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GLOSSARY OF MODERN UNDERSEA-WARFARE TERMS AND CONCEPTS
by JOE BUFF, copyright 2000, 2001, 2002, 2003

Note: Each of my novels includes a glossary at the back. This JoeBuff.Com Website glossary is a merger of all those different entries. Personally, I love to read glossaries! I hope you find this one here interesting and fun. If you have any questions, please do e-mail me by hitting the CONTACT button. All of these terms and definitions are real, by the way, not made-up “fiction,” and are based on open-source naval reference literature -- see the Bibliography info on my website for further reading...

Yours truly, from a fellow submarine enthusiast and SEAL fan,

--Joe Buff


GLOSSARY:

A / B / C / D / E / F / G / H / I / J / K / L / M / N / O / P / Q / R / S / T / U / V / W / X / Y / Z

-A-

Acoustic holography:
A technique for studying in detail a complex three-dimensional sound field using sophisticated signal processing mathematics applied to data from a rigid planar array of hydrophones. The sound field from a target source or sources may vary over time as to both spatial arrangement and signal strength. A submarine’s wide-aperture arrays (see below) may serve as the planar array of hydrophones.

Acoustic Intercept:
A passive (listening only) sonar specifically designed to give warning when the submarine is “pinged” by an enemy active sonar. The latest version is the WLY-1.

Active out-of-phase emissions:
A way to weaken the echo which an enemy sonar receives from a submarine’s hull, by actively emitting sound waves of the same frequency as the ping but exactly out of phase. The out-of-phase sound waves mix with and cancel those of the echoing ping.

ADCAP:
Mark 48 Advanced Capability torpedo. A heavyweight, wire guided, long-range torpedo used by American nuclear submarines. The Improved ADCAP has even longer range, and an enhanced (and extremely capable) target homing sonar and software logic package.

AIP:
Air Independent Propulsion. Refers to modern diesel submarines that have an additional power source besides the standard diesel engines and electric storage batteries. The AIP system allows quiet and long-endurance submerged cruising, without the need to snorkel for air, because oxygen and fuel are carried aboard the vessel in special tanks. For example, the German Klasse 212 design uses fuel-cells (see below) for air independent propulsion.

Alumina casing:
An extremely strong hull material which is less dense than steel, declassified by the U.S. Navy after the Cold War. A multi-layered composite foam matrix made from ceramic and metallic ingredients.

Ambient sonar:
A form of active sonar that uses, instead of a submarine’s pinging, the ambient noise of the surrounding ocean to catch reflections off a target. Noise sources can include surface wave-action sounds, the propulsion plants of other vessels (such as passing neutral merchant shipping), or biologics (sea life). Ambient sonar gives the advantages of actively pinging but without betraying a submarine’s own presence. Advanced signal processing algorithms and powerful onboard computers are needed to exploit ambient sonar effectively.

Antarctic convergence:
The area in the Great Southern Ocean (the name given to the seas surrounding Antarctica) where warmer water from more temperate climates first meets the cold water nearer the Antarctic. The result is a zone of dramatically unpredictable weather, and of confusing sonar conditions.

ARCI:
Acoustic Rapid COTS Insertion. The latest software system designed for Virginia-class fast attack submarines (see below) -- COTS stands for commercial-off-the-shelf. The ARCI system manages sonar, target tracking, weapons, and other data, through an on-board fiber optic local area network (LAN). The ARCI replaces the older AN/BSY-1 systems of Los Angeles-class submarines, and the AN/BSY-2 of the newer Seawolf-class fast-attack subs.

ASDS:
Advanced SEAL Delivery System. A new battery-powered minisubmarine for the transport of SEALs (see below) from a parent nuclear submarine to the forward operational area and back, within a warm and dry shirtsleeves environment. This permits the SEALs to go into action well-rested and free from hypothermia, real problems when the SEALs must swim great distances or ride on older free-flooding SDVs (see below).

ASW:
Anti-Submarine Warfare. The complex task of detecting, localizing, identifying, and tracking enemy submarines, to observe and protect against them in peacetime, and to avoid or destroy them in wartime.

