World War II led to grievous (and unanticipated) losses among all major naval powers and their merchant navies, and in turn, threatened both their economic lifelines (the sea lines of communication, or SLOC) and their only means of deploying troops to distant foreign shores.

The danger now posed by submarines to what had been, up to that point, somewhat of a grand surface war, was not particularly welcomed by those on the receiving end. In the words of the First Sea Lord, Admiral Lord Charles Beresford, circa 1900, submarines are "under-handed, under-water, and damned un-English"!

World War II was a watershed event for several major developments in undersea warfare. The Battle of the Atlantic saw the Kriegsmarine's U-boats pitted against emergent (and increasingly potent) ASW technologies as American and Allied forces herded their convoys to Europe. In the Pacific, US Navy submarines waged their own offensive war against Japanese SLOCs. The development of active sonar or ASDIC, believed to be an acronym for "Allied Submarine Detection Investigation Committee" (but in any event, with Canadian roots. Ahem! ), is a prominent example of emerging ASW technology during WWII.

Submarine performance during WWII was optimised for surface operations, and accordingly, the submarines of the era were more properly termed "submersibles". The first true submarine did not emerge until the end of the war (too late to be of any consequence in affecting the outcome) in the form of the German Type XXI. The new boat sought to address shortcomings in previous designs that were being vigorously exploited by Allied ASW efforts after 1943, most particularly the low submerged speed and endurance of the U-boat. (The Type XXI design had fallen into Soviet hands at the conclusion of WWII, leading in due course to the Project 611 (NATO codename Zulu) class submarine). The number of operational German U-boats peaked at some 240 hulls in March 1943, but by this time the Kriegsmarine force faced - in the British Royal Navy alone - some 875 ASDIC equipped surface escorts, 41 escort aircraft carriers, and 300 Coastal Command patrol aircraft. The tide had turned.

The final moments of German Type IXC-40 U-boat U-185

Image credit: S. Burbridge, "Final Moments", subart.net

In the post war era, and throughout the Cold War, as the hard lessons (and promising technologies) of WWII were developed and improved upon, the punch and counter-punch of ASW continued to develop at a fervent pace.

This included such rapid post war developments as the teardrop hull form (derived from the USS Albacore (AGSS-569) design, circa 1948), the emergence of nuclear powered propulsion (from Admiral Hyman G. Rickover's USS Nautilus (SSN-571), circa 1951), and the arrival of the ballistic missile submarine in the mid 1950s.

The evident teardrop hull form of the USS Albacore

Image credit: US Navy.

Submarine warfare in the modern era has been much less exciting, or perhaps more accurately, much less outside the gaze of the public eye. The examples of successful engagements, both by submarines and against them, are fairly well publicised.

For example, the sinking of the Argentine cruiser General Belgrano by the British Royal Navy nuclear attack submarine HMS Conqueror during the 1982 Falklands war and, on the other side of the equation, the destruction of the Pakistani Navy submarine PNS Ghazi during the 1971 conflict with India.

Much less well known are the countless times during the Cold War when submarines have attempted to affect, directly or indirectly, the course of geo-political events by their very presence, without a shot ever having been fired.

For example, the deployment of HMS Dreadnought to the Falklands in November 1977 under the auspices of Operation Journeyman; or the report that a Dutch Walrus class sub was stationed off Kotor during the 1999 Kosovo conflict and tasked to engage any Yugoslavian submarines that might emerge to threaten NATO ships.

In modern times, submarines have been more notable for tasks that defy traditional undersea warfare, such as launching cruise missiles against distant shore targets, or delivering special operations forces ashore to conduct clandestine small unit operations.

Suffice to say, undersea warfare - both from the point of view of the submariner, and from that of those attempting to hunt him down - has been a complex, fluid, and militarily important affair. It remains so today. And, notably, the see-saw battle of how to find, hunt and destroy submarines - and on the other side, how submarines evade, hunt and attack their enemies - continues to push technological boundaries.

B. SONAR: THE MAINSTAY OF ANTI-SUBMARINE WARFARE (ASW)

Stealth is arguably the defining characteristic of the submarine. The foremost, and by far the most difficult task in the ASW cycle, therefore, is actually finding them.

The acronym ASW has sometimes been translated as "awfully slow warfare", and this is probably a good description. A couple of anecdotal references serve to highlight this portrayal of ASW: Being in a submarine is like "being stuck in the boiler room of your high school for several weeks"; or, that the weather on a "sealed people tube" is always the same, “69 degrees and fluorescent”.

One can rest assured that it is an equally mind numbing exercise for the crew of the ships and aircraft that are scouring the ocean for a hint of an enemy submarine, the proverbial search for a "needle in a haystack".

When ASW does get exciting, however, and perhaps more than a little nerve wracking, is when you finally do get that "contact". So, the question naturally follows: How does one get (and ultimately prosecute) a contact?

1. Active Sonar

As we learned in Part 1 of this discussion, active sonar operates much like active radar, sending out a burst or pulse of sound energy (the "ping") through the water, which then reflects off a target and returns as a reflection (or echo) to the sonar transducer.

A single transducer has little directional control over the ping, but by arranging or stacking multiple transducers in an array, and using special signal processing techniques, it is possible to use "beam forming" to send active sonar pings in specific directions.

The Sonar 2051 transducer array aboard Oberon class submarines

Image credit: web.ukonline.co.uk.

Generally speaking, the power of the transducer will dictate its maximum range. It also directly impacts signal frequency, since the longest ranges will be achieved with the lowest frequencies, and therefore only the largest transducer array will be able to produce the necessary long wavelengths.

All of this means that, in order to increase the transmitted power for a given array configuration, it is usually necessary to increase the size of the array. The necessity of a large sonar array obviously leads to physical limitations, especially when space aboard a warship or submarine is at a premium. Large arrays can add significant drag and require large power sources together with their supporting electrical and electronic equipment.

Hull mounted sonar arrays are typically cylindrical in shape, to give good coverage outward and downward, while those fitted to modern submarines are often spherical in shape, providing a much wider vertical field of view (useful in the subsurface environment, where you need coverage both above and below).

Active sonar arrays are generally mounted in the bow or keel, giving good coverage ahead of the ship, except in the area directly behind the array (the "baffles"; more on those later). Sound absorption materials are mounted directly behind the array, both to protect the occupants of the ship or submarine from the very loud sound energy (recall, up to 250 decibels!) and to prevent reflections back into the system. Streamlining and shielding around the array is done both to reduce drag and to reduce self-noise (including flow noise created by the passage of water around the array).

Modern surface ships and submarines are also likely to have a "suite" of active and passive sonars (a collection of bow, hull, flank, and/or towed systems) rather than relying on a single system.

Sonar suite on the Type 212 submarine

Image credit: Gerwalk, subpirates.com

As we discussed in Part 1, surface ships or submarines performing ASW will only rarely employ their active sonar, as it may be heard at over twice the distance (and given the proper acoustic conditions, much further) it can effectively return an echo. Banging away with active sonar in the blue water ocean environment typically serves only as a beacon for lurking enemies or, worse, invites a spread of torpedoes or cruise missiles in your general direction.

There are circumstances, however, where active sonar is useful or even preferable. (Always keeping in mind the drawbacks already mentioned, of course). For example, in shallow water, where your towed array is unavailable (due to the risk of it hanging up on the bottom) or where the topography prevents good acoustic conditions (e.g. no CZ formation).

Active sonar may also be your only useful choice when dealing with diesel-electric submarines, since these are extremely quiet when running on their batteries. Here's an (alarming) analogy to get an idea of just how difficult (with thanks to Scott Gainer): Try to locate a refrigerator by listening for it from outside the house. :o

2. Passive Sonar

The preferable approach for detecting enemy submarines has historically been the use of passive sonar. It is the instrument of choice for ASW, and as stated, active sonar is most often relegated to the attack phase or special circumstances.

As already discussed, passive sonar involves an array of dedicated hydrophones, or the receiver portion of an active sonar array, being used to "listen" for the acoustic signals generated by a target. As with active transducers, hydrophones can be arranged in an array to improve beamwidth and directivity.

A dedicated passive hydrophone array is much lighter and considerably less complex than an active transducer array, generally because it does not have the same high power requirements. Conformal arrays, placed alongside the length of the ship or submarine (often the latter), take advantage of this reduced weight and complexity.

As mentioned in Part 1 of this discussion, a ship or submarine's self-noise, both narrowband and broadband in combination, forms its "acoustic signature". This signature can be exploited by an enemy's passive sonar to identify the target. For example, the broadband noise from a target's propellers (generally of low frequency, less than 1000 Hz) can be detected and demodulated to measure the shaft or propeller blade rate (blade rate tonals) - a useful identifier. (Narrowband noise is typically plotted and shown on the famous "waterfall display").

3. Variable Depth Sonar (VDS)

To improve the ability to hunt for submarines that might be hiding in shadow zones or below the thermocline (the "layer"), the Variable Depth Sonar (VDS) was developed in the 1950s. A VDS employs a streamlined body (the "fish") which contains the transducer and is towed behind the ship. In conjunction with the speed of the ship and the length of the tow cable, and by employing control vanes and depth sensors, the fish can be deployed at depth.

The principal advantages of VDS, of course, are the ability to move the transducer away from the ship's self-noise, penetrate the layer, and provide 360 degree coverage by placing the transducer behind and below the baffles.

VDS does have its limitations, however. Early models (such as the SQS-9) were streamed over the side, a rather cumbersome method, though in later practise the fish was streamed from the fantail via a hoist or winch system. The system does restrict ship maneuverability while in operation, and its bulky equipment is difficult to handle during inclement weather.

Thales TSM 2640 Salmon VDS aboard Royal Danish Navy frigate Thetis (F357)

Image credit: Naval Technology, SPG Media Limited.

In recent years, the VDS has been supplanted by the linear towed array for ASW work, while VDS systems have become more specialised tools in the field of mine countermeasures (MCM). In this role they are often equipped with high frequency side scan sonars which are short ranged but provide excellent resolution, sufficiently high in most cases to perform imaging of the bottom and object classification.

4. Towed Arrays

During the Cold War, when the North Atlantic was the main hunting ground of both NATO and Warsaw Pact submarine forces, the low frequency passive towed array sonar emerged as the prime ASW sensor for surface ships. Exploiting advances in signal processing, and taking advantage of the excellent acoustic propagation characteristics in the deep sound channel (DSC), towed arrays were capable of detecting and holding contacts at ranges at dozens of miles.

Unlike a VDS, in which the sonar array is encased in a streamlined body or "fish" at the end of a relatively short tow cable, the towed array or "streamer" comprises a hose or sheath of an elastomeric material (such as rubber) between 2 and 4 inches in diameter and containing numerous transducers (typically of ceramic piezo-electric design) or receivers arranged in a linear fashion. The array can be thousands of feet (even miles) in length.

The towed array's sheath is filled with an acoustically transmitting material (typically a fluid) that provides structural integrity, dissipates internal heat, and provides some isolation from flow noise. Since the array's diameter has a direct correlation with flow noise, it is desirable to reduce the diameter of the array to as small as possible. This is difficult from an engineering point of view, but even so, modern "thin line" towed arrays have been able to achieve a minimum diameter of about one inch.

In addition to the engineering and design obstacles, towed arrays have their own characteristic problems: these include boundary layer noise; internal self-noise caused by waves propagating between transducers inside the array; or external self-noise or "cable strumming" caused by towing the cable through the water. The cable may vibrate, that vibration is passed into the array, and is picked up by the transducers.

A fundamental problem with towed arrays is that the position of the hydrophones or "nodal points" within the array is inherently unstable. Because of ocean currents, their position in the array, the speed of the host platform, or other factors, their relative positions are changing continuously. Because of the negative effect this can have on acoustic properties, it is important to monitor the relative positions of the nodal points of the array at all times. One common method is to use multiple "birds" clipped onto the tow lines, each comprising a transducer used to calculate the range between nodal points. The results are used to determine the shape of the array, which in turn is critically important to its performance.

Towed array

Image credit: Federation of American Scientists.

Surface Ship Towed Arrays

Notably, towed array systems have offered the surface ship, for the first time, the possibility of parity with the submarine in passive sonar capability. The long range detections made possible by towed arrays are of limited value to surface ships, however, without some means of localizing and prosecuting an enemy submarine contact. And, of course, this is the function performed by aircraft acting in conjunction with the surface ship.

From an ASW point of view, the US Navy surface fleet evolved slowly from a sensor/weapon suite based on the SQS-26 hull mounted active/passive sonar set of the 1960s, the RUR-5 ASROC (Anti-Submarine Rocket) stand-off weapon, and the SH-2 Seasprite LAMPS I (Light Airborne Multi-Purpose System) helicopter.

By the mid 1980s, it had become a system based largely on variants of the SQS-53 hull mounted sonar first introduced in 1972, the SQR-19 TACTAS (Tactical Towed Array) passive towed array, and the longer range SH-60B Seahawk LAMPS III helicopter. (There never was a LAMPS II system).

Along the way, during the period of this evolution, the US Navy conducted:

(1) Its first experiments and deployment with the ITASS (Interim Towed Array Surveillance System) towed from VDS fish on the Dealey class destroyer escort USS Van Voorhis (DE-1028) in 1970;

(2) Its first experiments and deployment with an interim tactical towed array design (the SQR-18) on the Knox class frigate USS Moinester (FF 1097). The SQR-18 was a passive towed array some 800 feet long streamed from the SQS-35 IVDS (Independent Variable Depth Sonar) body on a tow cable nearly 5,600 feet long. (The SQS-35 was evolved from the EDO Model 983, a 13 kHz VDS).

(3) Its first experiments and deployment of a higher speed tactical array (the SQR-19) on the Spruance class destroyer USS Moosbrugger (DD 980) in the early 1980s.

The most recent, new generation towed arrays, such as the British Sonar 2087 and the US SQR-20 Multi-Function Towed Array (MFTA), employ both active and passive arrays, exploit the range advantages of the low frequency range, and are more tolerant of high speeds.

Submarine Towed Arrays

Submarines can deploy "fat line" towed arrays using a process known as flushing, wherein water is pumped into the tube to exert pressure upon and hence deploy the array. The US Navy's TB-16 is an example of a fat line towed array, which consists of an acoustic detector array weighing some 1,400 lb, measures about 3.5 inches in diameter, and 240 feet long. The TB-16 array is towed at the end of a cable some 2,400 feet in length.

