Military technology influences warship layouts dominantly. This has been obvious since the first warships got a ramming bow. It became even more obvious with guns, and later aviation. Technology is much more influential in the air and at sea than on land (warfare). Thus I'll approach the topic of anti-submarine warfare (ASW) mostly from the technological angle.
The submarine threat
First, submarines and anti-submarine warfare. The short version of this story is that submarines have nowadays a monopoly on heavyweight torpedoes which can destroy frigates and destroyers with one hit, but scoring such a hit on a moving and somewhat silent or target is difficult when the torpedo is launched at long distances. Other munitions of submarines include sub-launched anti-ship missiles (rather rare), sub-launched anti-submarine missiles (lightweight torpedo-tipped, even more rare), naval mines (very rare because munition storage is scarce and previous), land attack cruise missiles (common among American, British, Russian and Israeli submarines) and submarines may be equipped with short range air defence missiles. This would be an easily kept secret, but such efforts have been known since the 70's and the utility is obvious in light of the extreme importance of ASW helicopters.
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| (c) Naval Graphics |
Submarine survivability
The survivability of submarines rests on their stealth, which makes them an ideal naval platform for underdog naval powers that would see their surface fleet destroyed on short notice.
Stealth depends on being silent and on not reflecting much acoustic energy when in practical range of active sonars. The latter can be achieved by small size, by minimising the silhouette by pointing bow or aft at the emitter and by using anechoic tiles that absorb much of the acoustic energy and reflect little. The latter have a tendency of developing defects that create noise when the sub is moving and are reported as being not so good at dampening the low frequency (3 kHz and lower) acoustic waves.
Modern submarines can be considered extremely silent, even nuclear-powered ones. Warships cannot be expected to detect modern submarines at useful minimum distances when relying on passive sonars alone. Anechoic tiles and long ranges of munitions have on the other hand made old style active sonars unsatisfactory.
Low frequency active sonars can often achieve good detection ranges and can tell wrecks and natural objects apart from submarines by noticing the latter's movements and comparing sensor data with pre-war undersea maps. They're present in some rather new ASW frigates and few other units.
Stealth depends on being silent and on not reflecting much acoustic energy when in practical range of active sonars. The latter can be achieved by small size, by minimising the silhouette by pointing bow or aft at the emitter and by using anechoic tiles that absorb much of the acoustic energy and reflect little. The latter have a tendency of developing defects that create noise when the sub is moving and are reported as being not so good at dampening the low frequency (3 kHz and lower) acoustic waves.
Modern submarines can be considered extremely silent, even nuclear-powered ones. Warships cannot be expected to detect modern submarines at useful minimum distances when relying on passive sonars alone. Anechoic tiles and long ranges of munitions have on the other hand made old style active sonars unsatisfactory.
Low frequency active sonars can often achieve good detection ranges and can tell wrecks and natural objects apart from submarines by noticing the latter's movements and comparing sensor data with pre-war undersea maps. They're present in some rather new ASW frigates and few other units.
Submarine detection & contact confirmation
There are thus four approaches for the detection of modern submarines to be considered:
- low frequency active sonars (LFAS)
- lots of sonar-equipped drones swarming the sea and detecting subs when they get close
- fixed wing aircraft relying on sonobuoys
- helicopters with a unique dipping sonar capability in addition to sonobuoys
(1) faces extreme opposition by Greenpeace etc. due to the alleged damage done to maritime mammals, but militarily speaking the main disadvantage is that the emitter gives away its position (or at least bearing) with its noise. The smallest LFAS is a dipping sonar for helicopters, and a LFAS exists for surface ships ">300 tons", so LFAS can be employed by de facto all ocean-going units.
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| example (part) of a variable depth (towed) LFASS |
Towed LFAS appear to be limited in terms of maximum cruise speed (~17 kts) and maximum sea state (~ sea state 6).
(2) is unproven and in my opinion not practical at high seas. It may be relevant for coastal waters, but convoys at sea would want to cruise at 15...25 kts, and I doubt that many small drones could achieve the necessary endurance to accompany a convoy at such cruise speeds. They would need to be recovered and refuelled all the time, which would require a huge effort in addition to the purchase costs of dozens if not hundreds of drones per convoy. Lone slow patrol drones with a LFAS would attract attacks with long-range munitions that they would rather not survive on their own.
(2) is unproven and in my opinion not practical at high seas. It may be relevant for coastal waters, but convoys at sea would want to cruise at 15...25 kts, and I doubt that many small drones could achieve the necessary endurance to accompany a convoy at such cruise speeds. They would need to be recovered and refuelled all the time, which would require a huge effort in addition to the purchase costs of dozens if not hundreds of drones per convoy. Lone slow patrol drones with a LFAS would attract attacks with long-range munitions that they would rather not survive on their own.
(3) is a very expensive and in my opinion also laughable concept - hundreds of thousands of sonobuoys would need to be expended per month, which would be unsustainable in all but the shortest wars. I suppose naval conflicts might be rather long ones if both sides have safe harbours, for the loss of life and damage to economy and society would be much less extreme than with mobile land warfare. The effects would also be much less decisive in the short term.
