Current technologies
There are a large number of technologies used in modern anti-submarine warfare:
Sensors Acoustics particularly in active and passive sonar, sonobuoys and fixed hydrophones and in the reduction of radiated noise.
Pyrotechnics in the use of markers, flares and explosive devices
Searchlights
Radar
Low frequency spread-spectrum electromagnetic surface wave devices
Active spread-spectrum magnetic techniques
Hydrodynamic pressure wave detection
Blue-green laser airborne and satellite LIDAR
Electronic countermeasures and Acoustic Countermeasures such as noisemakers
Passive acoustic countermeasures such as concealment and design of sound-absorbing materials to coat reflecting underwater surfaces
Magnetic anomaly detection (MAD)
Active and (more commonly) passive infra-red detection
In modern times Forward looking infrared (FLIR) detectors have been used to track the large plumes of heat that fast nuclear-powered submarines leave while rising to the surface. FLIR devices are also used to see periscopes or snorkels at night whenever a submariner might be incautious enough to probe the surface.
The active sonar used in such operations is often of "mid-frequency", approximately 3.5 kHz. Because of the quietening of submarines, resulting in shorter passive detection ranges, there has been interest in low frequency active for ocean surveillance. However, there have been protests about the use of medium and low frequency high-powered active sonar because of its effects on whales. Others argue the high power level of some LFA (Low Frequency Active) sonars is actually detrimental to sonar performance in that such sonars are reverberation limited.
Weapons
Mines,
Torpedoes, acoustic, wire-guided, and wake homing
Anti-submarine Weapons
Anti-submarine weapons can be divided into three categories according to their mode of operation: Guided weapons, Non-guided weapons and Rocket- and Mortar weapons.
Guided anti-submarine weapons, such as torpedoes, seek out the submarine, either via its own sensors or from the launching platform's sensors. The advantage with this type of weapon is that it requires a relatively small payload since it detonates in direct contact or within a very close proximity of the submarine. The disadvantage is that this type of weapon can be decoyed and is adversely affected by stealth features of the submarine.
Non-guided anti-submarine weapons, such as mines and depth charges, are "dumb" weapons that has to be carried to the submarine or that the submarine has to come in close proximity of. This is to some degree compensated by a heavy payload, in some mines exceeding half a metric ton, but since the effect of an underwater explosion decreases with a factor of the distance cubed, an increase in payload of a depth charge from 100 to 200 kg would not result in more than a few meters in killing radius.
Rocket- and mortar weapons, such as anti-submarine grenades and anti-submarine rockets, main advantage is the rapid response time since they are carried through the air to the target. Once dropped on top of the target, they also have the advantage of not being sensitive to decoys or stealth features. A hybrid of this category is the rocket launched torpedo, which is carried to the proximity of the target via a rocket and therefore reduces the response time and gives the submarine less time to undertake countermeasures or evasive maneuvers.
Finally, a submarine can of course also be destroyed by means of artillery fire and missiles in the rare case that a modern submarine surfaces, but these weapons are not specifically designed for submarines and their importance in modern anti-submarine warfare is very limited.
Guns/Missiles
Gunfire has been used to disable submarines from the First World War onwards, while a helicopter missile attack was used to disable the "Santa Fé" in the Falklands War
Depth charge
The simplest of the anti-submarine weapons, the depth charge is a large canister filled with explosives and set to explode at a predetermined depth. The concussive effects of the explosion could damage a submarine from a distance, though a depth charge explosion had to be very close to break the submarine's hull. Air-dropped depth charges were referred to as 'depth bombs'; these were sometimes fitted with an aerodynamic casing.
Surface-launched depth charges are typically used in a barrage manner in order to cause significant damage through continually battering the submarine with concussive blasts. Depth charges improved considerably since their first employment in World War I. To match improvements in submarine design, pressure-sensing mechanisms and explosives were improved during World War II to provide greater shock power and a charge that would reliably explode over a wide range of depth settings.
Aerial-launched depth bombs are dropped in twos and threes in pre-computed patterns, either from airplanes, helicopters, or blimps. Since aerial attacks normally resulted from surprising the submarine on the surface, air-dropped depth bombs were usually timed to explode at a shallow depth, while the sub was in the process of making a crash dive. In many cases destruction was not achieved, but the submarine was nonetheless forced to retire for repairs.
Early depth charges were designed to be rolled into the water off of the stern of a fast ship. The ship had to be moving fast enough to avoid the concussion of the depth charge blast. Later designs allowed the depth charge to be hurled some distance from the ship, allowing slower ships to operate them and for larger areas to be covered.
Today, depth charges not only can be dropped by aircraft or surface ships, but can also be carried by missiles to their target
Anti-submarine mortar
With the discovery that depth charges rarely scored a kill by hitting a submarine, but instead were most effective in barrages, it was found that similar or better effects could be obtained by larger numbers of smaller explosions. The anti-submarine mortar is actually an array of spigot mortars, designed to fire off a number of small explosives simultaneously and create an array of explosions around a submarine's position. These were often called Hedgehogs after the name given a World War II British design. Later ASW mortar shells were fitted with impact detonators that fired only after actual contact with the hull of the submarine, allowing sonar crews to maintain a constant sound track until a hit was achieved.
The Hedgehog fired twenty four 14.5 kg charges whereas a later development called the "Squid" fired three full-sized depth charges. A further development called "Limbo" was used into the 1960s, and this used 94 kg charges.
A development of the Anti-submarine mortar, designed primarily for the exceptionally challenging task of littoral anti-submarine operations, utilizes a shaped charge warhead. An example of this is the Saab Dynamics ASW-600 and the upgraded ASW-601.
