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IP: RE: passive radar in Risks Digest 21.49
From: David Farber <dave () farber net>
Date: Tue, 19 Jun 2001 07:12:24 -0400
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.htmlDate: 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] - ******************************DISCLAIMER****************************** This message is intended only for the use of the person(s) (\"Intended Recipient\") to whom it is addressed. It may contain information, which is privileged and confidential. Accordingly any dissemination, distribution, copying or other use of this message or any of its content by any person other than the Intended Recipient may constitute a breach of civil or criminal law and is strictly prohibited. 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- IP: RE: passive radar in Risks Digest 21.49 David Farber (Jun 19)