Scouting For Surveillance
Detection of the B-2 Stealth Bomber And a Brief History on “Stealth”
“Cell phones uncover stealth bombers.”
In early June, the news was filled with headlines such as this one. Newspapers put them at the top of the front page, magazines printed colorful diagrams, and television networks ran the story as the lead on their evening news broadcasts.
And why not? The story was irresistible. Stealth technology is the most potent symbol of America's military supremacy in the post-Cold War world.Though other nations have worked on similar technology, so far none have been as successful as the United States. For something as commonplace as cellular telephones to bring down this symbol of America’s military-industrial complex was simply too ironic for the media to resist. In almost all accounts, the technology was described as new and revolutionary, and numerous analogies to David and Goliath were drawn.
Within a week, though, the story had practically disappeared from the media. The U.S. military did not launch any crash program to counter this threat.
No systems were sold. We are left wondering: “What happened?”
An overview of stealth technology
Stealth technology was developed at Lockheed Martin’s legendary Skunk Works research facility. This facility had produced aircraft such as the P-80, America’s first jet fighter; the U-2, the high-altitude reconnaissance aircraft made famous by photographing Soviet nuclear missiles being installed in Cuba in 1962; the SR-71, still the fastest operational jet aircraft ever built; and the F-117 Nighthawk, the Stealth Fighter that captured the world’s attention.
Even before the Stealth Fighter’s existence had been publicly announced, rumors circulated in the aerospace and defense community. Tom Clancy featured the Stealth Fighter in his novel Red Storm Rising, a political-military thriller describing a conventional war between the Warsaw Pact and NATO. Testors, maker of accurate scale models of cars, ships, and aircraft, even went so far as to sell a model, based upon alleged sightings of the F-19, the logical designation for this new aircraft.
When the F-117 was publicly announced, more than just its designation was surprising. The plane itself simply didn’t look like a modern jet fighter. Instead of a sleek, aerodynamic profile optimized for supersonic performance, the F-117 was blocky and featured many flat surfaces. Its wing was swept so sharply back that the plane had difficulty developing enough lift to take off.
There was a reason for this. Stealth technology had begun with coatings that reflect less radar than the aluminum commonly used on airplanes. In fact, the now thirty-year-old SR-71 reconnaissance aircraft made use of radar-absorbent coatings to help lower the risk of detection. But there is no perfect absorber of radar. Skunk Works went a step further by shaping the F-117 so that a radar beam would be bounced in direction different from the one in which it originated.
Due to the limited computing power available in the 1970s, the plane was designed using flat surfaces to reduce the number of calculations needed. Each flat surface would add an extra direction in which radar could be reflected, so the number of surfaces used was kept to a minimum. This made the plane aerodynamically unstable about all three axes, so fly-by-wire capability was required to allow the pilot to control the airplane. Enclosed bomb bays, special pilot canopies, special seals at all joints, and special cooling vents for the engines also helped make the plane stealthier.
The F-117 had a radar signature about a hundredth as large as that of conventional airplanes, making it appear little larger than a bird on radar scopes. The B-2 Stealth Bomber, which followed the F-117, benefited from greater computing power with a contoured shape that further reduced its radar signature. The newest fighter to enter the U.S. armada, the F-22, uses a still more advanced shape.
Countering stealth a difficult task
Stealth required years of research and massive computing power to develop. Defeating it was a similarly daunting task. F-117 Stealth Fighters flew over 1300 sorties in the Gulf War without a single one being shot down. A stealth airplane was not lost in combat until 1999, when Yugoslav forces in Kosovo shot one down. This feat was, however, not repeated.
Since the beginning, though, it has been recognized that stealth is not invulnerable. Stealth relies not only on its ability not to be detected by radar, but also on its ability not to be detected by other means. This is why stealth aircraft typically do not use radar or send any radio communications while in combat. However, the engines, while cooled to minimize their infrared signatures, still emit more heat than ambient air, a vulnerability that permitted Russian-made SA-3 infrared air-to-air missiles to lock onto the aircraft shot down over Yugoslavia. In addition, stealth aircraft show up visually over a bright sky, making them usable only at night.
Those problems can be solved operationally, though, by limiting the use of stealth warplanes to favorable military situations. A more serious problem is the inherent imperfection of the surfaces of the airplane. No matter how precisely they are manufactured, they will degrade naturally during flight as a consequence of atmospheric friction. Dust in the air and rain affect it even more. Despite special techniques for repairing nicks and scratches, and sealing joints where one manufactured part is attached to another, these are done by maintenance crews working under time pressure to get each plane out for another attack run. All of these contribute to the fact that a stealth plane will always reflect some amount of radar.
The Roke Manor system
The stealth-detecting system announced over the summer was developed at Roke Manor Research, a British defense firm based in Romsey, Hampshire. It does not try to detect emissions from careless stealth aircraft, a half-hearted and easily-countered move.
Instead, it attacks the stealth system itself by detecting the radar waves that do reflect off it.
John Hansman, a professor of Aeronautics and Astronautics at MIT, explains, “Some stealth aircraft, like the F-117, are specifically designed to have a low radar cross section to monostatic, or conventional, radars. They are not stealthy to some bi-static configurations.”
Conventional monostatic radar places the transmitter and receiver in the same location, making it simple to locate a plane when spotted. Bi-static, or multi-static radar, would position the receiver at a different position from the transmitter. This makes it more difficult to compute the location of the aircraft.
However, since stealth aircraft do reflect some radar, but away from the transmitter, bi-static radar could conceivably receive the reflection and detect the stealth aircraft.