Auxiliary Maneuvering Units:
Small propulsors at the bow and stern of a nuclear submarine, used to greatly enhance the vessel’s maneuverability. First ordered for the USS Jimmy Carter, the third and last of the Seawolf-class SSNs (nuclear fast-attack submarines) to be constructed.

-B-

Baud Rate:
The data transmission rate available over a particular communications link, whether radio of laser-beam or acoustic. The baud rate is often measured in bits (or bytes) per second. Higher is better, in that a given message or data dump can be sent or gotten faster, and/or a larger amount of imagery and other information can be transmitted or received in a given, fixed amount of time. To receive live color video imagery, for instance, a very high baud rate antenna is required, and the submarine must raise a special antenna mast above the surface, thus revealing its location. To receive simple Morse code very slowly, in contrast, a low baud rate suffices, and the latter can be achieved by a stealthy wire antenna trailed well below the ocean's surface by a submerged submarine, listening on extremely low frequency radio. (New types of ELF antennas are being developed which are small and fit on the submarine's hull, eliminating the need to trail this wire.)

Bi-polar sonar:
A form of active sonar in which one vessel emits the ping while one or more other vessels listen for target echoes. This helps disguise the total number and location of friendly vessels present.

Brilliant Decoy:
A device launched from a submarine's torpedo tube, which is intended to conspicuously mimic the maneuvering and noise signature of the parent sub. The purpose is to draw an enemy's attention away from the parent sub, allowing the parent sub to sneak away from the battle area undetected, or to escape enemy torpedoes because those torpedoes will be lured toward the decoy. A decoy contains a propulsion system and fuel supply like a regular torpedo. But the decoy is unarmed, and the space available because there is no explosive warhead is used instead for additional computers and sonar emitters. This gives the decoy the ability to be programmed -- before launching -- with complex instructions regarding how to behave regarding changes in depth, course, speed, etc., and also to give off appropriate noises like a real full-sized submarine. The decoy might be programmed to act like a different class of sub than the parent, to further confuse and distract the enemy. For instance, a Royal Australian Navy submarine near Chinese waters might launch a decoy programmed to sound and behave like a Russian Akula. The term "brilliant" refers to an advanced state of on-board artificial intelligence routines that allows the decoy to make autonomous real-time decisions once launched that further aid the tactical goals of its parent sub during combat or intelligence-gathering missions. (Although the primary concept of a decoy is defensive, i.e. enhancing escape-and-evasion by the parent sub, decoys can also be used offensively. They can trick an enemy sub or ship into an ambush, in which the parent sub then fires a live torpedo at the enemy from an unexpected location as a surprise attack.)

-C-

CACC:
Command and Control Center. The modern name for a submarine’s control room.

CAPTOR:
A type of naval mine, placed on or moored to the seabed. Contains an encapsulated torpedo, which is released to home on the target.

CCD:
Charge-Coupled Device. The electronic “eye” used by low-light-level television, night vision goggles, etc.

CERTSUB:
A certain hostile submarine contact.

COB:
Chief of the Boat. (Pronounced like “cob.”) The most senior enlisted man on a submarine, usually a master chief. Responsible for crew discipline, and for proper control of ship buoyancy and trim, among many other duties.

-D-

Deep scattering layer:
A diffuse layer of biologics (marine life) present in many parts of the world’s oceans, which causes scattering and absorption of sound. This can have tactical significance to undersea warfare forces, by obscuring passive sonar contacts and causing false active sonar target returns. The layer’s local depth, thickness, and scattering strength are known to vary by one’s location, the sound frequency being observed, the season of the year, and the hour of the day. The deep scattering layer is typically several hundred feet thick, and lies somewhere between 1000 feet and 2000 feet of depth during daylight, migrating shallower at night.

