快猫短视频

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 downwards out of control.

The pilot, a young US Air Force lieutenant, clawed desperately for the
ejection handles, but with the plane鈥檚 violent spinning, they remained just
beyond his grasp. 鈥淭he one fragment of this whole event I can鈥檛 remember is
pulling the handles,鈥� he recalls. Somehow, he ejected safely and, after six
hours shivering in a ditch just 200 metres from the burning wreckage of his
plane, he was scooped to safety by a heavily armed rescue team.

This event, which occurred during the Kosovo conflict on 27 March, was a
major blow to the US 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 this certainly wasn鈥檛 a fluke鈥攁 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 US planners. Night after
night, their stealth planes used the same route home. Worse still, NATO
mistakenly left three early warning radars intact. With these systems still
active, the Serbian defences were able to plot the flights of the stealth
aircraft for three nights before they finally shot an F-117 out of the sky.

In the technological battle to counter stealth, this was simply a skirmish.
But full-scale war is imminent. With stealth cruise missiles and even ballistic
missiles expected on the world market within a decade, researchers鈥攎ainly
in the US鈥攁re frantically designing radar systems to defeat stealth
technologies.

Some of these systems 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鈥攆or 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.

To work out how to defeat stealth technology, of course, you first have to
understand how it works. Aircraft give many clues to their presence鈥攖he
sound they make, infrared radiation from their hot engines, chemicals in their
exhaust and even the white vapour trails they leave in the sky. One of the best
methods of spotting them, however, is radar, which reveals distant objects such
as 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 receiver, usually mounted close by, keeps watch for any
reflections. Analyse 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鈥檚 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鈥攖hat is, to
reduce its radar 鈥渟ignature鈥�. So out go externally-mounted missiles and bombs,
prominent tailplanes and large vertical panels on the fuselage. These act like
mirrors, efficiently reflecting any radar pulses that hit them.

Just as bad are places in an aircraft鈥檚 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
vigilant radar receivers鈥攕ideways, upwards or straight down to the
ground.

The second key to stealth is a thick coating of radar-absorbing paint. For
example, the active ingredient in the coating used on the SR-71
Blackbird鈥攁 spy plane incorporating some of the earliest stealth
technology鈥攊s glass balls less than a micrometre across, each covered with
a magnetic metal ferrite coating.

These spheres behave like tiny, inefficient radio aerials, absorbing radio
waves and dissipating their energy before it can be re-emitted. The energy of
the radio waves is absorbed by the electrons in the magnetic coating. In a good
conductor like a metal aerial, the electrons can move freely and the radio waves
are re-emitted. But ferrite-coated spheres like those used on the SR-71 are poor
conductors, so the motion of the electrons is damped by the material鈥檚
electrical resistance
(快猫短视频, 6 December 1997, p 32).

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. And although existing radar-absorbing
coatings are rather delicate
(快猫短视频, 23 August 1997, p 5),
aircraft such as the F-22 raptor and joint strike fighter that will enter
service early next century should have far more robust radar-absorbing
coatings.

But 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 minimise these telltale signs by using their own radio
receivers to listen for radar. When the aircraft is 鈥減inged鈥� with a radar beam,
the pilot alters the plane鈥檚 orientation and direction to minimise 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鈥攌nown as bistatic
radar鈥攊s particularly good at catching the radar reflections that are
deflected away from the transmitter
(see Diagram). 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 anti-aircraft fire.

Old-fashioned edge

Bistatic radar systems also have other advantages. Split the transmitter and
receiver, and you can mount the two separately, in small pilotless drones for
instance. Since the transmitter is vulnerable to anti-radiation missiles that
lock onto the radar beam and follow it back to its source, this reduces the
danger to radar operators. It is also fairly simple to convert existing radar
into bistatic systems, although coordinating the signals they generate鈥攁nd
using them to plot a target鈥檚 movements鈥攔emains a challenge.

US commanders are keen to get bistatic radars operational within five to ten
years, prompted in part by fears that foreign manufacturers of medium-range
ballistic missiles will soon add stealth to their weapons. In the meantime,
researchers are also 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 of 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. But when it comes to spotting stealthy
aircraft however, longer wavelength beams still have an edge.

It turns out that with long-wavelength radar, the cloak of invisibility
begins to unravel rapidly. 鈥淭he changes you see on today鈥檚 electronic
battlefield are because we have finally awakened to the fact that the scientists
had it about right when they first built radar,鈥� says a US Navy official. When
the wavelength of a radar beam approaches the size of the structural elements of
an aircraft鈥攕uch as the tailplane, wings or fuselage, for
instance鈥攖hese 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 鈥渁erial鈥�. 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鈥攕o 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鈥攎ultiband radar鈥攁nd you can glean
useful data from specific points in the electromagnetic spectrum. Virtually
every target has an electromagnetic 鈥渟weet spot鈥� that can be used to identify it
unequivocally.

There are even plans to move anti-stealth radar into space. At the moment,
stealthy aircraft aren鈥檛 shaped or treated to be invisible from above so they
can be picked up by high-flying aircraft 鈥渟entries鈥� packed with high-power
radar. The next step is to move long-wavelength or multiband radars into space.
For example, the US military Discoverer 2 satellite constellation is expected to
grow from a system designed to track moving ground targets to one capable of
stealth detection.

Not surprisingly, multiband radar is also the key component of both the
Pentagon鈥檚 secret cruise missile defence scheme and an improved system for
gathering intelligence about foreign ballistic missile tests. This system, which
is under development by the US Defense Intelligence Agency, aims to use one
radar to search for missiles at long range while a much shorter wavelength radar
identifies and plots a target鈥檚 precise position.

For all their advantages, long-wavelength radars face a growing challenge,
not from the latest radar-absorbing material or electronic jamming device, but
from DJs, mobile phone users and television broadcasters. For long-wavelength
radars operate at the same frequencies as television and FM radio stations,
navigation aids and mobile phones. Such signals are creating an ever
intensifying soup of electromagnetic noise in which stealth aircraft and
missiles might reasonably hope to conceal themselves.

Soon, however, they may have no place to hide. One of the latest anti-stealth
technologies uses the electromagnetic noise that once protected stealth aircraft
to reveal them. After 15 years of research, Lockheed Martin Mission Systems of
Gaithersburg, Maryland, 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 through 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鈥檚
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 3 metres by 8 metres. 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.

Engineers at Lockheed Martin say they can use the broadcasts from many of the
world鈥檚 55 000 commercial FM radio and television stations, and in theory, any
normal radio transmission will do. To make the system work anywhere, they are
busy creating a huge database that lists the locations and frequencies of every
useful transmitter on the globe.

With no transmitter of its own, Silent Sentry can鈥檛 be detected and destroyed
by radar-seeking missiles. And since FM radio beams hug the earth, 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鈥檛 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鈥