Look Who's Talking Two-Way
The Royal Navy has tested successfully a towed antenna to provide two-way Link 11 service to a Trafalgar-class nuclear powered attack submarine (SSN). Many NATO submarines (including all U.S. SSNs) already have one-way Link 11 (receive only) installations, but a full two-way system obviously offers considerably more flexibility. Inboard electronics, moreover, are now so compact that the British installation reportedly provides space for future installation of Link 16 (more likely, Link 22, NATO Improved Link Eleven [NILE], which transmits Link 16 data using Link 11 radio media). That is no surprise, given developments in computer technology over the past two decades. For many years Link 11 has been the standard NATO naval data link, unifying battle groups and making international naval cooperation effective at the tactical level.
Link 11 normally is transmitted by both high-frequency (HF) surface waves for maximum uninterrupted range and by ultra-high frequency (UHF) for units within line of sight. HF originally was adopted so that ships of a battle group could disperse both to limit vulnerability to nuclear attack and to deceive satellites.
Tactical links become more and more important as submarines cooperate more closely with surface forces. Within a battle group, Link 11 carries updates to a shared tactical picture. Typically, units of a surface group build up the picture as they operate together. The link also carries commands; the group's antiaircraft commander, for example, can use it to order a unit to engage an incoming target. Units receiving Link 11 messages, then, are expected to share a common picture of the locations-and the identities-of friendly, enemy, and neutral submarines, ships, and aircraft. In the late 1960s, when the U.S. Navy became interested in using fast nuclear submarines as carrier battle group escorts (the "new" direct-support role), the Los Angeles (SSN-688) class was designed to accommodate a digital computer specifically to make sense of Link II messages; with a towed wire antenna, the submarine could receive HF signals carrying Link II messages. Link 11 itself was revised so that a submarine, which could receive messages only intermittently, could take in the whole tactical picture in a single burst.
The submarine's role was to detect enemy submarines approaching the battle group, but this often was complicated by the presence of surface shipping. If she were given the positions of the nearby surface ships, even intermittently, the submarine could hope to filter them out to expose her true quarry.
Direct support turned out to be impractical. Communication with the submarine was too spotty, and submariners always had a lively fear that, no matter how tight the tactical control, some battle group escorts would mistake them for the enemy. The only way to avoid such disasters was to maintain continuous two-way communication with the submarine-impractical at the time.
The submarine could receive HF signals using a wire antenna trailed at or just below the surface of the water, but that limited her maneuvers. A wholly submerged antenna could receive lower-frequency signals. Neither could usefully transmit. A submerged submarine could release a buoy to transmit a recorded message for an airplane, but that was a relatively inflexible one-way system. The same submarine could use a dish atop a mast for two-way communications, but only at periscope depth.
Some of the methods used for communication illustrate the limitations of submarine communications. U.S. warships use their SQS-53 low-frequency sonars to send alerting signals to submarines as part of a system called Probe Alert; the submarine then rises to periscope depth to listen. Antisubmarine warfare aircraft can drop special signal buoys for the same purpose. It also is possible to send the alerting message via the extremely low-frequency (ELF) system normally used to communicate with nuclear-powered ballistic missile submarines (SSBNs) after which the submarine can receive a satellite message, if no U.S. warship or airplane is nearby. There are also, of course, regular broadcasts to submarines via low-frequency radio receivable at periscope depth (but submarines rise to that depth mainly to await their scheduled signals). Because communication was inherently intermittent, the submarine could never be precise in her knowledge of battle group movements. Direct support was abandoned in favor of more distant support, for which intermittent information sufficed. In the new concept, submarines operating well ahead of a moving battle group could sanitize a lane down which it ran.
Now it may be time to revive direct support. In shallow seas, sonar range is limited; the convergence zone, used in deep water to detect targets as far away as 100 miles, does not exist. Reflection off bottom or surface may confuse sonars. Enemy diesel submarines may spend much of their time in ambush, lying silently on the bottom. Western nuclear submarines are still extremely effective means of detecting and killing opposing submarines, but if their own sonar ranges are very short, they may have to be used in greater numbers and closer to the supported battle or amphibious group. Their commanders will need the most current locations of surface ships.
If water is relatively shallow, submarine depth limitations usually associated with towed antennas may be almost irrelevant. Presumably, the towed antenna imposes few limits on submarine speed and maneuver.
A reliable two-way submarine radio link carries another interesting implication. If two submarines can exchange data reliably, they can triangulate noise sources rapidly. They also should be able to avoid attacking each other. Until now, it was assumed that such valuable communication would have to be acoustic. Submariners always were reluctant to emit unnecessary noise, so any acoustic link also had to be stealthy. Given long detection ranges, cooperating submarines might be tens of miles apart, which required a long-range acoustic capability.
None of this was easy. Sound can travel along multiple paths of different lengths from point to point through the sea. Signals traveling these paths tend to interfere at the receiver, smearing out digital signals. In the 1960s, multipath interference defeated the first U.S. attempts at covert acoustic communications. Conventional wisdom held that the Soviets were doing better, probably because their signals were far simpler. By the late 1970s, however, there was some hope that computers could correct for the interference problem. Apparently that worked better in theory than in practice, since as late as the 1990s a computer based acoustic data link was functioning only at the advanced technology demonstration level.