Deep Sound Channel:
A thick layer within the deep ocean in which sound travels great distances with little signal loss. The core (axis) of this layer is formed where seawater stops getting colder with increasing depth (the bottom of the thermocline, see below) and water temperature then remains at a constant just above freezing (the bottom isothermal zone, see below). Because of how sound waves diffract (bend) due to the effects of temperature and pressure, noises in the deep sound channel are concentrated there and propagate for many miles without loss to surface scattering or seafloor absorption. Typically the deep sound channel is strongest between depths of about 3000 and 7000 feet.

-E-

Elastomer membrane water slug:
A method, for quietly shooting a torpedo from a torpedo tube, which is powered by using a large, very strong, and stretchable plastic disk inside a tank within the submarine to store the potential energy of ambient sea pressure itself. This technique can be less noisy than current alternatives using electric or hydraulic turbines, or compressed air, to drive the slug of water which gives the torpedo a starting shove on its way. (The torpedo’s own engine then propels it to the target.)

ELF:
Extremely Low Frequency. A form of radio which is capable of penetrating several hundred feet of seawater, used to communicate (one-way only) from a huge shore transmitter installation to submerged submarines.

EMBT blow:
Emergency Main Ballast Tank blow. A procedure to quickly introduce large amounts of compressed air (or fumes from burning hydrazine) into the ballast tanks, in order to bring a submerged submarine to the surface as rapidly as possible. If the submarine still has propulsion power, it will also try to drive up to the surface using its control planes (called planing up).

EMCON:
Emissions Control. Radio silence, except also applies to radar, sonar, laser, or other emissions that could give away a vessel’s presence.

EMP:
Electromagnetic Pulse. A sudden, strong electrical current induced by a nuclear explosion. This will destroy unshielded electrical and electronic equipment and ruin radio reception. There are two forms of EMP, one caused by very-high-altitude nuclear explosions, the other by ones close to the ground. (Mid-altitude bursts do not create an EMP). Non-nuclear EMP devices, a form of modern non-lethal weapon, produce a similar effect locally by vaporizing clusters of tungsten filaments using a high-voltage firing charge. This generates a burst of hard X-rays, which are focused by a depleted-uranium reflector to strip electrons from atoms in the targeted area, creating the destructive EMP current.

ESGN:
The latest submarine inertial navigation system (see INS below). Replaces the older SINS (Ship’s Inertial Navigation System).

-F-

Fathom:
A measure of water depth equal to six feet. For instance, 100 fathoms equals 600 feet.

Firing solution:
Exact information (or best estimate) on an enemy target’s location, course, and speed, and depth or altitude if applicable. A good firing solution is needed to preprogram the guidance system of a missile or torpedo so that the weapon won’t miss a moving target.

Floating wire antenna:
A long, buoyant antenna wire which is trailed just below the surface by a submerged submarine, for stealth. Such an antenna can receive data at a higher baud rate than ELF radio (see above). Recently, floating-wire-antenna technology has been developed to the point where the wire is able to transmit as well as receive, allowing two-way radio communication while the submarine is completely submerged. (To transmit or receive radio data at a very high baud rate, such as live video imagery of a target, the submarine must come to periscope depth and raise an antenna mast out of the water, which might compromise stealth.)

Frequency-agile:
A means of avoiding enemy interception and jamming, by very rapidly varying the frequency used by a transmitter and receiver. May apply to radio, or to underwater acoustic communications (see gertrude below).

Frequency Power Spectrum:
A display of the relative strength of noise being detected by a sonar array at different sound frequencies. Such data can be valuable in locating and identifying passive sonar contacts, especially when tonals (see below) stand out within the display.

Frigate:
A type of ocean-going warship smaller than a destroyer.

Fuel cell:
A system for quietly producing electricity, for example to drive a submarine’s main propulsion motors while submerged. Hydrogen and oxygen are combined in a reaction chamber as the “fuels.” The byproducts, besides electricity, are water and heat.

-G-

Gertrude:
Underwater telephone. Original systems simply transmitted voice directly with the aid of transducers, and were notorious for short range and poor intelligibility. Modern undersea acoustic communication systems translate the message into digital high-frequency active sonar pulses, which can be frequency-agile for security. Data rates well over 1000 bits per second, over ranges up to thirty nautical miles, can be achieved routinely.

