A Vision Of Future ASW

Anti-submarine warfare (ASW) always has been and always will be a challenging endeavor.  The submarine has the overwhelming inherent advantage.  It’s clear that surface forces (to include aircraft and ships) need a breakthrough technological and/or operational advantage to tip the scales back in their favor or even simply even them up a bit.  The problem is that most new technology is just enhancements of old technology: a sonar with better sensitivity, a variable depth sonar, a new P-8 Poseidon ASW aircraft to replace the old P-3, etc.  Even the latest concept of multi-static sonar is just an enhancement of normal sonar and has, thus far, proven generally ineffective.

The latest and hottest craze in ASW is unmanned vehicles.  Western militaries have gone absolutely “all in” on unmanned vehicles of every type and description and for almost every function in the military – and all without any substantive proof that unmanned vehicles are viable and effective in peer combat.  Sure, we’re all familiar with UAVs that shoot Hellfire missiles at some hapless terrorist but what about when we try to send that same UAV over the battlefield against Russia or China?  Odds are that the life span of that UAV will be measured in minutes. 

But, I digress …

I was talking about unmanned vehicles being the latest, hottest craze in ASW.  For example, the US military’s research group, DARPA, is developing an Anti-Submarine Warfare (ASW) Continuous Trail Unmanned Vessel (ACTUV) unmanned surface vessel that is supposed to trail submarines for weeks on end – without explaining how the small, low powered sonars on the unmanned vessel, with no human guidance, insight, or interpretation of data are going to effortlessly find and track submarines that full size, high powered sonars on manned destroyers that are also equipped with towed arrays and undersea warfare software suites can’t.

But, again, I digress …

Let’s move away from specifics and, instead, consider general concepts.  I came across the following description of one blogger’s vision of future ASW operations.  His vision illustrates the fascination with unmanned and beautifully captures the essence of what, I suspect, most people would see as a reasonable operational concept.  Here it is,

“So, a vision of near-future ASW could look like this.  Ahead of operations in a particular area, a number of autonomous underwater sensors are deployed, mapping the environment and generating an updated model of the sonar conditions that exist.  A force then moves into the area, screened by a number of USV’s [ed. unmanned surface vessels] deployed ahead of the force.  The escorts could search a huge front, with the USV’s up [front] and able to detect the SM [ed. submarine] before it can get into firing position and MPAs [ed. Maritime Patrol Aircraft], if available, would link into the network and add their own sonobuoy data.  Once detected, the SM is fixed and the force can deal with it however it wishes – surging helicopters or MPAs to harry the SM, or long-ranged engagements with a weapon such as ASROC.  It’s a worthwhile vision of how effective a Type 31 frigate could be if both it and two SEAGULLs [ed. a specific USV] were equipped with an interlinked system like TRAPS [ed. a type of variable depth sonar] and a weapon such as ASROC.” (1)

Note:  SEAGULL is a 12 meter long unmanned powerboat being developed by Elbit Systems

Note:  SM – I don’t know what that stands for as it wasn’t defined in the post but from the context it is clear that it refers to a submarine.

Well, that’s a very appealing scenario, isn’t it?  I’m betting most of you think it’s a pretty realistic vision.  Let’s take a closer look and see if its appeal holds up.

One fascinating aspect of the author’s vision is the battlefield preparation.  An unspecified, autonomous sensor is deployed in large numbers to map the battlefield and generate sonar models based on water conditions.  This is a great idea … if we know the battlefield ahead of time.  How often does a surface force have the luxury of knowing where an encounter with a submarine will occur?  Not often, bordering on never.

Yes, there are some areas such as navigational chokepoints where submarine contacts can be anticipated with some degree of likelihood but those are relatively few and such chokepoints are likely to be avoided by any surface force.  Beyond that, most encounters will be unexpected or only vaguely anticipated.

Thus, the ability to pre-deploy sensors is extremely limited.  ASW battlefields are going to be a surprise rather than planned.

The next aspect of the ASW vision is the employment of small, 12 m USVs with some sort of small sonar.  The vision anticipates these boats searching “a huge front” and detecting submarines before they can get into firing position.  Given that modern torpedoes have ranges of 20-30 miles (the Chinese Yu-6 torpedo is the equivalent of the US Mk48 and has a reported range of 28 miles), that means that the USVs would have to be deployed 25-35+ miles in front of the ships they’re escorting.  The area to be searched would, therefore, be 25-35+ miles in front and 25 miles or so to either side of the surface group’s path of travel.  That requires a search box perhaps 15 miles in depth and 50 miles wide, at a minimum.  That’s 750 sq. miles. 

Assuming a rate of advance of the surface group of 20 kts, the search box would also be advancing at a rate of 20 kts.  How many 12 m boats with small sonars would be required to search a 600 sq. mile box while advancing at a rate of 20 kts?  If a small boat with a small sonar had a field of view of 1 mile, it would require 25 boats in a line abreast advancing at 20 kts to maintain the search – and we’re ignoring the detrimental acoustic effect of the 20 kts speed on the boat’s sonar – hence the need for some depth in the search box to allow some sprint and drift type of tactic.

Where are 25 such boats going to come from?  A typical destroyer, for example, might be able to accommodate a couple of deployable small USV boats.  It would require 13 escort destroyers to carry and deploy 25 USVs!

Of course, we can manipulate the numbers by making different assumptions. 