The problem then becomes one of scale and coordination.The stealth aircraft will be visible only if ideal alignment exists so that the transmitter bounces a signal off the stealth aircraft to the receiver. Stealth aircraft, however, are vulnerable from a very small subset of possible combinations of angles.
The Roke Manor system solves that problem with computing power and some creative thinking. Building a radar every few miles to solve the first problem is prohibitively expensive. However, radar is simply an application of radio, and in today’s wireless age, radio waves surround us. In particular, in industrialized nations, cell phone towers can be found every few miles, sometimes every hundred feet. Telephone companies also know exactly where the towers are located, and have telephone lines hooked up to them, facilitating communication.
In effect, the Roke Manor researchers have envisioned the use of cell phone towers as a extremely dense network of radar transmitters and receivers, interconnected via communications links. The sheer number of cell phone towers makes detection much easier than with solitary radar sites.
“A lot of stealth technology deals with redirecting radar waves,” said Greg Duckworth, a Principal Scientist at BBN working on underwater acoustics in an area very much analogous to radar.” It’s very effective against monostatic radars. However, if you have bistatic radars, in particular a very large number of sources, so that you excite the target from a wide range of angles, and you have a multiplicity of receivers in many locations, you essentially will get around the stealth target’s redirection capabilities. It is highly likely that an incident wave from a cell tower will be redirected towards one or more receivers.”
Having gotten around the stealth aircraft’s redirection capabilities, the system then puts together all the data from the cell phone towers. Until recently, this was not possible. However, increased computational power and advanced signal processing techniques have made it possible to sort through all the signals and form a coherent radar picture. Ironically, the further development of the same computing technology that originally made stealth possible has now made it possible to detect stealth aircraft.
Implications of Roke Manor
Given a cell phone network, massively parallel computers, and the Roke Manor software, how much can one determine about a plane? Quite a bit, as it turns out.
“If you can get a radar return, you can get all kinds of information from the return signal if you can process it sufficiently,” Hansman said. “For example, if you an look at the Doppler shift of the returned signal, you can get aircraft velocity. If you are sensitive enough, you can see frequency effects, such as engine rotation or structural vibration. If you have several receivers or different imaging angles, you can begin to reconstruct an image of the target.”
These data further reduce the effectiveness of stealth technology. While stealth has always returned a small signal, even to monostatic radars, that signal is so small that it is usually filtered out either by the radar scope or by the operator. However, with velocity and shape information, as well as software specifically designed to detect the inconsistencies that give away a stealth airplane, it becomes considerably easier to separate planes from birds in the sky.
Ernie Rockwood, a researcher for Sensis Corporation, a company that specializes in air traffic and air defense, said that he was “not surprised” by this development. “Some of my co-workers and I worked on novel bistatic battlefield radar techniques to improve survivability. We also submitted a proposal to Rome Labs for an operational concept using multistatic techniques.”
Defense researchers and experts in the defense industry also seem to agree that the technology is sound. Some believe this to be a natural development in radar technology.
“Underwater, they’ve already gone to multistatic systems because the reflectively of targets is such that they don’t naturally bounce stuff back,” said Greg Duckworth. “Not because they tried to, as was the case with stealth technology, but because the physics makes them do that naturally.”
Duckworth also drew an analogy between cell phone towers and television transmissions.
“Televisions have improved quite a bit, and comb filters have gotten better,” said Duckworth. “On older TV sets, though, when an airplane goes over your house, a reflective wave from the aircraft ends up interfering at your antenna, and you see lines and artifacts on your screen. To the extent that a stealth aircraft does not absorb the wave, the remnants of it still interact with the airplane and result in detectable interference patterns.”
The television analogy is particularly apt, since Lockheed has been working on a project that operates on the same principles as Roke Manor’s anti-stealth system. In this project, called Silent Sentry, FM radio stations and VHF television broadcasts are used to provide the dense network of radio waves that interacts with stealth aircraft. While there are fewer FM and VHF transmission towers than cell phone towers, each individual station transmits much more powerfully.The smaller number of stations would also reduce the computational requirements of the system.
Consequences of anti-stealth
How far-reaching are the implication of this anti-stealth technology? As with all military technologies, it depends on the particular application.
Owen Cote, Associate Director and Principal Research Scientist of MIT’s Security Studies Program, explained, “Even if this system works, it wouldn’t be useful if you couldn’t shoot the aircraft down. You’d have to find some way of guiding a missile very close to the target before an infrared or illuminating radar could achieve a lock on the aircraft.”
“This is not very mobile technology,” he continued. “Your cell phone towers are in fixed locations. While it would be close to impossible to destroy them all, they are susceptible to jamming just like conventional radar. Stealth might very well be a technology with a very short half life. However, against foes such as Serbia or Iraq whose technology is not yet competitive with ours, I see stealth as having a much longer life. As a proof of concept, this bistatic technology sounds right. The actual implementation, though, is another matter.”
Still, Dr. Cote saw some long-term effects of a successful system. “No offensive advantage lasts,” he said. “Often there is a relatively cheap defense counter to match new offensive technology. We may find ourselves moving further away from manned delivery platforms and focusing more on cruise missiles, tactical ballistic missiles, and short range missiles with incredible accuracy.”
The technology is widely acknowledged to be feasible, and Roke Manor claims to have working prototypes. However, bistatic radar is neither a miracle nor a disaster that renders worthless decades of stealth research. It is yet another battle in the war between armaments and armor.