How hard is it to find a submerged submarine?
An answer to one of the most commonly asked questions about anti-submarine warfare.
It is a common question often asked by most people about submarine and anti-submarine warfare. Especially when this article is made, an Indonesian submarine (KRI Nanggala, 402) was declared missing off the coast of Bali and it took the authority more than 72 hours to locate the location of the submarine using multiple assets before it was being declared sunk. On the surface, detecting a submarine is just a matter of turning on the chaser ship’s sonar and waiting until it detects the submarine. In reality, it is way more complex than that. In order to have a better understanding of submarine detection, we need to know the types of sensors and equipment to detect a submerged submarine, what the submarine has to avoid detection, and what are the necessary efforts to detect a submerged submarine.
Types of Sensors and Equipment
- Sonar
Sonar is the leading equipment to detect an underwater object, especially a submarine. Sonar can be mounted on a surface ship, a submarine, or even an aircraft. There are two types of sonar, active sonar and passive sonar.
Active sonar works similarly to radar but underwater. Unlike radar which transmits radio waves to detect an object, sonar transmits sound waves. Hence the name, sonar (sound navigation and ranging), is comparable to radar (radio detection and ranging). The main difference between them is that the sonar has a lesser effective range than radar because of the reverberation (disturbance in the water that prevents the sound waves from bouncing back to the sonar receiver). Active sonar can detect the bearing of a submarine, its heading, its speed, its depth, and its distance almost accurately. There are multiple applications of active sonar, such as HMS (hull-mounted sonar), VDS (variable depth sonar), dipping sonar (a sonar lowered by a rotary-wing aircraft), and active sonobuoy (a sonar dropped by a fixed-wing aircraft in the form of buoy).
The other sonar, the passive sonar, actually works differently than its active counterpart. Passive sonar is essentially listening for the sound made by objects underwater, in this case, a submarine, and does not transmit any sound waves. Although different than the active sonar, it is still complex to operate. Once the passive sonar detects a particular sound, the sonar operator must isolate it from ambient sounds and identify the isolated sound signature using a database to determine the origins of the sound. Whether a whale, a group of fish, a friendly or civilian ship’s propeller, or a submarine. If it is a submarine, the sonar operator could determine the submarine type using the same database because each submarine class creates a different pattern of sound signature (wavelength, decibels, sounds generated by the number of screws, the number of blades on each propeller, the depth of the screws in the water, the behavior of the submarine, the number of cylinders on its engine, et cetera). Fortunately for the sonar operator, in the present day, multiple technologies automate the process of sound identification, such as LOFAR (Low-Frequency Analysis and Ranging). Like active sonar, there are multiple passive sonar applications, such as SOSUS (sound surveillance system), towed array sonar (cable-like equipment containing multiple arrays of passive sonar and dragged from the back of a ship; also known as SURTASS, surveillance towed-array sensors), hydrophone (basically an ear mounted on a ship’s hull), dipping sonar, and passive sonobuoy.
- MAD (Magnetic Anomaly Detector)
Like sound, every submarine class has a distinct magnetic signature that MAD sensors could detect. MAD could detect a submarine because a mass of ferromagnetic material used by a submarine creates a detectable disturbance in the magnetic field. However, in order for the MAD to be effective, the target submarine must be near the surface of the ocean because effectiveness of MAD is greatly affected by distance (magnetic fields decrease as the inverse cube of distance). MAD is usually mounted on aircraft in the form of towed sensor.
- Infrared sensors
Infrared emissions or heat signature could give away a submarine position in multiple ways, such as heat signature from a snorkel, periscope, masts, and distinct temperature difference between a particular spot on the sea surface and its overall surroundings caused by a submarine propeller’s wake,
What submarines have to avoid detection:
- Teardrop hull design
In the two World Wars, most of submarine’s hull designs mainly based on their surface ships because, at that particular time, submarines do not have the endurance to stay underwater for a long time and spent most of its time on the surface. This hull design creates multiple problems, such as a lower submerged speed and a large sound signature. After the technological advancement in the mid 21st century, submarine designs start to use a “teardrop” design which is based on a teardrop shape. The teardrop shape helps the submarine achieve higher submerged speed, lower sound signatures, and deflect sonar waves. Teardrop design makes it harder for both active and passive sonar to detect a submerged submarine.
