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.