Fast fixed wing aircraft are in theory great to race to a contact, be it a SOSUS contact (unlikely with modern submarines), a contact made by navy assets or civilian ships reporting an attack by a torpedo in their area. I'm just unconvinced that ASW fixed wing aircraft can do much upon arrival. Their arsenal is largely limited to sonobuoys and lightweight torpedoes nowadays - nothing that a convoy couldn't employ on its own. Fixed wing ASW is in my opinion a remnant of Second World War fixed wing ASW - a result of path dependency, not a result of the state of the art and sciences. It's a deviation from the rule that land-based is more cost efficient than naval.
Fast fixed wing aircraft are in theory great to race to a contact, be it a SOSUS contact (unlikely with modern submarines), a contact made by navy assets or civilian ships reporting an attack by a torpedo in their area. I'm just unconvinced that ASW fixed wing aircraft can do much upon arrival. Their arsenal is largely limited to sonobuoys and lightweight torpedoes nowadays - nothing that a convoy couldn't employ on its own. Fixed wing ASW is in my opinion a remnant of Second World War fixed wing ASW - a result of path dependency, not a result of the state of the art and sciences. It's a deviation from the rule that land-based is more cost efficient than naval.
(4) is simply not practical in very rough seas; the limit is sea state 5 for NH-90 and Ka-27; that's 2.5-4 m high waves. The heavier AW.101 is claimed to be able to operate at sea state 6, but that claim may mean the lower bound of sea state 6.
Helicopters also face the threat of submarine-launched air defences (if they are present) once the sub commander believes he has been detected or is about to be detected. They lack terrain features at sea that their brethren over land could exploit to survive. ASW helicopters are furthermore terribly expensive, in a league with "generation 4.5" strike fighters.
Helicopters also face the threat of submarine-launched air defences (if they are present) once the sub commander believes he has been detected or is about to be detected. They lack terrain features at sea that their brethren over land could exploit to survive. ASW helicopters are furthermore terribly expensive, in a league with "generation 4.5" strike fighters.
My conclusion is thus that ship-operated low frequency active sonars should be the mainstay of anti-submarine sensors, with helicopters and drones being mere complementary asserts at high seas. Helicopters' low frequency dipping sonars may in fact be or become the main emitters (which allows keeping ships silent) if this provides enough effective detection range. This range variable is classified, of course.
Low frequency active sonars aren't necessarily as simple as old school active sonars, though: Techniques have been developed to address the problem of the treacherous emissions and small echoes from submarine bows. Bistatic and multistatic sonar networks have one or multiple emitter/receiver units and one or multiple units that don't emit, only receive (listen). Two well-spaced emitters would hit the submarine with acoustic waves from different directions, and even if due to shape the echo isn't strong enough to be detected by either emitter some other listener might pick it up. Active nodes can turn passive and passive nodes can turn active, which means that submarines could get but limited and quickly obsolete information about the warships by listening to those emissions. There's clearly a substantial advantage in quantity in modern ASW. The gold plated ASW destroyer or ASW frigate designed for self-reliance is no more the state of the art. Moreover, the extreme silencing of such ASW units is of no consequence during the active sonar operation. In fact, active sonars may give away even perfectly silenced ASW ships' bearing to submarines when said ships are being hit by the low frequency wave.
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| multistatic sonar (c)Topher200 |
The ideal submarine detection by a convoy at high seas would thus include a ~300° forward low frequency active sonar search, passive listening in the rear ~60° (as submarines become less silent when they are moving at 20 kts or faster, which would be required to catch up with a convoy from behind) with low frequency dipping sonar searches by ASW helicopters (360° around the convoy), not the least to have a second look at suspected contacts.
Some drones could be dispatched to investigate contacts as well, even helicopter-dropped and -recovered drones. Drones have the advantage that they would allow the scarce helicopters to stay at a safe distance to a suspected contact.
Finally, I'd like to mention about sonars that as early as in 1941 German passive sonars were able to detect a battleship or convoy well past the horizon of a battleship's mast. This over the horizon capability is still present against noisy ships, but it is of much-reduced importance due to the helicopters' radars that provide a much better look past the ship's horizon. Modern submarines are meanwhile too silent for passive long range detection.
Some drones could be dispatched to investigate contacts as well, even helicopter-dropped and -recovered drones. Drones have the advantage that they would allow the scarce helicopters to stay at a safe distance to a suspected contact.
Finally, I'd like to mention about sonars that as early as in 1941 German passive sonars were able to detect a battleship or convoy well past the horizon of a battleship's mast. This over the horizon capability is still present against noisy ships, but it is of much-reduced importance due to the helicopters' radars that provide a much better look past the ship's horizon. Modern submarines are meanwhile too silent for passive long range detection.
Destruction of the submarine & confirmation of the kill
Heavyweight torpedoes are almost extinct on surface warships, and the smaller (in the West usually 324 mm diameter) lightweight torpedoes (LWT) are badly outranged by the submarines' torpedoes. The dominant method for delivering a warhead onto a detected threat submarine is thus to drop lightweight torpedoes from helicopters. This means the helicopter has to come close (entering a hypothetical air defence zone of the threat sub) and it has to be able to operate at all. The latter is difficult not only in stormy weather, but also after weeks at sea with frequent patrols. The maintenance burden and the limited quantity of such expensive and thus scarce helicopters may lead to too few helicopters being available.
