Since World War I, submarines have been a key element of all navies to dominate the oceans. Littoral submarines can also support special forces and influence land battle. Due to the pivotal role of submarines, it is but natural for every navy to have anti-submarine warfare (ASW) systems and tactics.

Beginning of ASW Tactics

In the early days when modern sensors were non-existent, submarine once submerged became a ghost with no way to detect it. John Holland designer of the US Navy’s first commissioned submarine, said in 1900 that “the submarine is indeed a ‘sea-devil,’ against which no means we possess at present can prevail.” Early ASW tactics thus depended primarily upon static defences like underwater mines and chain-link nets to prevent the movement of submarines. Torpedo nets were also hung from ships to block torpedoes from reaching their hulls. To counter this submarines started using net cutting blades in front of the torpedoes. Other basic method were the towing of grappling hooks connected to explosive charges or throwing charges overboard in the direction of a suspected vessel. The use of smoke screens to obscure ships was also widely used. It was also tried to force the submarine to surface and then attack its periscope.

WWI

During WWI, ASW simply involved waiting for a submarine to surface and then attack it like a surface ship. In 1914 one of the first successful anti-submarine attacks was the ramming of the German submarine SM U-15, while it surfaced with engine troubles. The use of heavily armed anti-submarine ships, disguised as vulnerable targets decoy called “Q-Ships” also had some success. As threat from submarines became more dangerous, use of depth charges came into being with mines were dropped into the ocean attached to a string and sort of bobby trapped and exploded when a submarine contacted the string. The early US Navy Mark 1 Depth Charges weighed 100 lbs, contained 50 lbs of TNT. Later models such as the Mark 2 were detonated using a hydrostatic pistol controlled by water pressure to fire at a preselected depth and other innovations. However all naval minds were thinking as how to locate the submerged submarine? As usual US had to lead the way and in 1917 the Navy started working on echo-ranging technology (then called ‘supersonics’), which would finally emerge as a means to detect for underwater submarine detection. Magnetic sensitive mines also came in service due to the magnetic field of the submarine.

ASW during WWII

ASW continued to develop such as better depth charges and development of forward-throwing anti-submarine weapons. As depth of submarines increased so was the capability of depth charges. The defensive tactic of shipping convoys and escort ships was also improved.

Advent of Sonar

The Anti-Submarine Division from which the term ASDIC originated was behind the sound based detection devices. The first recorded use of the technique was by Leonardo da Vinci in 1490 who used a tube inserted into the water to detect vessels by ear. The device was known to the US Navy as Sonar (an acronym for sound navigation and ranging, was basically a transmitterreceiver sending out a highly directional sound wave through the water. If the sound wave struck a submerged object it was reflected back and picked up by the receiver. The range was calculated based on the time taken for the signal to be reflected from the objective. It was similar to radar. The combination of sonar technology and depth charges, became the foundation of ASW and were used to hunt submarines. There were also many improvements in detecting and identifying submerged submarines. New technologies in acoustic detection like the SOSUS (sound surveillance system) were developed, which used sea bottom mounted hydrophone arrays connected by underwater cables to facilities ashore, to detect submarine positions by triangulation. Acoustic interceptors such as sonar-buoys were also developed, in addition to towed passive devices such as the Magnetic Anomaly Detector (MAD), which can accurately track large metallic underwater objects.

Indigenous Effort. There are four systems including ‘Abhay - compact hull mounted sonar for shallow water crafts; Humsa UG - upgrade for the Humsa sonar system’; ‘NACS - Near-field Acoustic Characterisation System’ and ‘AIDSS - Advanced Indigenous Distress sonar system’ for submarines. The systems have been designed and developed by NPOL, a Kochi-based laboratory of DRDO.

Aerial Platforms. Aircraft were also initiated in ASW and fitted with ASW weapons such as the FIDO (Mk 24 ‘mine’) airdropped homing torpedo. These ‘smart’ torpedoes achieved a combat effectiveness of about 22 per cent, compared with about 9 per cent for Depth Charges. Airborne radar also evolved to became one the best ways to locate submarines. During the end of WWII, radar equipped aircraft were credited for most of Allied kills against U-Boats.

ASW Helicopters. Helicopters are very effective in ASW. India has Kamov Ka-25, Kamov Ka-28, and the Russian origin helicopters. India has contracted for 24 MH-60R helicopters at a cost of $2.6 billion. The MH-60R Romeo is a next-generation ASW and Anti-Surface Warfare helicopter produced by Sikorsky Aircraft Corporation. There are a large variety of ASW and anti- surface helicopters available globally to choose from.

