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Tactics 101: Anti-Submarine Warfare (ASW) - Part 2


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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.


War never changes (there's my Fallout 3 quote of the day :P), 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.


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”. :D

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: www.naval-technology.com, 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.

SQR-19 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
Image credit: National Oceanic and Atmospheric Administration (NOAA).

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.


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).


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.


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
Image credit: Unknown.

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.

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.


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: www.rusarmy.com

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.


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.

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.


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. :P

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.


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.

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Thank you.

You have a knack for writing solid articles.

I hope you keep at it,

Cheers !!

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