There should thus be a backup. The state of the art for this are LWT-carrying missiles which can be stored in and launched from vertical launch silos just like most other warship missiles. A salvo of three such missiles could be launched at a confirmed contact with impact point updates by radio for a triangular impact pattern around the contact that traps the submarine in a no escape zone. Suspected contacts would at most be shot at with a single missile, which would likely provoke a confirming reaction if the contact isn't a false one. I don't think that equipping submarines with such ASW missiles makes much sense - the costs and volume & mass required are not justified by the low probability of success. An interesting detail about such missile-delivered lightweight torpedoes is that they might be used to sink (finish off) crippled surface ships.
This might sound like an endorsement of VL-ASROC, but to my knowledge that missile has no datalink and doesn't get updates during the flight, which may be important to engage unpredictably sprinting (30+ kts) nuclear attack submarines at long ranges due to the short effective range of the LWT warhead. This and the range (that's shorter than the range at which submarines may choose to unleash a salvo of missiles at a merely heard convoy) seem to be critical limitations of VL-ASROC. The USN should not have aborted the Sea Lance program in the 90's.
This might sound like an endorsement of VL-ASROC, but to my knowledge that missile has no datalink and doesn't get updates during the flight, which may be important to engage unpredictably sprinting (30+ kts) nuclear attack submarines at long ranges due to the short effective range of the LWT warhead. This and the range (that's shorter than the range at which submarines may choose to unleash a salvo of missiles at a merely heard convoy) seem to be critical limitations of VL-ASROC. The USN should not have aborted the Sea Lance program in the 90's.
Long distances between the warship and the submarine target would allow for imprecise direction- and range-finding by active sonar only. The speed of sound in seawater is about 1,500 m/s. A 30 km distant submarine's echo from the warship's active sonar would thus arrive about 40 seconds after emission. A 30 kts fast nuclear-powered submarine would have travelled about 600 m in this time, and likely over 1,000 m until the impact of an ASW missile. The missile could thus create a miss distance of well over 1,500 m with a u-turn even if the sonar had zero degree bearing error (which no sonar can do without sheer luck). Additionally, the LWTs on the tip of the ASW missile would have no information about the depth the submarine is at, and would likely waste time searching in circles in the wrong layer of seawater.
Another sonar needs to be brought to the submarine's vicinity to get a more accurate location and also accurate information about the submarine's depth at long distances (= likely all distances outside of effective heavyweight torpedo range) from the warship. An engagement with a pattern of ASW missiles would often be a waste of munitions without such improved targeting data.
The destruction of a threat submarine could be confirmed with high resolution sonar by helicopter or drone. The use of a drone would be practical at short ranges only, due to its likely slow travel and afterwards slow catch-up with the convoy. The implosion of a submarine as it sinks beyond its destruction depth may also serve as indicator, but this doesn't necessarily happen. The waters of the continental shelf may not be deep enough and a hit may lead to a pressure equalisation between inside and outside, avoiding implosion.
The ASW topic also includes the defences against the threat posed almost exclusively by submarines: Heavyweight torpedoes.
Such torpedo threats can be divided into categories depending on propulsion and seeker
- high speed propulsion or electric propulsion
- acoustic homing or wake homing
High speed propulsion is meant to allow for extremely fast terminal approach to the target (reportedly even around 70 kts), which makes any attempt to run away futile if the torpedo was detected late or fired at short range. Even 40+ kts warships could not run away from a 70 kts torpedo that's close and has enough fuel left. The most extreme high speed torpedoes use supercavitation, but this appears to be practical only with nuclear warheads or at short ranges, for such a torpedo cannot have own search and track sensors (it can be wire-guided, though).
Electric propulsion torpedoes are sneakier and more versatile. They could for example be used to destroy an undersea cable from well over 50 nm away. They could also self-deploy by extremely long distances (even exploiting sea currents) and become a long endurance naval mine in the mission area.
Acoustic homing torpedoes are similar to the first ones in the Second World War. They carry an onboard sonar with passive and active modes (typically in the nose), but they are connected with the submarine by a copper cable or fibre-optic cable for most if not all of the approach to the target. The submarine can listen through the torpedo's sonar, which enables human intelligence to participate in the choice of a target. This is important, as there may be plenty acoustic decoys. Acoustic decoys have to be smart to properly simulate the warship or another attractive target, as torpedoes (just as acoustic naval mines) can process sounds since the 80's so well that they can tell one ship class from another.
A wakehoming sensor is a common secondary sensor in modern heavyweight torpedoes, to be used in the ship's vicinity. Torpedo fuses vary, but contact fuses are uncommon nowadays though there may be a contact fuse mode present as backup.
A wakehoming sensor is a common secondary sensor in modern heavyweight torpedoes, to be used in the ship's vicinity. Torpedo fuses vary, but contact fuses are uncommon nowadays though there may be a contact fuse mode present as backup.