Gravimeter:
A device which measures the gravity field gradients around a submarine. Using special mathematics, a detailed, three-dimensional map of local sea floor terrain is then displayed in real time on a computer display screen. Gravimeters make no emissions, and their operation cannot be detected by an enemy. Gravimeters cannot detect moving objects, but they are immune to bad sonar conditions.

-H-

Halocline:
An area of the ocean where salt concentration changes, either horizontally or vertically. Has important effects on sonar propagation and on a submarine’s buoyancy.

Hertz (or Hz):
Cycles per second. Applies to sound frequency, radio frequency, or alternating electrical current (AC).

Hole-in-ocean sonar:
A form of passive (listening only) sonar which detects a target by how it blocks ambient ocean sounds from further off. In effect, hole-in-ocean sonar uses an enemy submarine’s own quieting against it.

Hydrophone:
An underwater listening device. In essence a hydrophone is a special microphone placed in the water. The signals received by hydrophones are the raw input to passive (listening-only) sonar systems -- active sonar return echoes off targets (“pings”) are also picked up by hydrophones. Signal-processing algorithms then continually analyze this raw data to produce meaningful tactical information -- such as a firing solution (see above).

HUD:
Head-Up Display. Laser holography is used to project tactical information onto a transparent plate within the user’s field of view.

-I-

IFF:
Identification Friend or Foe. A radar or sonar system for identifying one’s own aircraft or vessel to friendly units, for tactical coordination and to help avoid friendly fire. Encrypted pulses are transmitted when the IFF system is “interrogated” by properly-coded pulses from another friendly IFF. Of course, the IFF can be switched off when at EMCON (see above).

INS:
Inertial Navigation System. A system for accurately estimating one’s position, based on gyroscopes and accelerometers which determine from moment to moment in what direction one has traveled, and at what speed.

Instant range-gating:
A capability of the new Wide-Aperture Array sonar systems (see below). Because each wide-aperture array is mounted rigidly along one side of the submarine’s hull, sophisticated signal processing can be performed to “focus” the hydrophones at different ranges from the ship. By focusing at four ranges at once and comparing target signal strengths, it is possible to instantly derive a good estimate of target range. The target needs to lie somewhere on the beam of the ship (i.e., to either side) for this to work well.

IR:
Infrared. Refers to systems to see in the dark or detect enemy targets by the heat which objects give off or reflect.

ISLMM:
Improved Submarine-Launched Mobile Mine. A new type of mine weapon for American submarines, based on modified Mark 48 torpedoes, and launched through a torpedo tube. Each ISLMM carries two mine warheads which can be dropped separately. The ISLMM’s course can be programmed with waypoints (course changes) so that complex coastal terrain can be navigated by the weapon, and/or a minefield can be created by several ISLMMs with optimum layout of the warheads.

Isothermal:
A layer of ocean in which the temperature is very constant with depth. One example is the bottom isothermal zone, where water temperature is just above freezing, usually beginning a few thousand feet down. Other examples are 1) a surface layer in the tropics after a storm, when wave action has mixed the water to a constant warm temperature, or 2) a surface layer near the Arctic or Antarctic in the winter, when cold air and floating ice have chilled the sea to near the freezing point.

-J-

-K-

Kampfschwimmer:
German Navy “frogman” combat swimmers. The equivalent of U.S. Navy SEALs and the Royal Navy’s Special Boat Squadron commandos. (In the German language, the word Kampfschwimmer is both singular and plural.)

Knuckle:
"Make a knuckle"
A knuckle is a defensive technique used by a submarine under attack. A submarine makes a knuckle by a sudden, sharp movement back and forth of the rudder. This leaves an especially tubulent spot in the water behind the submarine, and then the sub continues moving away. Because of how sound waves are bent and reflected in water, this "knuckle" can confuse an inbound enemy torpedo's target-homing sonar, and sometimes even fool the torpedo into thinking the knuckle is the actual submarine's hull. The result is thus hopefully that the torpedo (or the enemy weapons techs controlling the torpedo thru its guidance wire) will lose contact on the submarine, or else the torpedo will actually detonate against the knuckle, with little or no harm to the escaping sub itself. Knuckles can also be used to confuse/defeat active sonar being used by surface ships, sonobuoys, ASW helicopters with dipping sonars, etc.