- Hull materials
In order to deceive MAD and magnetic mines, multiple submarines are: made with nonmagnetic hull materials, running electric currents through the hull to manipulate the magnetic signature, or still using a magnetic material but coated by a rubber coating (as found of the modern Soviet and Russian submarines).
- Decoys
Decoys are used to deceive sonar sensors by creating sounds or movement signatures and lure the submarine hunters away from the actual location of the submarine. There are many forms of decoy, such as bubble decoy (creates bubbles in the water), hammer and explosive decoy (creates explosions to disable or disorient passive sonar for a few moments), signature decoy (mask the submarine’s signature), towed decoy, and decoy torpedo (a specially made torpedo that mimick the submarine’s behavior and creates a large sound signature).
- Passive sonars
Submarines are also equipped with hydrophones and towed array sonar. With the presence of passive sonar suites onboard, submarines could detect any active sonar emitted by the hunting ships twice the range the active sonar could detect the submarines. By being able to know earlier that they are being hunted, submarines can easily avoid detection by simply change its course so it would not cross paths with the hunter.
- Reduced sound signature
Early submarine designs have large sound signatures caused by their propulsion system and propeller design. Modern submarines have a reduced sound signature due to multiple efforts by designing a quieter propeller or even completely replacing the propeller with a water jet system, creating a sound-proof compartment for its engine room, and designing a quieter propulsion system. A quieter propulsion system for the submarine is actually a nuclear propulsion system. Overall, submarines with nuclear propulsion systems have much fewer decibels than submarines with a diesel-electric propulsion systems. This greatly hinders the ability of passive sonar to detect the submarine. For example, USS Ohio (SSGN-726), due to its overall “stealth” design, was virtually undetectable and presumed sunk during her first sea trial because the escorting ships could not detect her while she was underwater.
- Swim-out torpedoes
Conventional torpedoes need air compression in the torpedo tube in order to launch. When launched, this air compression would create enormous bubbles, also known as a torpedo launch transient, which the passive sonar could easily detect. So, every time a submarine launch its torpedoes, it would reveal its locations to the submarine hunters. However, the invention of the swim-out torpedo, in the example Mk. 48 ADCAP eliminates the launch transient. The swim-out torpedo could swim out of the torpedo tube by itself and does not require any compressed air.
- Playing dead
Highly trained submariners could maximize the evasion effort by just shutting off all electronic systems, including the propulsion system, to avoid emitting any signature. By playing dead, the submarine could not be detected by passive sonar and even active sonar. Passive sonar would not be able to detect any signature, and active sonar would mistaken the submarine for some obstacle on the seabed. During World War II, several submarines from both sides actually managed to avoid being sunk by the submarine chasers using this method. By being invincible to the sensors, the chasers/hunters would assume that the submarine has been sunk or has escaped and thus ending the sinking efforts. This is also one of the reasons why searching for a sunken submarine is much more complicated.
The effort needed to detect a submarine: cooperation
A submarine can only be effectively hunted by using multiple assets at the same time. There has been no successful attempt to hunt a submarine using only one ship or asset. It requires a combined anti-submarine warfare task force consisting of hunter-killer submarines, surface ships, and aircraft. They could combine and coordinate the search using active and passive sonars and MAD sensors with multiple assets joining forces.
Successful ASW operations typically involved a combination of sensor and weapon technologies, along with effective deployment strategies and sufficiently trained personnel. Typically, sophisticated sonar equipment is used for first detecting, then classifying, locating, and tracking a target submarine. Sensors are therefore a key element of ASW. Common weapons for attacking submarines include torpedoes and naval mines, which can both be launched from an array of air, surface, and underwater platforms. ASW capabilities are often considered of significant strategic importance, particularly following provocative instances of unrestricted submarine warfare and the introduction of submarine-launched ballistic missiles, which greatly increased the perceived lethality of submarines.
Conclusions
With all of the equipment and factors mentioned above, it is actually very complex to hunt a submerged submarine. Especially “stealth” or even sunken submarines. Hunting submarines also require patience, time, and dedication because anti-submarine warfare is basically a highly electronic and deadly hide-and-seek game on the sea. In the end, the successful outcome of submarine hunting mission is determined by not only the utilization of the right sensors but also the skill of the operators behind it and the skill of the submariners themselves.
