Seeing through walls was always a fiction and even shown in an Arnold Schwarznegger’s movie more than a decade ago. But the technique which was most-awaited by the defense and security forces all over the world has finally come true partially, if not fully, with a radar developed by MIT’s Lincoln Laboratory.
The researchers have developed a new radar technology using the reflection of radio waves which can penetrate the walls and bounce back to the radar’s receivers. It is similar to the way humans see through waves of visible light that bounces off objects and then strikes our eyes’ retinas.
The main drawback in this new radar technology is the distance. The scene behind walled interiors can be detected by these radas from a distance of 20 to 60 meters, say the MIT researchers who have developed the technology.
Secondly, the radar can only detect moving targets, not inanimate objects such as furniture. But even a slight movement by humans can be detected on this radar.
The researchers have arranged an array of antenna into two rows — eight receiving elements on top, 13 transmitting ones below, besides some computing equipment, all mounted onto a movable cart.
The experiment on a solid or a four-and-eight-inch-thick concrete wall from a distance of 20 feet and it received a real-time movement of what happened behind the wall in the form of a video at the rate of 10.8 frames per second. It is more useful in “urban combat situations,” says Gregory Charvat, leader of the project at MIT’s Linclon Lab.
When transmitters emitted waves of a certain frequency in the direction of 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.
But according to Charvat, signal loss from the wall is not the main challenge here. “[Signal] amplifiers are cheap,” he says. What has been difficult for through-wall radar systems 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 says.
Filtering for frequencies
As longer wavelengths are better able to pass through the wall and back, which makes for a stronger signal and 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.
But this means more signal loss, and the need for amplifiers and the hosrt distance, here it was kept to about eight and a half feet long. “This, we believe, was a sweet spot because we think it would be mounted on a vehicle of some kind,” Charvat explains.
The next hurdle was the wall — whether it’s concrete, adobe or any other solid substance. It will always show up as the brightest spot blurring the image inside. So, the researchers have used an analog crystal filter, which exploits frequency differences between the modulated waves bouncing off the wall and those coming from the target.
“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 says. 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.
“It’s a very capable system mainly because of its real-time imaging capability,” says Robert Burkholder, a research professor in Ohio State University’s Department of Electrical and Computer Engineering 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 says, but agrees that mounting it on a truck would be appropriate and useful.
Captures only movement
In a recent demonstration, Charvat and his colleagues showed how the radar was able to image two humans moving behind solid concrete and cinder-block walls, as well as a human swinging a metal pole in free space. The project won best paper at a recent conference, the 2010 Tri-Services Radar Symposium.
Because the processor uses a subtraction method — comparing each new picture to the last, and seeing what’s changed — the radar can only detect 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.
The system digitizes the signals it receives into video. Currently, humans show up as “blobs” that move about the screen in a bird’s-eye-view perspective, as if the viewer were standing on the wall and looking down at the scene behind. The researchers are currently working on algorithms that will automatically convert a blob into a clean symbol to make the system more end-user friendly. “To understand the blobs requires a lot of extra training,” Charvat says.
With further refinement, the radar could be used both domestically and for military use. Charvat says, “This is meant for the urban war fighter … those situations where it’s very stressful and it’d be great to know what’s behind that wall.”