KT:
Kiloton. A measure of power for tactical nuclear weapons. One kiloton equals the explosive force of 1000 tons of TNT.

Krytrons:
Extremely fast-acting electrical switches used to detonate all of the implosion lens components in a nuclear warhead at exactly the same time.

-L-

LIDAR:
Light Direction And Ranging. Like radar, but uses laser beams instead of radio waves. Undersea LIDAR uses blue-green lasers, because that color penetrates seawater to the greatest distance.

Littoral:
A shallow or near-shore area of the ocean. Littoral areas present complex sonar conditions because of bottom and side terrain reflections, and the high level of noise from coastal shipping, oil drilling platforms, land-based heavy industry, etc.

LMRS:
Long-term Mine Reconnaissance System: A remote-controlled self-propelled probe vehicle, launched from a torpedo tube and operated by the parent submarine. The LMRS is designed to detect and map enemy minefields or other undersea obstructions. The LMRS is equipped with forward and side-scanning sonars and other sensors. Each LMRS is retrievable and reusable.

-M-

Mach stem:
A phenomenon resulting from a nuclear explosion at an optimum height in the air. The Mach stem produces an extremely destructive shock wave moving along the ground. It results when the blast’s initial shock wave bounces off the ground, and then moves quickly through the now-heated air to catch up with and merge with the original shock front still moving outward from the air burst itself. This merging multiplies the overpressure greatly, and is an important factor in the effectiveness of tactical nuclear weapons.

MAD:
Magnetic Anomaly Detection. A means for detecting an enemy submarine by observing its effect on the always-present magnetic field of the Earth. Iron anywhere within the submarine (even if its hull is nonferrous or de-Gaussed) will distort local magnetic field lines, and this can be picked up by sensitive magnetometers in the MAD equipment. Effective only at fairly short ranges, often used by low-flying ASW patrol aircraft. Some naval mine detonators also use a form of MAD, by waiting to sense the magnetic field of a passing ship or submarine.

MEDEA:
A study group comprised of civilian scientists and U.S. Navy oceanographers, formed at the request of METOC (see below), to study classified Navy oceanographic databases, and then report on the value of possibly making public some of that data so as to support advances in oceanography, understanding of the ocean environment, and related public policy.

Megaton:
A measure of power for strategic nuclear weapons. One megaton equals the explosive force of one million tons of TNT. (A megaton also equals one thousand kilotons.)

METOC:
Meteorology and Oceanography Command. The part of the U.S. Navy which is responsible for providing weather and oceanographic data, and accompanying tactical assessments and recommendations, to the Navy’s operating fleets. METOC maintains a network of Centers around the world to gather, analyze, interpret, and distribute this information.

-N-

Naval Submarine League (NSL):
An professional association for submariners and submarine supporters. See or call (703) 256-0891.

Network-Centric Warfare:
A new approach to warfighting in which all formations and commanders share a common tactical and strategic picture through real-time digital data links. Every platform or node, such as a ship, aircraft, submarine, Marine Corps or Army squad, or SEAL team, gathers and shares information on friendly and enemy locations and movements. Weapons, such as a cruise missile, might be fired by one platform, and redirected in flight toward a fleeting target of opportunity by another platform, using information relayed by yet other platforms -- including unmanned reconnaissance drones. Network-centric warfare promises to revolutionize command, control, communications, and intelligence, and greatly leverage the combat power of all friendly units while minimizing collateral damage.

NOAA:
National Oceanic and Atmospheric Administration. Part of the Department of Commerce, responsible for studying oceanography and weather phenomena.