Fixed Wing Aircraft. IN also operates Il-38 planes, which are currently being upgraded with the the Sea Dragon suite. The latest ASW fixed wing aircraft is Boeing’s P-8I which is a long-range ASW, antisurface warfare, intelligence, surveillance and reconnaissance aircraft capable of broad-area, maritime and littoral operations. Eight P-8(I) are in service with four more likely to join in 2020. Ten more have been approved this year. They have replaced Tu-142.

Air dropped ASW weapons. First used in World War I, air-dropped torpedoes were used extensively in World War II, and still remain in limited use. The Mark 54 lightweight torpedo has been adapted for this role with the High Altitude Anti-Submarine Warfare Weapon Capability (HAAWC for short). The folding-wing kit contains a flight control computer and GPS, and is attached via a set of collars to a Mark 54 torpedo. Once launched, HAAWC’s wings spring out and it heads for its target many km away. On arriving at the target area, the wing kit jettisons the torpedo, with a parachute gently lowering it into the water. Once in the water, the Mark 54 torpedo then activates to autonomously hunt for and kill a submarine.

Rise of the Torpedo

The submarines became stronger, stealthier, were able to operate independently thus became difficult to hunt. The development of nuclear submarines capable of carrying ballistic missiles, provided greater challenges to ASW. Depth charges were replaced by more sophisticated guided torpedoes, such as the Mark 48 and its improved Advanced Capability (ADCAP) variant which are of US origin heavyweight submarine-launched torpedoes. They are capable of sinking deep-diving nuclearpowered submarines and high-performance surface ships. They were designed by Gould, Inc. and Naval Surface Warfare Centre, and manufactured by Gould/Honeywell (Model1), Hughes Aircraft (ADCAP) and Westinghouse Naval Systems. Later models are still in service. The acoustic torpedoes were designed to locate the targets by the noise characteristics or signature through active or passive sensors. Later versions included wire guided torpedoes, which received steering commands through a wire. More recently the metal wire was replaced by optical fibre, which can transmit much more information. These are similar to the wire guided anti-tank missile. Once these torpedoes acquired a target, they changed over to their internal guidance system and the wire was cut. Submarine hunting torpedoes are now also quieter and harder for submarines to detect and avoid.

Some examples are:

• Raytheon’s Mark 54 LHT. The Mark 54 Lightweight Torpedo (formerly known as Lightweight Hybrid Torpedo, or LHT) is an ASW used by the US Navy. It can be launched from surface ships and also from most ASW aircraft. The P-8 Poseidon uses the High-Altitude Anti-Submarine Warfare Weapons Capability (HAAWWC) GPS-guided parachute kit to drop torpedoes from high altitude.
• Mk 50. The Mk 50 torpedo is a US Navy advanced lightweight torpedo for use against fast, deep-diving submarines. The Mk 50 can be launched from all anti-submarine aircraft and surface combatant ships. Contractor is Alliant Techsystems and Westinghouse.
• Varunastra. India has produced Varunastra which is an advanced heavyweight anti-submarine torpedo, developed by Naval Science and Technological Laboratory of the DRDO for the IN with a range of 40 km. They have also developed The Advanced Light Torpedo (TAL) Shyena which is the first indigenous advanced lightweight anti-submarine torpedo with a range of 19 km.
• Other Weapons. Anti-submarine mortars and grenades have a shorter range and are use with certain navies.
• ASW Shallow water craft. These are meant for ASW in the littorals. India has signed a contract with GRSE in April 2019 for eight such craft at a cost of 6,311.32 crore (about $0.9 billion). The first craft will be delivered within 42 months and the remaining within 84 months.

Unmanned Underwater Vehicles (UUVs). UUVs are employing active sonar to detect, characterise, and track adversary submarines. They augment existing surface and aerial platforms like the P-8(I). The DARPA-sponsored Submarine Hold at RisK (SHARK) UUV, developed by Applied Physical Systems, serves as an example of this type of emerging technology. The SHARK UUV is an active sonar platform designed to detect and track stealth submarines. Another initiative of DARPA is the Sea Hunter ASW Continuous Trail Unmanned Vessel (ACTUV), as well as swarms of AUVs (autonomous underwater vehicles) which could be used to hunt and maybe even destroy submarines. Bluefin Robotics is developing the SandShark micro-autonomous underwater vehicle designed for ISR and ASW with decoy capabilities to be launched from larger UUV systems.