Alternatively, submarines may deploy "thin line" towed arrays using mechanical handling systems. A thin line array comprises an outer sheath or hose that contains the hydrophones and supporting wiring and electronics. When the array is deployed or retrieved, it is fed through a guide by a handling system. The US Navy's TB-23 is an example of a thin line towed array.

Winch system for the Type 212 submarine's TAS-3 towed array

Image credit: Gerwalk, subpirates.com

Advances gained from commercial off the shelf (COTS) computer processing (such as that made available through the Advanced Rapid COTS Insertion, or ARCI, program) has substantially reduced cost while significantly improving processing power, which in turn permits the use of powerful new algorithms for better towed array detection ranges. The US Navy's TB-29 thin line towed array, for example, is a version of the legacy TB-29 array utilising commercial off the shelf (COTS) telemetry.

Towed array technology has advanced rapidly with longer, multiple line systems that provide increasing number flexibility for submarine based ASW. Many existing Navy tow cable systems have single coaxial conductors, 1-2 kilometers in length, within which power, uplink data, and downlink data are multiplexed. These systems typically run at uplink data rates of less than 12 Mbit/sec due to the bandwidth limitations of a long coaxial cable.

Operational beam formers for towed arrays have traditionally assumed the array geometry to be straight and horizontal aft of the host platform, but in reality there is always some deformation in this geometry. "Chain link" and "stiff stick" models have been used to estimate towed array position and heading, but the newer arrays (such as the TB-29) are equipped with their own sensors to accurately determine the position of the array and its heading. This is more accurate than either of the previous methods, and will be used to optimise TMA solutions.

5. Dipping Sonars

Dipping or dunking sonars are a variant of the VDS concept. While principally fitted aboard helicopters, they can also be found aboard some small patrol craft and surface ships (such as the MGK-345 Bronza (NATO Rat Tail) dipping sonar found aboard Project 1241.2 (NATO Pauk) class corvettes).

While operating in the hover, a helicopter can use a winch and cable to lower a sonar transducer into the water and to the desired depth. Again, self-noise is minimal because the array is isolated from both the noise and vibration of the power source and the helicopter.

AQS-13 dipping sonar deployed from a Sea King helicopter

Image credit: www.solarnavigator.net

A helicopter dipping sonar typically consists of a "wet end" or transducer; a cable (at least 1,000 feet in length) and reel/winch assembly; and a "dry end", consisting of the power supply and sonar processing systems.

There are, of course, no baffles or blind spots with a dipping sonar. It has full 360 degree capability, and for the size of the transducer, most are quite capable, with source levels exceeding 200 decibels (dB) and an effective range of several thousand yards. Newer systems also have a sonobuoy interface, allowing them to process the returns from three or more sonobuoys, and the ability to communicate with friendly submarines via underwater telephone.

Power was supplied to the transducer via a lead-acid battery in most older systems, and recharged between transmission cycles, but newer dipping sonars are powered directly by the helicopter rather than by a battery.

Helicopter dipping sonars have also joined the drive to enhance capabilities in the littorals, both in terms of locating ultra quiet diesel subs and in the mine countermeasures role. Examples of the latter include the AQS-14 and AQS-20 systems.

The new AQS-22 FLASH (Folding Low Frequency Active Sonar for Helicopters) system is being fitted to the new MH-60R Seahawk (as well as other types), and is claimed to provide submarine detection, tracking, localization and classification; acoustic interception; underwater communications; and environmental data acquisition, both in blue water and littoral zones. The AQS-22 transducer/receiver weighs only 100 lb and has a cable length of over 2,500 feet.

6. Jezebel, Julie, and Company: Sonobuoys

Sonobuoys are essentially expendable sonar devices typically deployed by aircraft, though they may also be hand deployed over the side of a ship. In fact, the first sonobuoys of early WWII were an expendable sensor towed behind convoy escorts and used to detect German U-boats attempting to approach from the rear. Their usefulness as an aircraft delivered sensor, however, was not fully appreciated until late in the war, by which time the US Navy finally ordered some 150,000 examples (mostly the CRT-1 type).

Sonobuoy development languished somewhat immediately after WWII, that is, until the quick pace of Soviet submarine development became an increasing concern and the SOSUS project began in earnest (more on SOSUS later).

In 1951, Project Jezebel combined LOFAR (Low Frequency Analysis and Recording) equipment with fixed hydrophone arrays (such as SOSUS), and aircraft became the means of rapidly prosecuting the SOSUS detections. Whereas SOSUS provided only the initial detection and a general area of probability, sonobuoys could be used for tactical level search and localization. Passive sonobuoys thus became known as Jezebel buoys.

Whether sonobuoys are parachute delivered from the air or dropped over the side of a ship, the launching platform must be equipped with the electronic equipment needed to receive and process the data being gathered and returned by the sonobuoy (typically via VHF transmitter).

The sonobuoy's own battery (often silver chloride) power is activated by contact with seawater (though some types are now switching to lithium), and a mechanism for inflating a flotation device is activated, such that the transducer or receiver is suspended below the surface to a certain specified depth while the buoy (and antenna) remain floating.

Sonobuoys are classified by their size (A, B, C, etc.) and their type (active or passive, or measurement). Most US manufactured sonobuoys are A size, measuring about 4 7/8 inches in diameter and 36 inches in length. (An A/2 size buoy is a half size A type buoy).

A sonobuoy being dropped from a P-3 Orion

mage credit: National Oceanic and Atmospheric Administration (NOAA).

 

[Manticore]

Passive Sonobuoys

Passive sonobuoys have traditionally been the type of choice for open ocean ASW efforts by aircraft. By relying on passive sonar detection, a passive sonobuoy denies an enemy submarine the knowledge that it is being tracked or that an aircraft is searching for it.

The earliest passive LOFAR sonobuoys (such as the SSQ-23) provided only target detection with no indication of range or bearing. The CODAR (Correlation Detection And Ranging) method was developed to provide localization, wherein two pairs of sonobuoys were used to compare signal arrival times and resolve directional ambiguity. This was time consuming, however, and not particularly effective when the submarine went quiet. Another method was called Julie, more on that later.

The development of a LOFAR sonobuoy with directional capability, called a DIFAR (Directional Frequency Analysis and Ranging) buoy, combined omnidirectional hydrophones with compass information to provide target bearing.

The US Navy's SSQ-53 DIFAR series of passive sonobuoys have directional detection capabilities, and offer selectable depths (between 90, 200, 400, and 1,000 feet, depending on type) and selectable endurance (between 0.5, 1, 2, 4 or 8 hours). Another example, the SSQ-77 VLAD (Vertical Line Array) combines the directional capability of the DIFAR with a vertical line array of omnidirectional hydrophones for improved tracking in a noisy, high traffic environment.

Active Sonobuoys

The acoustic pulse (ping) from an active sonobuoy more readily provides directional information (range and bearing) on a potential submarine target. Moreover, when two or more fixes are obtained, it is possible to establish the speed and course of the submarine. Active sonobuoys are typically deployed at deeper depths than passive buoys, and because of their power requirements, have typically shorter endurance.

The early SSQ-47 Ranger active sonobuoy produced a free running acoustic pulse every 10 seconds at one of six frequencies. It had a selectable depth, but a short life span, and provided only range information. It was replaced in due course by the SSQ-50 CASS (Command Activated Sonobuoy System), which pinged on command (in four fixed frequencies) from the launch platform rather than pinging continuously.

In the late 1970s, the US Navy introduced the SSQ-62 DICASS (Directional Command Activated Sonobuoy System) series active sonobuoys, with directional capability and frequency modulated (FM) sonar sweeps.

The proliferation of very quiet diesel-electric submarines among world navies (too quiet for effective use of passive sonobuoys) has led to increased interest in active sonobuoys and other shallow water detection techniques. This has included a return to the early concepts of broadband acoustics and echo ranging.

Explosive Echo Ranging

Acoustic propagation experiments conducted by Ewing and Worzel during WWII and the years thereafter used explosives as impulse sources to discover and exploit the Sound Fixing And Ranging (SOFAR) channel (now called the Deep Sound Channel). In 1960, for example, they dropped 100 kg depth charges off Perth, Australia, and discovered that the low frequency sounds could be heard by hydrophones near Bermuda some three and a half hours later.

Like CODAR, another operational mode saw the use of small explosive charges that could be detonated near passive sonobuoys to create a broadband acoustic pulse that would (hopefully) reflect off a nearby enemy submarine. This explosive echo ranging technique, called Julie, was introduced into the US Navy in 1956. The Soviets made extensive use of explosive echo ranging.

In the late 1960s, the Julie approach was replaced by active sonobuoys, but the use of explosives as an acoustic sensor continues today with the SSQ-110 Explosive Echo Ranging (EER) sonobuoy and the more recent SSQ-101 ADAR (Air Deployed Active Receiver) sonobuoy. The latter uses a multiple element planar hydrophone array to further improve detection capability in shallow littoral waters.

Special Purpose Sonobuoys

Specialized measurement sonobuoys can also be used to detect electric fields, magnetic anomalies, and bioluminescence (the light emitted by microscopic organisms disturbed by a passing submarine), as well as measuring important environmental parameters like water temperature, air temperature, barometric pressure, and wave height.

Although incapable of detecting submarines directly, measurement sonobuoys are critical to the ASW cycle. Prior to the deployment of detection sonobuoys, it is advisable to obtain a vertical temperature profile of the local ocean environment. This information can then direct the ASW operator as to what depths it might be useful to place detection sonobuoys.

The SSQ-36B bathythermograph (typically abbreviated BT), for example, is widely used by the US Navy for this purpose. It places a temperature probe at 800 meters depth and has an operating life of about 12 minutes. Submarines can also release bathythermograph probes (abbreviated XBT), which rise to the surface before then sinking and returning temperature data to the submarine. Obviously if the submarine is very deep, it may take awhile to start receiving data.

The SSQ-71 and SSQ-86 sonobuoys are used to establish two-way communications between an aircraft and a submarine.

7. SOSUS

The story of Project Caesar and SOSUS is perhaps one of the most significant and secretive stories in the history of the Cold War. SOSUS would become a key early warning asset against the perceived threat of Soviet submarines and in providing vital long range cueing information for open ocean ASW.

The culmination of several research efforts in the early 1950s on the topic of low frequency acoustic propagation, including Projects Hartwell, Jezebel and Michael (and including the findings of such researchers as Maurice Ewing, discussed earlier), was brought together to design, engineer, and eventually deploy a broad area surveillance system under the unclassified designation Project Caesar and the highly classified acronym SOSUS (Sound Surveillance System).

Simply described, SOSUS (designation FQQ-1/2/3/9) is comprised of passive hydrophone arrays hung vertically from the sides of underwater mountains. It takes advantage of the very long range propagation of low frequency noise in the deep sound channel (DSC) we described in Part 1, and "looks" into the deep ocean basins of the Atlantic. (It is not located at the chokepoints of the Greenland-Iceland-United Kingdom (GIUK) Gap, contrary to popular belief).

The low frequency noise emanating from a submarine, arriving at several different arrays, is triangulated and a SOSUS Probability Area (SPA) can be plotted as an ellipse ranging in size from 10 x 20 nautical miles (nm) up to 100 x 200 nm, depending on the range, angles, and numbers of arrays in contact with the target source.

By late 1957, several shore based monitoring stations, rather innocously termed "naval facilities" (or NAVFACs), were placed along the eastern seaboard of North America, forming a huge semi-circle from Barbados to Nova Scotia and opening toward the deep water west of the mid Atlantic Ridge. SOSUS was also extended into the Eastern Pacific, with NAVFACs spanning from California to Washington state, and with arrays later terminating at Guam, Midway, Adak (in the Aleutians), and at Hawaii. In 1959, another NAVFAC was established at Argentia, Newfoundland (less than a couple of hours from where I live), to monitor a number of shallow water arrays placed south of the Grand Banks. Others were strategically placed at locations like the North Cape (off northern Norway) and the Strait of Gibraltar.

SOSUS Naval Facilities (NAVFACs) in the Atlantic

Image credit: Edward C. Whitman

Though initially aimed at detecting snorkeling diesel-electrics, SOSUS proved even better at finding deep running nuclear powered submarines. The Soviets remained largely oblivious to the sensitivity and success of the SOSUS network until the criminal espionage of one John Walker Jr.

The end of the Cold War has seen the mothballing of much of the SOSUS network, with several monitoring stations closed (including, for example, the NAVFACs at Keflavik in Iceland, Adak, and Bermuda). The hydrophone arrays are still there, of course, but they have been placed in a stand-by status in which the data is available but not is no longer being continuously monitored.

A kind of "son of SOSUS" has since emerged in the Advanced Deployable System (ADS), a rapidly deployable net of broadband passive sensors that could be laid to detect submarines (particularly ultra quiet diesels), surface ships, and mine laying activities in shallow littoral waters. ADS is still in the development stage, and if procured and deployed, could be fitted to the Littoral Combat Ship (LCS).

8. SURTASS

While SOSUS is the fixed site portion of the Integrated Undersea Surveillance System (IUSS), the Surveillance Towed Array Sensor System (SURTASS) was developed in the early 1980s as the mobile, tactical element.

Several types of tactical auxiliary general ocean surveillance (T-AGOS) ships, namely the Stalwart, Victorious, and Impeccable classes, were deployed for the SURTASS mission, beginning in the mid 1980s. These ships employed the UQQ-2 passive sonar array, which was streamed for miles behind the ships and used for very long range submarine detection.

With the mothballing of the SOSUS network, SURTASS has been called upon to provide the undersea surveillance necessary to support regional conflicts and sea lane protection. Even SURTASS, however, has been the victim of a shift away from open ocean, deep water ASW toward the littoral, shallow water environment.

By 2004, only five of the US Navy's 22 Cold War era T-AGOS ships continued to perform the ASW mission, comprising only one of the 18 original Stalwart class ships and all four Victorious class vessels, together with the ocean surveillance ship MV Cory Chouest.

