Photographs of the Indian (Russian-built) Talwar-class frigates nd of the new Chinese missile destroyers show a familiar Cold War radome, the Russian "Band Stand." The technology it represents potentially is quite significant for the more dispersed U.S. fleet now in prospect. band Stand covers a variety of electronic systems, all of them involved in targeting antiship missiles against targets well beyond a ship's normal radar horizon. The version first seen, on board Nanuchka-class missile corvettes, employed an L-band troposcatter radar. This was secret technology, and it was not exported. Thus, when the Indian Navy received Nanuchkas, their band Stands concealed nothing more impressive than the Square Tie targeting radar used on board Soviet Osa-class missile boats. (The Indian version carried the same Styx missile as the Osas, rather than the longerrange SS-N-9 of Soviet Nanuchkas.) The current version of Band Stand exported to India and to China presumably is the same as that on board Sovremennyy-c\ass destroyers sold to China.
These systems arose from work the Soviets did in 1959-1961 in both the Atlantic and the Baltic when they were about to field antiship missiles with ranges well beyond the horizons of their ships. It turned out that a variety of phenomena, both ducting and scattering off layers in the atmosphere, could carry signals with wavelengths between 3 centimeters (X-band) and 20 centimeters (L-band) well beyond the horizon to ranges as great as 400 kilometers (220 nautical miles), albeit intermittently in many cases. Work on the first system of this type, Molniya, began in 1962, and it entered service in 1967. It combined a passive element with its over-the-horizon radar. Presumably, the idea was that the firing ship would use the passive element to detect and identify a target, then ping actively to establish target range and bearing. Ducting was incorporated in a series of fire-control systems: Dubrava (for early Nanuchkas), Titanit (in service in 1973 on board Nanuchkas), Monolit (a modular system for Tarantuls), and Mineral, the latter being installed on board Sovremennyy destroyers. Titanit apparently was the first fire-control system to combine independent passive and active channels in a single system.
The phenomena involved create what Western navies call anomalous propagation. Under some circumstances, evaporation above the sea forms a duct that acts as a waveguide for some radar signals. Similar ducts also can form over land, although they are rarer. The effect of the duct is to carry signals within it over very long distances. Signals from outside tend to reflect off the duct. Ducting initially was associated mainly with X-band radar, but apparently it can affect signals at other frequencies. For example, investigation of the USS Vincennes (CG-49) incident (in which the Iranian Airbus was shot down) showed that L-band IFF (identification, friend or foe) signals generated by an Iranian F-14 on the runway at Bandar Abbas were carried (by a duct, apparently) to the ship, 48 nautical miles away, where, because of an error, they were confused with signals that might have come from the Airbus.
Anomalous propagation usually is treated as a nuisance, at least in Western navies, because it creates confusion in radar systems. A radar measures range by the delay between emitting a signal and receiving its echo. As long as the echo returns before the next signal goes out, there is no problem. Radar designers typically choose pulse intervals such that signals returning after the next pulse goes out will be so small (as a result of power loss over a great range) that they probably will not be detected at all. The main exception to this practice is pulse Doppler radar, which uses much higher pulse rates, hence must be designed to take such late pulses into account. Ducting ruins such calculations because the signal retains much more of its power at long range. A classic radar text, for example, recounts the story of a radar on the Arabian coast that normally had a range of about 20 miles. In the ducting season, however, it could see mountains in Pakistan, more than 1,000 miles away.
Ducting seems to be a littoral phenomenon, common in places such as the Eastern Baltic, the Persian Gulf, the Indian Ocean, and the South China Sea. For years, there have been claims that navies that spend most of their time in such areas habitually exploit it so that they can engage targets beyond their theoretical horizons. For example, the Indians found that their Square Tie sets (on board Nanuchkas) often could detect long-range targets, and they modified the radars accordingly. This modification was shared with the Soviets, who classified their Garpun (NATO Plank Shave) radar as an over-the-horizon sensor. Garpun is associated with weapons such as later versions of Styx and SS-N-25 (Harpoonski), which fly beyond the horizon but do not reach as far as the weapons associated with band Stand.