Countermeasures to heavyweight torpedoes
Surface warships have some measures to silence them (rather silent diesel-electric propulsion, water bubble systems, screws and cruise speeds meant to avoid cavitation that transport ships usually lack and cannot be retrofitted with. Warships also have a reduced magnetic signature, albeit the utility of this is questionable nowadays.
Acoustic countermeasures to torpedoes are the reason why wake homing was introduced, usually as a second homing mode for a torpedo that's also got a sonar. Wake homing torpedoes sense the wake of a ship and zig-zag along it to the ship. The one most effective countermeasure to this is to stop moving, but even then wake homing would bring the torpedo close to the only recently stopped warship.
There would be no countermeasure available against an IR sensor on a torpedo. IR doesn't help underwater, but all torpedoes can reach 40+ kts in the terminal approach, which is enough to jump out of the water by several metres and make a 180+° snap shot of the surroundings. The problem with this would be to steer the torpedo into a suitable angle for the re-entry (this could be achieved with minute charges). No such thing has ever been published to my knowledge, and military patents are usually classified, so I can't even tell if anyone ever came up with the idea before I did.
There would be no countermeasure available against an IR sensor on a torpedo. IR doesn't help underwater, but all torpedoes can reach 40+ kts in the terminal approach, which is enough to jump out of the water by several metres and make a 180+° snap shot of the surroundings. The problem with this would be to steer the torpedo into a suitable angle for the re-entry (this could be achieved with minute charges). No such thing has ever been published to my knowledge, and military patents are usually classified, so I can't even tell if anyone ever came up with the idea before I did.
Countermeasures to wake homing might include hull vanes for reduced wakes, towed mines along the wake that blow up an approaching torpedo and anti-torpedo munitions, including guided lightweight torpedoes (might be the same as the ones used on submarines). I have not read of any modern towed mines and the anti-torpedo torpedo thing is according to published infos still largely in an experimental stage (just as submarine air defences). Hull vanes for wake reduction are experimental as well, apparently.
Some more remarks
The use of sonar is not a simple affair. There are layers of water in oceans with different salinity and temperature that keep acoustic waves from travelling straight. Variable depth sonars that could be lowered on a tether down into such layers (or have microphones in different layers at the same time) were very fashionable in the 1970's and 1980's for this reason.
Passive sonar is furthermore affected by the ship platform itself - the only passive sonars that work fine to the rear 30° or so are towed sonars. They are speed-limited, though. I saw published operating speeds in the 15...17 kts range and maximum survival speeds a little short of 30 kts. A ship that cruises at 20...25 kts would rather not make use of a towed sonar. Hull-mounted sonars on the other hand have no business trying to listen for what's behind the ship and can't enter lower water layers themselves.
Bubbles underwater can create an active sonar echo that may be mistaken for a submarine (by an active sonar) - this was used for deception back in the Second World War already. Bubbles don't move horizontally (save for the general drift of the water) as submarines often do, though.
Sonars can be used to create imagery of objects, and this is done when looking for naval mines or sunk shipwrecks. This is de facto not available at long ranges, though.
There are some rather elaborate acoustic 'stealth' technologies; the old prairie - masker has become publicly known, for example.
The thermal and salinity layers as well as the currents of the sea can be sensed with tiny drone or single use sensors. They could also be surveilled by slow-moving drones that travel along expected convoy routes and make regular reports by satellite uplink.
Ship identification by passive sonar is possible if the ship's acoustic profile was recorded before, and naval mines even use this to prefer real targets over simulators used by minesweepers. It's quite safe to assume that modern threat torpedoes have the same capability, which rules out some primitive acoustic decoys.
Cavitation is the forming of water vapour bubbles in water, caused especially at ship screws. This is noisy, damages the screws and is to be avoided if possible, but it's hard to avoid with sprinting torpedoes and sprinting surface warships. Cavitation depends on the vessel design and the water pressure (depth) mostly. Submarines may sprint silently at great depth even if they would be loud due to cavitation at a lesser depth. That's in part the appeal behind diving deep.
Blast is less effective at great depths than at shallow depths, so lightweight torpedoes usually have shaped charges to pierce a submarine hull at any depth. This helps especially against large double-hulled submarines, as these may very well survive a single blast hit. A single hit by a lightweight torpedo is still not necessarily a kill on a large submarine. It's an almost guaranteed flooding of one possibly critical compartment, though. There is very little if any experience with what lightweight torpedoes do to large submarines at depth in practice.
Lightweight torpedoes are not as fast as the fastest heavyweight torpedoes and may be outrun by nuclear attack submarines particularly at great depths. Light weight torpedoes also have quite small sonars and face various countermeasures deployed by the targeted submarines. We shouldn't assume a respectable probability of hit or kill by the employment of a single lightweight torpedo. A single torpedo is a suitable approach for testing whether a contact is a target or a false contact (provoking a reaction), and while -as mentioned before- a pattern of three or four lightweight torpedoes is a promising approach for engaging a high confidence contact.