Despite significant curtailment of the SURTASS fleet, however, the remaining assets have seen some significant upgrades in their capability, principally among them the Low Frequency Active (LFA) sonar array, and the TL-29A twin line array. Both are primarily aimed at detecting ultra quiet diesel-electric submarines.

The LFA is a powerful active sonar array system, designed to exploit low frequency acoustic propagation in the deep sound channel, and intended for use against targets too quiet to be detected by the passive system alone. Its extremely loud acoustic source is blamed for causing the deaths of numerous marine mammals during development and testing of the system, and has been the subject of considerable litigation. Presently installed only aboard the Cory Chouest, the LFA will eventually be fitted aboard the USNS Impeccable (T-AGOS 23).

The Low Frequency Active (LFA) system

Image credit: Pacific Whale Foundation (how's that for irony).

The prototype TL-29A twin line array (which incorporates a pair of arrays towed side by side) is aimed at use in the littoral zone. It was tested during 1996 in a variety of world locations, with positive results, and was certified for deployment in November 2005. It is reportedly far superior to any other shallow water passive towed array system previously employed. Six TL-29A systems will support SURTASS vessels operating in the Western Pacific.

C. NON-ACOUSTIC DETECTION

1. Visual Detection

Submarines operating at or near the surface are extremely vulnerable to visual detection. Anything that protrudes above the surface, such as a periscope, antenna, or mast will leave a significant wake if the submarine is moving at any speed over a few knots. And, since depth control and steerage is quite difficult at low speeds, it is not uncommon for submarines to be traveling at least 4 or 5 knots just below the surface.

The periscope (for example) will create a wake, called "feather", which is quite visible, and will also leave a remnant of its passage, called a "scar". The scar is a long streak of foam or bubbles left behind after the object passes. The feather may be just a few meters, but the scar may be tens of meters long. Either may be visible for up to 10 miles, and are easily spotted by low flying aircraft in the vicinity.

Periscopes and other protuding masts and antennas are also often painted in dark or camouflage colors to reduce their visibility.

If the water is especially clear, the submarine hull or its shadow may be visible for a few hundred feet under water, but is usually not distinguishable unless the water is shallow with a light colored bottom (like white sand).

The vulnerability of the submarine at shallow depth visibly demonstrated

2. Radar

As their radar cross section (RCS) is generally very small, exposed periscopes and masts cannot usually be detected by ordinary surface search radars. Furthermore, the interference from sea clutter (e.g. waves) near the target will generally obscure it.

To be effective against a periscope or mast, a radar needs to have high resolution (both in range and bearing). Inverse synthetic aperture radar (ISAR) systems, such as the Raytheon APS-137B(V)5 set fitted to the P-3C Orion AIP variant, have proven very effective against submarine periscopes and masts.

Traditional submarine periscopes, masts, and antennas have been of the hull penetrating type, i.e. a column or pole of a length that typically extends the vertical length of the sail and the diameter of the submarine. New technology, however, will use non-hull penetrating systems by exploiting COTS advances in photonics (especially fibre optics). In addition to eliminating an opening in the pressure hull (always a good idea), eliminating the periscope column will potentially permit submarine designers to build longer, flatter sails than was previously possible, thereby improving hydrodynamic performance.

Taking advantage of this technology, the US Navy's Virginia (SSN-774) class attack submarines have the Universal Modular Mast (UMM) - eight of them, in fact. Two will house a photonics sensor package, with high resolution color CCD TV, monochrome HDTV, thermal imager, eye safe laser rangefinder, ESM sensor, and a communications and GPS receiver. Two more will have high data rate satcom antennas.

Some synthetic aperture radars have also demonstrated the capability to detect the presence of a submarine by the change in the surface water height as it passes, known as the "Bernoulli hump". This effect is greatest when the submarine is at shallow depth and moving rapidly. These is not a real time asset, however, since the necessary signal processing requires significant computing power and may take several hours to complete. Also, few submarine commanders would be so silly as to cruise at speed just below the surface in any situation where stealth was a priority (and it most often is).

3. Infra-red (IR) Detection

Submarines are vulnerable to passive infra-red (IR) detection when they are snorkeling, since the diesel exhaust is released close to the surface (as it must be because of the back pressure limitation). The exhaust gases give off a sufficiently strong IR signature as to be detectable. However, this is only useful if the submarine is snorkeling, which is only a few hours a day for diesel-electric submarines. Periscopes and masts are also susceptible to IR detection, though much less so than diesel exhaust.

Factors which play to the favor of the submarine, and limit the usefulness of IR sensors, include bad weather and high humidity levels, which reduce the effective range of an IR sensor.

Modern naval helicopters and maritime patrol aircraft are often equipped with forward looking IR (FLIR) systems, not just because they are useful tools for spotting submarines, but make for excellent aids in detecting, classifying and identifying surface ships as well. More recently, the FLIR systems of maritime patrol aircraft, such as the P-3 Orion, have proven useful in the overland reconnaissance mission.

4. Magnetic Anomaly Detection (MAD)

Magnetic anomaly detection (MAD) systems use ionized noble gas (such as helium) to measure changes in the Earth's magnetic field due to the presence of a large amount of ferrous material found in most submarines. While ionized, the atoms in the gas are aligned, but when they pass by a localized magnetic disturbance (that pesky submarine), they change their alignment. This change in alignment is measurable, and can be recorded and displayed on a scrolling paper loop like an electrocardiogram (ECG) monitor or seismometer.

MAD gear is typically mounted aboard fixed wing aircraft and helicopters, and will indicate the presence of a ferrous metallic object when the aircraft overflies it.

MAD detection only works if the submarine is relatively shallow, and therefore is not a great long range detection system, being effective to within about 1,000 to 2,000 feet of the aircraft's path. It can however, provide a precise location of the submarine of sufficient accuracy to permit weapons delivery, which is its main use.

ASQ-81 MAD system in the tail boom or "stinger" of a P-3C Orion

Image credit: Maryu, 2005.

 

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5. Electronic Surveillance Measures (ESM)

There are two sides to submarine electronic surveillance measures (ESM): firstly, as a means of self-defense, and secondly, as a means of targeting or situational awareness.

Typically a submarine has an omni-directional radar warning antenna atop the periscope, which functions much like aircraft's radar warning receiver (RWR), and indeed, some submarine radar warners are just versions of airborne systems. For diesel-electric submarines, in particular, radar warning is vital since the submarine must spend considerable time either surfaced or snorkeling and therefore exposed to enemy radar.

During World War II, when submarines still hunted their prey on the surface, it became evident to the German Kriegsmarine that their U-boats were being detected by Allied airborne radar. The first basic RWR, the FuMB-1 Metox (FuMB, or Funkmess-Beobachtungs-Gerat, means "radar warning apparatus"), was rushed into service in mid 1942 in an effort to give U-boats adequate warning of the presence of Allied radar. However, it was eventually discovered that Metox itself emitted a signal that could be detected and tracked by the Allies, with the result that it was much a beacon as it was a warning device.

Several successors were developed and pressed into service, but the Germans found it difficult to field a reliable system that could accurately match Allied radar frequencies and provide more than a very short warning time (sometimes only one minute). By the time they had seemingly perfected the technology (with the FuMB-35 Athos), the Battle of the Atlantic was over.

Modern day submarines are still equipped with radar warning and direction finding (DF) systems, but these have grown significantly more complex, and have evolved into tools of electronic surveillance rather than simple warning devices. The attack submarines deployed by Western navies bear physical evidence of the importance being assigned to the electronic surveillance mission. The submarine's sail houses the upper portion of each periscope, mast, and antenna. When a submarine extends these devices while at periscope depth (roughly 60 feet), its steadiness in the water and the degree to which the boat is affected by surface wave action, depends on how deep in the water the hull remains. With a tall sail, the submarine's hull can remain deeper in the water and thus remain steadier.

However, sail height has a price. It creates considerable drag, increases flow noise, and is prone to a dangerous phenomenon called "snap roll", in which the sub can unintentionally dive while attempting to execute a turn (because the sail acts like a plane). Because of these problems, there have been attempts over the years to reduce or eliminate the sail, but beginning with the Sturgeon (SSN-637) class, the US Navy actually enlarged the sail, specifically to expand electronic surveillance capabilities (a testament to the importance of the mission). The Soviets, meanwhile, have traditionally used considerably lower sail structures (presumably due to significantly less interest in using their submarines in the electronic surveillance mission).

Submarines that use active emitters while at periscope depth or when surfaced (such as the surface search radar or communications mast, the latter exploiting HF, VHF, UHF and SATCOM channels) are advertising their position to enemy assets that have the ability to detect or intercept those emissions, even if the submarine is not otherwise detectable by other traditional methods (sonar, radar, IR, MAD, or the good old Mark I human eyeball). This is because, just like sound waves, radio frequency (RF) waves travel much further (and can be intercepted much further) than the range at which they can effectively be used by the emitter.

Because of the risks associated with coming to a shallow or periscope depth to conduct communications, and the generally inhospitable relationship between salt water and most radio frequency communications, the US and Soviet navies explored alternate methods. Only extremely low frequencies (ELF) in the range of 3-100 Hz are able to penetrate to any appreciable distance under the ocean, and for many years, ELF was the principal means of long range covert communications with their deployed submarines. The US Navy used an ELF system in the 76 Hz range, while the Soviets had a 82 Hz system.

The ELF system had certain advantages: worldwide range, low power requirements (only a few watts), and the ability to communicate with submarines at deep depths. However, it also had certain, significant disadvantages: one way transmission (submarines were receive only), very slow data transmission and limited bandwidth (meaning a typical ELF message comprised only a short coded message), and multi-path distortion (the result of energy arriving at the receiver over more than one path). The end of the Cold War and the availability of many more communications options spelled the decline of the importance of ELF. In September 2004, the US Navy announced it was turning off its ELF transmitters at Clam Lake, in the Chequamegon National Forest, in northern Wisconsin, and in Upper Michigan’s Escanaba State Forest near Republic.

Extremely low frequency (ELF) concept of submarine communications

Image credit: R. Romero and V. Lehtoranta, 22 November 1999.

Submarines also sometimes use buoys to establish communications while remaining submerged. Examples in US Navy use include the BRC-6 XSTAT (Expendable Submarine Tactical Transceiver) for two-way UHF communications between a submarine and aircraft; the BRT-1 SLOT (Submarine Launched One-way Transmitter), which broadcasts a pre-recorded message of up to four minutes duration; the BRT-3/4/5 identification buoys; and the BRT-6, which transmits one-way UHF communications to a satellite.

5. Lasers

Lasers have long been a favorite subject of military related research, and its no surprise, then, that they also have application in undersea warfare.

In the 1980s, the US Defense Advanced Research Projects Agency (DARPA) studied the possibility of developing laser based communications between satellites and submerged submarines. The system would use complex blue-green lasers to accomplish this task, since the attenuation of light in sea water is minimized in the blue-green region of the electromagnetic (EM) spectrum. The technological challenge was steep, however, since the satellites would need to be fitted with a highly efficient, solid-state laser with a high power output, and the submarine receiver would need a filter to remove the background solar radiation that would otherwise interfere with their detection of the very faint laser signals. The project was eventually abandoned. (It is notable that optical data relay via satellite was eventually achieved and successfully deployed, as exemplified by ARTEMIS (Advanced Relay and Technology Mission Satellite) launched in 2001).

Blue-green lasers have since, however, found a niche in the mine countermeasures role, again due to their ability to penetrate water. The AES-1 ALMDS (Airborne Laser Mine Detection System), the first of which was fielded in 2007, uses a blue-green LIDAR (Light Detection And Ranging) laser to rapidly detect, localize, and classify near surface, moored and floating sea mines. Housed in a 9 foot long, 820 lb pod hung from a MH-60S helicopter, the laser images the entire near-surface volume of water that might potentially contain mines.

AES-1 ALMDS pod aboard MH-60S helicopter

Image credit: Northrop Grumman.

C. ANTI-SUBMARINE WARFARE (ASW) WEAPONRY

1. Depth Charges

Perhaps the oldest anti-submarine weapon is the depth charge, developed during World War I as a relatively simple "barrel" of TNT detonated by a water pressure activated "pistol" at a selected depth. The first (and most basic) means of delivery was to simply roll the depth charges off the stern of a surface ship as it passed over the guesstimated location of the enemy submarine.

Scoring a direct hit with a depth charge was improbable, of course, but the shock of a nearby explosion could cause serious and accumulating damage, and did tremendous psychological damage to the crew of the submarine being subjected to the barrage. The slow speed and limited endurance of early submarines meant that their only means of defense was to conduct evasive maneuvers at the last possible moment and to try and get as deep as possible (a depth charge sinking above a deeply submerged submarine would take longer to reach its depth, thereby giving the submarine more time to move away). If a surface ship could catch a submarine in shallow water, however, depth charges were a cheap and effective means of damaging it or at least driving it off. (A kill was unlikely. The fact that U-427 is reported to have survived an onslaught of some 678 depth charges during an attack in April 1945, is a good illustration of the same).

Eventually depth charge projectors (like the British K-gun) were developed, which could hurl a depth charge out to the side of the ship, and when used in combination with stern racks, could lay a pattern of depth charges over an enemy submarine, hoping to bracket it.

An Allied destroyer dropping a pair of depth charges

Image credit: Mary Bellis.


2. Ahead Thrown ASW Weapons

Ahead thrown ASW weapons fire a salvo of charges or bombs in a pattern over an enemy submarine's position. The most famous, the Royal Navy's Hedgehog system of World War II fame, fired a salvo of 24 bombs from a spigot mortar (essentially derived from a type of infantry trench mortar where the mortar launching tube is actually built into the tail of the bomb). Each bomb weighed about 65 lb and contained 30-35 lbs of explosives. The pattern, dropped ahead of the ship at ranges approaching 200 yards, could be either elliptical or circular in shape (depending on variant) and measured 120 to 200 feet across.

The principal advantages of Hedgehog over ordinary depth charges lay in the fact that the bombs were contact fuzed rather than working on a time or depth fuze; their smaller, streamlined shape allowed them to sink faster than depth charges (able to sink 200 feet within 30 seconds of launch); and the depth of the targeted submarine did not need to be known. Hedgehog rapidly achieved favor over the depth charge, and a much more impressive success rate, approaching 25 percent. A very similar American system, called Mousetrap, used four or eight rails for rocket propelled charges.

