X-Ray Vision is Here

Dina Katabi isn’t faster than a speeding bullet. Her physical strength pales in comparison to that of a locomotive. As for leaping over buildings, the diminutive electrical engineer prefers to take the stairs. Like Superman, however, Katabi possesses an uncanny ability to see through walls.

Her super power is on display inside MIT’s Stata Center, a monument to architectural fantasia that itself seems to have sprung from the pages of a comic book.

Near the base of the shiny, yellow cylinder at the heart of the Stata Center, Fadel Adib, a PhD student in Katabi’s lab, leads me through their latest creation. Sprawling across the top of a humble AV cart sit a jumble of circuit boards, signal processors, and high-precision clocks. Adib explains their individual functions as I dutifully nod my head with only the vaguest of understanding. He then walks to a nearby wall where the cables terminate at a pair of antennas, each of which bear a striking resemblance to the plastic fins or “flights” at the end of a dart on a dart board.

Adib walks past the antennas through a doorway to an adjoining room. As he closes the door behind him, the antennas beam radio signals through the wall that track his every move. The device is able to zero in on changes so subtle that I wouldn’t be able to see them with my own eyes even if Adib were still standing right in front of me.

Taking a seat next to Hongzi Mao, another student in Katabi’s lab, I watch a computer monitor plot a sine wave that crests and troughs at regular intervals. “Fadel, can you hold your breath?” Mao shouts to his lab mate on the other side of the wall.

Before Mao can finish his request the wave on the screen flatlines. “That was his breath,” Mao says of the waves as he pantomimes an exaggerated expansion and contraction of his own chest. Then, Mao points to smaller squiggles that continue to punctuate the otherwise flat line. “This is his heartbeat,” he says.

Seeing Through It All

Drywall, cement block, even solid concrete are no longer impenetrable barriers to what we can see. Recent advances using radio signals to track movement allow researchers to peer not only through walls but also through human flesh with such clarity that they can actually monitor the movement of one’s heart as it beats. Such “X-ray vision,” once the province of superheroes, has obvious military applications for urban combat. Other unexpected uses, however, are emerging with technology that surpasses even the most advanced warfighter capabilities.

In 2011, a study funded by the United States Air Force showed it was possible to peer through concrete or cinder block walls to monitor the movements of people on the other side. “It was strictly a military problem,” says former MIT researcher Gregory Charvat who led the study. “We were in these conflicts in Iraq and Afghanistan where there was a lot of searching of buildings.” If, before entering a building, U.S. soldiers could see who was inside and what they were doing, it would give them a tremendous tactical advantage.

prism-200
Through-wall radar, like this Cambridge Consultants Prism 200 device, has grown more capable in recent years.

Using a powerful radar, Charvat and colleagues could see how many people were on the other side of walls and could track their movements. “It could help in the searching of buildings—you may not even have to [physically] search the buildings,” Charvat says.

Using radar to see through walls works much like conventional radar: a transmitter emits a radio signal in a certain direction and then an antenna records what bounces back. The time and strength of the signal that returns allows the device to infer how far away and how big an object is. Imaging through walls, however, requires a bit more finesse. Unlike visible light, the shorter wavelength “microwaves” used in radar imaging, can pass through solid walls, but just barely.

When Charvat and colleagues pointed their radar at a four-inch-thick concrete wall less than one percent of the signal made it to the intended targets on the other side. When the remaining signal bounced off the target back toward the radar more than 99 percent of this already weakened signal was again lost before making it back to the radar. “Only about 0.005 of the original signal makes it through the wall and back again,” Charvat says. Still, the returning signal allowed them to monitor in real time the movements of four people on the other side.

And that’s not all their radar could see. Radar imaging can tell if a person on the other side of a wall is carrying a large weapon like an AK-47 or a rocket propelled grenade launcher, according to a 2009 study by the United States Army Research Laboratory. In 2013, a study by the Air Force Institute of Technology found the technology could also determine, with more than 95% accuracy, whether individuals on the other side of a wall were children or adults.

Over the past decade the U.S. Department of Defense spent millions developing technology that could peer through walls. From 2005 to 2012, the Defense Advanced Research Projects Agency, known as DARPA, alone spent over $60 million on Visibuilding, a program whose goal was to “detect personnel within buildings, determine building layouts, and locate weapons caches.”

