Driving home from work, you suddenly remember that a few of the T-ball kids are supposed to come over after the game. Should you pick up a couple of pizzas on your way? You pull out your cellular phone and call home to check.
Eight miles above you, unseen and unheard, a B-2 stealth bomber is cruising along on a practice run. The pilot believes that even radar can't detect his plane, but he's wrong. That call you're making, along with thousands of other innocent cellphone conversations taking place all over town, has inadvertently unmasked the bomber-defeating stealth technology that cost $40 billion to develop.
At least, that is the claim recently made by Roke Manor Research, a small research institute housed in an 1850s manor house in a quiet English town. Roke Manor, a subsidiary of the German electronics industry giant Siemens, announced earlier this year that its engineers had "rendered stealth aircraft useless." By listening for the echoes of cellphone signals bouncing off a stealth plane, the engineers say, it's possible not only to detect the plane but also to determine its exact location.
Conventional radar works by pointing a powerful radio beam at the sky and listening for the reflections from flying objects. But today we live in a sea of radio waves that are continuously broadcast from cellphone towers, television transmitters, and other sources. With this wireless revolution has come a potential new spy tool: a radar system that exploits existing radio signals rather than generating its own.
The Roke engineers came up with the idea for their cellphone-based radar as something of a lark. "We were brainstorming blue-sky ideas," recalls managing director Paul Stine. Can the system that emerged from the brainstorming session prove better than traditional radar at detecting stealth planes? Possibly, but the researchers haven't yet built a working model, and some experts question the system's practical military value, since analyzing cellphone echoes accurately is a very tricky business.
Modern warfare has been shaped to a large extent by radar. Before radar, there was no way to detect bombers until they were already overhead. But in the 1930s, British researchers began experiments that changed all that -- and the course of World War II. When the Luftwaffe bombed London in 1940, the British saw them coming, thanks to radar beams that swept the skies and bounced off any incoming planes.
It was the start of a decades-long cat-and-mouse game between airplanes and radar. Early on, engineers tried to camouflage airplanes using special paints and coatings. It didn't work. In 1958, the CIA sent a camouflaged U-2 on a spy flight across Russia. Attached to the subsequent protest note from Moscow was a detailed radar plot of the airplane's flight path.
Seventeen years later, teams at Lockheed's Skunk Works and Northrop cracked the problem. There was no point trying to camouflage a conventional airplane. Instead, the engineers realized, they had to come up with an entirely new type of airplane that would not reflect radio signals. The secret, Lockheed engineer Alan Brown would later say, "was to design a very bad antenna and make it fly."
In the eyes of an aerodynamicist, the first successful stealth airplane, Lockheed's Have Blue prototype, was a misshapen monster. The multifaceted plane had no curved surfaces, even on the wing, which was angled back so sharply that the craft could barely get off the ground. But all that really mattered was that to a radar system, the 6-ton jet looked no bigger than a small bird.
Have Blue took advantage of the fact that the radar systems of the time were monostatic, meaning that they employed a single antenna both to transmit radar signals and to listen for their echoes. The airplane's odd shape caused radar signals to scatter, instead of bouncing back toward the antenna.
The first operational stealth planes -- the F-117 Nighthawk and the B-2 Spirit, both introduced in the 1980s -- relied on the same principle. Their sloped upper and lower surfaces deflect radar energy upward or downward, away from the radar antenna. The F-117 and B-2 also have long, straight edges that focus radar reflections into single, concentrated beams. The way the plane's edges are angled, the beams shoot off to the side, rather than directly back at the antenna that sent the signal.
But though stealth aircraft can fool monostatic radars, they may not be as good at fooling so-called bistatic radar, a system in which the transmitter and receiver are placed in separate locations. Because a bistatic system does not rely on a single antenna, it may be able to pick up some of the radio signals that are scattered by a stealth plane. And when a stealth plane ventures between a bistatic system's receiver and transmitter, the system may even detect the "shadow" created when the airplane blocks the radar beam. Most experts agree that conventional stealth aircraft will look different and possibly larger on a bistatic radar screen.
Known but long ignored, bistatic radar has been getting a second look in recent years. For example, bistatic radar could potentially enable the U.S. military to spot and track enemy aircraft with greater precision. The system might consist of a central surveillance aircraft carrying a large radar transmitter, as well as several small, unmanned craft carrying radar receivers. The transmitter plane could hang back while the receiver-bearing planes ventured far into enemy territory.
Still, monostatic radar retains some advantages. For example, target location with monostatic radar is a snap: Once a target has been detected somewhere along the narrow beam of the radar, it's simply a matter of measuring the time that elapses between sending a radio signal and detecting its echo. Since the speed at which radio waves travel through the atmosphere is known, it's simple to calculate the distance of the plane. Bistatic radar, by contrast, must employ sophisticated computer analysis to perform this basic task.