Hedgehog on the forecastle of the V&W class destroyer HMS Westcott (D47), 28 November 1945

Image credit: IWM Collections, UK.

Ahead thrown weapons like Hedgehog and Mousetrap were so successful that the concept was immediately copied by the Soviets in the post war period, as the MBU and RBU series of ASW rocket launchers. The RBU series system, the latest of which is the RPK-8 variant of the RBU-6000, is still in use today despite the fact that it entered service nearly 50 years ago. The RPK-8 can launch a pattern of twelve 90R actively guided projectiles to a distance of over 2 nautical miles, each of which is equipped with a shaped charge. The system is believed to have secondary capability as a hard kill torpedo defense and anti-diver system.

RPK-8 launch from the Project 133.1 (Parchim) class ASW corvette Bashkartostan (MPK-228)

Image credit: : , , , ,

The UDAV-1/RPK-5 Leevyen (Heavy Rain) or RBU-10000/12000 series system optimises the torpedo defense role, creating an anti-torpedo barrier by launching rockets that deploy an outer layer of acoustic decoys, a middle layer of depth charges suspended by buoys, and an inner layer of explosives.

3. Torpedoes

The favoured, and most effective, ASW weapon remains the torpedo. The earliest "torpedoes" were probably better described as mines, demolition charges, or underwater booby traps, and the modern torpedo, a self-propelled underwater projectile, did not see widespread use until World War I. (Although the first victim of such a device is believed to have been the Turkish steamer Intibah on 16 January 1878). Torpedoes are generally divided into two categories or types:

(1) Heavyweight torpedoes, typically launched only from submarines (with the notable exception of some Soviet surface ship systems), usually 21 inches (533mm) in diameter (but sometimes larger, as with the 25.6 inch (650mm) Soviet Type 65), up to around 20 feet long, and may weigh several thousand pounds.

(2) Lightweight torpedoes, typically launched from aircraft and surface ships (again, with some exceptions), usually about 12.75 inches (324mm) in diameter (though Soviet types are often 16 inches), are less than 10 feet long, and weigh about 500 lbs.

Torpedoes are often also categorised by their method of guidance. More on that in just a bit.

Propulsion

The first truly successful torpedo designs used compressed air fed into a piston engine for propulsion, which was later modified to use a liquid fuel injected into an oxygen combustion chamber.

In the "wet heater" design, perhaps most popular among torpedoes of the two world wars, introducing water into the combustion chamber produced cooling and steam for even greater power (and speed). The principal drawback was that the steam left a visible trail of bubbles in the torpedo's wake. (The German G7a torpedo was eventually relegated to use at night because of this).

Wakeless torpedoes came with the introduction of electrical (battery) power, but at the cost of reduced range and speed.
Electric propulsion is relatively cheap and is still used with some modern designs, though with more efficient, long lived batteries (often silver-oxide or magnesium types), such as the British Royal Navy's Mk 24 Tigerfish or the German DM2 series. Electric propulsion also tends to have the advantage of being very quiet.

Other modern choices include gas turbines and swashplate piston engines powered by a high density (and often highly toxic) fuel, such as Otto Fuel II, for a so called "thermal" engine propulsion system.

As with submarines, torpedoes have sought to modify their propellers in an effort to improve propulsion and silencing. Pump jet propulsors encase the propellers in a shroud or duct, taking in water through an inlet in the front and pushing it out the rear, which both reduces noise and boosts speed.

Propulsor on Mk 50 Barracuda Advanced Light Weight Torpedo (ALWT

Image credit: US Navy.

Speed is of critical importance in a torpedo engagement. For early straight running torpedoes, it meant reducing the margin of error, where the target might maneuver at any time and ruin your firing solution. For modern undersea warfare, it means the ability to engage a fast nuclear powered submarine or surface warship, and the possibility of a rapid re-attack.

As a reliable rule of thumb, an ASW torpedo needs a 50 percent margin of superiority in speed over its target in order to assure that the target cannot escape once alerted to the attack.

Nuclear submarines were expected to achieve 30 kt speeds at 1,000 ft depths, thus creating the need for an ASW torpedo capable of at least 45 kt. A US Navy development program to achieve this for both light and heavyweight torpedoes was begun in 1956. (The later appearance of the Soviet Project 705 Lyra (NATO Alfa) class SSN was particularly alarming to NATO and repeated the process in some respects, spawning both the US Mk 48 ADCAP and British Spearfish torpedo programs).

In the first case, this effort resulted in the Mark 46 lightweight torpedo, which began deployment in 1965. In the latter case, requirements for a heavyweight torpedo were established in 1960, aiming for a 55 kt speed, a range of 35,000 yd, and a depth capability of 2,500 ft. In 1967, it was decided to make the heavy weapon also capable of anti-surface attack, which in turn mandated a larger warhead. In 1971, production of the Mark 48 began and the first examples were delivered in 1972, about 7 years after the first lightweight Mk 46 was delivered.

Many torpedo designs (of both today and yesteryear) have the option of trading range for speed. The German G7a (T1) steam driven torpedo of WWII was capable of an impressive 44 knots over about 6,500 yards, but at a leisurely 30 knots it could potentially achieve over 15,000 yards (though the chances of hitting anything at that distance was slim to none). A modern example, for comparison, would be the aforementioned Spearfish. It can reach 24,000 yards (almost 12 nm) at a blistering speed of 65 knots, or cruise to a distance of 50,000 yards (nearly 25 nm) at 29 knots.

 

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There is another, rather radical option for torpedo propulsion: rocket power. The advantage? Pure, unadulterated speed. B) The Soviets near monopolized the technology, starting with the introduction of the 70 knot RAT-52 anti-ship torpedo in 1953 (and copied by the Chinese as the Yu-2). They continued the trend with the APR-1, APR-2 and APR-3 rocket powered anti-sub lightweight torpedoes (though, in the case of the APR-3, the rocket motor drives a pumpjet), all capable of impressive speeds in the 62 to 65 knot range.

The most famous (or infamous) rocket powered torpedo is the Shkval (Squall) supercavitating projectile. Supercavitation is a phenomenon by which cavitation effects are exploited to create a bubble of gas under the water. An object within the gas bubble is able to escape the tremendous drag ordinarily created by traveling in water, and thereby move at high speed. By diverting some of its rocket exhaust through a nozzle on the nose, which helps to form the gas bubble, Shkval can achieve speeds approaching 200 knots. This eliminates the possibility of an acoustic homing head, leaving the weapon unguided, but as the Shkval can reach its maximum range of about 11,000 yards in less than two minutes, this may not matter.

VA-111 Shkval - note the opening at the nose for the rocket exhaust

Image credit: One half 3544.

Guidance

The earliest "guided" torpedoes were straight running, relying only on the crew's skill in using a slide rule and protractor, and gyroscopes in the torpedo, to send the weapon on what they hoped would be a collision course.

The German T4 or Falke, the first homing torpedo, was fitted with a passive acoustic homing device. Introduced in March 1943, it was quickly replaced by the T5 or G7s Zaunköning (Wren), which came into service that autumn. The T5 was faster, had superior range, and could use either magnetic or contact fuzing.

Again, however (as was the Kriegsmarine's curse, it seems), these advances in ASW technology really came too late to have significant impact on the war, and by this time the Allies' own technological pace was rapidly increasing. The Foxer towed acoustic decoy, for example, which was really just a noise maker that produced a more attractive target, condemned German passive homing torpedoes to a life of harmlessly circling behind their target until they ran out of fuel.

Passive sonar homing has traditionally been used as an anti-surface weapon. Although the Soviets had been pursuing the development of a domestic homing torpedo, that study had been interrupted by World War II. The capture of German T5 torpedoes got that project back on track, and by 1950, the first Soviet passive homer, the SAET-50, was in service. This was followed in 1961 by the electrically propelled SAET-60.

Active sonar homing has, on the other hand, been traditionally an anti-submarine weapon. The acquisition range of an active homing torpedo depends largely on its "ping rate" or "interrogation rate", which is typically increased as the torpedo nears the target, in order to improve accuracy. The seeker's aspect angle will define the band depth which the seeker can "see" at any given moment. Seekers with narrow depth bands tend to follow helical search patterns, while those with broader depth bands often use a snake like search pattern.

British Royal Navy Stingray lightweight torpedo

Image credit: Royal Navy.

Flow noise over the acoustic seeker head of a homing torpedo tends to interfere with its operation, but better signal processing and seeker dome shaping has brought improvements. Even so, active homing faces several obstacles. Near the surface, or in shallow water, reflections and reverbations can interfere with the seeker, and so active homing must rely on the Doppler effect to distinguish the real target from clutter. Considerable effort has been expended in recent years to improve active seeker performance in shallow littoral zones. Moreover, since active sonar can be heard at some distance, any submarine hearing an incoming torpedo can be expected to try to escape at high speed.

The generally accepted solution has been to combine active and passive homing, and moreover, to add wire guidance. This allows the torpedo's guidance package to receive course correction commands, benefiting from the superior sensors of the launch platform to update the weapon on the target's position. The guidance wire is dispensed from both the torpedo and the submarine to avoid stress on the wire due to their relative motion. Furthermore, with the advent of two-way or dual wire guidance, the torpedo becomes a self-propelled offboard sensor.

This way, a torpedo can be run out to the target's vicinity under wire control and without alerting the target (except by its own propulsion noise). Upon reaching that area, the torpedo can carry out the terminal attack phase with its active sonar seeker. If the wire is broken (either accidentally or intentionally), the weapon can immediately switch to acoustic homing.

Wire guidance has typically been reserved for heavyweight torpedoes, with the exception of some Swedish and Russian lightweight torpedoes that use wire control to help overcome the difficult acoustic conditions found in the Baltic and other shallow water seas.

There is another, unique torpedo guidance option: wake homing. The Soviets have exploited wake homing for quite some time, firstly with its Type 53-57M in the mid 1960s, and it is only in recent years that the method has found its way into some Western torpedoes as an adjunct seeker capability (e.g. the French F17 Mod 2, Italian A.184 Mod 3 and Black Shark).

Wake homing uses an upward looking sonar beam to detect a ship's wake boundaries. The control vanes are set to automatically turn the torpedo through a fixed angle each time the weapon crosses the wake, allowing it to follow a snake like track to the apex of the wake (the target). Since a ship's wake extends for quite some distance (see the depth charge picture above for reference), it is a relatively simple matter for the seeker to determine its boundaries. One notable advantage of wake homing is that it is practically immune to countermeasures.

A further refinement of wake homing, cutely termed "wake nibbling", seeks efficiencies by having the torpedo follow the boundaries of the wake rather than expend time (and fuel) crossing the wake.

The most well known wake homer is the Soviet Type 65-76 heavyweight torpedo, a monster 650mm weapon carrying a huge warhead capable of devastating the largest US Navy supercarriers. The Type 65-76 entered service with the first Project 671RT (Victor II) class attack submarine in 1972. It was retired in February 2002 after the tragic loss of the Kursk, an event widely blamed on an explosion related to the torpedo's High Test Peroxide (HTP) fuel.

4. Stand-off ASW Missiles

As sonar performance steadily improved in the post World War II years, the US Navy perceived the need for a stand off weapon that could more closely match the range at which new sonars could detect enemy submarine targets. The objective was to be able to deliver an acoustic homing torpedo (or, alternatively, a nuclear warhead) to a point where the targeted submarine could not escape. This could be difficult, especially when engaging a submarine that was moving quickly, and so the process was largely dependent on an accurate prediction of where the target was going to be when the missile arrived. Absolutely precise accuracy was not required (particularly when using a nuke, of course), but it was necessary to drop the torpedo within acquisition range of its own acoustic seeker. What was needed was a quick reaction weapon with a short flight time.

Development of the RUR-5 ASROC (Anti-Submarine Rocket) began in the early 1950s, based on a relatively simple concept of using a solid rocket motor to deliver a Mk 44 homing torpedo or nuclear depth charge (the 10 kiloton W44) to a specific water entry point right above the enemy submarine. Range (up to 5 nautical miles) was determined by a timer set before launch, such that when the timer expired in flight, the rocket motor separated and the payload fell on a ballistic trajectory into the ocean.

Another program was soon launched to provide a similar weapon for underwater launch from submarines. Several technical problems delayed the program, but the UUM-44 SUBROC (Submarine Rocket) entered service with the USS Permit (SSN-594) in 1965. SUBROC was launched from a standard 21 inch torpedo tube, igniting its solid rocket motor once it safely cleared the submarine and breaching the surface. An inertial guidance system directed the missile toward the target location up to 30 nautical miles away, where, like ASROC, the motor separated from the payload. The only payload option for SUBROC was a 200 kiloton nuclear depth charge, leaving virtually no chance of escape for any enemy submarine within a radius of about 4 nautical miles.

In the early 1980s, the US Navy initiated the RUM-139 Vertical Launch ASROC (VLA) program to take advantage of the new Mk 41 Vertical Launch System being fitted in US Navy warships. However, another more promising program intervened - the RUM/UUM-125 Sea Lance - which promised a common successor to both ASROC and SUBROC. The Sea Lance would be capable of launch from either a submarine or surface ship, and would deliver either the new Mk 50 Barracuda Advanced Light Weight Torpedo (ALWT) to a range of 30 nm or a nuclear depth charge to 60 nm. Progress with the program was slow, and the end of the Cold War also spelled the end of Sea Lance.

RUR-5 ASROC launch

Image credit: David Schmitt.

Fortunately for the old reliant, this meant a revival of the RUM-139 VLA. It uses a new solid rocket booster and combines a digital autopilot with the inertial guidance system, improving maximum range to 12 nautical miles, making the missile more maneuverable, and permitting it to fly a lower altitude trajectory. Range is controlled in the usual manner, and the payload is the Mk 46 Mod 5A or Mod 5AS lightweight homing torpedo. (The originally plan to add the Mk 50 Barracuda was shelved, and instead the newest variant will use the Mk 54 Lightweight Hybrid Torpedo (LHT), which combines the guidance and warhead of the Mk 50 with the propulsion system of the Mk 46).

In the face of a growing Soviet submarine threat in the 1950s, NATO and Allied nations were quick to jump on the bandwagon.