Consumption of lightweight torpedoes is a major problem due to many false contacts and apparently lacking ability to recover & refuel/recharge those torpedoes in wartime for repeated use. This is done in exercises even with dedicated torpedo recovery boats, of course. NATO had a serious problem with North Atlantic convoy escorts because operational research suggested that the lightweight torpedo expenditure on false contacts would be so high that the escorts may have run out of munitions before completing a single crossing of the North Atlantic. On the one hand improved sensors have no doubt become better at identifying natural source signatures as no threat, but submarines might deploy entire swarms of decoys and even a torpedo might be mistaken as a submarine and trigger the expenditure of one lightweight torpedo for probing and three for the kill.
Heavyweight torpedoes might deploy submunition torpedoes that serve as moving decoys. This should be kept in mind when one thinks of hard kill defences.
Lightweight torpedoes can usually be launched by 324 mm torpedo tubes onboard of warships. These tubes are usually close to or even in the helicopter hangar for a reason; the torpedoes are the same as used by the helicopters. To add this last ditch defence (which is badly outranged by the submarines' heavyweight torpedoes) costs hardly anything in terms of money and weight. Such LWT launchers might become more relevant if hard kill defence against heavyweight torpedoes by lightweight torpedoes really is or becomes a relevant.
Throughout this text I assumed a silent nuclear attack submarine (SSN) or a conventional yet air-independent submarine (SSI) as the threat that needs to be defeated. These are the high ends among submarine threats - the vast majority of today's submarines are noisier, and most conventional submarines need to penetrate the surface with a snorkel from time to time in order to recharge their batteries with a (relatively loud) operation of diesel engines.
Some submarine types have small torpedo tubes in addition to larger diameter torpedo tubes. The smaller tubes are supposedly for anti-submarine torpedoes, which makes sense as the detection range in a submarine-on-submarine encounter would likely be short. These smaller tubes may still become more important as decoy and anti-torpedo torpedo (hard kill defence) launchers. The same can also be had by loading a container with multiple mobile decoys or small anti-torpedo torpedoes into a single large torpedo tube, of course.
The use of the periscope (for surface target identification) and snorkel isn't nearly as treacherous as one might believe. Submarines can first extend a tiny ESM antenna to sense any radar activity, then extend an electro-optical periscope for a few seconds to record a digital 360° panorama which can then be interpreted by humans while the periscope is back under the water surface again. Such E/O periscopes are apparently not standard yet, but they can be retrofitted quickly. Finally, snorkels are still small objects in an ocean with lots of floating trash and it's easy to minimise their radar return. The bigger concern is the wake that periscope/snorkel and even the submarine itself at periscope depth create (the wake causes a recognisable pattern in radar returns and may even be visible to the naked eye). This forces the submarine to be very slow when it's close to the surface.
Submarines may have towed sonars, too. This is common among nuclear submarines only, especially SSBNs and SSGNs (nuclear-powered submarines with a primary armament of ballistic or cruise missiles).
I wrote a lot about heavyweight torpedoes as threat munitions because they are de facto unique to ASW. I will write about defence against missiles in the AAW part. That will include submarine-launched missile threats.
Non-nuclear submarines are capable of sprints (about 20 kts), but their cruise speed is poor (about 4...10 kts). Nuclear submarines can be fast (20+ to almost 40 kts) at all times, but they choose to be slow much of the time in order to be less noisy.
ASW fixed wing aircraft and some ASW helicopters used to employ magnetic anomaly detectors (MAD) and diesel smoke detectors, but both are prone to false alarms and largely obsolete today.
I neglected the employment of manned submarines as convoy escorts so far. The U.S.Navy uses nuclear- attack submarines to support carrier battlegroups. This is way too expensive (more than € 2.5 bn per nuclear submarine) for escorting transport ship convoys of any practical size in my opinion. Non-nuclear submarines lack the cruise speed to escort any convoy. Submarines may be used to stealthily scout straits at neutral countries ahead of a convoy as a secondary mission of theirs, but I don't see much else they could cost-efficiently do for convoys. Their greatest asset - stealth - is lost once they employ the only effective long range sensor against other modern submarines - a LFAS.
Maybe some submarines do indeed find a hostile submarine and succeed at killing it even if both submarines are very silent/stealthy. This may add to a general naval campaign by attrition of the hostile submarine force, but is not likely enough for convoy security.
A torpedo is really small, and especially its frontal silhouette is tiny. There is thus a potential for active noise cancelling, both to hide the torpedo's propulsion noises and to cancel out acoustic waves sent by active sonar. Torpedoes have already an active sonar nose, so this could be used for the noise cancelling within a design-dependent frequency range. This technical possibility hints that very high quality torpedoes may be practically undetectable by the targeted ship itself. It would then depend on other sources for detection. That's another good point in favour of multistatic sonar networks.
Submarines are not really good at radio traffic. Some can deploy a tethered, floating radio antenna without leaving a deep altitude, but generally submarines need to maintain radio silence to not give away their location. This lesson was learned in the Second Battle of the Atlantic (in WW2). Submarines can receive radio messages while at depth without a tethered antenna, but only really long wavelengths penetrate the sea well. These wavelengths have great ranges, but low data rates. A submarine at 300 m depth will typically receive no long messages. It would rather receive a ELF or SLF radio signal with the message to be ready (at a lesser depth) for the real message later on. Submarines at a small depth on the other hand may receive VLF radio signals that are of greater use. They may tow an antenna that stays for this purpose at such a shallow depth even while the sub is cruising deeper to avoid cavitation, stay below helpful water layers and make MAD detection even less likely by sheer distance to the MAD sensor.
