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By Norman Friedman, Author, Naval Institute Guide to World Naval Weapons Systems
Detecting Stealth Aircraft
In April, it was reported that a U.S. Navy satellite, launched in the fall °f 1989, had successfully detected and tracked both U.S, Air Force f'H7A “stealth” fighters and Soviet Blackjack bombers, using their ■nfrared signatures. The satellite was reportedly a prototype of a Navy- sPonsored wide-area air surveillance system; the Air Force supports an alternative radar system. The Defense Acquisition Board is to decide between the two during the spring of 1990. Both systems are probably ‘Mended largely to support the Air Defense Initiative (ADI), a program designed to detect air-breathing vehicles and so complement the Strategic defense Initiative (SDI), which is designed to counter ballistic missiles.
There may be some question as to just how far the country should go in Pursuing such strategic defensive programs in an era of lessening United States-Soviet tensions. From a naval point of view, however, wide-area air surveillance is likely to be extremely important, both in conflict with the Soviets and in the Third World. After all, air attacks are probably the smgle greatest threat presented by Third World powers.
. Wide-area air detection is particularly valuable as a means of counter- lng enemy air attacks well before the attackers can launch antiship mis- s'les. The faster the attackers, the greater the need for early detection, a requirement enshrined in the outer air battle studies of the early 1980s. Several systems of radar surveillance satellites were proposed at the [line, but they were rejected because of their cost. A radar satellite must be quite large, since its antenna must be wide enough to produce a very narrow beam that can localize targets hundreds of miles away. Each satellite provides only fleeting coverage of any particular area, so full- hme coverage requires a large number of costly satellites. In the early I^80s, a ground-based relocatable over-the-horizon radar (ROTHR) was developed as a more affordable alternative and the first of these is now entering service. The radar covers a swath out to a maximum range of 1600 nautical miles, over a 63° field of view. One is now being installed ln Wales, to cover the North Sea and Norwegian Sea areas through which carrier battle groups would approach the Soviet Union.
The system has some major limitations. It cannot provide tracking accuracy comparable to that of a satellite viewing an approaching airplane directly. Relatively little of the radar signal returns to the radar; Presumably that limits its performance against stealthy targets. Perhaps .Most important in the current context, it cannot be redeployed quickly to Meet contingencies in the Third World. The radar can be relocated, but it Must be emplaced on land; in many parts of the world, few countries Might be willing to provide that land. In any case, relocation takes several weeks. Third World crises are characterized by their unpredictabil- "V- The Navy’s great virtue, the reason it is likely to suffer cuts much shallower than those affecting the other services, is that it alone can respond to such surprises without the consent of local powers. Naval forces can appear and remain in a crisis or latent-crisis area without local assistance, while providing their own support. To the extent that support Mcludes air surveillance, the surveillance system too must be mobile, or eIse global—so that it is always available.
A global system has an important advantage. A deployed carrier battle group can carry only so much aircraft fuel, and only so many spare parts, •he more intense its operating schedule, the shorter its self-contained endurance or, alternatively, the greater its dependence on logistics ships Md bases. In a world in which bases are less and less available, anything vvhich reduces the necessary operating tempo helps enormously. A car- JMr group in contested waters must keep her early warning E-2Cs airborne constantly, and may decide to maintain a fighter combat air patrol 0r> station. An external air surveillance system, however, may be able to detect attackers early enough so that the E-2Cs and fighters can remain °n call on deck alert, with attendant fuels savings. It may also provide so much alert that air counterstrike tactics, which use up less fuel than combat air patrols, can be used. These possibilities are valuable enough in general war, but they should make a great difference in the new conditions of the future.
It seems likely that the naval satellite was practicable because of recent advances in infrared (IR) sensor technology. An IR sensor is much more compact than a radar sensor operating at the same range because IR wavelength is so much shorter than radar wavelength—the ratio is much greater than 1,000:1. Assuming that the IR image can be processed properly, the IR satellite should be so much smaller that several can be launched by a single booster, considerably reducing overall system cost. The technology i" question is probably that of large focal plane arrays, developed in part for the Strategic Defense Initiative (SDI). A large array can probably operate at a much worse overall signal-to-noise ratio than the much smaller arrays current in use; hence the success against the F-l 17A. It is very unlikely that any airplane or missile can operate at so low a temperature (above the background) as entirely to avoid such detection.
It the F-l 17A aircraft were indeed detected due to improvement in IR technology, the implication is that stealthy airplanes will also be easy to detect from the ground. The space-based IR sensor sees a slightly warm airplane against a relatively warm earth. A similar sensor, looking up, may see the warm airplane against a cold sky, where it presents a much better signal-to-noise ratio. It may well be, then, that a network of large focal plane array IR sensors will be quite sufficient to defeat stealthy aircralt. Recent statements that the IR signature of the airplane had been minimized refer to its exhaust plume, not to the temperature of the airplane itself. Infrared reconnaissance systems already can detect airplanes on an airfield merely by measuring the slight temperature difference between them and the ground. Of course, weather will sometimes reduce IR effectiveness.
That the satellite managed to track the F-117A must add to the skepticism surrounding stealthy aircraft. The original stealth concept was to reduce an airplane’s observable signatures just enough to let it get by amid the confusion of a combat area, at little or no cost in combat capability. This modest goal was perverted into a hope that an airplane or missile could virtually escape detection altogether, a much costlier proposition. It appears that very considerable compromises of combat capability were cheerfully accepted to do this—along with enormous unit costs. The F-l 17A, with its single laser-guided bomb, is a case in point. Almost certainly, too, the airplane lacks sufficient performance to survive an engagement with a more conventional airplane. Once the sacrifices have been made, the value of the stealthy airplane rests entirely on its ability to escape detection. The naval satellite test suggests that much of this effort has been wasted. A cynic might even suspect that the Air Force has proposed a radar satellite because it thought mainly in terms of radar detection; its entire stealth effort has focused on ways to detect enemy radars.
Without invisibility, an F-l 17A is merely an expensive way to deliver a single 2,000 pound laser-guided bomb. Much the same argument applies to the B-2, which also has a very limited bomb load. The Navy’s approach to stealth, represented by the A-12, seems to have been much more pragmatic. The Navy, after all, bought the predecessor airplane, the A-6, specifically to attack in a stealthy manner—at night or in bad weather, at low altitude beneath radar coverage—while carrying a large bomb load. As the A-6 successor, the A-12 must also be able to deliver a useful bomb load. At the same time, it is likely to be much stealthier than its predecessor. It is, then, probably much closer to the original stealth concept, a relatively moderate application of reduced-observables technology. It seems unlikely that the A-12 carries the sort of belief in a cloak of invisibility that marks the Air Force programs. Just such a compromise is very well described in Stephen Coonts’s novel, Minotaur.
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Proceedings / July 1990