Another means of long-range transmission, probably the one the Soviets first used, is troposcatter. Many radar signals, from S-band up, scatter off the troposphere, an electrically charged (and hence reflecting) layer above the earth. The U.S. Army, for example, uses troposcatter as a reliable means of transmitting radio signals about 200 miles. Presumably, troposcatter is substantially more reliable than ducting. From a radar point of view, its main deficiency is that it leaves a gap around the transmitting ship, between the usual radar horizon and the nearest point at which troposcattered signals come to earth. The solution adopted in Mineral was to combine ducting and troposcatter. Inside the radome are two back-to-back antennas, one for ducting and the other for troposcatter. The troposcatter antenna appears to be a planar array, so presumably electronic beam-forming is used to set the angle at which the signals are sent into the troposphere.
Other than such anomalous propagation, the only hope for over-the-horizon detection has been to exploit the properties of HF (high frequency) radio signals. HF radio gains very long range because its signals bounce off a charged layer high above the earth, the ionosphere. Because the surface of the sea conducts, the surface and the ionosphere form the boundaries of a kind of waveguide. For about 40 years, researchers have periodically proposed using this waveguide to create an HF radar that could see beyond the horizon. This type of radar is in service in Canada, for surveillance of the Grand Banks (as an antipoacher measure), and it has been bought by Australia as a supplement to the longer-range Jindalee over-the-horizon radar. Attempts to place HF radars on board ships have had limited success, partly because such devices require considerable signal processing. As computers have become much more powerful, HF shipboard radar has become more practicable. Moreover, because the wavelength involved is so long, HF radar might be unaffected by the shaping used to make missiles and aircraft stealthy. This last is now a major selling point for the main Western producer of such radars, Alenia Marconi Systems (AMS). Shore-based HF surface wave radar also has been offered by the Russians (the system is called Irida), and by others (the Canadian version, developed by Raytheon, is unrelated to those offered by AMS).
Some of the current Chinese surface combatants offer another interesting radar example. China is the only navy to uses UHF (ultra-high frequency) airsearch radars, in its case with characteristic antennas featuring long beams and transverse dipoles (Yagis). These radars were descended from Soviet prototypes. At one time, UHF radars also were common in the West. These radars are extinct because they fell afoul of civilian use of the frequency spectrum, in their case for television. From a technical point of view, for an antenna of a given size, UHF means that the radar beam is broader and hence that target definition is considerably worse. The Chinese seemed to follow the example of other navies in abandoning UHF for much higher frequencies. In recent years, the characteristic box Yagi antennas have reappeared. Why?
The answer might be depressing for us. UHF wavelengths are relatively long, so it can be argued that UHF radars are less affected by the shaping that, for centimetric radars, makes airplanes stealthy.
Stealth is most likely defined in terms of the radars we ourselves use. Because we do not use naval UHF radar, quite possibly we do not test against it, particularly over water. Chinese UHF radars might thus be an unpleasant surprise. Almost certainly they cannot achieve sufficient resolution to track targets well enough to engage them. Given the land orientation of the Chinese military, however, it seems plausible that surface combatants using UHF radar have some sort of link back to the Chinese national air-defense system.
What does this mean for the U.S. Navy? In the past, it was expected that virtually all operations would be conducted under the umbrella provided by a carrier and her E-2C Hawkeyes. The Hawkeyes could see well beyond the fleet's surface horizon and they contributed to a tactical picture shared by the surface combatants. As long as the Hawkeye was there, the surface ships did not really need any special over-the-horizon radar. There was some interest in long-range passive detection of Soviet surface ships during the Cold War, but that relied on HF technology and was not associated with a U.S. radar system.
After the terrorist attacks of 11 September 2001, the U.S. Navy proposed a new concept of operations. In the Navy's view, the most important implication of the attack was that the United States now faced a dispersed enemy. Its operations likely would not be closely coordinated; it might be necessary to field mobile forces in many places more or less simultaneously. The Navy proposed, and the Department of Defense accepted, the argument that it was vital to deploy more self-contained forces. Part of the solution was to increase the fraction of carriers and amphibious groups that could deploy quickly, with consequences for the process of electronic modernization. Another part was to transform the amphibious groups into independent striking forces called expeditionary strike groups. The Navy also expects to form surface action groups.
Neither of the new groups, however, can support airborne radar aircraft. It seems arguable that over-the-horizon radar coverage becomes crucial in this case. Perhaps the fact that these groups are most likely to operate in littoral areas will make a solution possible, in the form of ducting radar. Maybe we should be taking a leaf from the Russians' book.