Nicknamed the “Silent Service,” submarines are considered the most survivable weapons-delivery platform.
Submarine detection and monitoring was traditionally the exclusive domain of highly classified military units specializing in naval anti-submarine warfare (ASW). Military ASW employs technologies such as magnetic anomaly detectors (MAD), which detect tiny disturbances to Earth’s magnetic field caused by metallic submarine hulls, passive and active sonar sensors that use sound propagation to detect objects underwater, as well as radar and high-resolution satellite imagery to detect surfaced submarines. Recent advances in commercial tools and technologies now give open-source researchers some ability to monitor submarine fleets. With commercial satellite imagery, synthetic aperture radar (SAR), hydro-acoustic sensors, and even social media analysis, open-source researchers can better understand the size and composition of countries’ submarine fleets, monitor the construction of submarines and submarine bases, and potentially learn about patrol patterns and behaviors.
A submarine can be detected by a number of different sensors and methods. Let’s check some of them:
This is the most commonly overlooked method of locating a submarine: experience and common sense. If you constantly monitor another country’s submarine forces’ operational methodology, you can make a fairly good educated guess at roughly where their submarines will be at any particular moment in time. Regular satellite monitoring of submarine bases will let you know when submarines leave harbor or return to port. Meticulous record-keeping of past detection events will give you a database of submarine locations.
Readily accessible high-resolution commercial satellite imagery is one of the essential tools for open-source analysis of submarine activity. Imagery enables researchers to visually monitor naval shipyards and activities, such as those in China and North Korea.
The rise of social media through platforms such as Facebook, Twitter, Snapchat, and Instagram and crowd-sourcing websites (e.g., www.liveuamap.com) has made maintaining operational security far more difficult for militaries. Some seemingly harmless Tweets and Instagram pictures have revealed themselves as significant security breaches in recent years. Interestingly, searches on Twitter and Instagram show many individuals who “sub spot” as a hobby.
A modern submarine may pop up the periscope to verify its position or visually ID a surface ship. It may also have to raise a radio mast to communicate via radio or raise the radar mast to make a couple of radar sweeps. It is not easy to detect a periscope/mast with a radar, especially if there are any waves. The periscope is usually not up for very long. The submarine often pops the periscope up, sweeps the horizon 360 degrees, takes it down, and analyzes the sweep’s video recording with the periscope down).
If the sea is calm and the periscope is up long enough, a radar can pick it up. A diesel-electric submarine without AIP (Air Independent Propulsion) has to raise the snorkel to run the diesels and charge the batteries. The snorkeling submarines can be detected by radar if the waves are not too high.
Active sonar is like underwater radar, except it transmits sound waves instead of radio. The difference is that sonar has far, far less range than a radar since there is so much more reverberation (things that bounces the wave back) in water than in air.
Not only that, while a radar signal goes pretty much straight and is pretty much only limited by the horizon (in certain weather conditions, it can even see beyond the horizon), the sound waves will bounce up and down and bend through different layers of temperature and salinity in the water. A submarine can “hide” under a layer of cold water where an active hull-mounted sonar (HMS) can’t reach since the sound wave will bounce off the layer up against the surface. For this reason, surface ships have variable depth sonars (VDS) that they can dip below the layers and search. There are often multiple layers, which makes it even harder.
The primary advantage of active sonar is that you can get the submarine’s exact location required for a successful attack. However, the submarine can detect an active sonar at 2-5 times the sonar’s detection range vs. the submarine, so it can fairly easily move out of the way when it hears the surface ship coming.
Active sonar could be effective after the submarine is detected with passive sensors and/or intelligence reports.
This is the primary method to detect a submarine. It is essentially very advanced microphones that listen to sound under the water. You don’t necessarily have to be able to hear the submarine with the naked ear; there are computers (such as LOFAR, LO Frequency Analysis and Ranging) that can analyze the sound and find patterns that can tell a variety of information about the ship that is the source of noise: The number of screws, the number of blades on each propeller, the depth of the screws in the water (deep or not deep), if the ship is turning, etc. If the source has the diesels running, you can determine the number of cylinders, if it is turbocharged, the gearing, etc. This is the case for a snorkeling SSK submarine.
An SSK running on battery power, and an SSK with Air Independent Propulsion (AIP), basically doesn’t get picked up on passive sonar unless it’s going fast enough to cavitate (which submarines don’t do unless they have to). They are quieter than the surrounding water.
Surface ships can tow a passive sonar behind the vessel in a mile-long array (can’t be hull-mounted, the ship is too noisy for that). It can also deploy passive sonar buoys from its helicopter or airplanes.
The primary advantage of passive sonar is that the target doesn’t know it’s been detected.
Electronic Support Measures (ESM)
A radio transmission, even though it only takes a split second to send, can be picked up and give the submarine’s bearing.
Magnetic Anomaly Detectors (MAD)
A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the Earth’s magnetic field. The magnetometers used to detect submarines (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field).
It is generally used by aircraft. The aircraft will fly a grid search pattern over a given area; if it passes directly over a large mass of metal (i.e., a submarine) within the detection range (not too deep), it will be detected by the searching aircraft.
Navies are trying to reduce the magnetic signature by running currents through the hull and using non-magnetic hull materials. The Russians have built submarines out of non-magnetic titanium, and the new Swedish A26 class will be partially built out of carbon fiber reinforced vinyl that is non-magnetic (and 5x stronger than steel).
There are some other sensors and methods to detect the submarines like a green laser, electrooptic systems (detects heat emissions from a snorkeling SSK), hydrodynamic water pressure analyses, etc.
The submarine can hear a surface ship long before the surface ship can detect the submarine, and therefore it can take evasive action before its detected.
Because of water physics, sound is the most effective way to detect a submerged submarine. The oceanographic conditions can significantly reduce the effectiveness of the sensors. Many factors are affecting how sound is propagated through water, including the water column itself. However, the biggest problem is that the ocean itself is noisy, and we have to distinguish the submarine’s noise from all the other noise in the water.
New advanced submarines can stay submerged for weeks or even months and move relatively undetected while on patrol. Many submarines emit less noise than ambient water noise.
So finding the submarine in the ocean is similar to looking for the proverbial needle in a haystack.
To summarize, to detect a submarine, nations need detailed intelligence (long time obtained data), active and passive sensors, many assets (MPA, Satellites, ASW units, Helo, UAVs, etc.), and adequate sources to sustain search efforts.
The navies are working to reduce these disadvantages with new sensors like Low Frequency Active Sonars, Active and Passive Sonobuoys, Unmanned ASW units (USV, UUV, and UAVs). But on the other hand, submarines are becoming stealthier. It looks like the submarines will protect their advantageous role in the near future as well.
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