Winning the ASW Technology Race

Huge technical efforts are needed to provide ASW platforms with sensors good enough to detect new and stealthier generations of submarines.

Antisubmarine warfare is one of the longest running races in naval warfare— a race between the technologies of find­ing submarines and hiding them. With the hiders now pulling ahead, it is time for renewed ASW technology efforts. The United States can sidestep some mil­itary problems, but not ASW.

The oceans cover about 105 million square nautical miles. In war, perhaps only a few tens of millions of square miles would be militarily significant at any one time. But for most of the 74-year history of ASW, average effective opera­tional search rates for an ASW ship, sub­marine, or aircraft have been well under 1,000 square miles per hour, making comprehensive searches impossible and giving submarines ample opportunities to hide. Success in ASW always has hinged on narrowing the areas to be searched with convoying, intelligence, and sur­veillance as major tools. But such means never have had enough certainty or preci­sion to allow a missile to be dispatched to some indicated point with much assur­ance that it would find a hostile subma­rine there.

Thus, ASW forces have needed large numbers of “platforms” that seek to gain more precise information about the loca­tions of submarines whose general pres­ence is already known or suspected, and then to deliver weapons against them. Because detection ranges are so short, it is essential to have a great many plat­forms to achieve anything approaching adequate coverage. There are other fac­tors as well—for example, ASW forces are required to cover widely separated areas—so it is misleading to suppose that increases in detection ranges can translate directly into reducing numbers of plat­forms needed. But it is clear that if the ranges of tactical ASW sensors are re­duced significantly, then the current forces of ASW platforms will become seriously inadequate.

Open-literature sources suggest that radiated noise levels of Soviet attack sub­marines have been cut by about 25 deci­bels (dB) since the late 1970s, which means that the actual sound pressures are about 0.3% of their original values. This can be expected to slash the range of any given passive acoustic sensor by roughly 95%. Because it takes years to replace an entire submarine fleet, we have not yet felt the real effects of Soviet quieting. But we will, soon enough.

Because passive acoustic sensors gen­erally have been our most effective sub­marine detectors, a steep decline in pas­sive acoustic ranges is a particular concern. Must we abandon passive acoustic sensors? What will take their place? Is there any way to achieve effec­tive ASW capabilities against quiet threats?

The answer is that technology holds reasonable promise for bringing the anti­submarine side back to the point where it is neck and neck with the prosubmarine side in the race. But this is not technology in the guise of the cheap and easy maker of miracles. ASW technology will have to take major strides in scale and com­plexity.

It must be emphasized that the problem is that submarines are hiding too well, and we need the technology to find them better. We must not expend major re­sources in solving minor problems, sim­ply because they are easier or because they lend themselves to solutions that are “fun,” or pleasing to some interest. Some subsidiary problems also need sig­nificant attention, but anyone who sug­gests putting anything in ASW ahead of improving detection capabilities might as well be in the pay of the Soviets.

Since platforms are the most expensive factor in ASW and do figure prominently in finding submarines, it is necessary to think about platform improvements in terms of how they can improve detection. ASW platforms use many different meth­ods to find submarines, but most fall into two classes: platform sensors or distrib­uted sensors. A sonar mounted on or trailed by a ship or submarine is an exam­ple of a platform sensor, as is a magnetic anomaly detector (MAD) on an aircraft; there are many others. Today, the sono- buoy is virtually the only example of a distributed sensor.

* The detection capacity of platform sen-

i sors usually can be represented in a sim-

I pie way as the product of three factors:

the platform's speed, the width of the search swath swept by the sensor, and the average probability of detecting a subma­rine that falls within this swath. This is the search rate, and it also can be calcu­lated for distributed sensors and platform sensors that operate in a dipped or sprint- and-drift fashion, although in a somewhat more complicated way.

Currently, ships and submarines de­pend exclusively on platform sensors; aircraft nonacoustic sensors also belong to this class. Obviously, the search rate can be increased by increasing the plat­form’s speed, but this tends to involve major costs for minor gains. Fixed-wing aircraft are the only class of ASW plat­form for which greatly increased speeds are possible, but even in the air, speed is costly beyond a certain point. Moreover, aircraft depend mostly on deployed sen­sors for ASW, and in such cases, in­creases in platform speed do not translate directly into increases in search rate.

The speeds of spacecraft are so great that if the spacecraft could search even a relatively small swath width with a low probability of detection, it could have a large search rate. For instance, a space­craft in a 90-minute orbit that could de­tect submarines lying ten nautical miles to either side of its track with a probabil­ity of 35% would have a search rate of 100,000 square nautical miles per hour! Ideas for space-based ASW sensors de­serve serious consideration, therefore, even if they will be difficult to imple­ment.