The takeaway from all this is that submarines are rather loners that fight on their own, rarely as a part of a combined arms team.
We shouldn't forget about the crews among all this technocentric stuff that coins naval warfare. The difference between a rested well-trained crew and an exhausted green crew is stark. ASW operators may also become desensitised by false contacts. False contacts could even be created artificially to this end; aircraft could disperse ten thousands of swimming radar reflectors over an ocean to make snorkels and periscopes indistinguishable, for example. A crew that works three times four hours per day in a watch system would also be exhausted and less alert after a week-long wartime patrol / convoy action than at its beginning. The crew would also be less alert at the end of a watch, so only stupid captains use a known watch rhythm in wartime (they sure shouldn't change watches at 0/4/8/12/16/20 o'clock!).
Some more remarks
The use of sonar is not a simple affair. There are layers of water in oceans with different salinity and temperature that keep acoustic waves from travelling straight. Variable depth sonars that could be lowered on a tether down into such layers (or have microphones in different layers at the same time) were very fashionable in the 1970's and 1980's for this reason.
Passive sonar is furthermore affected by the ship platform itself - the only passive sonars that work fine to the rear 30° or so are towed sonars. They are speed-limited, though. I saw published operating speeds in the 15...17 kts range and maximum survival speeds a little short of 30 kts. A ship that cruises at 20...25 kts would rather not make use of a towed sonar. Hull-mounted sonars on the other hand have no business trying to listen for what's behind the ship and can't enter lower water layers themselves.
Bubbles underwater can create an active sonar echo that may be mistaken for a submarine (by an active sonar) - this was used for deception back in the Second World War already. Bubbles don't move horizontally (save for the general drift of the water) as submarines often do, though.
Sonars can be used to create imagery of objects, and this is done when looking for naval mines or sunk shipwrecks. This is de facto not available at long ranges, though.
There are some rather elaborate acoustic 'stealth' technologies; the old prairie - masker has become publicly known, for example.
The thermal and salinity layers as well as the currents of the sea can be sensed with tiny drone or single use sensors. They could also be surveilled by slow-moving drones that travel along expected convoy routes and make regular reports by satellite uplink.
Ship identification by passive sonar is possible if the ship's acoustic profile was recorded before, and naval mines even use this to prefer real targets over simulators used by minesweepers. It's quite safe to assume that modern threat torpedoes have the same capability, which rules out some primitive acoustic decoys.
Cavitation is the forming of water vapour bubbles in water, caused especially at ship screws. This is noisy, damages the screws and is to be avoided if possible, but it's hard to avoid with sprinting torpedoes and sprinting surface warships. Cavitation depends on the vessel design and the water pressure (depth) mostly. Submarines may sprint silently at great depth even if they would be loud due to cavitation at a lesser depth. That's in part the appeal behind diving deep.
Blast is less effective at great depths than at shallow depths, so lightweight torpedoes usually have shaped charges to pierce a submarine hull at any depth. This helps especially against large double-hulled submarines, as these may very well survive a single blast hit. A single hit by a lightweight torpedo is still not necessarily a kill on a large submarine. It's an almost guaranteed flooding of one possibly critical compartment, though. There is very little if any experience with what lightweight torpedoes do to large submarines at depth in practice.
Lightweight torpedoes are not as fast as the fastest heavyweight torpedoes and may be outrun by nuclear attack submarines particularly at great depths. Light weight torpedoes also have quite small sonars and face various countermeasures deployed by the targeted submarines. We shouldn't assume a respectable probability of hit or kill by the employment of a single lightweight torpedo. A single torpedo is a suitable approach for testing whether a contact is a target or a false contact (provoking a reaction), and while -as mentioned before- a pattern of three or four lightweight torpedoes is a promising approach for engaging a high confidence contact.
Consumption of lightweight torpedoes is a major problem due to many false contacts and apparently lacking ability to recover & refuel/recharge those torpedoes in wartime for repeated use. This is done in exercises even with dedicated torpedo recovery boats, of course. NATO had a serious problem with North Atlantic convoy escorts because operational research suggested that the lightweight torpedo expenditure on false contacts would be so high that the escorts may have run out of munitions before completing a single crossing of the North Atlantic. On the one hand improved sensors have no doubt become better at identifying natural source signatures as no threat, but submarines might deploy entire swarms of decoys and even a torpedo might be mistaken as a submarine and trigger the expenditure of one lightweight torpedo for probing and three for the kill.
Heavyweight torpedoes might deploy submunition torpedoes that serve as moving decoys. This should be kept in mind when one thinks of hard kill defences.
Lightweight torpedoes can usually be launched by 324 mm torpedo tubes onboard of warships. These tubes are usually close to or even in the helicopter hangar for a reason; the torpedoes are the same as used by the helicopters. To add this last ditch defence (which is badly outranged by the submarines' heavyweight torpedoes) costs hardly anything in terms of money and weight. Such LWT launchers might become more relevant if hard kill defence against heavyweight torpedoes by lightweight torpedoes really is or becomes a relevant.
