Anti-subm arine warfare (ASW) is defined as a branch of underwater warfare that employs surface warships, aircraft or submarines to find, track and deter, damage or destroy enemy submarines. The first step is to detect, track and identify followed by destruction/degradation of the submarine.

Detection Technology and Systems

The doctrine of submarine operations is based on total secrecy and stealth while on an operational mission, own submarine’s detection by the enemy could prove fatal. Thus having effective sensor systems for detection of enemy’s submarines first, provide a great advantage to own submarines and surface ships. Details of sensors in use are given in succeeding paragraphs.

Visual. Earlier method of making visual detection is not that effective with modern submarines due to their low ‘indiscretion rate’. Modem conventional submarines are generally powered by diesel-electric propulsion systems. Diesel engines are used on the surface or with a snorkel to charge batteries which are used to power electric motors for propulsion. While underwater, oxygen is stored in high pressure tanks and released slowly to sustain the crew. The ‘indiscretion ratio’ is thus defined as the ratio of the time spent charging the submarine batteries, to the time elapsed for the complete discharge/charge cycle of the batteries. In a typical diesel-electric submarine, the indiscretion ratio is about 10 per cent, which increases rapidly with increasing speed.

Radio interception. Radio interception was possible during the two earlier World Wars as the German submarines operated in a pack and the encryption techniques were not that sophisticated. Modern submarines transmit through methods that make the transmission difficult to detect.

Radar. This was an effective sensor during World War II for detecting surfaced submarines. With the development of the snorkel and the advent of nuclear-powered submarines; submarines rarely surfaced outside their home port thus it was not possible to detect them with radar.

Sonar

Sonar (sound navigation and ranging) has emerged as the primary means of detecting submerged submarines. It has multiple uses for underwater detection as sound waves can effectively move underwater. Sonar can be passive or active.

Passive sonar. Passive sonar systems do not emit any signal and are used to detect noise emanating from a submarine. Being silent thus it is stealthy and cannot be detected. Passive sonar cannot measure the range of an object unless it is used with multiple passive listening devices.

Active sonar is also dropped from aerial platforms in the form of disposable sonobuoys for detection in the zone of interest

Active sonar. Active sonar has transducers which emit an acoustic signal which get reflected when they meet an object. The reflected signal is then picked up, which allows measurement of range and azimuth, similar to a radar. As active sonar will reveal the source of emission which will give out the position of the operator thus it is used by fast moving platforms like aircraft and helicopters, and by noisy platforms like surface ships but rarely by submarines. In all cases active sonar is activated in short bursts to avoid detection. Due to the limitation of not having stealth features, active sonar is normally considered a backup to passive sonar.

Towed Sonar. Because of the problems of ship noise, towed sonars are also used. These also have the advantage of being able to be placed deeper in the water but have a problem in shallower waters. They can be towed arrays (linear) or variable depth sonars (VDS) with 2/3D arrays. They require a winch to deploy and recover, which is large and expensive. VDS sets are primarily active in operation while towed arrays are passive. An example of a modern active/passive ship towed sonar is Sonar 2087 made by Thales Underwater Systems. Sonar 2087 is designed to provide a manageable source of high quality sonar data. It is high source level, low complexity, omnidirectional acoustic projector deployed in a hydrodynamic towed single ping contact bearing ambiguity resolution; single receive towed array and very low frequency passive detection and tracking. Active/passive mode depends upon the countermeasures taken by the submarine.

Variable Depth Sonar. In the recent past sonar arrays were hull mounted on the ship but it was found out that they required means of reducing flow noise. This was done by a canvas cover dome which was later on made of steel and then of reinforced plastic or pressurised rubber. Such sonars are primarily active in operation. An example of this is the modern hull-mounted sonar AN/SQS-56, which features digital implementation, system control by a built-in minicomputer, and an advanced display system. The sonar is an active/passive, digital sonar providing panoramic echo ranging and panoramic passive surveillance.

Sonar on Aerial Platforms

Sonobuoy. Active sonar is also dropped from aerial platforms in the form of disposable sonobuoys for detection in the zone of interest. The term Sonobuoy is formed from Sonar and buoy. It is a small equipment which can be dropped or ejected from an aerial platform or surface ship to detect a submarine and transmit the received signal by radio to a designated receiver.

