You no longer have to be from Krypton to see through walls, thanks to new radar technology developed at MIT’s Lincoln Laboratory. Radar “sees” by sending out radio waves that bounce off targets and return to the special receivers. But just as visible wavelengths can’t pass through solid objects in quantities large enough for the eye to detect, it’s hard to build radar that can penetrate walls well enough to show what’s happening behind them. The Lincoln Lab researchers, however, have built a system that can see through walls from some distance away, giving an instantaneous picture of the activity on the other side. The system has powerful implications for military operations, especially “urban combat situations,” said Gregory Charvat, the leader of the project. The novel system is an unassuming array of antennas arranged into two rows — eight receiving elements on top, 13 transmitting ones below — and some computing equipment, all mounted onto a movable cart. At first blush, the radar functions as any other — the transmitters emit waves of a certain frequency in the direction of the target. But in this case, each time the waves hit the wall, the concrete blocks more than 99 percent of them from passing through. And that’s only half the battle: Once the waves bounce off any targets, they must pass back through the wall to reach the radar’s receivers — and again, 99 percent don’t make it. By the time it hits the receivers, the signal is reduced to about 0.0025 percent of its original strength. MIT researchers have developed a novel radar system that can detect movement behind solid concrete walls. (Photo: Gregory Charvat and John Peabody, MIT) However, signal loss from the wall is not the main challenge, according to Charvat, because signal amplifiers are inexpensive. However, what has been difficult for through-the-wall radar imagers, which have existed for years, is achieving the speed, resolution and range necessary to be useful in real time. “If you’re in a high-risk combat situation, you don’t want one image every 20 minutes, and you don’t want to have to stand right next to a potentially dangerous building,” Charvat said. One consideration for through-the-wall radar, Charvat said, is which wavelength to use. Longer wavelengths are better able to pass through the wall and back, which makes for a stronger signal; however, they also require a correspondingly larger radar apparatus to resolve individual human targets. The researchers settled on S-band waves, which have about the same wavelength as wireless Internet — that is, fairly short. That means more signal loss; hence, the need for amplifiers. The actual radar device can be kept to about 8.5 ft long. “This, we believe, was a sweet spot because we think it would be mounted on a vehicle of some kind,” Charvat said. Even when the signal-strength problem is addressed with amplifiers, the wall always shows up as the brightest spot by far. To get around this problem, the researchers use an analog crystal filter, which exploits frequency differences between the modulated waves bouncing off the wall and those coming from the target. Lincoln Laboratory researchers Gregory Charvat (rear) and John Peabody stand before the solid concrete wall through which they successfully detected human movement. (Photo: Melanie Gonick) “So, if the wall is 20 feet away, let’s say, it shows up as a 20-kilohertz sine wave. If you, behind the wall, are 30 feet away, maybe you’ll show up as a 30-kilohertz sine wave,” Charvat explained. The filter can be set to allow only waves in the range of 30 kilohertz to pass through to the receivers, effectively deleting the wall from the image so that it doesn’t overpower the receiver. The system can be used up to 60 ft from the wall. Demonstrations were performed at 20 ft, which Charvat said is realistic for an urban combat situation. In these tests, it gives a real-time picture of movement behind 4- and 6-in. walls in the form of a video at 10.8 fps. “It’s a very capable system mainly because of its real-time imaging capability,” said Robert Burkholder, a research professor at Ohio State University, who was not involved with this work. “It also gives very good resolution, due to digital processing and advanced algorithms for image processing. It’s a little bit large and bulky for someone to take out in the field,” he added, but agreed that mounting it on a truck would be appropriate and useful. Because the processor uses a subtraction method — comparing each new picture to the last, and seeing what’s changed — the system can detect only moving targets, not inanimate objects such as furniture. Still, even a human trying to stand still moves slightly, and the system can detect these small movements to display that human’s location. For more information, visit: www.mit.edu