Throughout this text I assumed a silent nuclear attack submarine (SSN) or a conventional yet air-independent submarine (SSI) as the threat that needs to be defeated. These are the high ends among submarine threats - the vast majority of today's submarines are noisier, and most conventional submarines need to penetrate the surface with a snorkel from time to time in order to recharge their batteries with a (relatively loud) operation of diesel engines.
Some submarine types have small torpedo tubes in addition to larger diameter torpedo tubes. The smaller tubes are supposedly for anti-submarine torpedoes, which makes sense as the detection range in a submarine-on-submarine encounter would likely be short. These smaller tubes may still become more important as decoy and anti-torpedo torpedo (hard kill defence) launchers. The same can also be had by loading a container with multiple mobile decoys or small anti-torpedo torpedoes into a single large torpedo tube, of course.
The use of the periscope (for surface target identification) and snorkel isn't nearly as treacherous as one might believe. Submarines can first extend a tiny ESM antenna to sense any radar activity, then extend an electro-optical periscope for a few seconds to record a digital 360° panorama which can then be interpreted by humans while the periscope is back under the water surface again. Such E/O periscopes are apparently not standard yet, but they can be retrofitted quickly. Finally, snorkels are still small objects in an ocean with lots of floating trash and it's easy to minimise their radar return. The bigger concern is the wake that periscope/snorkel and even the submarine itself at periscope depth create (the wake causes a recognisable pattern in radar returns and may even be visible to the naked eye). This forces the submarine to be very slow when it's close to the surface.
Submarines may have towed sonars, too. This is common among nuclear submarines only, especially SSBNs and SSGNs (nuclear-powered submarines with a primary armament of ballistic or cruise missiles).
I wrote a lot about heavyweight torpedoes as threat munitions because they are de facto unique to ASW. I will write about defence against missiles in the AAW part. That will include submarine-launched missile threats.
Non-nuclear submarines are capable of sprints (about 20 kts), but their cruise speed is poor (about 4...10 kts). Nuclear submarines can be fast (20+ to almost 40 kts) at all times, but they choose to be slow much of the time in order to be less noisy.
ASW fixed wing aircraft and some ASW helicopters used to employ magnetic anomaly detectors (MAD) and diesel smoke detectors, but both are prone to false alarms and largely obsolete today.
I neglected the employment of manned submarines as convoy escorts so far. The U.S.Navy uses nuclear- attack submarines to support carrier battlegroups. This is way too expensive (more than € 2.5 bn per nuclear submarine) for escorting transport ship convoys of any practical size in my opinion. Non-nuclear submarines lack the cruise speed to escort any convoy. Submarines may be used to stealthily scout straits at neutral countries ahead of a convoy as a secondary mission of theirs, but I don't see much else they could cost-efficiently do for convoys. Their greatest asset - stealth - is lost once they employ the only effective long range sensor against other modern submarines - a LFAS.
Maybe some submarines do indeed find a hostile submarine and succeed at killing it even if both submarines are very silent/stealthy. This may add to a general naval campaign by attrition of the hostile submarine force, but is not likely enough for convoy security.
A torpedo is really small, and especially its frontal silhouette is tiny. There is thus a potential for active noise cancelling, both to hide the torpedo's propulsion noises and to cancel out acoustic waves sent by active sonar. Torpedoes have already an active sonar nose, so this could be used for the noise cancelling within a design-dependent frequency range. This technical possibility hints that very high quality torpedoes may be practically undetectable by the targeted ship itself. It would then depend on other sources for detection. That's another good point in favour of multistatic sonar networks.
Submarines are not really good at radio traffic. Some can deploy a tethered, floating radio antenna without leaving a deep altitude, but generally submarines need to maintain radio silence to not give away their location. This lesson was learned in the Second Battle of the Atlantic (in WW2). Submarines can receive radio messages while at depth without a tethered antenna, but only really long wavelengths penetrate the sea well. These wavelengths have great ranges, but low data rates. A submarine at 300 m depth will typically receive no long messages. It would rather receive a ELF or SLF radio signal with the message to be ready (at a lesser depth) for the real message later on. Submarines at a small depth on the other hand may receive VLF radio signals that are of greater use. They may tow an antenna that stays for this purpose at such a shallow depth even while the sub is cruising deeper to avoid cavitation, stay below helpful water layers and make MAD detection even less likely by sheer distance to the MAD sensor.
The takeaway from all this is that submarines are rather loners that fight on their own, rarely as a part of a combined arms team.
We shouldn't forget about the crews among all this technocentric stuff that coins naval warfare. The difference between a rested well-trained crew and an exhausted green crew is stark. ASW operators may also become desensitised by false contacts. False contacts could even be created artificially to this end; aircraft could disperse ten thousands of swimming radar reflectors over an ocean to make snorkels and periscopes indistinguishable, for example. A crew that works three times four hours per day in a watch system would also be exhausted and less alert after a week-long wartime patrol / convoy action than at its beginning. The crew would also be less alert at the end of a watch, so only stupid captains use a known watch rhythm in wartime (they sure shouldn't change watches at 0/4/8/12/16/20 o'clock!).