Leaving aside the possibility of subma­rine-detecting spacecraft (pending devel­opment of suitable sensors), none of the foreseeable platform technologies can provide significant increases in search speed per dollar. But another way to in­crease search rate is to increase the sensor-carrying capacity of platforms that use distributed sensors. With little in­crease in the P-3C’s size or cost, for in­stance, it would be possible to provide capacity for twice its 84 sonobuoys, plus enough endurance to permit dispensing and monitoring them all in a single mis­sion. This would increase the search rate by a factor of two or so for a small incre­ment in life-cycle cost. This appears to be one of the major goals of the Navy’s long-range air ASW-capable aircraft (LRAACA) program. It is more difficult to make such gains with shipboard air­craft, whose weight and size are severely constrained.

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The details of the prospects for p^a: sonars (and other sensors) are too te< cal and too sensitive than is approp¹ for this discussion. Very broadly, bo ever, substantially better perform³” can be obtained at the price of ^vaS greater complexity in terms of nunm¹- of hydrophones, amounts of electron¹ “ and volume of computations, as wel better exploitation of the acoustic e” ronment. To develop these vastly ^c°

ballistic missiles, major space sys> or even the Strategic Defense Initw At best, it may be impractical for P ₍ sive sonars to regain all of the rang⁶ to quieting. One partial solution may the greater use of distributed sens __t widely scattered to ensure that the I comes within detection range.

Given the difficulty of increasing

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sive sonar performance, active

Technology has increased the effec­tiveness of this P-3C tactical coordi­nator a hundredfold. But in time, his successors will need near-human computerized assistants, to help handle the llood of data from advanced sensors.

Many proposals for platform technol­ogy center on improving survivability through more hardening, greater stealth, improved countermeasures, or other means. Recently, specific concern has been expressed over the vulnerability of ASW aircraft, even though sober reflec­tion on the performance of Soviet air de­fenses in intercepting large airliners that have wandered over Soviet territory would not lead to excessive anxiety over the safety of ASW aircraft hundreds of miles out at sea. Another aspect of air­craft vulnerability is the danger of coun­terattack by submarines. It is widely sup­posed that submarines can hear and localize aircraft noises, but the submarine

would have to carry a torpedo-tube-size missile to attack any aircraft more than a mile or so from the launch point, and without real-time guidance updates, kill probability is bound to be poor against an aircraft equipped with good warning and countermeasures systems. Also, launch­ing missiles tends to attract the sort of attention submarines normally seek to avoid, so it is questionable whether the submarine would choose to sacrifice scarce weapon space and run an increased risk of being attacked by ASW forces for a relatively small chance at killing an ASW aircraft that might otherwise never detect, localize, or classify the sub. Air­craft, after all, are the only ASW units that are a great deal less expensive and more numerous than the submarines they hunt, have far smaller crews, and can be more easily and rapidly replaced.

The value of reducing the vulnerability of ASW units depends in large part on how great their vulnerability is in the first place. It is obviously important that the attrition of ASW forces be at a slower

rate than that which they inflict on th<j submarines, and ASW crews shou ^ never be exposed to needless risk. But makes no sense to reduce the risks to °^ne class of ASW platform at the cost of >ⁿ creasing the losses to others—or to o prive the troops ashore of reinforcerns and resupply because fear of risks ASW forces prevents them from mee^Iin" the threat.

If platform technology cannot impr⁰'^L ASW search rates significantly, we ^a forced to refocus on sensor and syste ■ technologies. There are three broad ^cate gories: passive acoustic, active acousd > and nonacoustic (known to acoustic¹³ as “un-sound” sensors). There has be speculation that passive acoustic sens will pass from the scene altogether be supplanted by active sonars. F^orI nately, the future of passive sonar not appear quite so dark as that.

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plex systems at all will take major eff° ^ to package them in a feasible size and' practical cost for ASW use will more. Altogether, the prospect is f°^rS ,_£ entific and engineering efforts on a s hitherto associated with programs ^sUC_fnSi

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probably will have greater promin^e,K Certainly this would be desirable sin” would limit U. S. vulnerability to su unanticipated gains in quieting by , Soviets. But we must remember also ^ active-sonar stealth may be possibl well. .„J

Long-range active sonars must ^ advanced passive sonars in size and ^c ._fl plexity. Because sound travels bett^e|.^, the lower-frequency range, the ^aC sources must be large and, of ^c°¹' ^ high power is also essential fo^r _lS range. At the same time, these sy^st” will need receiving arrays with ¹¹¹ hydrophones and very powerful P^r0L i ing; indeed, it is possible that advnⁿ