Roke Manor's system is known as "passive bistatic" radar because it makes use of existing radio signals rather than generating its own. Using a cellphone tower for a transmitter, the system listens with its receivers for the echoes of the cellphone signals, then analyzes those echoes to detect flying objects. The idea arose when an engineer recalled that the first British radar experiments had relied on the BBC's main transmitter in London to "illuminate" the target.
"What if a stealth aircraft had flown across London in 1934?" managing director Stine recalls one of his colleagues asking. "Would that radar have detected it?" Roke's passive bistatic radar is also similar to a system called Silent Sentry that was recently developed by Lockheed Martin -- though Silent Sentry relies on radio and television signals, not those from cellphones.
Is cellphone -- based radar a true threat to national security? John Shaeffer, co-founder of stealth consultants Marietta Scientific and co-author of the standard textbook on radar detection, has doubts. "I'm not sure," he says, "that there's a real pony in there." Shaeffer points out that a bistatic radar system has the best chance of defeating stealth when the receiver is on the opposite side of the airplane from the transmitter, which means the airplane is already inside enemy territory before the radar has a chance of picking it up.
An even bigger question has to do with power. Conventional monostatic radars focus hundreds of kilowatts into a pencil beam, like a bright searchlight. Cellphone towers, by contrast, put out only tens of watts, and in all directions, more like a household lightbulb. Like ripples on a pond, the radio waves lose energy as they spread, and they scatter farther when they hit a target, so the signal at the receiver is weak. Although TV and FM radio signals are stronger than those from cellphones, they are still much weaker than those emitted by a focused radar transmitter.
In a March 2000 report on critical military technologies, the Pentagon's Defense Threat Reduction Agency wrote that TV-based bistatic radars like Silent Sentry have some potential against stealth targets. Even so, the Pentagon's Low Observables/Counter Low Observables Executive Committee, or Excom, which controls the export of any technology that could compromise stealth, does not mention bistatic radar in a long list of potential counter-stealth techniques. And the Pentagon has permitted Lockheed Martin to go public with its Silent Sentry system, which the company is pitching as a way for one nation to keep a discreet eye on another's airspace, perhaps to stop smuggling or other illegal traffic.
Roke Manor researchers have been close-lipped about how their radar system would work, but the company has released a diagram showing that it depends on multiple receivers. When a cellphone tower sends out a signal, each receiver hears it twice. The first signal comes directly from the tower and the second is an echo from the target. If three or more receivers measure the time difference between the two signals, using GPS to provide precise synchronization of the arrival times, they should be able to pinpoint the target.
But as the Roke Manor scientists roll up their sleeves, stealth engineers continue to refine the shapes of their all-but-invisible aircraft. Today's technical papers describe planes whose radar cross-sections are the size not of small birds but of mosquitoes. Moreover, work is progressing on stealth- improving methods that have nothing to do with a plane's shape. Edges and other "hot spots" on stealth aircraft can be treated with plastics or paints that contain radar-absorbing inks, powders, or mineral compounds. Notably, those materials are most effective in the microwave band where cellphones operate.
And so, even in our signal-strewn, wireless world, the 60-year-old game of cat-and-mouse between radars and their targets continues.
The small British research institute Roke Manor made headlines earlier this year when engineers there announced the invention of a new radar system capable of detecting stealth aircraft. Called bistatic radar, the system uses separate antennas to transmit and receive radio signals. (Conventional radar relies on a single antenna.)
The U.S. Air Force may already be deploying a bistatic radar system. In April, one of our sources spotted the warty nose of an airplane in Air Force markings sticking out of a shop at Goodyear airport in suburban Phoenix. Experts speculate that the strange-looking plane contains an experimental bistatic radar system aimed at detecting stealth aircraft.
Known as the Radar Test Bed (RTB), the plane is a T-43, the Air Force version of the Boeing 737. The prime contractor is Denmar, a company specializing in stealth technology. The "Den" stands for President Denys Overholser, the former Skunk Works engineer credited with devising the shape of the first stealth aircraft.
RTB seems to be more than just a flying laboratory. In a purely experimental program, there would usually be no need to provide a radar system with a full 360-degree field of view, but the shape and size of the nose and tail radomes-6.5 feet in diameter-suggest they contain moving antennas.
The Air Force's center for airborne surveillance radar, Rome Laboratory in New York, has worked closely with MIT's Lincoln Laboratory on bistatic radar for many years. Technical papers from Rome Lab refer to a bistatic Advanced Airborne Surveillance system-originally due to be demonstrated in 2000-and a graphic in a Lincoln Lab briefing paper shows a bistatic radar with its transmitter mounted on a 737.
The Lincoln paper describes a radar operating in the UHF television band, which is seldom used for airborne radar because the long wavelength requires a big antenna. Lincoln Labs has also worked on laser radars to detect airborne targets, possibly explaining the optical domes above each radar housing.
In action, the 737 would orbit safely to the rear of a battlefield, sending out UHF signals. Unmanned planes would fly above the battle area, fitted with receivers to pick up target echoes. These receivers would require less power than a radar-transmitting system, so they could be carried on small, cheap planes that would be "attritable." That is, nobody gets killed or fired if one is shot down.