Indian Navy Tests Missile
In 11 April, the Indian Navy test-fired its new surface-to-surface missile, Dhanush, from the converted patrol craft Subhadra, a unit of the 1,650-ton Sukanya class. Indian press reports described the shot as "partially successful," which presumably means that it left its launcher but failed to hit its target. Dhanush is a naval version of the 250-kilometer (about 135 nautical miles) Prithvi missile. Plans call for all seven units of the Sukanya class to have pairs of stabilized Dhanush launchers on their helicopter decks. Presumably, the missiles will be housed in the present helicopter hangars.
The liquid-fueled Dhanush is hardly a Polaris, and a small patrol boat is hardly an invisible submarine. The test is significant, however, because once such missiles have been deployed India will have something not too different from the secure second-strike strategic force the superpowers have built. As in most Third World countries, Pakistan's wealth and economic muscle are concentrated near the sea, so even a short-range missile like the Dhanush is a real strategic threat (it may, however, lack the range to deal with Pakistani troops inland, threatening the Indian position in Kashmir).
More important, the Indian test reminds us that there is still a sea sanctuary, that once ships vanish beyond the horizon they are rather difficult to find, even in an era of commercial imaging satellites. During the Cold War, the United States and the Soviet Union spent very heavily to develop space-based ocean reconnaissance systems to target enemy surface combatants for long-range antiship missiles. Each found it difficult. The radar satellites the Soviets developed could indeed find ships at sea, but there are so many such ships that the satellite produces a confusing overload of data. Even sea clutter can be mistaken for ships. To some extent the Soviets simplified their problem by excluding ships below a fixed (rather large) size, which made it difficult for them to spot U.S. surface combatants armed with land-attack cruise missiles. The Indian Navy's ships are smaller still, hence even more difficult for a radar to distinguish in sea clutter, let alone to identify. The U.S. system relied heavily on using the ships' own distinctive emissions to identify them—but the Indians probably will equip their strategic surface ships with commercial-type navigational radars nearly indistinguishable from many others in the area.
With no obvious commercial uses for such satellites, it would seem to follow that no commercial supplier will be able to offer a country without considerable resources any equivalent service. As for the new commercial imaging satellites, the sea is still very broad, and the chance that a ship will occupy the very narrow strip the satellite images just when the satellite is overhead is very small. Thus it is unlikely that the new commercial space technology will eliminate the sea sanctuary that navies enjoy. Certainly ships can be found by radar-equipped search aircraft, but a small patrol boat generally will look, on radar, much like a big fishing boat or a small freighter. It can be identified optically, but to do so the airplane must close to within a few miles. That is very difficult in even so small a water area as the Bay of Bengal; there are just too many potential targets. It seems unlikely that imaging synthetic-aperture radar (ISAR) will do much better.
In this sense, India is buying a deterrent not too different in concept than the systems built by the great powers; of course, it is not as satisfactory. A submarine waiting off the patrol ship's base probably can pick it up and track it easily, yet quite covertly; at the appropriate moment the patrol ship is relatively easy to dispatch with a torpedo. Nor is the missile itself altogether satisfactory. It has only a short range, so the ship carrying it must operate from within a small offshore "box;" presumably it patrols farther out and runs in (on command) to shoot. The missile uses liquid fuels; its twin engines are derived from that of the old Soviet SA-2 surface-to-air missile. Shipboard safety is likely to be questionable. Moreover, the missile is unlikely to be accurate, particularly if it must be launched from a moving, rolling, pitching ship (albeit one with fm stabilizers).
All of this having been said, the Indian achievement is notable for having side-stepped the most difficult problems the United States, the former Soviet Union, Britain, France, and China all had to solve. The Indians may provide inspiration to other Third World countries interested in developing their own secure or reasonably secure deterrents. The Israelis, for example, who apparently have land-based nuclear missiles of their own, surely fear that once their more or less hostile neighbors acquire both nuclear weapons and medium-range ballistic missiles, they will begin to imagine the pleasures of a first strike. It can certainly be argued that Israeli territory is too constricted to make any form of land-basing really secure, and that the Israelis themselves would hardly want to adopt a launchon-warning policy, with all the undue reliance on sensors that implies. Perhaps they think that antimissile weapons, such as their new Arrow missile, will solve the problem. Alternatively, perhaps they are interested in something like the Indian shipboard missile, presumably with much better range.