This leads us to our next point of interest.  As we previously noted, if we’re going to believe that a handful of small boats are going to be effective then have to explain how our very best, highest powered sonars and towed arrays mounted on full size destroyers and operated by highly trained crew can’t find submarines with any significant degree of success but 12 m unmanned boats with tiny sonars powered by batteries are going to find submarines with unerring success.  If we actually had sonars that effective wouldn’t we be engaged in a crash program of retrofitting them to our existing destroyers?  Of course we would!  Since we’re not, it’s obvious that no small boat with a tiny battery powered sonar is going to find a submarine.

What becomes apparent is that such a boat is equivalent to a mobile sonobuoy.  Recognizing that, we see that such a small sonar CAN detect a submarine but the field of view is extremely limited (recall our earlier discussion about field of view and numbers).  Also, sonobuoys are static, once deployed.  As such, they are immune from self-generated acoustic “noise”, unlike the moving small boats which would likely be, essentially, deaf.

Perhaps we could deploy the small boats much farther in front – say, 30-50 miles.  That would allow some time to drift or move slowly while searching.  The problem there is how to get the boats there and then how to control/communicate with them since their communications are line-of-sight.  Communications limits the deployable distance to 20 miles, max – that’s not very far out in front and may not even exceed the operating host ship’s own sonar, depending on conditions.

Finally, note that the various platforms the author calls for in his scenario:

• small unmanned boats,
• an undefined field of autonomous sensors
• maritime patrol aircraft
• helos

What do these platforms have in common?  Answer: they’re all utterly defenseless and their effectiveness and survivability depends on the enemy offering absolutely no resistance.  So many modern US military (Western militaries, in general?) operational and tactical plans seem to depend heavily on the enemy’s cooperation.  For example, our plans to use UAVs and large, slow P-8 Poseidons depends on zero enemy opposition.  Would we allow the Chinese that kind of freedom to operate against us?  Of course not!  So, why do we persist in thinking the Chinese will allow us to freely operate all our shiny new toys?  I guess scenario planning is a lot easier when you start with the assumption that the enemy won’t do anything to hinder your actions!

Okay, I’ve critiqued the author’s vision of future ASW and without him being able to respond – an unfortunate and undesirable situation for which I apologize to the author – so what’s a better proposal?  Criticizing is fine but it’s preferable to offer an alternative at the same time.

We noted that ASW needs a breakthrough technology to level the battlefield.  Here’s some possibilities.

Unmanned “beaters” – torpedo-like vehicles that can travel ahead of a ship and carry active sonar.  The small size allows for many dozens to be carried on a ship as opposed to one or two unmanned surface boats.  The torpedo size/shape allows for simple launch from the host ship.  The purpose is less to detect a submarine than to flush a submarine from hiding.  The sub can either retreat – a mission kill – or attempt to close and risk increased odds of detection.

Wake Homing Anti-Submarine Torpedo – Submerged submarines leave wakes – large, long trails of turbulent, disturbed water which an ASW torpedo with a suitably designed sensor should be able to detect just as conventional wake homing torpedoes can detect surface ship wakes.  In fact, submarines impact their environment in many ways (thermal wake, Debye magnetic wake effect, eddies, chemical trails, minute radiation trails, etc. (2,3)) and those impacts can be sensed and tracked.  We need a brand new generation of homing technology based on previously impossible detection methods.

Limpet Particles – Small particles spread over large, suspect areas with properties of attachment and detectability.  Consider a submarine sailing through particle-seeded waters and slowly building up concentrations of attached particles that can be detected and tracked.  The attachment could be magnetic or some other attractive force that sea creatures would be immune to (it would do no good to track whales!).  The detection mechanism might be low level radiation, thermal due to induced friction, chemical (some unique, not naturally occurring chemical or isotope), or some other mechanism.

Penetrating Wavelengths – Acoustic wavelengths have been the traditional means to penetrate the protective water and detect subs but there is an entire spectrum of wavelengths that be able to penetrate water to useful depths.  Bear in mind that we don’t necessarily need to penetrate from the surface down to a thousand feet below the sea.  If the wavelength generator were mounted on a torpedo or some such device, the device itself could start at a hundred feet or a thousand.  The sensor would only need to penetrate the sphere immediately around itself to whatever range it can.

Bioluminescence – Many marine life forms generate light which is referred to as bioluminescence.  The passage of submarines disturbs the normal background light levels and studies have postulated the ability to find useful signals from the changes. (4)

Vortex – Similar in concept to wake detection, the vortices that submarines leave as the travel and maneuver can be tracked by mobile, underwater sensors.

And so on.

The purpose of this post is not to advocate for any specific technology but, rather, to note that we need to alter the current submarine/anti-submarine warfare balance which decidedly favors the submarine.  Unfortunately, with the end of the Cold War the Navy foolishly allowed its ASW capabilities and research to atrophy and we are now scrambling to catch up.  The Navy needs to begin conducting serious ASW research (not the warmed over obsolete version of ASW that they’re trying to install on the LCS but something truly effective) and developing new ASW tactics. 

We also need to cut the cord linking us so solidly to unmanned vehicles and expand our thinking to many other areas.  Until we do, submarines will continue to dominate the naval battlefield.

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(1)Verdigris blog, “Unmanned Systems and Anti Submarine Warfare”, 30-Dec-2017,

https://verdigris.blog/2017/12/30/unmanned-systems-and-anti-submarine-warfare/

(2)https://www.economist.com/science-and-technology/2016/11/12/hunting-submarines-with-magnets

(3)https://www.popularmechanics.com/military/navy-ships/a28724/submarine-sonar-soks/

(4)“The Anti-Submarine Warfare Potential of Bioluminescence Imaging”, Strand, Pautzke, and Mitchell, 1-Jan-1980,

http://www.dtic.mil/dtic/tr/fulltext/u2/a084124.pdf