IP: RE: passive radar in Risks Digest 21.49

[David Farber <dave () farber net>]

> From: Ben.Crystall () rbi co uk
> To: dave () farber net
> Subject: RE: passive radar in Risks Digest 21.49
> Date: Tue, 19 Jun 2001 10:13:53 +0100
>
>
> Dave
> thought you might be interested to know that New Scientist magazine ran this
> story over 18 months ago (4 December 1999 issue). This story has been known
> about since at least summer 1999. The telegraph has just dredged up old
> stuff--as often happens--probably by accident. For your information here is
> the copy we ran:
>
> @Title:SEEK AND DESTROY
> @Intro:America's stealth planes once ruled the skies. But now their
> inventors need to spot the whispery traces of stealth weaponry in enemy
> hands, reports David Fulgham.
> @Body: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, 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's violent spinning, they remained
> just beyond his grasp. "The one fragment of this whole event I can't
> remember is pulling the handles," he recalls. Nevertheless, he did somehow
> eject 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. For the aircraft was special: a F-117
> Nighthawk stealth bomber that should have been all but invisible to the
> Serbian air defences. And this certainly 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 well-integrated 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 American planners.
> Night after night, their stealth planes used the same route home. Worse
> still, NATO mistakenly left three early warning radars intact. With these
> critical systems still active, the Serbian defences were able to plot the
> flights of American stealth aircraft for the three nights before they
> finally shot a 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--mainly in the US--are frantically designing radar systems
> designed to defeat stealth technologies and uncloak enemy aircraft and
> missiles.
>         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--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.
>         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--the 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 that bounce back. 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'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 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'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 vigilant radar receivers--sideways, 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--a spy plane incorporating some of the earliest stealth
> technology--is 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 such as a metal aerial, the electrons can move
> freely and the radio waves are re-emitted. But the ferrite-coated spheres
> like those used in the SR-71's radar-absorbing paint are poor conductors, so
> the motion of the electrons is quickly damped by the material's electrical
> resistance (New Scientist, 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 (New Scientist, 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, yet their radar signatures will make them seem
> about the size of a golf ball.
>         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 "pinged"
> with a radar beam, the pilot alters the plane's 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--known as bistatic
> radar--is particularly good at catching the radar reflections that are
> deflected away from the transmitter (see Diagram, p ??). 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.
>         Bistatic radar systems also have other advantages. Split the
> transmitter and receiver, and you can mount the two separately, in small
> unmanned 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--and using them to plot a target's
> movements--remains a challenge.
>         Senior US military 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 slow-moving biplanes. 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. "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 US
> 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 exactly
> twice the size of the "aerial". In this situation, the radio waves are
> absorbed and re-emitted very efficiently, with these in-phase reflections
> making the aircraft appear far larger than it really is. (Precisely the same
> phenomenon is exploited by chaff, the metallic ribbons dispensed by planes
> under attack 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 among the easiest things to
> destroy on the battlefield since they are large and hard to move or
> camouflage, their signals are difficult to jam. And some Soviet-made
> long-range surveillance radars operate at precisely the right wavelengths to
> spot stealth aircraft such as the F-117.
>         On the other hand, long-wavelength radar is usually accurate only to
> between 30 and 50 metres--so air defences must still rely on shorter
> wavelength radar to guide a fast-moving 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 electronic "sweet
> spot" that, if recorded and catalogued, will identify it unequivocally.
>         There are even plans to move anti-stealth radar into space. At the
> moment, stealthy aircraft aren't shaped or treated to be invisible from
> above so they can be picked up by high-flying aircraft "sentries" packed
> with high power radar. The next step is to move long-wavelength or multiband
> radars into space. For example, the American 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's 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's 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 cellular telephones.
> 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
> altogether 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'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 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's 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, the Silent Sentry can't 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't yet good enough to accurately 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 . . .
> @Biog:David Fulghum is military editor at Aviation Week & Space Technology
> @Further reading:Further reading: The US Intelligence Community by Jeffrey
> T. Richelson, (Westview Press, 1999)
> The Invention That Changed The World by Robert Buderi (Simon & Schuster,
> 1996)
> For more information see:
> www.airpower.maxwell.af.mil/airchronicles/apj/cunn.html
> www.afa.org/magazine/0299radar.html
>
>
>
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> Date: Sat, 16 Jun 2001 10:39:31 -0400
> From: David Farber <dave () farber net>
> Subject: Passive radar?  Removing the cloak of invisibility (What's New)
>
> So just how stealthy is the $3.6B stealth bomber?  Radar would need to look
> straight up at the bomber's flat bottom surface.  Tracking would therefore
> require a vast array of antennas.  But according to a story early this week
> in the *London Daily Telegraph*, such arrays already exist: Roke Manor
> Research in Britain claims that stealth aircraft can be tracked by their
> effect on ordinary mobile phone traffic.  News media in the US did not
> discover the story until last night.  The Pentagon is taking it seriously,
> and other nations, including China, are now developing such a system.
> [Source: What's New, 15 Jun 2001, from Dave Farber's IP distribution]
>
> -
>
>
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