Torpedo
The early anti-submarine torpedoes were straight-running types and usually a group was fired in case the target manoeuvred. They can be divided into two main types, the heavyweight, fired from submarines, and the lightweight which are fired from ships, dropped from aircraft (both fixed wing and helicopters) and delivered by rocket. Later ones used active/passive sonar homing and wire-guidance. Pattern running and wake-homing torpedoes have also been developed.
The first successful homing torpedo was introduced by the German Navy for use by its U-boat arm against Allied shipping. After capturing several of these weapons, along with independent research, the United States introduced the FIDO air-dropped homing torpedo (also called the Mark 24 'mine' as a cover) in 1943. FIDO was designed to breach the steel pressure hull of a submarine but not necessarily cause a catastrophic implosion, forcing the now-crippled submarine to surface where the submarine and crew might possibly be captured. After World War II, homing torpedoes became one of the primary anti-submarine weapons, used by most of the world's naval powers. Aircraft continued to be a primary launching platform, including the newly available helicopter, though homing torpedoes can also be launched from surface ships or submarines. However, the torpedo's inherent limitations in speed of attack and detection by the target have led to the development of missile-borne anti-submarine weapons that can be delivered practically on top of the enemy submarine, such as ASROC.
On ships the torpedoes are generally launched from a triple-barreled launcher by compressed air. These may be mounted on deck or below. On submarines torpedoes have been carried externally as well as internally. The latter have been launched in the past by stern tubes as well as by the more normal forward ones.
Aircraft delivery platforms have included both unmanned helicopters, such as the US DASH, and manned ones such as British Westland Wasp. The helicopter may be solely a weapons carrier or it can have submarine detection capabilities
Mine
Similar to those designed to defeat surface ships, mines can be laid to wait for an enemy submarine to pass by and then explode to cause concussive damage to the submarine. Some are mobile and upon detection they can move towards the submarine until within lethal range. There has even been development of mines that have the ability to launch an encapsulated torpedo at a detected submarine. Mines can be laid by submarines, ships, or aircraft.
One of the latest anti-submarine weapons, Anti-Submarine ROCkets (ASROCs), SUBROC, the Ikara, the French Malafon, and the Italian MILAS differ from other types of missiles in that instead of having a warhead which the missiles delivers to the target directly and explodes, they carry another anti-submarine weapon to a point of the surface where that weapon is dropped in the water to complete the attack. The missile itself launches from its platform and travels to the designated delivery point.
The major advantages of a missile are range and speed of attack. Torpedoes are not very fast compared to a missile, nor as long-ranged, and are much easier for a submarine to detect. Anti-sub missiles are usually delivered from surface vessels, offering the surface escort an all-weather, all-sea-conditions instant readiness weapon to attack time-urgent targets that no other delivery system can match for speed of response. They have the added advantage that they are under the direct control of the escort vessel's commander, and unlike air-delivered weapons cannot be diverted to other taskings, or be dependent on weather or maintenance availability. Aircraft-delivery can be further compromised by low-fuel-state, or an expended weapon load. The missile is always available, and at instant readiness. It allows the torpedo or Nuclear Depth Bomb to enter the water practically on top of the submarine's position, minimizing the submarine's ability to detect and evade the attack. Missiles are also more rapid and accurate in many cases than helicopters or aircraft for dropping torpedoes and depth charges, with a typical interval of 1 to 1.5 minutes from a launch decision to torpedo splashdown. Helicopters frequently take much longer to just get off the escort's deck
Anti-submarine Rocket
One of the latest anti-submarine weapons, Anti-Submarine ROCkets (ASROCs), SUBROC, the Ikara, the French Malafon, and the Italian MILAS differ from other types of missiles in that instead of having a warhead which the missiles delivers to the target directly and explodes, they carry another anti-submarine weapon to a point of the surface where that weapon is dropped in the water to complete the attack. The missile itself launches from its platform and travels to the designated delivery point.
The major advantages of a missile are range and speed of attack. Torpedoes are not very fast compared to a missile, nor as long-ranged, and are much easier for a submarine to detect. Anti-sub missiles are usually delivered from surface vessels, offering the surface escort an all-weather, all-sea-conditions instant readiness weapon to attack time-urgent targets that no other delivery system can match for speed of response. They have the added advantage that they are under the direct control of the escort vessel's commander, and unlike air-delivered weapons cannot be diverted to other taskings, or be dependent on weather or maintenance availability. Aircraft-delivery can be further compromised by low-fuel-state, or an expended weapon load. The missile is always available, and at instant readiness. It allows the torpedo or Nuclear Depth Bomb to enter the water practically on top of the submarine's position, minimizing the submarine's ability to detect and evade the attack. Missiles are also more rapid and accurate in many cases than helicopters or aircraft for dropping torpedoes and depth charges, with a typical interval of 1 to 1.5 minutes from a launch decision to torpedo splashdown. Helicopters frequently take much longer to just get off the escort's deck.
Weapon Control Systems
The readiness of weapons was at first determined manually. Early fire control consisted of range measurements and calculation of the submarines course and speed. The aiming point was then manually determined by rule. Later, mechanical computers were used to solve the fire control problem with electrical indication of weapon readiness. Today the weapon firing process is carried out by digital computer with elaborate displays of all relevant parameters.
ASW Countermeasures
The main countermeasure the submarine has is stealth, that it tries not to be detected. Against the ASW weapon itself, both active and passive countermeasures are used. The former may be a noise making jammer or a decoy providing a signal that looks like a submarine. Passive countermeasures may consist of coatings to minimise a torpedo's sonar reflections or an outer hull to provide a stand-off from its explosion. The Anti-submarine weapon has to overcome these countermeasures.