Conclusions
The construction and operation of dedicated surface warships is expensive, and they are thus scarce. To secure a single convoy with enough low frequency sonars and helicopters would require at a minimum two, more likely four such warships. Dozens if not hundreds of convoys would need to be secured if navies were to meet their claim that they're the service that protects maritime trade. Additionally, aircraft carriers need to be protected against submarines as well (at least until they're lost or damaged beyond repair for the remainder of the conflict).
The very minimum to come close to proper anti-submarine protection is to replace such scarce dedicated warships by other means in friendly coastal regions where land-based assets and boats can take over the ASW role .
A second requirement is that every frigate and destroyer has to be fully ASW-capable. The quantity of ships needed to secure a convoy grows badly when one employs dedicated ASW and AAW (anti-air warfare) warships. The AAW article will add to this case by showing that a mixed ASW escort and AAW escort group isn't cheaper than a smaller escort group of GP (general purpose) warships.
Third, there seems to be no way around using low frequency active sonar systems (LFASS) in ASW. So either we agree with Greenpeace and give up on ASW (which actually is an option and sensible for the vast majority of countries with a coastline) or we go all-in and do ASW properly, with effective LFASS technology. To keep antiquated ASW units in service that are ineffective against modern submarines helps only if the opposing forces employ about equally antiquated submarines. That's actually true regarding North Korea, Russia and the vast majority of navies in the world, but it can change quicker than the West could reform its ASW arsenal.
The scarce missiles with lightweight torpedo as payload (such as VL-ASROC) cannot be the primary means of probing underwater contacts. The much more cost-, weight- and volume-efficient helicopter-dropped LWT has to be available and be used much. Maybe the helicopters or dinghys should be tasked with torpedo recovery, but that's difficult to tell without knowing R&D results on the subject. Surface and underwater drones might be used for confirming underwater contacts as hostile, but this capability is not quite available off the shelf (unless you use minehunting drones) and may be a too slow approach, especially compared to a helicopter closing in and using a dipping sonar.
Submarines are much more different from civilian ship designs than any dedicated surface warships are. Civilian shipyards would thus have a harder time to mass-produce submarines than to mass-produce surface warships - especially in the first few years of a naval arms race. This is a good reason to believe that an all-out arms race in East Asia would feature a sub-proportional rise in submarine tonnage compared to the rise in surface warship tonnage.
This doesn't help ASW much, though; the quantity of false contacts (on which munitions will be expended) and the necessary quantity of escorts would not be affected by this. It only helps insofar as escort losses might be moderate and auxiliary warships (armed merchantmen) might be used as ASW platforms.
Anti-torpedo countermeasure kits should be available for equipping cargo ships despite their acoustic signatures. There's little point in giving comprehensive countermeasure kits to the protecting ships, but not to the ships that shall be protected. To spend on stocks of countermeasure kits and for their storage is not exactly something that naval bureaucracies want to do by instinct, though. They desire many impressive warships to cruise with.
A comprehensive countermeasure suite would include towed decoys/mines towed in on the wake to defeat wake-homing torpedoes, signature reduction measures on the ship hull and screws, mobile and stationary decoys to be launched when a torpedo attack is imminent or ongoing (and ideally recovered later) and anti-torpedo torpedoes. The latter could also be effective as ordinary lightweight torpedoes against submarines. The Russians also appear to trust multiple rocket launchers in the role of decoy launchers - these rocket launchers originally used to project depth charges on submarines and may actually still be relevant in this role at least at very short range in shallow waters.
Minesweeping knows simulator vehicles that recreate the acoustic and magnetic signature of ships to trigger naval mines. Short-distance convoys might make use of such boats (some of which are optionally manned) as anti-torpedo defence for the real ships. I doubt that transoceanic convoys would find it practical to have such boats cruise thousands of nautical miles at 15...25 kts.
Anti-torpedo countermeasure kits should be available for equipping cargo ships despite their acoustic signatures. There's little point in giving comprehensive countermeasure kits to the protecting ships, but not to the ships that shall be protected. To spend on stocks of countermeasure kits and for their storage is not exactly something that naval bureaucracies want to do by instinct, though. They desire many impressive warships to cruise with.
A comprehensive countermeasure suite would include towed decoys/mines towed in on the wake to defeat wake-homing torpedoes, signature reduction measures on the ship hull and screws, mobile and stationary decoys to be launched when a torpedo attack is imminent or ongoing (and ideally recovered later) and anti-torpedo torpedoes. The latter could also be effective as ordinary lightweight torpedoes against submarines. The Russians also appear to trust multiple rocket launchers in the role of decoy launchers - these rocket launchers originally used to project depth charges on submarines and may actually still be relevant in this role at least at very short range in shallow waters.
Minesweeping knows simulator vehicles that recreate the acoustic and magnetic signature of ships to trigger naval mines. Short-distance convoys might make use of such boats (some of which are optionally manned) as anti-torpedo defence for the real ships. I doubt that transoceanic convoys would find it practical to have such boats cruise thousands of nautical miles at 15...25 kts.
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