The ability to see through walls is no longer the stuff of science fiction, thanks to radar technology developed at Lincoln Laboratory. The new system provides a real-time picture of what’s happening on the other side of a solid wall, which has powerful implications for urban combat situations, says project leader Gregory Charvat, a member of the technical staff at Lincoln Lab.
Just as most visible light is reflected by solid objects, so too are radar waves; in fact, more than 99 percent of a radar signal is blocked when it hits a concrete wall. And that’s only half the battle: when the tiny fraction that does penetrate hits objects on the other side and bounces off, it must then pass back through the wall, resulting in another 99 percent loss. By the time it reaches the radar receivers, the signal is reduced to about 0.0025 percent of its original strength.
But as Charvat points out, signal amplifiers are cheap. A bigger challenge is achieving the speed, resolution, and range necessary for through-wall radar 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,” he says.
The Lincoln Lab system works up to 60 feet from the wall. It can generate images through eight-inch-thick concrete at the rapid rate of 10.8 frames per second.
The system relies on an analog filter crystal that exploits frequency differences between waves reflecting off the wall—which would normally show up as the brightest spot on the radar, since it’s the closest, densest object—and those reflecting off moving objects behind the wall. “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 frequencies that correspond to targets behind the wall to pass through to the receivers, effectively deleting the wall from the image. That makes it easier to home in on the subtler signals from the targets behind it.
A digital-to-analog converter turns the signals into video. Currently, humans show up as “blobs” on a screen that offers viewers a bird’s-eye perspective, as if they were on the wall looking down at the scene. The researchers are working on converting the blobs into symbols, such as crosses or squares, to make the images easier to interpret. The radar now detects only moving targets, not inanimate objects such as furniture. But even people trying to stand still shift slightly, and the system registers these movements.
Charvat says the radar could eventually be used for search-and-rescue missions and other domestic applications, but it was developed primarily “for the urban war fighter … those situations where it’s very stressful and it’d be great to know what’s behind that wall.”