-O-

OBA:
Oxygen Breathing Apparatus. A self-contained respirator pack used on submarines to move around freely during emergencies such as fires. (The crew are also supplied with breather masks which plug into nozzles in special air lines, for use while manning their stations or lying in their racks.) Note that OBAs are being replaced by Scott-type air packs, like those used by civilian firefighters. The compressed air tank can then be recharged from the nearest compressed air line in the submarine or ship, so that damage control parties need not leave the immediate area of the fire scene to maintain a good continuous breathing supply.

Ocean Interface Hull Module:
Part of a submarine’s hull that includes large internal “hangar space” for weapons and off-board vehicles, to avoid size limits forced by torpedo tube diameter. (To carry large objects such an ASDS minisub externally creates serious hydrodynamic drag, reducing a submarine’s speed and increasing its flow noise.) The first Ocean Interface has been ordered as part of the design of the USS Jimmy Carter, the last of the three Seawolf-class SSNs to be constructed.

-P-

PAL:
Permissive Action Link. Procedures and devices used to prevent the unauthorized use of nuclear weapons.

Photonics Mast:
The modern replacement for the traditional optical periscope. The first will be installed in the USS Virginia (see below). The photonics mast uses electronic imaging sensors, sends the data via thin electrical or fiber optic cables, and displays the output on large high-definition TV screens in the control room. The photonics mast is “non-hull-penetrating,” an important advantage over older ‘scopes with their long, straight, thick tubes which must be able to move up and down and rotate.

Piezo-rubber:
A hull coating which uses rubber embedded with materials that expand and contract in response to varying electrical currents. This permits piezo-rubber tiles to be used to help suppress both a submarine’s self-noise and echoes from enemy active sonar (see active out-of-phase emissions, above).

PROBSUB:
A probable (but not certain) enemy submarine contact.

Pump-jet:
A main propulsor for nuclear submarines which replaces the traditional screw propeller. A pump-jet is a system of stator and rotor turbine blades within a cowling. (The rotors are turned by the main propulsion shaft, the same way the screw propeller’s shaft would be turned.) Good pump-jet designs are quieter and more efficient than screw propellers, producing less cavitation noise and less wake turbulence.

-Q-

Q-ship:
An anti-submarine vessel disguised as an unarmed merchant ship, to lure an enemy submarine into a trap. First used by the Royal Navy in World War I, in actions against German U-boats.

Quieting:
The combination of design and engineering features, and disciplined crew behavior, which makes a submarine as silent as possible -- and thus as stealthy as possible -- while at sea on operations. Quieting is vital to any sub's mission success, because the primary method of detecting a submerged submarine is by detecting the noise it gives off. (Noise can be transmitted through seawater for great distances, to be picked up by sensitive "enemy" hydrophones many miles away.) For instance, vibrating or rotating machinery will be mounted on "rafts," which are themselves supported by special pads made of rubber or of strong and flexible bags containing oil, to isolate the piece of machinery from the submarine's hull and the outside waters. Entire decks may be constructed so as to "float" on springs or other cushioning. (The latest details of quieting design are highly classified.) In addition, the crew is careful to make as little noise as possible. This can include avoiding use of loudspeakers, and care to not drop tools or food plates on the deck. At "rig for ultraquiet," the crew will take further precautions, such as not flushing the sanitary facilities, and lying still in their sleeping racks as much as possible when not on watch. (Any submarine will be inherently more quiet if is moves more slowly, since propulsion-plant noise and the hull's hydrodynamic-resistance "flow noise" will be lessened. In addition, non-essential equipment, such as some fans and pumps, can be switched off, at least temporarily, to minimize emitted noise.)

-R-

Radiac:
Radiation Indications and Control. A device for measuring radioactivity, such as a Geiger counter. There are several kinds of radiac, depending on whether alpha, beta, or gamma radiation, or a combination, is being measured.

ROEs:
Rules of Engagement. Formal procedures and conditions for determining exactly when weapons may be fired at an enemy.

-S-

SDV:
Seal Delivery Vehicle. A battery-powered underwater “scooter” used by SEALs, wearing scuba gear, to approach and depart from their objective.