Dipping sonar. Wide area, underwater battlespace surveillance, using the active and passive functions of Sonar 2087 is complemented by airborne acoustics suite which includes the Sonar 2189 low frequency dipping sonar (based on Flash) and a sonobuoy detection system. For closer range surveillance and self-protection the Type 23 also operates the hull mounted Sonar 2050. All of this sonar equipment has been supplied by Thales Underwater Systems. The Royal Navy employs the sonar from their Maritime Force Protection and airborne anti-submarine warfare Merlin helicopter. Another example is the AQS-13 series systems used by the US Navy. This was manufactured by a division of Bendix Corporation which went through multiple ownerships and name changes over the years and is currently L-3 Communications Ocean Systems.

Some developments. Synthetic Aperture Sonar measures the slight differences in a bunch of acoustic ‘pings’ off the same location to acquire details of the subsurface object. Another new technique uses low-frequency sonar (less than 1,000 Hz) to increase the system’s range but which reduces accuracy.

Magnetic anomaly detection (MAD). MAD is simply an electronic magnetometer which can measure magnetic field variations due to large metal objects, such as the steel hull of a submarine. Before the development of sonar buoys, MAD system was commonly installed on aerial platforms to pick up shallow-submerged submarines.

Anti-submarine weapons

They can be guided, unguided and rocket/mortar weapons.

Guided ASW weapons. The modern torpedo can be loosely called the sea version of a missile as it is self-propelled, carries a warhead, navigates itself to the target with its own sensors or from the launching platform’s sensors and detonates on contact with the target or in close proximity to it. They can also be launched from aerial platforms. Torpedo is the most effective weapon for ASW and also against surface ships. There are many types of torpedoes in service with the navies of the world. Mark 48 and its improved advanced capability variant are US heavyweight submarine-launched torpedoes. Mark 46 aerial torpedo is the backbone of the US Navy’s lightweight anti-submarine warfare torpedo inventory. India’s TAL is a lightweight torpedo (LWT) which was India’s first production grade torpedo. Advanced LWT is successor programme of TAL which has some minor changes and major improvements and is likely go for trials this year. The Black Shark is a heavyweight torpedo developed by WASS of Italy and is specially meant for Scorpène diesel-electric submarines produced by France.

Non-guided ASW weapons. These are mines and depth charges. Mines are laid in critical sea lanes and harbours to deter intruders. Depth charges are fired in the close proximity of submarines to damage them.

Rockets and mortar. Anti-submarine grenades and anti-submarine rockets have a short response time as they are fired through the air onto the target. An improvement in the response time is achieved by launching a torpedo via a rocket which gives the submarine less time to take countermeasures.

India’s Conventional Submarine Force level

Scorpene under Project 75. At present the force level of submarines with the Indian Navy (IN) is just 13 which includes nine of Sindhughosh class and four of Shishumar class. The IN had formulated a 30-year Submarine Perspective Plan in 1999 which envisaged 12 submarines by 2012 and the number was expected to double by 2029. The delivery schedule of the new submarines was to be dovetailed with the phasing out of the existing Shishumar and Sindhughosh classes of submarines. The two distinct submarine designs were named as Project 75 and Project 75(I). For Project 75, a contract with DCNS was signed for six French submarines Scorpene to be made in the Mazagon Dock Limited, and to be delivered between 2012 and 2016. There has been a delay of about four years and the first Scorpene submarine, named INS Kalvari, was undocked in April 2015 for starting sea trials. It is hoped that the delivery starts this year and the remaining five delivered by 2020.

Project 75(I). This was to follow-up of Project 75 and accordingly a request for information (RFI) was issued in 2008 for procuring of six AIP equipped submarines with high degree of stealth and land-attack capability. The projected cost then was $10 billion ( Rs. 50,000 crore). Now it is Rs. 64,000 crore but this is just an indicative cost and actual cost will only be known when the contract is signed. These were also to be built at an Indian shipyard, public or private, with special emphasis on full transfer of technology. Now these have come under the ‘Make in India’ programme of the NDA Government but there seems no progress.

Nuclear Submarines

India has ambitious plans to acquire sea component of the nuclear triad. Towards this aim it has leased INS Chakra from Russia which is nuclear-powered attack submarine. India’s own nuclear-powered ballistic missile submarine project is progressing well and the first submarine INS Arihant is expected to be commissioned soon.