The Marine Nationale (French Navy) introduced the Malafon in 1964, which could deliver the 533mm L4 acoustic homing torpedo to a distance of about 11 nm. A turntable launcher would slew to the target bearing and launch the missile at a fixed elevation of 15 degrees. Upon launch, two solid rocket boosters accelerated Malafon to about 450 knots before falling away, allowing the weapon to glide toward the target under radio control, dropping its torpedo payload in the usual manner about 800 meters from the target submarine. Malafon also had a secondary anti-ship attack capability. The Australian Ikara (from an Aborigine word for "throwing stick"), meanwhile, which entered service around 1966, delivered a Mk 46 acoustic homing torpedo to a range of nearly 10 nm.

The Soviet Union did not take long to recognize the inherent advantages of a stand off ASW missile, and developed its own version of ASROC. The 82R Vikhr (NATO Free Rocket Anti-Submarine, or FRAS-1) was essentially a navalised version of the FROG-7 (Free Rocket Over Ground) battlefield artillery rocket. Launched from the SUW-N-1 twin arm launcher, it was capable of delivering a 200 kiloton nuclear depth charge to a range of about 16.5 nm. The FRAS-1 was fitted to the Project 1123 (Moskva) and Project 1143 (Kiev) class aviation ships (except Admiral Gorshkov).

The RPK-3 Metel (Snowstorm) (NATO SS-N-14a/b Silex) appeared in 1970 aboard the new Project 1134A (NATO Kresta II) class guided missile cruiser. The 60R version carried a 5 kiloton nuclear depth charge, while the 70R carried a 450mm homing torpedo. Rocket propulsion and radio command guidance carried the weapon out to a maximum range of 28 nm, whereupon the payload was released.The SS-N-14 saw further variants developed in the early 1980s, including the 85R Metel-M (SS-N-14c), which carried a heavier 533mm homing torpedo; and the 85RU Rastrub (SS-N-14d), which returned to a lightweight (and more advanced) 400mm homing torpedo. Most interestingly, the SS-N-14d was capable of anti-ship attack, made possible by the addition of an IR guidance mode and a 185 kg warhead ahead of the torpedo payload.

The Soviets also did not fail to appreciate the utility of a submarine launched, stand off ASW missile. The RPK-2 Viyuga (Blizzard) (NATO SS-N-15 Starfish) introduced in or about 1972 performed the same way as the US Navy SUBROC, launched from a 21 inch torpedo tube, utilising a solid rocket motor and delivering a nuclear depth charge to a distance of about 25 nm.

Unlike the US Navy and the failed Sea Lance program, however, the Soviets have expanded their stand off ASW missile arsenal.

The 83R Vodopad and 84R Vodoley versions of the SS-N-15 (RPK-6), introduced in 1981, delivered a homing torpedo and nuclear depth charge, respectively, and moreover, could be launched from a surface ship. A further system, the SS-N-16 Stallion (RPK-7), takes advantage of the 650mm torpedo tube to achieve even greater range, at nearly 60 nm. The 86R Veter (SS-N-16a) and 88R Vsplesk (SS-N-16b) versions of the weapon deliver a homing torpedo and a nuclear depth charge, respectively. The latest Soviet stand off ASW weapon, the RPK-9 Medvedka (NATO SS-N-29), is intended for launch from small surface combatants, delivering a homing torpedo to a little over 12 nm.

Comparison of stand off and airborne ASW weapon delivery methods

Image credit: Brian Burnell.

In recent times, the US Navy has pursued a new concept in airborne stand off ASW, namely the High Altitude ASW Weapons Concept (HAAWC). (The effort was largely born out of a desire to reduce P-3 Orion airframe fatigue, and improve P-8 Poseidon endurance, by obviating the need to drop the torpedo at low altitude). HAAWC mates a low cost wing adaptor kit, nicknamed "LongShot", with a lightweight torpedo and uses GPS based navigation to accurately deliver a torpedo to ranges of 50 nm. In May 2007, a HAAWC equipped Mk 54 torpedo was successfully deployed from a P-3 Orion flying at 8,000 feet.

5. Naval Mines

Mines, cheap and easily deployed, are perhaps the single most effective weapon available to a combatant who seeks to prevent the projection of naval power onto his shores. More than 50 nations currently possess naval mine inventories and are able to deploy more than 300 varieties of mines. Mine countermeasures (MCM), therefore, remain a critical enabler for maritime expeditionary forces. Like the weather, everyone likes to complain about mines but there's not much we can do about it. If you think ASW is mind numbing, try MCM.

Mines can be offensive or defensive, that is, placed in enemy territorial waters to destroy or hamper the movement of enemy shipping, or alternatively, placed in defensive fields to protect friendly areas from intrusion by enemy shipping and to force them elsewhere. That being the case, mines can be either a deadly nuisance or a useful tool from an ASW point of view.

Mines can be classified in either of several ways:

(1) Method of delivery: aircraft laid, submarine laid, or surface laid;
(2) Method of deployment: bottom, moored, or drifting; and
(3) Method of actuation: contact, influence, controlled, or rising.

Method of Delivery

Whether offensive or defensive, any force intending to lay mines must prepare and maintain minefield plans for all geographic locations where it is intended to deploy mines. The cost and time necessary to remove mines (even your own) is many times more than the cost and time to deploy them. And, of course, hitting your own mines would be entirely unsatisfactory.

Just about any aircraft that can drop a simple iron bomb can also lay a mine. In fact, some mines are simply converted iron bombs. The US Navy Destructor (DST) series, introduced in the late 1960s, are a prime example. With a Mk 75 modification kit, any Mk 80 series iron bomb becomes a naval mine: the 2,000 lb Mk 84 becoming the Mk 41 DST, the 1,000 lb Mk 83 becoming the Mk 40 DST, and the 500 lb Mk 82 becoming the Mk 36 DST.

The newer US Navy Quickstrike series mines of the early 1980s work on the same principle, converting the 500 lb Mk 82 iron bomb into the Mk 62, the 1,000 lb Mk 83 into the Mk 63, and the 2,000 lb Mk 84 into the Mk 64. A fourth version, the Mk 65, uses a Mk 84 bomb with a thinner walled case and some other improvements.

The advantage of a submarine laid mine is related, unsurprisingly, to the submarine's ability to reach waters usually inaccessible to other platforms, and to do so covertly.

The US Navy's Mk 67 Submarine Launched Mobile Mine (SLMM), introduced in 1987, is essentially a modified Mk 37 torpedo that can be launched from a distance and delivered to a bottom laid deployment. Once deployed, it sits on the bottom until actuated by magnetic/seismic or magnetic/seismic/pressure sensors. It was retired in or about the year 2000. The Russian SMDM mine works on the same principle, using a 650mm heavyweight torpedo.

Many types of mines that can be laid from aircraft or submarines can also be laid from surface ships, much like depth charges, by rolling or sliding them off the ship from a rack. The advantage of laying mines in this manner is that a ship can typically carry and deploy much larger numbers of mines than an aircraft or sub. The principal disadvantage of surface mine laying is that the ship can only undertake the operation while in relative safety, making the task difficult to accomplish in enemy waters and thereby relegating surface laid mines largely to the defensive mission.

Many moored mines are laid from surface ships, including, for example, the Russian MAG and UGM types.

Method of Deployment

Drifting mines are perhaps the most feared, not because they're particularly effective but rather because they tend to be indiscriminate. A drifting mine, carried anywhere by the wind and the currents, is just as liable to sink your own ship, or a non-combatant, as the enemy. Drifting contact mines were used on occasion during the world wars, but this limitation was eventually recognised. Sometimes a moored mine may break its cable and become a drifting mine, but modern types are usually equipped with electronics to sense the problem and deactivate or scuttle the mine.

The US Mk 19 and Russian APM (circa 1955) are examples of drifting contact mines.

A moored mine is connected to the seabed by a cable and anchor to keep it in place. The mine itself will float at a predetermined depth, the latter being determined by the tides in the area and more importantly, the intended target. Shallow moored mines are typically intended for use against surface ships, while deep moored mines seek submarine targets. Moored mines are unavailable, of course, where the water is too deep to result in effective deployment.

The US Navy's Mk 56 and Mk 57 are examples of moored mines intended for the anti-submarine mission.

Early in the morning of 18 February 1991, the Iwo Jima class amphibious assault ship USS Tripoli (LPH 10) hit a moored contact mine that ripped a 16 x 20 foot hole in her hull below the waterline. The ship had been operating for some 11 hours in the previously undetected Iraqi minefield.

Bottom mines are generally used where the water is either very shallow or too deep for moored mines. As the name suggests, a bottom mine is deployed on the ocean floor and is typically difficult to detect and sweep. Bottom mines are useful in shallow waters where an enemy amphibious assault might be expected, or where you need a large warhead.

The Destructor and Quickstrike series mines described earlier are all examples of bottom mines, as is the Italian Manta which also haunted US Navy warships in the Persian Gulf during the first Gulf War. A few hours after the incident with Tripoli, the Bunker Hill class guided missile cruiser USS Princeton (CG 54) struck a Manta mine in about 16 meters of water, and about three seconds later, was hit by the sympathetic detonation of another about 350 yards away. The damage was substantial, including a cracked superstructure, severe deck buckling, and a damaged propeller shaft and rudder.

A Manta bottom mine just a few hours after deployment

Image credit: SEI.

Method of Actuation

The earliest and simplest type of actuation was the contact mine, which in many respects typifies the traditional image of a naval mine. Early versions of the contact mine had mechanical detonators, but later evolved into chemical (such as the Hertz Horn, where contact with a target's hull cracked a vial of acid that poured into a lead-acid battery) and electrical fuzes (where a copper wire attached to a buoy created a voltage and activated the detonator when in contact with a target's hull).

Contact mines are increasingly rare in the modern era. The Russian AG, AGSB, and AMAG 1 are all examples of WWII era contact mines.

Influence mines are actuated by the "influence" of a target, that is, triggered by the presence of a ship or submarine itself, through the use of such sensors as magnetic, passive acoustic, or water pressure displacement. Early influence mines used simple magnetic sensors or broadband hydrophones. Advances in electronics and computer processing have seen the introduction of increasingly sophisticated sensors; for example, the introduction of "total field" magnetometers and narrowband passive sensors that can listen for certain acoustic signatures. These sophisticated systems permit a mine to be programmed to ignore all passing ships and submarines except for a very specific target.

When first introduced, pressure actuated mines were virtually immune to sweeping. There was no device that could simulate a ship's pressure signature, except, well, a real ship. <_< The only protection against them was to move so slowly as to not cause a change in water pressure sufficient to set off the mines.

In practise, however, very few mines have been equipped with pressure actuation only, and in fact, most modern influence mines use multiple influence sensors as to avoid premature actuation by some natural phenomenon and to defeat simple countermeasures. (Most pressure mines, therefore, can be swept by using an acoustic noisemaker or magnetic field).

Examples of influence mines include the German DM 61 (G2), British Stonefish, and the Russian MDM-3/4/5/6 series of bottom mines.

Remotely controlled mines are typically found in defensive minefields, where the mines can be deactivated or detonated on command, usually by electrical signal from shore (usually when you can visually observe enemy targets), or switched over to normal (contact or influence) methods. Examples of controlled mines include the Chinese EM-57 and Swedish M/9 (GMI 600).

There is a fourth category of naval mine actuation, although in most cases it is a sub-variant of influence mines. The rising mine can lay very close to the ocean bottom, usually in moored fashion and in very deep water, and then launch a torpedo or projectile when a ship or submarine passes by. The influence sensor needs to have long range (1,000 feet or more), which generally necessitates a passive acoustic or magnetic system, because of the lag time between triggering the mine and the warhead or payload reaching the target.

The classic example of a sophisticated rising mine is the US Navy's Mk 60 CAPTOR (Encapsulated Torpedo), which is a deep water (up to 3,000 ft) moored mine that uses "reliable acoustic path (RAP) sound propagation" as its influence method and the Mk 46 lightweight torpedo as its payload. It is strictly an ASW mine, ignoring surface ships and (reportedly) friendly submarines, and may be laid by air, ship, or submarine.

Mk 60 CAPTOR method of operation

Image credit: Federation of American Scientists.

CONCLUSION

Hopefully this rather lengthy discussion has given you a useful foundation in the tools of anti-submarine warfare (ASW).

In Part 3 we'll move to the ASW cycle, the specific methodology of detecting, locating, and attacking submarines.

Source references:
Jane's Navy International, November/December 1995, November 1997.
Naval Institute Guide to World Naval Weapons Systems, 1997-98.
Military Parade, Stanislav Proshkin and Valery Marinin, 1998.
The Third Battle: Innovation in the US Navy's Silent Cold War Struggle with Soviet Submarines, Dr. Owen R. Cote Jr., March 2000.
Jane's Information Group: August 1999; June 2003; September 2005.
Journal of Electronic Defense, March 2001.
ASW after the Cold War, Owen Cote and Harvey Sapolsky, MIT Security Studies Program, April 2001.
Proceedings, June 2002.
National Defense, January 2003.
Undersea Dragons: China's Maturing Submarine Force, Lyle Goldstein and William Murray, International Security, Spring 2004.
World of Defence, UDT, Issue No.2, 2004.
U.S. Destroyers, Norman Friedman, 2004.
Naval Institute Guide to the Ships and Aircraft of the US Fleet, Norman Polmar, 2005.
Navy Times, August 2005.
SOSUS: The "Secret Weapon" of Undersea Surveillance, Edward C. Whitman, Undersea Warfare, Winter 2005.
Principles of Naval Weapon Systems, Craig Payne, 2006.
Not Ready for Retirement: The Sonobuoy Approaches Age 65, Holler et al., Sea Technology, November 2006.
Harpoon 3 Sonar Model, AGSI, 2007.
Proceedings, June 2007.
ES310, Introduction to Naval Weapons Engineering.
Ocean Talk, Naval Meteorology and Oceanography Command.
Federation of American Scientists.
GlobalSecurity.org - Reliable Security Information
The U-boat Wars 1939-1945 (Kriegsmarine) and 1914-1918 (Kaiserliche Marine) and Allied Warships of WWII - uboat.net
Designation-Systems.Net

 

[Manticore]

Part III

TACTICS 101: ANTI-SUBMARINE WARFARE: PART 3 - THE ASW CYCLE

In Part 1 of our discussion about ASW, we learned the basics of naval oceanography, and in large part, how sound behaves in the subsurface ocean environment.