If the British two-way link works, matters become far simpler. Because it operates at frequencies at least 1,000 times higher than acoustic links, a radio link carries far more information, and can still operate on a stealthy basis-i.e., spread spectrum. Not surprisingly, it is reported that the U.S. Navy is interested in the new British system.
Link 11 itself is almost obsolete. Its likely successor, Link 16, offers a much richer vocabulary and better precision, since it transmits far more information. Link 22 (NILE) is a NATO project to use Link 11 media to transmit Link 16 data. Since NATO warships already have radio transmitters and receivers suited to Link 11, NILE is a natural follow-on. Whether it can be adapted depends entirely on whether a ship has adequate computer capacity-and that is becoming less and less a problem as commercial computers evolve far beyond standard military types. Thus the real limit on adoption of NILE is probably signal propagation, which is a real issue for submarines. It also is the issue the British experiments have apparently put to rest.
Soviet-Built Kilos Getting SS-N-27s?
The Russians have begun showing animated videos of Kiloclass submarines firing the Novator Alpha (NATO SS-N-27) antiship missile from their torpedo tubes-and reportedly have conducted actual firing demonstrations for the Iranians on board Iranian Kilos. The missile is apparently an adaptation of the earlier SS-N-21 torpedo-tube-launched cruise missile. In tactical form, the missile cruises toward a target subsonically; once within range (about 20 kilometers), it fires a supersonic weapon at the target. Unlike other new Russian anti-ship missiles, such as Raduga's Moskit and Chelomey's Yakhond, Alpha can be fired from a standard 21-inch torpedo tube, just like the SS-N-21. The main internal adaptation is presumably a computer modification. The Project 877 Kilos already have a computer-driven weapons control system, hence is presumably easy to adapt to a new weapon. Similarly, Alpha is probably easily adaptable to existing Russian attack submarines.
In the past, Chelomey's Yakhond (now NATO-designated SS-N-26) and an associated missile (confusingly called Alpha) have been advertised for submarine launch, but they apparently require larger-diameter tubes. That might mean the 65-centimeter tubes of many existing submarines or the vertical tubes of the new Severodvinsk, or the big inclined tubes of surviving Charlie-class antiship cruise missile submarines. For the Kilo export submarines, the Chelomey design bureau (Mashinostroyeniye) has displayed sketches showing four inclined vertical tubes in a special section installed abaft the sail. Although Yakhond is now being offered as ready for shipment, no one yet appears to have bought a stretched Kilo.
Novator's missile is a much more interesting proposition, because it can be adopted by any nation that operates standard Kilos. It probably cannot be fired by older submarines with analog fire control systems (the Russians have offered digital replacements, but as yet there are no reported takers).
The missile was shown first at Abu Dhabi in 1993; it then appeared at the 1993 Moscow Air Show. In both cases the Russians apparently hoped that the Chinese, who were buying Kilos, also would buy Alphas. This may be the missile sometimes reported on board Chinese submarines. At the shows, Alpha always was described as a coast-defense weapon, but it showed torpedo-tube lugs and, therefore, was clearly something more. Novator ("leader" in Russian) has been responsible for a variety of innovative missiles, including ballistic ASW missiles fired from submarine and surface-ship torpedo tubes (NATO SS-N-15 and -16).
Alpha disappeared from Russian arms shows after 1993. Later it was learned that Novator itself was one of a very few companies forbidden to exhibit at such shows; its projects were, in effect, black. Given its ancestry, the Alpha missile can best be seen as a Russian equivalent to the antiship version of Tomahawk, with a shorter range (a little more than 100 nautical miles). The main difference is that it ejects a supersonic submissile intended to defeat its target's terminal defenses.
Surface ships probably can fire Alpha. Novator's 53- and 65centimeter ballistic ASW missiles were fired initially from submarine torpedo tubes, such as the defunct U.S. SUBROC. Eventually the 53-centimeter version appeared in a surface-ship version, fired from conventional surface torpedo tubes (for example, on board the later Kirov-class nuclear-powered cruisers). It would not seem farfetched to imagine that the designers (or, more likely, those who wrote the Alpha specification for the old Soviet Navy) envisaged an alternative firing mode, from standard surface torpedo tubes.
Alpha certainly has been tested, but its production status is less certain. Much the same can be said of the new Yakhond. Each is ready for production, but money is scarce in Russia; it is not clear whether anything exists beyond the prototypes.
The new status accorded Novator, one of a handful of companies now banned from shows, suggests that its designs are perceived as the next generation, which will equip the Russian forces likely to emerge from the ashes of the breakup. A conscious decision may well have been made to sacrifice the security of the current generation of weapons (including some not yet in service, such as Yakhond) in hopes of securing enough orders to finance a future. Another possibility is that the Chinese demanded that Russian companies supplying them keep their wares secret-to ensure Chinese security, not Russian. In that case, Novator may have decided (or may have been assigned) to work exclusively for the Chinese.
As for Yakhond, which received considerable publicity at the 1997 Moscow Air Show, it apparently began as an alternative to the Moskit missile (SS-N-22) which arms the Sovremenny, modified Udaloy, and Tarantul classes (see the 1997-1998 edition of World Naval Weapons Systems).