SEAL:
Sea Air Land. U.S. Navy Special Warfare commandos. (The equivalent in the Royal Navy is the SBS, Special Boat Squadron.)

SSGN:
A type of nuclear submarine designed or adapted for the primary purpose of launching cruise missiles, which tend to follow a level flight path through the air to their target. An SSGN is distinct from an SSBN, which launches strategic (hydrogen bomb) ballistic missiles, following a very high “lobbing” trajectory that leaves and then reenters earth’s atmosphere. Because cruise missiles tend to be smaller than ballistic missiles, an SSGN is able to carry a larger number of separate missiles than an SSBN of the same overall size. Note, however, that since ballistic missiles are typically “MIRVed”, i.e. equipped with multiple independently-targeted re-entry vehicles, the total number of warheads on an SSBN and SSGN may be comparable; also, an SSBN’s ballistic missiles can be equipped with high-explosive warheads instead of nuclear warheads. (A fast-attack submarine, or SSN, can be thought of as serving as a part-time SSGN, to the extent that some SSN classes have vertical launching systems for cruise missiles, and/or are able to fire cruise missiles through their torpedo tubes.)

7MC:
A dedicated intercom line to the Maneuvering Department, where a nuclear submarine’s speed is controlled by a combination of reactor control rod and main steam throttle settings.

SOSUS:
Sound Surveillance System. The network of undersea hydrophone complexes installed by the U.S. Navy and used during the Cold War to monitor Soviet submarine movements (among other things). Now SOSUS refers generically to fixed-installation hydrophone lines used to monitor activities on and under the sea. The Advanced Deployable System (ADS) is one example: disposable modularized listening gear designed for rapid emplacement in a forward operating area.

Sound Ray Traces:
A display of the paths in which spreading sound waves will be bent and reflected underwater in a particular area. Ray traces are estimates, based upon calculations using information on local ocean temperature and salinity at different depths. Sound ray trace information can be used to help a submarine find the best place to hide from enemy detection platforms. In addition, this information can be applied in interpreting noises detected coming through the water from an enemy submarine, to help determine the hostile sound source’s likely bearing, range, depth, and even its course and speed.

Sound Short:
A failure of a submarine’s quieting (see above), in which noise from within the sub is transmitted into the surrounding sea. Sound shorts are very serious matters, since they can ruin stealth and lead to being detected and attacked by an enemy. A submarine’s own sonars are able to check itself for sound shorts, and if any are found the crew will give a priority to correcting them. Often this can be done by repairing or replacing faulty quieting gear, or if necessary by switching off the machinery which is causing the unwanted noise -- although the latter may put the submarine at a bad tactical disadvantage, if the errant machinery is needed for full warfighting readiness.

Subtropical Convergence:
The area in the South Atlantic Ocean where currents of warmer water from near the equator meet and clash with other currents of colder water from near the Antarctic. The result is a zone of unpredictable and confusing sonar conditions. The subtropical convergence does not extend across the South Atlantic as a well-defined straight line, but rather is a broad area that snakes across different latitudes in different places, and varies over time.

Synchrolift:
A kind of gigantic fork lift or elevator used to move an entire submarine at a shore base or a shipyard.

-T-

Thermocline:
The region of the sea in which temperature gradually declines with depth. Typically the thermocline begins at a few hundred feet and extends down to a few thousand feet, where the bottom isothermal zone is reached (see above).

TMA:
Target Motion Analysis. The use of data on an enemy vessel’s position over time relative to one’s own ship, in order to derive a complete firing solution (i.e., the enemy’s location, course, and speed, and depth or altitude if applicable). The TMA mathematics depends on what data about the enemy is actually available. TMA by passive sonar, using only relative bearings to the target over time, is very important in undersea warfare.