In Part 2, we moved to a basic examination of the tools of anti-submarine warfare (ASW), the sensors, the platforms, and the weapons.

Now, in Part 3, we'll examine the ASW cycle, the methodology of detecting, locating, and attacking submarines.

A. DETECTION, LOCATION AND TARGETING

1. Detection

Initial detection of a subsurface contact, whether by active or passive means, is your first clue that there is a submarine in the area, or in the old parlance, a POSSUB (Possible Submarine) report is generated.

For passive sonar, confirmation of submarine detection is often made through receiving and analyzing the lowest frequency sounds, such as propeller noises, and will give you a general idea that the target is in fact a submarine, rather than say, a biological or just "magma displacement". It will not tell you which navy the submarine belongs to, or what type it is.

Broadband noise is produced by a target's propulsion machinery, its propellers, the hull passing through the water, external fittings, etc. As a contact gets closer to the receiver, more and more information is gleaned as progressively higher frequency and lower strength sound becomes available, including narrowband sources. This combination of a ship or submarine's self-noise, both narrowband and broadband, form its "acoustic signature" and can be exploited by to identify a contact as a submarine.

Initial detection by active sonar provides even less information, simply telling you that there is something there underwater. The determination of the identity of that "something" is a function of operator training and experience.

Direct Path

As discussed in Part 1, the direct path is the simplest acoustic propagation path and is essentially a straight line path between the sonar source and the target.

Due to the reduced opportunity for signal propagation loss and the fact that there is no reflection of the sound, a direct path sonar contact may be the easiest to obtain and analyse, but it also presents the shortest range passive detection opportunity (typically less than 10 nautical miles).

Because it is so short ranged, any surface ship obtaining a direct path detection of a submarine target is already in significant danger. It is extremely likely that the submarine has already been tracking the ship for some time, working a fire control solution, and is now approaching for the kill. It is the "oh, crap" detection scenario.

Bottom Bounce

With the bottom bounce technique, active sonar energy is directed towards the bottom. Steeply inclined sound ray paths are affected to a lesser degree than the shallower (in some cases, nearly horizontal) paths of other methods (e.g. surface duct, convergence zone, etc) and so there is less transmission loss and less potential for shadow zones.

Bottom bounce is only useful, however, where the ocean floor has a hard, flat surface (where you can get a good reflection) and its a poor choice where the bottom is rocky or soft mud (where sonar is dispersed by reverberations or absorbed). The Barents Sea, for example, has excellent conditions for bottom bounce sonar technique.

Because it typically involves the use of active sonar, however, bottom bounce carries a significant amount of risk in the ASW role and is a technique generally reserved for very specific scenarios where passive sonar simply isn't sufficient.

Convergence Zone (CZ) Detections

As discussed in Part 1, convergence zones (CZ) are areas where refracted sound in deeper water can focus at the surface, typically at predictable intervals. The first CZ, for example, typically develops at about 30-33 nautical miles. The second CZ might form at 60-66 nm, a third at 90-99 nm, and so on.

The extent of CZ formation, or whether a CZ forms at all, is naturally dependent on whether sound can propagate far enough or deep enough, which in turn is dictated by environmental factors, notably water depth, temperature, etc.

In most cases, you are perhaps unlikely to get a detection beyond the first CZ. This is, under most conditions, a quite distant sonar contact in any event (keeping in mind, of course, that the CZ phenomenon can result in some pretty extraordinary sonar ranges). A contact would often have to be very loud and/or your passive sonar equipment very sensitive to achieve second and third CZ detections.

Also, a CZ detection will not usually provide any positional information on the target (i.e. you just get a bearing), negating the possibility of employing a long range ASW weapon against it. And, finally, a CZ detection tends to be a fleeting one. Once the target moves out of the CZ, or into a shadow zone, contact is often lost.

Sonobuoy Fields and Search Patterns

Using sonobuoys alone for an acoustic search is known as a "cold pattern". Since sea temperature, pressure (a function of depth) and salinity all affect the way in which sound travels through the water, those factors need to be determined for an effective cold pattern.

The first sonobuoys dropped from a P-3 Orion might therefore be, for example, a SSQ-36 and a SSQ-57, in order to establish local ocean conditions. The SSQ-36 is a bathythermograph sonobuoy used to get a temperature profile, while the SSQ-57 is a passive, omnidirectional "calibrated collection" sonobuoy, used to acquire ambient noises roduced by marine life and other ocean sources.

Once local conditions have been established, the aircraft might move to laying a field of active and/or passive sonobuoys.

The layout of a sonobuoy field will depend on what kind of search pattern is employed. The following are some examples of search patterns, which could be used to sow sonobuoy fields or conduct a radar/FLIR/MAD search, each of which indicates a commence search pattern (CSP) point.

The parallel pattern is most desirable when the target is assumed equally likely to occupy any part of the search area.

Image credit: Journal of Simulation (2006) 1, 29-38.

The creeping line pattern is typically employed when the target is more likely to be in a particular end of the search area.

Image credit: Journal of Simulation (2006) 1, 29-38.

When the point of last contact is well known, or established within close limits, the square or sector search pattern is preferred. The square pattern is used when uniform coverage of the search area is desired.

Image credit: Journal of Simulation (2006) 1, 29-38.

The sector search pattern is used where the target is difficult to detect.

Image credit: Journal of Simulation (2006) 1, 29-38.

Finally, when a target is fast moving or when a strong current is present in the search area, the barrier patrol search pattern is preferred (its CSP is either starting point). This was the pattern used extensively during the actual Bay of Biscay U-boat war.

Image credit: Journal of Simulation (2006) 1, 29-38.

Although the greatest threat to a submarine is likely that posed by aircraft, it is by no means a sure thing that the aircraft will detect, locate, and successfully prosecute a submarine contact.

The sound of an aircraft coming close enough to attack (or the splashes from its dropped sonobuoys) can be detected by a submarine, particularly if the sub has a very sensitive passive sonar, such as a towed array.

And, since the submarine has better, more constant access to the ocean's environmental conditions, it can use this information to exploit weaknesses in the aircraft's sensor net. For example, by changing speed and direction, or by dropping below the thermal layer. Actively pinging sonobuoys can be mapped and avoided, or alternatively, hide directly beneath an active sonobuoy. This trick takes advantage of the fact that the active sonar pings move outward from the buoy like the rings from a tossed pebble, and so sitting underneath the buoy can shield the submarine to some extent.

2. Location and Identification

The next step is localizing the target. With active sonar, this is largely a simply exercise, but of course employing active sonar will almost always alert the enemy to your presence and so carries with it more than a little risk. Using passive sonar only, localization can still be accomplished by a number of means, including:

Target Motion Analysis (TMA)

An initial passive sonar contact typically provides only a bearing to the target, and tells you nothing about distance (range) or speed. The contact may be close by, creeping along at only 5 kt, or twice as distant, and moving at 10 kt.

Using only passive sonar, target motion analysis (TMA) is a mathematical process by which a contact's course and range can be estimated using timed readings of the contact bearing and an estimate of its speed. Each estimate is (hopefully) more accurate than the one previous, and eventually the estimation will be accurate enough to reliably predict the next bearing. TMA is thus, essentially, an initial approximation of target range, course, and speed. The closer the ensuing predictions come to reality, the more likely it is that the approximation is an accurate one.

Suffice to say, any target whose bearing is changing rapidly is quite close and needs to be localised in a hurry. Conversely, a distant target will show almost no change in bearing over time. (The same can be said, of course, for target that is exactly parallel, but its bearing will change as you change course even slightly).

TMA is a tedious procedure, one that can consume a lot of time and create considerable frustration, but it is certainly more stealthy than energizing an active sonar and can eventually produce a suitable firing solution. Initially, TMA calculations involved only a paper plot, used to determine short range fire control, and later evolving into a computerised system using software target tracking that can project a target's position ahead in time and be sufficently accurate to control wire guided weapons.

Limitations to the TMA technique include: contact must be maintained over a period of time; all contact bearings must be assumed to come from the same target over that period of time; and maneuvers must be built into own ship track in order to resolve ambiguities.

Beyond a certain range, of course, passive TMA techniques cannot reliably produce an accurate solution. For example, for convergence zone (CZ) detections, the only data typically available from passive sonar is a bearing. Range, course, and speed are practically impossible to determine. And, since a CZ contact is easily lost, the TMA solution clock would have to be reset and a new solution started from scratch each time.

LOFARGRAM

A Low Frequency Analysis and Ranging Record Gram (LOFARGRAM) plot can be generated where signal strength data can be gathered from non-directional passive sonobuoys.

The plot looks much like a waterfall display, and can be used be determine such identifying information as blade rate. This is a fairly complex exercise, however, and requires a detailed understanding of target signatures.

An example of a LOFARGRAM

Image credit: Defence Science and Technology Laboratory (Dstl), UK Ministry of Defence.

Triangulation using DIFAR Sonobuoys

Here, each DIFAR buoy transmits a compass direction for each of its hydrophones and the operator can then determine which element on the buoy has the highest signal strength.

Target Identification

Although precise identification of the target may matter considerably less during wartime, when any unknown submarine contact found outside a recognized patrol area or transit lane may be assumed hostile, it is still of some value, especially when multiple targets present themselves. Identifying the contact can provide information about the threat it poses and help determine the application of rules of engagement (ROE) or an engagement priority. For example, identifying one of two submarine contacts as an Oscar II class SSGN, and the other as a Charlie class SSGN, may be convincing evidence of their threat priority.

At longer ranges, narrowband sources can be used to begin generating a picture of what the target might (or might not be). For example, during the earlier days of the Cold War, Soviet nuclear submarines could be broadly identified by their propulsion plants as follows:

Type 1 reactor plant: First generation nuclear subs, including the Hotel (Project 658) SSBN, Echo (Project 659/675) SSGN, and November (Project 627/645) SSN classes
Type 2: Victor (Project 671) SSN and Charlie (Project 670) SSGN classes
Type 3 (twinned Type 2 plants): Yankee (Project 667) and Delta (Project 667B) SSBN classes
Type 4: the Papa (Project 661) class SSGN
Type 5: the Alfa (Project 705) class SSN
Type 6: Sierra (Project 945) SSN, Oscar (Project 949) SSGN, and Akula (Project 971) SSN classes
Type 7: Typhoon (Project 941) class SSBN

More precise identification will require increasing analysis of a target's noise signature and usually requires fairly close quarters proximity between the target and the sensor, so that your classification becomes more refined over time.

Improved sonar processing techniques now make it possible to detect and demodulate target noise to identify it by measuring its shaft or propeller blade rate (blade rate tonals). Also, as a note of interest, the more blades, the more likely the contact is a submarine (as commercial merchant ship propellers have only three or four blades, while warships often have five).

The twin shafts and propellers of the Arleigh Burke class Aegis destroyer USS Curtis Wilbur (DDG-54)

Image credit: Federation of American Scientists.

Many older Soviet/Russian nuclear powered submarines, such as the Yankee (Project 667), Delta (Project 667B), Papa (Project 661) classes, have five blades. So do some older diesels, including the Swedish Vastergotland (A17) class. The Soviet/Russian diesel-electric Kilo (Project 877) class has six, while the later Project 636 variant of the Kilo and the newest Lada (Project 677) class SSK both have seven blades.

Many other modern submarine designs employ seven blade propeller systems, including, for example, the Soviet/Russian Typhoon (Project 941) SSBN, the Victor (Project 671) SSN (though some of the Victor III (Project 671RTM) variant were unique in having two four-blade props mounted in tandem), and the Akula (Project 971) SSN; the US Navy's Los Angeles (SSN-688) attack sub; the Chinese Type 039 (Song) and Type 039A (Yuan) class SSKs, and its Type 093 (Shang) SSN; and most newer European diesels, such as the Dutch Walrus SSK.

The 7 blade, skewed propellers of the Chinese Type 039 (Song) class SSK (this is the type that shadowed the US Navy's carrier Kitty Hawk in October 2006, surfacing within 5 miles - uncomfortably close - apparently without having been detected).

Image credit: Chinese Military Aviation.

It probably should be noted that blade noise among Soviet/Russian submarine types dropped dramatically after Japanese electronics company Toshiba and Norwegian company Kongsberg sold advanced milling machinery and control equipment to the USSR in the mid 1980s. (And, of course, the treachery of John Walker).

Listening to and recording a submarine's unique sonar signature in peacetime can permit a navy to develop a library of what the other guy's submarines sound like, so much so that it is eventually possible to identify a submarine contact not only by its class or type, but by name!

3. Targeting

The final step in the ASW cycle is targeting, that is, developing a sufficiently accurate picture of the submarine&#8217;s location and movements to mount an attack that has a high probability of success.

In the targeting phase, final localization of a submerged target is often accomplished using active sonar (either active sonobuoys, echo ranging, or an active dipping sonar), or magnetic anomaly detection (MAD) gear. An aircraft that has localised a target will typically drop a smoke flare to mark its position, and then come around for a second pass to drop the weapon.

Also, as discussed in Part 2, there are a number of other sensors typically available to an ASW aircraft that can provide both detection and location of a submarine contact when it is contact is surfaced, snorkeling or traveling at a shallow depth, including visual means, radar, and infra-red (IR).

Active means of detection are not usually exercised during encounters between opposing submarines, where an attacking submarine will typically want to maintain stealth throughout the engagement.

B. ENGAGEMENT

1. Airborne Attack

Aircraft, and in particular, helicopters, are a God send to the ASW surface ship. They provide a way to search for, detect, locate, engage and destroy submarine targets at ranges well beyond the ship's own sensor envelope, while simultaneously avoiding the submarine's own weaponry.

Modern ASW helicopters also have their own suite of dedicated sensors, including surface search radar (useful for spotting periscopes and snorkels), MAD gear, dipping sonar, and/or sonobuoys, not to mention the means to destroy a submarine with lightweight torpedoes or depth charges.

