Submarine detection in the open seas involves several strategies, in which noise frequencies are not overly relevant.
Submarine hunting takes place in three ways, generally. Firstly, through ships using active sonar transmission to try and get an echo from the submarine. Secondly, by helicopters attempting to do the same way with their dipping sonar. And lastly, by fixed wing aircraft and helicopters using sonobuoys to listen to the submarine’s noise.
The first two are the most used methods and the Scorpene data leak doesn’t affect the efficiency of these two methods since they depend on bouncing a sound beam off the submarine’s hull. Nothing in the data that was leaked adds to the science of these two methods.
The third method of listening to the submarine can be marginally improved by knowing the frequency and amplitude of the submarine’s radiated noise. This information will only be available once the submarine’s noise is measured after its construction and is not included in the leaked data.
In peacetime exercises, the submarine’s presence is usually first known when it fires its torpedo at one of the hunting ships. The torpedo firing is indicated by the submarine firing up a green flare. The position of the green flare is taken as the submarine’s last known position (datum) and a search commences, the methods of which are worked out by operational analysis and presented to the hunting ship as a ready reckoner.
The speed with which the datum can be approached matters critically and hence the advantage of the helicopter. In open oceans, the transmission on sonar of the hunting ship can be heard by the submarine at considerably longer ranges than it can be detected. The sub captain can use this advantage either to close in and attack the ship or to use the thermal underwater layers skillfully, for evasion.
Once the sub decides to evade, it is virtually impossible to detect it unless it runs out of battery power and is forced to come to periscope depth to raise its schnorkel and charge its batteries. During this time the submarine’s masts protrude above the surface and it can be detected by radar. This period when the sub can be detected is called the indiscreet period, and the merit of a conventional submarine is measured by how low the indiscretion rate is. For a modern submarine, it is generally around 6%. Even during the indiscreet period, a submarine uses its electronic listening device to detect the presence of enemy radars. If the intensity of the radar is high, it indicates that the enemy is near and approaching, in which case the submarine stops schnorkeling and goes passive.
Nuclear vs conventional
So, the odds are heavily in the submarine’s favour in the third method of detecting submarines. The advantage the sub-hunter has in this method is that the submarine’s sensors are unaware that it is being hunted and is therefore potentially dangerous for the submarine. This method became the primary source of detection of ballistic missile submarines (SSBNs) during the cold war. A missile submarine of the American variety carries 24 missiles and each missile can carry 8 warheads, making a total of 192 warheads, often as much as the entire arsenal of a medium nuclear power. Tracking such a submarine becomes a strategic mission and comes under the heading of strategic anti-submarine warfare (ASW). In strategic ASW, no effort however costly is considered a waste as the stakes are so high. Detecting a missile sub is therefore an achievement for which any effort is thought to be worthwhile.
The sound characteristics of SSBNs are so important that they are considered national secrets and millions of dollars may be spent on detecting one missile sub. A conventional diesel sub doesn’t have the same importance since it is only a tactical platform. The same aura and myth does not surround a conventional submarine and hence its radiated noise characteristics don’t convey the same importance as those of an SSBN. In any case, the radiated noise characteristics of INS Kalvari – the first of the six Scorpenes being built in India – can be known only after its noise ranging trials, which are yet to take place.
The use of sonobuoys to listen passively against diesel submarines is a tactic that would be used if the submarine is ‘localised’ by electronic or other means while it was being indiscreet; then fixed wing aircraft are quickly sent out to the datum to lay sonobuoy barriers or fields, of which the submarine may be unaware. In such a case, the sonobuoy field relies on the ‘amount’ of noise made by the submarine to detect it. The frequencies of the radiated noise are not so relevant. During the Cold War, sensors that listened to submarines passing through choke points were laid for Soviet subs by the Americans. The sound picked up was transmitted to a computer thousands of miles away and a ‘tail’ in the form of a nuclear hunter – a killer submarine was attached to the Soviet SSBN. In the Indian ocean, strategic ASW has not reached such levels and Chinese and Pakistani ASW is relatively primitive in comparison.
A submarine always listens both underwater and above water. Hence it can detect the presence of submarine hunting aircraft and ships at considerably longer ranges than can be detected, because the electro-magnetic wave goes only one way for the submarine to detect it. So even when the submarine has its schnorkel up, it is listening in on radar frequencies which are generic and known all over the world. So a submarine’s listening frequencies are of no use to the enemy because the listening envelop is wide and can include any radar of any country.
In any case the radiated noise levels of the Scorpene are so low that it would practically have to be at one mile or so for it to be detected underwater and such encounters are mathematically improbable. A nuclear sub has no need to schnorkel and therefore has no indiscretion rate. However, it has machinery that has to run all the time to keep its reactor going. The machinery makes very low frequency radiated noise and it is this noise that espionage is all about. When passive, a conventional sub is more silent that a nuclear sub and is virtually undetectable. During the 1971 war, the same amount of commercial and contractual information concerning the Pakistani Daphne class submarine was obtained by the Indian Navy, but it made no difference to its operations. Similarly, the Pakistani and Chinese navies will find the information leaked by The Australian (or whoever the original source is) quite useless when it actually comes to sub-hunting.
Admiral Raja Menon was a career officer and a submarine specialist in the Indian Navy. He commanded seven ships and submarines before retiring in 1994 as assistant chief of naval staff (operations).
Categories: Armed forces