Stealth Threat

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.