When used in concert with shipboard sensors, especially long range passive towed arrays, the helicopter is a formidable ASW tool, perhaps the most formidable of them all. Its ability to hover precisely over a contact, dip a sonar, or tend a sonobuoy field, and operate from just about any ship that can host a helipad, is a testament to its flexibility. About the only weaknesses of the helicopter are its limited endurance, relatively slow speed, and small payload, but these are just small factors in comparison.

Where helicopters are lacking, fixed wing maritime patrol aircraft (MPAs) make up for it. They invariably have long endurance, considerable payload, and better speed.

2. Stand-Off Attack

A stand off attack against a hostile submarine is always the preferable method of engagement, whether this is having your helo dump lightweight torpedoes from a position far ahead of the formation; sending an ASROC on its ballistic trajectory; or launching a heavyweight torpedo on a dogleg course that exploits wire guidance and approaches from a different bearing.

The advantage of stand off attack lies principally with being able to rapidly engage a submarine outside the effective range of its own weapons, and sometimes, beyond the effective range of its own sensors.

3. Close Range Attack

A close range engagement with a submarine is a dangerous affair, one that more often than not ends up with both the hunter and the hunted switching positions.

An attack from a position in the target's baffles is one exception, in which case the attacker is already in the best possible position - the undersea equivalent of six o'clock high. If your opponent has a less capable passive sonar, maneuvering into an advantageous position is considerably easier and you may get much closer to him without being detected. If your passive sonar capabilties are about equal, however, then its really going to be quite a long, careful dance as you approach, followed by a frantic, desperate getaway.

The other close range option is a shipboard, "over the side" torpedo launch, in which case you are likely already in a worst case scenario. A hostile submarine has somehow managed to penetrate your screen and you are now trying to take last ditch, desperation measures to force him to maneuver by shooting your own torpedoes down the bearing of the contact (or worse, the bearing of an incoming weapon).

C. ANTI-SUBMARINE WARFARE (ASW) TACTICS

1. Tin Cans: The Surface Ship Sub Hunter

Since the noise of a surface ship formation (aka the "thundering herd") produces a lot of ambient noise, and may drown out or mask the noise of an enemy submarine, or otherwise degrade own passive sonar performance, the surface ships with the best passive sonars (typically those equipped with towed array sonars) typically operate at some distance away from the main body of the formation.

This stand off distance is typically about 10 to 30 nautical miles (nm), and frequently, out to the range of the first convergence zone (CZ). Units with less capable passive gear (e.g. a hull sonar but no towed array) tend to be stationed closer to the main body, with those ships equipped with only active sonar the closest of all.

ASW escorts often employ "sprint and drift" tactics. The "sprint" involves racing ahead at high or flank speeds to the leading edge of their assigned patrol sector, and then slowing to creep speed (typically below 10 knots, or slower) to conduct the "drift".

The sprint phase, of course, is noisy, but necessary in order to keep pace with the formation. The drift phase, however, cuts self-noise produced by revving engines, racing propeller screws, cavitation, and the noise of the hull passing through the water. In this fashion the ship can optimise the performance of its own passive sonar.

One might wonder why ASW ships bother to sprint at all, but rather, why not just creep from place to place to both optimise your own passive sonar and, at the same time, make the submarine's job of finding you harder?

Well, firstly, of course, the naval surface ship group is typically on a timetable and more often than not, a strict one. HQ wants you to put that group on station in a particular place, establishing force presence, conducting sea control, or pounding the enemy, and the guy right above you in the pay scale no doubt wants it done yesterday. And since the primary objective of your surface group is rarely blue water ASW, there are other more pressing concerns that require you to move at best speed. Sprinting will allow your ASW ship to thus keep up with the "thundering herd".

Secondly, there is truly an ASW method to the sprint and drift madness. Placing the most capable ASW assets on stations that are well removed from the noise of the main body, and in locations that either flank or are ahead of your group's intended path, puts those assets in the best possible position to detect enemy submarines.

In most cases, diesel-electric submarines have little opportunity for engaging a surface group from the sides or rear. They simply lack the submerged speed and endurance to do so. And, while nuclear powered submarines have both the speed and the endurance, approaching a surface group from the sides or rear would necessitate high speed maneuvering.

Speed, as we have already discussed, is noisy and degrades own sonar performance. Simply put, in the subsurface battlefield, noise kills. Therefore, "nuke boats" will also try to place themselves in the path of an enemy surface group, again right where your ASW escorts hope to find them.

 

[Manticore]

2. Knife Fight in a Phone Booth: Submarine versus Submarine

The baffles are the area around a vessel in which its sonar is ineffective, forming a blind spot or dead zone. For most ships and submarines, this involves a cone about 15 degrees wide extending aft from the stern, in water disrupted by the screws and the vessel’s own passage.

Image credit: Dan Short (DanMS).

An effective tactic for submarines, especially, is to close on a target, in the area of its baffles, and then shoot a torpedo from close range. There is virtually no opportunity for counter-attack or escape. Nuclear submarines, with their near limitless endurance and high speed capability, are especially good at exploiting this tactic. Even when you have no intention of attacking, placing your vessel in the baffles of the enemy can permit you to track him, trail him, and remain undetected.

There are a few tactics which can help defeat this approach. Turning your ship, or submarine, suddenly through 90 to 360 degree turns (the famous "Crazy Ivan") will place your sensors so as to listen down your former baffles.

In the following image, a Soviet Project 671RTM (NATO Victor III) class nuclear attack submarine has turned to port to check his baffles. The trailer, a US Navy Sturgeon (SSN-637) class boat, having detected the "Crazy Ivan" maneuver, has stopped his screw and gone quiet in an effort to maintain the subterfuge. If he is successful, the Victor III will make a complete 360 degree turn, around the Sturgeon, and then return to his original course, having never detected the drifting American sub. (Also visible is the Sturgeon's streamed towed array).

Image credit: Jim Christley, "Trailing", subart.net

On the down side, any abrupt course change will also produce a pocket of disturbed water, called a "knuckle" that will act as a blind spot for both active and passive sonar systems. Scott Gainer says it still beats a torpedo enema anyday, though, and its difficult to disagree.

If more than one ship is traveling together, they can periodically turn and search each others' baffles using a crossing maneuver. And, when on the attack, one ship can stand off and conduct a search while the other deploys ASW weaponry, in order to avoid the submarine slipping under and escaping through the attacker's baffles, or during "blue outs" caused by weapon detonations (a "blue out" is a disruption of the sound path caused by the loud and sudden release of acoustic energy and bubbles in an underwater explosion).

Sonobuoys, dropped from the air or from ships, and helicopter dipping sonar employed to the rear of a formation, in the area of its baffles, can together provide a good plan to counter any approach from the rear.

3. Most Feared: the ASW Aircraft

The aircraft - whether fixed wing or rotary wing (helicopter) - is without a doubt one of the best tools in the ASW arsenal, perhaps the penultimate "force multiplier" from an ASW perspective. While the traditional ASW hunters - surface ships and submarines - have an equal (or better) chance of becoming the "hunted", the aircraft is perhaps the only weapon against which the submarine has little recourse (though even this is changing, more on that later).

Even for aircraft, however, the task of searching a vast ocean for a small, likely submerged, target is a hugely daunting task. And, as I've mentioned before, it can be pretty boring until you actually find something.

The principal advantage of an aircraft in the ASW role is speed, as compared to other platforms: speed in getting to the patrol area, speed in searching that area, and speed in prosecuting a contact.

Helicopters

Commensurate with the expanding size of the Soviet submarine fleet in the 1950s came a realization within the US Navy that its own increasing sonar detection ranges were outstripping its ASW engagement range. In particular, the RUR-5 ASROC (Anti-Submarine Rocket) then under development would not have the range to take advantage of the capability of the SQS-26 hull sonar.

A solution was found in the QH-50 DASH (Drone Anti-Submarine Helicopter), a small coaxial rotor equipped and unmanned helicopter which could operate from ships too small to have extensive aviation facilities, and moreover, operate in bad weather, even up to Sea State 6. (Something helo pilots did not (and do not) look forward to). Spooling up and taking off within two minutes of engine start, the DASH could deliver two Mk 44 lightweight homing torpedoes or a single Mk 17 nuclear depth charge to a sonar contact up to 22 miles away.

A QH-50A DASH operating off the Fletcher class destroyer USS Hazelwood (DD-531)

Image credit: Gyrodyne Helicopters.

To illustrate the urgency of the Navy's desire for ASW stand-off weapons, production of the DASH was authorised a full year before the first model had ever taken to the air. By late 1963, funding had been approved for production of three QH-50C aircraft for each of the Navy's 240 FRAM I and II destroyers.

High attrition rates and the intervention of a non-ASW war in Southeast Asia (Vietnam, of course) led to the premature demise of the program in 1970, but the versatility of the shipboard ASW aircraft had clearly been established.

Ship based helicopters continue to be one of the most useful and flexible of ASW platforms, especially in terms of their multi-mission nature and ability to move out ahead of their host ship, thereby extending the range of the defensive zone. A modern naval helicopter can be packed with ASW relevant sensors - sonobuoys, a dipping sonar, radar, FLIR, ESM, etc - and can carry enough weaponry for one or two engagements, typically a pair of lightweight torpedoes.

The dipping sonar is the ASW helicopter's forte - the ability to hover right above a suspected enemy submarine, hammer it with active sonar, and thereby precisely determine its position as a prelude to a weapon drop. As we've learned throughout this Tactics 101 discussion, a submarine whose stealth has been compromised can quickly find itself in a world of hurt.

Active dipping sonar deployed from helicopters are the answer to the threat from quieter submarines in coastal areas by employing much lower frequencies, coupled with new transducer and beamforming technology.

Cold War ASW Icon: the P-3 Orion

The Lockheed P-3 Orion is perhaps the iconic symbol of airborne ASW during the Cold War era, certainly setting the standard for NATO maritime patrol and reconnaissance. A testament to the importance placed on the Orion's role by the US Navy was the fact that, at the height of the Cold War, the Navy operated about 26 active Patrol Squadrons (each with nine aircraft) and 13 Reserve Squadrons.

Although it first flew in 1958, the Orion continues to form the mainstay of US Navy fixed wing ASW and maritime patrol and reconnaissance capabilities, although in recent years the emphasis has shifted more toward the intelligence, surveillance and reconnaissance (ISR) role. (By 2001, for example, the previously mentioned squadron numbers had been cut to less than half). More on this in a bit.

The P-3 Orion with its weapons bay doors open

Image credit: Global Aircraft.

A P-3 Orion crew typically consists of the following members:

* Three pilots, designated 1P, who is the aircraft commander and makes the tactical decisions; 2P, who monitors the aircraft's systems; and 3P, who is a new Orion pilot and comes straight from the Fleet Readiness Squadron;
* Two flight engineers;
* Tactical coordinator (TACCO);
* Navigator/communications operator;
* Two acoustic operators (Sensor 1, responsible for active sonobuoys, and Sensor 2, responsible for passive sonobuoys, although there is a lot of cross over);
* Non-acoustic operator (Sensor 3); and an
* In-flight tech/ordnance crewman.

Why three pilots? Flying at low level over the ocean for long periods of time, monitoring a myriad of instruments and maneuvering constantly to put the aircraft in a favourable position for the detection of submarines, is hard work. Three pilots are in fact probably the minimum necessary for most missions. Fortunately, the cockpit is spacious. During night missions, a curtain separates the cockpit from the remainder of the cabin, allowing the flight crew to concentrate on their instruments.

Moving toward the rear, the TACCO's station just outside the cockpit on the left. He recieves information from the three sensor operators and passes it to the cockpit so that the pilots can position the aircraft advantageously for an attack or sonobuoy drop. The TACCO is responsible for search tactics and tactical control. His console is dominated by a large screen showing the locations of sonobuoys, surface contacts, and the aircraft, as well as possible submarine contacts.

Sonobuoy information used to be recorded on two 16 channel AQH-4 analog magnetic recorders, each weighing nearly 300 lbs and providing only two hours' recording time, but this was replaced by an 80 lb AQH-13 system using Digital Tape Format (DTF) cassettes that offer four hours of recording time per cassette.

On the TACCO's right hand side is the Navigator/Comms, who is responsible for all navigation and communications duties.

Further down on the right side sits Sensor 3, the operator who processes information gathered by the radar (the APS-137 is the most recent type), forward looking infra-red (FLIR), the MAD and ESM. His console has a large radar display for the APS-137, which has four modes: periscope; weather; surface search; and navigation; and can effectively track 32 contacts. Right above the radar display is the FLIR display.

Behind Sensor 3 on the left and seated facing “out” of the P-3 (all other crew members sit facing forward) are Sensors 1 and 2, who process sonobuoy data from the sonobuoys. All of the information received from the sonobuoys is channeled into the UYS-1 Single Advanced Signal Processor (SAPS). Although Sensor 1 is technically responsible for passive buoys and Sensor 2 for active types, both can process the information from either type. A maximum of 32 sonobuoys can be monitored simultaneously (it used to be only four on the original P-3A), but only if they are all passive. Returns from the buoys are displayed on a screen, looking much like a “green snow blizzard”. When not actively engaged in ASW efforts, both Sensors 1 and 2 act as observers looking out of the aircraft’s windows and as such, both are also qualified photographers.

The locations of sonobuoys dropped by the aircraft must be constantly updated, since getting a target bearing from a buoy is only useful if the correct location for the buoy is known. In order to correctly plot a buoy's position for the benefit of the TACCO, it is necessary to take into consideration the aircraft's altitude, air speed, and the wind characteristics. The drift of the sonobuoy is reported by an On Top Position Indicator (OTPI), which reports the information back to the aircraft. Information collected by the buoy, either from active (“pinging”) or passive acoustic means, is transmitted to the aircraft in the VHF band (over a number of channels). The TACCO uses the ASQ-114 digital computer, with its memory loaded with a large number of submarine acoustic profiles and radar and radio signals for ESM, to identify targets.

Halfway along the aircraft is the sonobuoy rack and chutes, with an observation position on either side of the fuselage. This is the domain of the In-flight Tech and Ordnance crew member, who is responsible for in-flight repairs of the electronics, and for preparing sonobuoys for drop. During the aircraft's transit to the patrol area, he inserts the Cartridge Actuated Device (CAD, a pyrotechnic firing device that launches buoys from their tubes, leaving behind the smell of cordite and a little smoke) and "channelizes" (assigns VHF channels) the sonobuoys in their rack.

