World Naval Developments

By Norman Friedman

The Indian missile program includes a submarine launched weapon, Sagarika, with a 200-mile range. The Russians probably are helping develop it. It is not clear whether Sagarika is a cruise or ballistic missile, only that it is intended for submerged launch. It may be connected with Indian interest in building a nuclear missile submarine.

For years, the U.S. government has been trying to dampen enthusiasm for new ballistic missile projects, on the theory that the mere presence of such weapons encourages arms races.

There is, however, another side to this story. At present, the United States can threaten most Third World countries with relative impunity. But once they have acquired long-range missiles with nuclear or chemical warheads, they may impose countervailing threats, greatly reducing our freedom of maneuver. It would be tragic if the old arms-control mantra defense is destabilizing—left Americans no way of dealing with the likely threats of the next decade, or left us no alternative counter except our own nuclear attacks on other countries.

With the end of the Cold War, the strategic meaning of missile defense surely has changed.

Active Sonar Makes a Comeback

A new littoral-warfare submarine detection system, Distant Thunder, was displayed at this year's Navy League show. Developed by BBN Inc. under a Defense Advanced Research Projects Agency contract, Distant Thunder is related to projects for "enhanced echo ranging" (EER) developed for patrol aircraft.

In each case, listeners use echoes from the sounds created by small explosions to detect small, quiet submarines. The echoes are detected by existing passive sensors: sonobuoys in the case of aircraft; towed arrays for surface ships. The sound sources are distributed by air, in sonobuoy-size packages (SSQ-110s), which are detonated on command. Now, more powerful signal processor are available to make sense of the sounds (including echoes) received by the existing passive arrays. Past practice capitalized on the steadiness of the sounds generated by submarines, particularly nuclear ones. The new technique exploits the sharp, short irregular sound made by an explosive.

The approach is part of a larger trend away from passive detection and back to active methods. Passive detection was extremely effective against nuclear submarines because they had to run their machinery constantly, generating steady sounds at very nearly fixed frequencies. Even a very weak sound at a constant frequency can often be picked up if the sensor can listen for long enough; the surrounding random sound of the sea can be made to cancel itself out. The key is the stability (in frequency terms) of this narrow-band noise. The more stable the frequency, the longer the sensor can integrate and thus pull the signal out of the surrounding noise. The discovery of this phenomenon of stable frequency "lines" in submarine signatures was the key to much of Western Cold War antisubmarine warfare. Initially the lines appeared in the signatures of snorkeling submarines; they corresponded with cylinder firing rates. Although the rotating machinery of nuclear submarines produced weaker signals, they were purer, hence easier to detect. Moreover, they could not be turned off if, for example, the submarine slowed down. The sound-analysis technique, which was classified until the mid1970s, was concealed by its made-up designation, LOFAR (typically rendered as low-frequency analysis and ranging, but actually intended to show its kinship to radar and sonar). Moreover, only the last generation of Soviet nuclear submarines was quiet enough to cause any problems with this technique.

A diesel-electric submarine is a different proposition. Although relatively noisy when snorkeling, it is inherently quiet when running on its electric motor. The stable sounds it makes are either relatively weak or are emitted at very low frequencies, below those normally used in passive systems (blade rate). Moreover, the submarine can change signature, for example, by lying quietly on the bottom with all machinery stopped (a nuclear submarine almost never stops all of her machinery, since she generally has to keep turbo-generators and coolant pumps running). Most of the sound radiated by a diesel submarine running through the water is caused by flow over her hull. At least using current standard techniques, this broad-band noise is difficult to distinguish from the random noise of the sea.

That need not be true permanently. Mathematically, LOFAR breaks down noise by frequencies, but there are other mathematical ways to characterize noise. The noise of water flowing over a submarine hull could reflect the shape of that hull. Noise broken down according to a series of functions tuned to different shapes might show peaks corresponding to the shape of the submarine making that noise, and those peaks might behave much like LOFAR lines. LOFAR used frequency because it was easy to mechanize frequency analysis, using filters. Now that the analysis is digital, it need not correspond to any set of electronic filters—and other kinds of functions may be usable.

All of that, however, is for the future. For now, if diesel submarines are the main threat, passive techniques are not nearly as attractive as they were in the past. If the submarine does not produce enough of a signature, then the detector can impose one by pinging. Distant Thunder, however, is only one of two distinct approaches to this problem.