Nor should Americans smugly imagine that a shipboard missile is no more than a Third World threat. The Coast Guard's current Deepwater modernization project includes a substantial investment in command, control, and surveillance. Perhaps the Indian example suggests that Deepwater is a necessary part of any national strategic defense.
Communicating Underwater
At a recent conference on military communications, Lockheed Martin-Sanders described work it had been doing for the Naval Underwater Systems Center on acoustic communications. Underwater communication has long been the bugbear of underwater warfare. The sea distorts any signals put into it, as anyone who has heard an underwater telephone will attest, because there are so many paths through the sea.
Yet underwater links are very desirable. At the least, effective links would make submarine-surface unit cooperation practicable, where in the past it has been, at the least, quite dangerous. The British, who were very interested in such cooperation, tried to develop a link which surface ships could use to communicate with submarines. Indeed, one justification the British cited for developing a computerized submarine fire-control system was to improve such coordination. Again, the link seems not to have been successful. Without a link, the best that can be done is bell-ringing. A coded sonar ping goes out, and submarines either come to periscope depth (to communicate by radio) or they launch buoys or floating cables. There is no quick and reliable way, for example, of informing an accompanying submarine that a battle group has made an unexpected turn, and therefore that the submarine should turn accordingly. Identification friend or foe (IFF) is entirely out of the question, with unacceptable consequences for any submarine in close support of a surface group. Yet close support is more, rather than less, likely to be valuable in littoral waters, where sonar ranges are often very short and where a submarine in direct support may be the group's best underwater sensor platform. For a submarine to have to come to periscope depth to communicate is generally for her to lose her sonar picture altogether, which is hardly desirable.
The solution to multipath is computing power. In a simpler case of multipath, high-frequency (HF) radio, the solution is to insert test messages periodically among the real messages. The receiver uses the test messages to measure the degree and nature of distortion, and to correct what it receives accordingly. This technique is currently used to increase the rate of transmission about 30-fold, from 75 to about 2,250 bits per second. Further increases come by stacking bits, transmitting at multiple energy levels. In this way experimental high-frequency radios can reach data rates as high as 28,800 bits per second, which would have been unimaginable in the past. Note that the original Link 11 used multiple parallel channels to go from the usual 75 to 2,250 bits per second; applying similar techniques to the new computer-corrected systems might make megabit transmission rates possible.
Transmission through water is more complicated. Sound travels at much slower speeds, so signal frequencies are far lower than for radio. A signal cannot be transmitted at a large fraction of overall carrier wave frequency, so the basic signalling rate is far slower than for radio. That is, HF radio operates at 3 to 30 megaHertz (mHz), which is 3 to 30 million cycles per second. A good long-range sonar operates at about 1.5 to 3.5 kiloHertz (kHz), 1.5 to 3.5 thousand cycles per second. Because sound travels slowly, about 1/200,000 the speed of radio (or light) waves, a sound message takes far longer to cover a given distance. During that time it undergoes considerable distortion. One way of saying this is that distortion covers many more symbols in an acoustic than in a radio message. Too, distortion is very dependent on frequency. Yet the basic idea, that computer correction based on sampling can overcome distortion, is still valid. Lockheed Sanders realized that modern computers offered sufficient capacity to take much longer sequences into account. The idea was not new; about 1980, Sperry claimed that by using a UYK-20 computer it could provide a valid underwater link. It failed, because the computer had nothing like enough capacity. To some extent, the new technique employs a separate probe of the communications channel, which is used for coarse adjustment of the receiver; that is an entirely new technique.
Lockeed Sanders succeeded spectacularly. It tried two different links, one operating at high frequency (20-30 kHz) and high data rate (10 kbits/sec) and another at medium frequency (2-4 kHz, as in a standard destroyer or submarine active sonar) and medium data rate (2.4 kbps, as in Link 11). In fiscal year 1999, both in deep and in shallow water, it demonstrated a 35-nm communication range between a surface ship and a submarine at the low frequency. It demonstrated submarine-to-unmanned underwater vehicle (UUV) communication at 2.5 nautical miles using the high-frequency link; this rate was sufficient to transmit pictures. On the basis of this success, the Naval Underwater Weapons Center has become interested in a UUV, which it calls Manta, and which it sees as the stand-off sensor/weapon system of a future kind of submarine. The submarine's bow would accommodate several Mantas, each with its own short-range sonar and its own torpedoes. Operating off a port, for example, the submarine would launch Mantas, which could report back via the new kind of underwater link—and which could be commanded to attack ships identified by their sonars or to do other remote tasks. Without the links, Manta would be impossible.