The P-3C can carry 84 sonobuoys, of which 48 are pre-loaded from the outside before takeoff and 36 are carried in the cabin. In the cabin there are three "A size" launch tubes and one larger "B size" tube (for use without cabin pressure).

Behind the sonobuoy rack area is the crew rest position and galley.

So how has the Orion's mission changed since the Soviet submarine threat has all but vanished?

Turns out the P-3 Orion is more valuable than ever, but in a different role than traditional ASW and maritime patrol, as illustrated by the fact that it was responsible for shooting perhaps a couple dozen Standoff Land Attack Missiles (SLAMs) over the Balkans during Operation Allied Force in 1999 and during Operation Enduring Freedom in 2001; and as well, during Operation Iraqi Freedom in 2003, Orions were engaged in supporting the advance of ground forces toward Baghdad, warning them of enemy activity ahead, locating enemy armored vehicles at night, and making the initial detection of the burning oil fields at Ramallah; supporting US Navy SEAL and British Royal Marine commando operations to seize Iraqi oil terminals before they could be sabotaged; were involved in the rescue operation for captured soldier Pfc Jessica Lynch; aided in intercepting ships trying to smuggle oil out of Iraq; and provided targeting to a USAF AC-130 gunship so it could destroy some Iraqi patrol boats.

Submarine Self-Defense against Air Attack

How can a submarine defend itself from air attack?

Historically, or at least since the tide turned during the Battle of the Atlantic, a submarine's only defense has been to "run silent, run deep". But in any case, run. Gone are the days of the U-boat's Turmumbau flak guns.

The idea of submarine self-defense against aircraft never went away, however. The Soviets, for example, have been known to equip their diesel-electric types, particularly the Kilo (Project 877/636) class, with shoulder launched, short range heat seeking missiles, due to concern that they might be caught on the surface. The launcher and missiles are typically stored in a watertight container located between the snorkel and the radio antenna masts in the sail. The earlier Type 877 had a SA-N-5 (Strela) launcher and 8 missiles, while the Type 636 has the more capable SA-N-8 (Igla-1M) launcher and six missiles.

There have also been several efforts aimed at coming up with a way of launching a missile from a submerged submarine against a hostile aircraft. For example, the American DARPA program of the late 1970s for a Self-Initiated Anti-aircraft Missile (SIAM). More recently, the German Navy looks to adopt the IDAS (Interactive Defence and Attack System for Submarines): a fibre optic guided adaptation of the air launched IRIS-T short range missile, which can be launched from a torpedo tube and is due to arm Germany's Type 212 submarines from 2014.

The IDAS missile breaking the surface after launch from the Type 212 sub U-33

Image credit: Aviation Week.

D. ENDGAME: EVASION, DECOYS, AND COUNTERMEASURES

It goes without saying, of course, that a submarine's best defense is to stay undetected. And if detected, a submarine hopes to break contact as soon as possible and disappear once more. You can't kill what you can't find, and all that.

However, once firmly discovered, a submariner's life can become a frantic race for survival. (Many hours of Silent Hunter 3/4 have reinforced that point with me ). As with many other ASW technologies, the techniques and tools of evasion, decoys, and countermeasures have their origin in World War II.

1. Submarine Decoys and Countermeasures

From about 1942 onward, just as Germany beginning to lose its iron grip on the Battle of the Atlantic, the Kriegsmarine introduced the Pillenwerfer (or BOLD) decoy. This was a metal can or tube about four inches in diameter and filled with a alkaline metal (calcium or lithium) hydride. When released from a U-boat and exposed to seawater, a chemical reaction released large amounts of hydrogen that poured out of the container in thousands of gas bubbles and thereby, created a false sonar target. A hydrostatic valve held the device at a depth of about 100 feet, and permitted the effect to continue for about 20 to 25 minutes.

The principal limitation of Pillenwerfer, however, was that if an attacker could see both the stationary decoy and the moving submarine at the same time, it could differentiate between the two (the decoy lacking Doppler shift). Furthermore, the slow moving U-boats found it difficult to quietly slip away before the decoy expired.

A solution was found in a further German submarine decoy technology called Sieglinde. Powered by electric motors that allowed it to move at about 6 knots, as well as change depth, this decoy more accurately simulated a moving submarine. Used in combination with Pillenwerfer, this was a more effective method of allowing the real U-boat to escape.

In the post war period, the US Navy began work in earnest on developing a suite of submarine countermeasures, including acoustic intercept receivers which would automatically detect sonar signals, including the ping of actively homing torpedoes, over the full frequency range. Sonar jamming was also developed, much in the same way as electronic warfare is aimed against radars and radios, using both noise and deception (echo repeater) techniques. The most success has been obtained, however, in the field of expendable countermeasures.

In the modern era, a submarine facing an air dropped, acoustic homing torpedo has little opportunity to sneak away and must act quickly just to survive. An airborne lightweight torpedo may be dropped only a few hundred yards away, or less, and in many cases, will immediately go into active search and homing mode. Today's submarines are therefore typically equipped with a comprehensive expendable countermeasures system that combines both decoys (i.e. acoustic jammers) and mobile submarine/target simulators.

The decoys or jammers are ordinarily accommodated within their own individual launch tubes, and are ejected by compressed air, in either a manual or automatic (computer controlled) mode.

The US Navy's ADC (Acoustic Device Countermeasure) Mk 1, essentially an improved version of Pillenwerfer, was an expendable acoustic countermeasures device (an "ensonification bubbler") running on a saltwater battery, and weighing about 19 kg. It was introduced in the early 1970s. It has been followed by electronic decoys that actively emit an acoustic signal as a counter to homing torpedoes, beginning with the ADC Mk 2. These use a small, shrouded propeller to permit the decoy to "hover" in the water at a pre-selected depth. The Mk 2 has been followed (predictably) by the Mk 3, Mk 4, and Mk 5, which offer increasingly advanced signal generation. The British Royal Navy's submarine service, meanwhile, has used such decoys as the Type 2042 and Type 2066 Bandfish.

The Mk 57 Mobile Submarine Simulator (MOSS) is a 10 inch wide, mobile decoy that weighs about 1,000 lb and can only be launched through a torpedo tube rather than through a dedicated decoy launcher. Entering service in 1979, it was originally intended to protect ballistic missile submarines, with the Trident SSBNs carrying six decoys and the attack subs carrying four. And, because a MOSS might need to be launched at any time, one torpedo tube was usually kept empty and available. The MOSS has since been replaced by the six inch EX-10 Mobile Multi-function Device (MMD), which can be fired from a countermeasures tube. Soviet/Russian equivalents include the MG-74 and Berilly systems.

2. Surface Ship Decoys and Countermeasures

If an enemy submarine cannot be killed or avoided (including by such means as reducing one's acoustic signature, as with Prairie Masker described in Part 2), then for the surface ship it becomes a matter of torpedo defense. This can include maneuvers to complicate a submarine's fire control (such as the zig zag course taken by Allied convoys during WWII) and deployment of decoys and countermeasures.

The first of these was the British Foxer decoy of World War II, introduced in reply to the introduction by Germany's U-boat force of the G7es (T-5 Zaunkoning) homing torpedo in late 1943. There was nothing terribly sophisticated by Foxer. It was an arrangement of hollow metal pipes with holes cut into them, which was then towed about 500 feet behind a host Allied ship. To an early generation passive acoustic seeker, the noise produced by water rushing through the holes, and by the pipes banging together, made for a more attractive target than the ship's propellers.

The obvious disadvantage of Foxer, of course, was that constantly towing the decoy created more noise than might otherwise be produced by the ship (or the convoy it was escorting), thereby potentially attracting the attention of U-boats. After the war, the T.Mk 6 Fanfare was introduced, which more accurately simulated the noise of a ship's propeller rather than just produce broadband noise.

The most widely deployed towed torpedo decoy since that time has been the SLQ-25 Nixie (or variants thereof), which first entered service in 1974 and introduced improved deceptive countermeasures. The decoy or "fish", measuring about 37 inches long, six inches in diameter and weighing about 46 lb, is towed at the end of a 1,600 foot cable, and can receive the incoming torpedo's active sonar pings, amplify them, and then return the signals to the torpedo to lure it away from the ship. As with towed array sonars, it is generally unwise to tow the decoy at high speeds. Typically two SLQ-25 decoys are ready to be deployed (or "streamed"), in case one is destroyed by a successfully decoyed torpedo.

SLQ-25 Nixie torpedo countermeasures equipment aboard the USS Iowa (BB-61)

Image credit: US DoD.

Some torpedoes aren't easily decoyed by acoustic means - for example, the wake homers. For this reason, US Navy aircraft carriers can have a pipe lattice structure at their stern to produce a larger than normal wake: this reduces the effectiveness of torpedoes like the giant Russian Type 65 by forcing them to track back and forth across a much larger area, burning up valuable fuel (and thereby reducing their range) in the process.

Wake homing torpedo guidance

Image credit: US DoD

Other torpedo countermeasure programs have have sought to take advantage of the ubiquitous Mk 36 Super Rapid Blooming Offboard Chaff (SRBOC) launcher system fitted fleet wide in the US Navy, and widely exported to other NATO and allied navies. Ordinarily used to deploy chaff and infra-red decoys to defeat radar and anti-ship missiles, these 130mm launchers can now also use the Mk 13/14 Launched Expendable Acoustic Device (LEAD), a deploying a pattern of these decoys to seduce homing torpedoes.

3. The Hard Kill Option

In more recent years, navies have sought to develop a suite of integrated torpedo countermeasures that could automatically detect incoming torpedoes, deploy decoys or jammers to seduce or confuse them (the "soft kill" method), and if necessary, deploy anti-torpedo systems that could destroy them (the "hard kill" method). Examples of these efforts include the European SLAT (Systeme de Lutte Anti-Torpille) and the US-UK Surface Ship Torpedo Defense (SSTD) system.

The most difficult aspect of these kinds of programs has invariably been the hard kill element. One might wonder why it is tough to destroy an incoming torpedo. After all, we have developed the means to destroy supersonic sea skimming missiles, ballistic missiles, even satellites moving in orbit at 18,000 mph.

The answer: it is a fire control problem. Fire control depends on accurately predicting the position of an incoming weapon at the time of intercept by the defending projectile. That said, the detect to engage sequence for an underwater weapon is vastly more complex than that for airborne missiles. The ocean environment makes it significantly more difficult to receive and process data at a sufficiently high fidelity because of the speed at which information can be transferred underwater. Propagation speeds (radio frequency or infra-red signals in the air versus acoustic signals in the water) are nearly 200,000 times slower, so information is received significantly more slowly for a torpedo than for an airborne threat.

The development of an effective hard kill anti-torpedo system in the US Navy has been long and tortuous, evolving from the aforementioned US-UK SSTD program of late 1988 into what may eventually be achieved with the WSQ-11 Tripwire in fiscal year (FY) 2011. It is worthy of note, in comparison, that the Russian UDAV-1/RPK-5 Leevyen (Heavy Rain) or RBU-10000/12000, which combines a torpedo defense system of acoustic decoys, depth charges suspended by buoys, and explosives, has been around since 1989. The Israeli Scutter/Torbuster is another emerging system.

One unique system that has been under consideration came out of the DARPA Water Hammer mine countermeasures program in 2005. This would use explosives to generate a low frequency acoustic pulse in a sequence of shock tubes, which would in turn focus, amplify, and direct the pulse into the surrounding water over a narrow bearing, thereby creating a high pressure (around 2,000 psi) directional shock wave. This pulse could not only disrupt mines but would probably disrupt or destroy incoming torpedoes.

CONCLUSION

To quickly summarize, then, the ASW cycle requires detection of a submarine, location of its position, identification, targeting, and engagement. Its not at all as simple as that, of course, since each step in the process has its own quirks and is influenced by myriad factors, including the type of sensors at work, the nature of the target, and the tactics employed by both the hunter and the hunted.


Source references:
U.S. Submarines Since 1945, Norman Friedman, 1994.
Jane's Navy International, November/December 1995.
Naval Institute Guide to World Naval Weapons Systems, 1997-98.
"What me worry? - The current state of surface ship torpedo defense". Vining, P. USNI Proceedings, 1999.
The Third Battle: Innovation in the US Navy's Silent Cold War Struggle with Soviet Submarines, Dr. Owen R. Cote Jr., March 2000.
Jane's Information Group: August 1999; June 2003; September 2005.
Journal of Electronic Defense, March 2001.
ASW after the Cold War, Owen Cote and Harvey Sapolsky, MIT Security Studies Program, April 2001.
Air Forces Monthly, May 2001; December 2003.
Proceedings, June 2002.
National Defense, January 2003.
World of Defence, UDT, Issue No.2, 2004.
U.S. Destroyers, Norman Friedman, 2004.
Navy Times, August 2005.
SOSUS: The "Secret Weapon" of Undersea Surveillance, Edward C. Whitman, Undersea Warfare, Winter 2005.
Principles of Naval Weapon Systems, Craig Payne, 2006.
Not Ready for Retirement: The Sonobuoy Approaches Age 65, Holler et al., Sea Technology, November 2006.
Harpoon 3 Sonar Model, AGSI, 2007.
Proceedings, June 2007.
ES310, Introduction to Naval Weapons Engineering.
Ocean Talk, Naval Meteorology and Oceanography Command.
Gyrodyne Helicopters.

 

[Manticore]

credits

Tactics 101: Anti-Submarine Warfare (ASW) - Part 1 - General - HarpGamer
Tactics 101: Anti-Submarine Warfare - Part 3 - General - HarpGamer
Tactics 101: Anti-Submarine Warfare (ASW) - Part 2 - General - HarpGamer
Modern ASW

 

[Penguin]

Great series!

 

[Manticore]

yes! I found it by chance

 

[Jguo]

Absolute Excellent Read, thank you for posting!

 

[Johnlaw]

Nowaday submarine doesn't need to get any where near its target. It can simply launch cruise missiles tens of kms away. It can also dive much deeper. So traditional sonar won't be able find it. Even if it is found, it can dive and depth charge won't be any use. It would be submarine vs submarine I guess.

 

[AUSTERLITZ]

Hardcore stuff.Thanks.

 

[jaibi]

@Manticore great read.