ASW Helicopters

Sikorsky’s S-70B Seahawk helicopter

NH90 NFH advanced ASW helicopter

Russian Ka-27 helicopter

Helicopters with ASW systems are normally deployed by all naval forces to destroy submarines at long ranges. There are many type of ASW helicopters in service and a few examples are as follows:

• MH-60R Romeo. This is a next-generation ASW and ASuW helicopter produced by Sikorsky Aircraft Corporation. It is currently one of the most advanced naval helicopters available.
• NH90. NH90 NFH, offered by NHIndustries, is an advanced ASW helicopter built by Thales.
• Ka-27 or Ka-28 (export designation). This is a Russian ASW and ASuW helicopter which is in service in India. The Ka-27 helicopter is equipped with VGS-3 dipping sonar and sonobuoys to track and detect submarines. It can fire torpedoes, anti-submarine missiles PLAB-250-120 anti-submarine bombs and OMAB bombs.
• S-70B Seahawk. The S-70B Seahawk is an ASW and ASuW helicopter developed by Sikorsky Aircraft Corporation. The S-70B helicopter is fitted with sensors like search radar, sonobuoy launcher, Helicopter Long-Range Active Sonar Dipping Sonar, towed MAD and is suitably armed.

Gaps on India’s ASW Capability

Submarines. At present India has only 13 submarines which is no where near the force levels projected in the 30-year Submarine Perspective Plan of the Indian Navy. Six Scorpenes are likely to be inducted by 2020 provided no hitch comes up due to the leak in the design documents. Project 75(I) has yet to crystalise. Going by the experience of Scorpene, it may take about two decades before the induction starts. Meanwhile, the current submarines will get obsolescent, leaving a wide gap in the subsurface operational readiness.

It was reported earlier in the media that only one-fifth of helicopters are available for the high-end ships thus leaving a big gap in their ASW capability

ASW Helicopters. Indian Navy’s Ka-28 fleet has been reduced to just four operational helicopters thus India has signed a deal with Russia to modernise ten Ka-28 naval helicopters at a cost of $294 million. The Sea King helicopter fleet has been reduced to just 16-17 upgraded machines with a few capable of flying at any one time. The Navy has doubts of the ASW capability of indigenous Dhruv helicopter. The Indian Navy had originally planned to acquire 54 multi-role helicopters and 16 of these should have come in 2007 or so as replacement for the first lot of Sea Kings but nothing has happened. Sikorsky’s S-70BTM Seahawk ASW/ASuW was shortlisted for acquiring 16 as an interim arrangement but that project has not moved forward. All naval ships have multi-role helicopters onboard, e.g., all destroyers will have two each and INS Vikramaditya aircraft carrier can-carry 12 helicopters. It was reported earlier in the media that only one-fifth of helicopters are available for the high-end ships thus leaving a big gap in their ASW capability.

Sonar. The Naval Physical & Oceanographic Laboratory has developed a variety of indigenous sonars for surface ships Nagan was a towed array sonar system that was converted into demonstarator and the project closed in 2012. Other system for surface ships is the Hull Mounted Sonar Advanced–NG (HUMSA) which is an active-cum passive system. HUMSA UG is an upgraded version of HUMSA with new receiver electronics and an ultra-cool power amplifier system. Advance Light Towed Array Sonar (ALTAS) is a sonar system for the detection, localisation and classification of submarines operating especially the below sonic layer. It is useful in ASW operations and is the apt sensor for warships to locate silent submarines. For EKM class of submarines it has developed USHUS sonar suite. Mihir was a first-generation helicopter sonar system, comprising of dunking sonar and a four channel sonobuoy processor. It was designed for advanced light helicopter type platform. Low frequency dunking sonar is an advanced version of Mihir.

Meanwhile it is reported that India has imported six of Atlas Elektronik’s Active Towed Array Sonar (ACTAS) which is a lowfrequency ASW sonar system that operates simultaneously in active and passive modes and provides high-resolution target detection. ACTAS provides excellent performance up to very long ranges, which includes over-the-horizon surveillance. It is designed to detect, track and classify submarines, torpedoes, surface vessels, including fast boats. As per media reports these were imported for Kamorta class of ASW corvettes but three have been fitted on Talwar class frigates and three on Delhi class destroyers. The additional requirements will be met by the Bharat Electronics Limited manufacturing under transfer of technology from Germany.

Black Shark torpedoe. The Black Shark is a heavyweight torpedo developed by WASS of Italy and is specially meant for Scorpène diesel-electric submarines which are being made in India. It has an effective range of 50 km (27 NM) and speed of 50 kt (about 90km/h). Black Shark was to be fitted on India’s Scorpene Submarines but is waiting for a go-ahead from the Indian Ministry of Defence. In case Black Shark is not cleared, India will have to look for another torpedoe which could replace it without any design changes.