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