Then in 2013 the Department of Defense cut all funding for Visibuilding. The same year the US Army terminated its Sense Through The Wall program after determining it “was not mature enough to meet user needs and soldier’s requirements.”

One reason was likely researchers’ inability to generate anything more than a very crude approximation of individuals on the opposite side of the wall. As Charvat describes it, all you could see were “blobs.”

radar-images
These images show what through-wall radar imagery looks like. The outlines are the people, walls, and objects in the room (overlaid for context), while the heat map is the intensity of radar reflectance.

“When I see blobs I think, ‘Wow, that is amazing!’ ” Charvat says. “But when you take it to an end user they may not think it is that great. If you need a PhD electromagnetics person to interpret the data for you, it is not going to be used. There are some real limitations with the data.”

Another problem was accuracy. Moeness Amin, director of the Center for Advanced Communications at Villanova University in Pennsylvania, says proponents of through wall imaging promised more than the technology could deliver. “They were very ambitious in terms of what the physics could actually allow,” he says. “Most assumptions from the lab were unfortunately invalid because the real world is very complicated. When a frequency goes through a wall it hits not only you but the chair, the ceiling, the filing cabinets, the interior walls. There is a lot of clutter and the radar hits all of this.”

The bouncing of signals from object to object tended to have a multiplier effect on the results. “Instead of you showing up as one, you show up as four,” Amin says. “It generates ghosts.”

Slimming Down

Katabi can work with blobs and ghosts, but to meet her needs, the imaging equipment had to get a lot smaller. “Typically radar is very big, heavy, and expensive,” she says. “One and a half years ago, the state of the art had to be mounted on a truck. We want something you can attach to your baby monitor.”

In June 2013 Katabi showed it was possible to track people through walls using conventional Wi-Fi, the ubiquitous radio signals used to connect computers and other electronic devices to the Internet. Like conventional radar, the technique emitted radio signals that passed through a wall, reflected off objects on the other side, and passed back through the wall to a receiver antenna. Unlike military radar, however, the Wi-Fi signals were offset in such a way that reflections from static objects—things like walls, tables, and chairs—cancelled each other out. The only signals that made it back to Wi-Fi device were those from moving objects; people.

By focusing on moving things, Katabi was able to use significantly less power and a single receiver antenna instead of the vast antenna arrays typically used in military pilot studies. Instead of blasting an entire room with radio waves to get a detailed image of everything inside, Wi-Fi could shine a spotlight on only the moving objects.

The new technique, however, had its problems. Wi-Fi, it turned out, wasn’t ideal for pinpointing an individual’s exact location. And when people stood still, they often disappeared. “Wi-Fi signals are incapable of providing high resolution because they were not intended for localization to start with.” Katabi says. “There are so many other things going on, like the actual communication that the signal was intended for.”

Katabi took the Wi-Fi signal and modified it, increasing the frequency of its radio waves to better pinpoint individuals. She also developed more robust algorythms to better interpret the signals that made their way back through the wall. The changes not only allowed her to zero in on an individual’s exact location. They also allowed her to penetrate an individual’s chest cavity and record the very beating of his or her heart.

Her newest iteration, WiTrack, isn’t strong enough to peer through thick concrete walls, but the lower power signal it transmits, 100 times less powerful than Wi-Fi, would likely preclude any safety concerns about its use in consumer electronics.

It will likely be years before WiTrack appears in any household gadgets. Current prototypes still gets confused when more than one person enters a room. But, if Katabi and her labmates can make the system more robust, the potential applications, including infant monitors, fall detectors for the elderly, and interactive gaming consoles that allow users to hide behind furniture and walls, are limitless, she says. “Can I, for example, do EKGs remotely without anything on the person themselves?” What began as a failed military research project may have a promising future in health care and other consumer applications.

Just the Beginning

Long before the Stata Center’s shiny cylinders and leaning towers loomed over the MIT campus, researchers developed military radar on the site in a building known as the Radiation Laboratory. Toward the end of World War II, one of the researchers on the project noticed a candy bar in his pocket melted when he got too close to an active radar device. The “Radarange,” now better known as the microwave, was born.

Sitting in her office high above the former Rad Lab, Katabi says her recent experiments in radio wave sensing are just the beginning. “We can create a very smart environment that can help us as humans without interfering with our lives,” she says.