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Invisible and invincible, stealth bombers and fighters once ruled the skies. But now arms researchers are devising surprisingly simple new radar techniques for detecting an attack.

DAVID FULGHUM - New Scientist

Wednesday, January 19, 2000


New York -- He should have been untouchable. Heading for home in one of the most advanced bombers money can buy, the pilot had no reason to suspect that his enemy even knew he was there. But they did.

Out of nowhere, three or four bright explosions enveloped his plane, slicing chunks from its wings and smashing an engine. The next moment, the aircraft was tumbling downward out of control.

The pilot, a young U.S. Air Force lieutenant, clawed desperately for the ejection handles and shot out of the plane. Six hours later, found shivering in a ditch just 200 metres from the wreckage of his plane, he was scooped to safety by a rescue team.

This event, which occurred during the Kosovo conflict on March 27, 1999, was a major blow to the U.S. Air Force. The aircraft was special: an F-117 Nighthawk stealth bomber that should have been all but invisible to the Serbian air defences. And it wasn't a fluke -- a few nights later, Serb missiles damaged a second F-117.

There were several simple reasons for the loss. For example, the Serbians plugged powerful computers into their air-defence system to help generate rough route tracks from the faint, whispery radar returns of the American stealth aircraft. And the missiles they fired were optically sighted and automatically detonated to avoid giving off radio signals that would reveal their positions to the bomber.

But the real clincher was the mistakes made by U.S. planners. Night after night, their stealth planes used the same route home. Worse still, NATO mistakenly left three early warning radars intact, allowing Serbian defenders to plot stealth aircraft for three nights before finally shooting one out of the sky.

With stealth weaponry soon to be within the reach of almost any country, arms researchers are now frantically designing radar systems that will improve on the Serb techniques. Some are surprisingly simple. For example, among the best radar systems for revealing stealth aircraft are those based on designs dating back more than half a century. Others are mind-boggling -- for example, in the future, radar defences may rely on everyday radio and TV stations to detect a stealth attack. One day, even your local FM radio channel could be doing its bit to defend your country.

Until recently, radar detected any aircraft in much the same way that a torch lights up a face in a darkened room. Instead of light, a radar transmitter sends out pulses of radio waves or microwaves while a nearby receiver keeps watch for reflections. Analyze these and you can work out the position, altitude, speed and even the identity of your target.

So how do aircraft designers hide their creations from radar's all-seeing eyes? The most important trick is to shape an aircraft so that it reflects as little energy as possible back towards the radar receiver -- that is, to reduce its radar "signature." So out go externally mounted munitions, prominent tailplanes and large vertical panels. These act like mirrors, reflecting radar pulses that hit them.

Just as bad are places in an aircraft's structure where surfaces meet at right angles. These junctions act like the corners of a billiard table, bouncing radio waves straight back to their source. Instead, the fuselage and wings must be smoothly angled or curved so that they deflect radar signals well away from radar receivers. A thick coat of special paint also helps. The energy of the radio waves is absorbed by electrons in the magnetic coating.

With the right shape and coating, aerospace engineers can shrink the radar signature of an aircraft to tiny dimensions. For example, the B-2 Spirit bomber has a wingspan of 52 metres, yet its radar signature gives the impression that it is about the size of a large marble.

No matter how carefully stealth aircraft are crafted, they still reflect minute amounts of radiation back towards the electronic ears of the enemy. In flight, stealth aircraft minimize these telltale signs by using their own radio receivers to listen for radar. When the aircraft is "pinged" with a radar beam, the pilot alters the plane's orientation and direction to minimize the reflections bounced back towards the receiver. But as it banks or climbs, short bursts of radio waves are reflected in every direction, just as a mirrored sphere bounces light all over the place. If radar operators can detect and plot these ghostly traces, they may be able to track stealth aircraft or missiles.

One of the best ways to pick up these flickering signals is to separate the transmitter and receiver. This arrangement -- known as bistatic radar -- is particularly good at catching the radar reflections that are deflected away from the transmitter. With high-speed computers, defenders can use these fragmentary data to plot the path flown by stealth aircraft and predict their course with enough accuracy to saturate a given piece of the sky with conventional anti-aircraft fire.

Researchers are now working on a surprisingly simple way to tackle stealth attacks, using technology that dates back to the 1930s. At that time, radar researchers used radio waves with wavelengths on the order of metres to spot ships and slow-moving planes. Since then, the wavelength of radar has shrunk to less than a centimetre, mainly because short wavelength radio waves make radar far more accurate.

It turns out that with long-wavelength radar, the cloak of invisibility begins to unravel rapidly.

"The changes you see on today's electronic battlefield are because we have finally awakened to the fact that the scientists had it about right when they first built radar," says a U.S. Navy official. When the wavelength of a radar beam approaches the size of the structural elements of an aircraft -- such as the tailplane, wings or fuselage, for instance -- these elements start to act like aerials, absorbing and then re-emitting the radio waves.

The effect is enhanced when the wavelength of the radar is twice the size of the "aerial." In this situation, the radio waves are absorbed and re-emitted very efficiently, making the aircraft appear far larger than it really is. (The same phenomenon is exploited by chaff, metal ribbons used to confuse radar.)

Worse still for stealth pilots, there are large numbers of Soviet and Chinese-made long-wavelength radars in use all over the world. Enhanced with the latest computers, these can provide a powerful means to spot stealth planes. Although these radars are easy to destroy since they are large and hard to camouflage, their signals are difficult to jam. And some Soviet-made long-range surveillance radars operate at just the right wavelengths to spot stealth aircraft such as the F-117.

On the other hand, long-wavelength radar is usually accurate only to within 50 metres -- so air defences must still rely on shorter-wavelength radar to guide a missile to its target. Link two or more radar systems operating at widely separated wavelengths -- multiband radar -- and you can glean useful data from specific points in the electromagnetic spectrum. Virtually every target has an electromagnetic "sweet spot" that can be used to identify it unequivocally.

Newer anti-stealth technologies use the electromagnetic "noise" of our cluttered airwaves to hunt stealth aircraft. After 15 years of research, Lockheed Martin Mission Systems of Gaithersburg, Md., has released details of Silent Sentry. This system dispenses with conventional radar transmitters and instead exploits broadcasts from TV and FM radio stations.

Any aircraft flying through this soup of music and electronic chit-chat generates patterns of reflections. Using conventional radio receivers and powerful parallel processors, Silent Sentry sifts the soup looking for these reflections. From their angles of arrival, time delay and Doppler shift relative to the unscattered broadcasts, Silent Sentry can pinpoint a target's location and plot its position on a three-dimensional electronic map.

In tests around Baltimore-Washington international airport, for instance, Lockheed Martin researchers followed targets of less than 10 square metres at ranges up to 190 kilometres, using an antenna just three metres by eight. The system can even screen out stationary targets such as tall buildings or radio masts, while still picking out helicopters by the Doppler-shifted reflections from their rotating blades.

With no transmitter of its own, Silent Sentry can't be detected and destroyed by radar-seeking missiles. And since FM radio beams hug the globe, Silent Sentry should be good at detecting low-flying aircraft and cruise missiles, or even the high-speed boats favoured by drug smugglers. Although the technology isn't yet good enough to target an aircraft with a missile, there are plans to link it to a second, more accurate radar system.

So in the next conflict, even the radio waves carrying the pictures of the fighting and the voices of reporters may become a weapon. The term media war could be about to take on a whole new meaning.




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