Tonal:
Sound given off at a single frequency, similar to a pure musical “tone” or note. Tonals are important in detecting and identifying passive sonar contacts. This is because different equipment -- and thus different classes of friendly and enemy submarines carrying that equipment -- have unique sets of frequencies at which they emit tonals. One example of the source of a tonal might be an item of equipment which rotates at a particular rate per second, such as a turbogenerator, a reactor cooling-water circulation pump, or even a food blender in the ship’s galley (kitchen).

Towed array:
A long cable equipped with hydrophones (see above), trailed behind a submarine. Towed arrays can also be used by surface warships. The towed array has two advantages: Because it lies behind the submarine’s stern, aft of self-noise from the propulsion plant, the towed array is able to listen in directions where the submarine’s on-hull sonars are “blind.” Also, because the towed array is very long (possibly as lengthy as a mile), it is able to detect very long wavelength (very low frequency) sounds -- which smaller, on-hull, hydrophone arrays may miss completely. Recently, active towed arrays are being introduced. These are able to “ping” as well as listen at very low frequencies, which has significant tactical advantages in some sonar and terrain conditions. The next planned advance is a towed array with three or more separate parallel lines, in which the individual hydrophones use fiber optic coils and lasers. Tiny changes in the behavior of the laser light will result when the coils are influenced by sound waves in the surrounding ocean. Analysis of such data promises to greatly increase the sensitivity of the array to the presence of enemy submarines and other targets. (When not in use, the towed array is retracted by winches in the submarine’s hull. Towed arrays often need to be retracted if the submarine is in close proximity to bottom terrain or surface shipping, or if the submarine intends to move at high speed.)

Transducer:
An underwater device, similar in principle to a submersible loud-speaker, which emits noise into the water. Sonar transducers can serve different purposes: as the submarine or surface ship or sonobuoy’s main active-sonar “ping” emitter, or as the device that generates sound pulses to measure depth of water beneath the keel, or to search for enemy mines, or to detect (and measure the thickness of) surface ice in order to navigate safely while submerged.

-U-

Unmanned Aerial Vehicle:
A drone aircraft, which might be launched and/or controlled from a parent submarine. UAVs can be used for intelligence gathering and electronic signals intercept, visual and infrared and radar reconnaissance, or as a relay point for friendly radio or laser communications, among other things. (Sometimes more properly called “uninhabited” aerial vehicles, as the UAV may be under the active real-time control of someone on a sub or ship or other aircraft or on land, and in that sense is in fact manned -- or womanned!)

Unmanned Undersea Vehicle:
A probe vehicle launched and controlled from a parent submarine or surface ship or land station. UUVs can be used to map bottom terrain, listen for intruding enemy combat-swimmers, search for and map out minefields, or even locate and trail enemy submarines. UUVs can be designed to come to “periscope depth” and raise sensor masts above the surface, to gather data and/or to relay data by radar or laser to other friendly assets and platforms. Underwater, a UUV can be linked to its parent vessel by an acoustic link or by a wire or fiber-optic tether. UUVs are also being developed that are truly autonomous, i.e. they rely upon on-board computers with artificial intelligence to patrol, seek information, and communicate with higher headquarters. UUVs might even be armed. UUVs are generally powered by batteries or fuel cells (see above), but some are being designed to recharge their power source independently -- by harnessing the potential energy of the ocean itself, such as the temperature differences at different depths in the sea.

-V-

Virginia-class:
The latest class of nuclear-propelled fast-attack submarines (SSNs) being constructed for the United States Navy, to follow the Seawolf-class. The first of four currently on order, the USS Virginia, is due to be commissioned in 2004. (Post-Cold War, some SSNs have been named for states, since construction of Ohio-class Trident missile “boomers” has been halted.)

-W-

Wide-aperture array:
A sonar system introduced with the USS Seawolf in the mid-1990s, distinct from and in addition to the bow sphere, towed arrays, and forward hull array of the Cold War’s Los Angeles-class SSNs. Each submarine so-equipped actually has two wide-aperture arrays, one along each side of the hull. Each array consists of three separate rectangular hydrophone complexes. Powerful signal processing algorithms allow sophisticated analysis of incoming passive sonar data. This includes instant range-gating (see above).

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