One is to rely more heavily on existing medium-frequency active sonars, such as the massive SQS-53C of surface ships and the SSQ-62 DICASS (directional command-activated) sonobuoys. Early in the 1990s, for example, large orders were placed for DICASS buoys. It seemed that the earlier passive SSQ-53 DIFAR (directional LOFAR) buoys would be abandoned. The Navy announced plans to retire its standard surface-ship towed array (SQR-19) on the theory that henceforth the active SQS-53C would be the only important shipboard ASW sensor. These sensors had been designed to achieve considerable ranges in the open deep ocean. In very shallow water, their powerful pings can reflect off the complex bottom, confusing them. The frequencies selected to achieve long range can open the sonar to more confusion, since it receives echoes from more distant bottom features.

Distant Thunder exemplifies an alternative. The explosive sound source produces a very short pulse with a distinctive shape. Given a powerful enough signal processor, that shape can be used to distinguish echoes from the explosion from the surrounding noise. An array of sound sources, exploded in sequence on command, produces a beam that can be swept across the bottom. It can be tailored to bottom topography.

The developers of Distant Thunder argue that the combination of very short pulses with very low frequencies makes for long inherent range while avoiding the trap of confusion caused by unwanted reflections. To some extent this solution already has been achieved by the low-frequency adjunct (LFA) developed for the Navy's long-range sound surveillance (SURTASS) ships. What Distant Thunder offers, however, is a low-frequency system easily integrated with existing U.S. Navy surface combatants (SURTASS ships are specialized low-speed units that cannot deploy with the fleet, and their sound sources are quite massive). The ships` LAMPS helicopters lay the sound source arrays. Existing towed arrays receive the echoes; the only new equipment is a very powerful computer, which processes the signal and draws tactical inferences. Connecting it to the output of the existing towed array requires very little effort.

Historically, Distant Thunder and enhanced echo ranging fit an established pattern. When passive techniques must be discarded, the most natural solution is to retain the existing receivers but add a source of active signals. For example, when LOFAR was first discovered it offered so dramatic an improvement in passive range that the U.S. Navy began to build fixed sound surveillance (SOSUS) arrays, which it hoped might detect snorkeling submarines at ranges as great as 600 miles. In 1956, however, the Royal Navy discovered that, by careful attention to silencing, it could drastically reduce the range at which its new diesel submarines could be detected even when they snorkeled. The main British underwater sound research center published a classified paper: SOSUS, just being installed, was already obsolete. The U.S. Navy seriously considered emplacing massive pingers in the oceans (some readers may recall the big transducer experimentally carried on board the tanker Mission Capistrano ; it was a half-size prototype). As it happened, the Soviets did not quiet their diesel submarines, and LOFAR/SOSUS proved extremely effective against Soviet nuclear submarines.

At that time aircraft could not accommodate the analyzer required for LOFAR. Passive sonobuoys simply picked up sound in the water and transmitted it by radio link to aircraft. If the submarine was loud enough, it was detected. If, as the British predicted, it could be silenced effectively enough, it would become undetectable. The solution was explosive echo-ranging, a system code-named Julie. The operator had a stop watch. He heard the sound of the explosion itself, followed by the echo off the submarine. The time interval gave the range.

Although quite simple, Julie had severe disadvantages. Timing alone would not indicate range; data from several buoys (or directional data from one buoy) were needed. Also, it could be used only in deep water, since otherwise a reflection off the bottom might be mistaken for a submarine. Airborne LOFAR (Jezebel) solved the problem of submarine silencing.

Western navies retained the potential for Julie ranging, since their sonobuoys could send the sort of broadband data needed to describe a short, sharp explosion or its echo. The conversion to narrowband data (needed for LOFAR) was done only on board the airplane monitoring buoys. The Soviets also developed some important improvements in underwater sound sources. Strings of explosives spiraled around vertical poles formed a beam that scanned and depressed at the same time. Even without much improvement in signal processing on board the airplane, forming a beam from the sounds of the explosive sources could help solve the problem of reflection off the bottom.

For now, it seems that Distant Thunder and its airborne equivalent, enhanced echo ranging, give much better search rates than conventional medium-frequency active sonars. They are likely to cause the surface and air navies to retain the passive receiving arrays that, until recently, seemed doomed. Retention, moreover, would leave open the possibility of shifting back to passive operation should some new kind of analysis, better suited to broad-band noise emerge (which happened with Jezebel).


Norman Friedman is a prominent naval analyst and the author of more than thirty books covering a range of naval subjects, from warship histories to contemporary defense issues. He is a longtime columnist for Proceedings magazine and lives in New York City.

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