MIT researchers have developed radar technology that provides real-time video of what is happening behind solid concrete walls measuring four to eight inches (10-20 cm) thick. Just like any other radar, the device emits radio waves that bounce off objects and then analyzes the return signal, but to give it its X-ray spec capabilities, the newly developed radar technology disregards the 99.9975 percent of the signal that returns to the radar after bouncing off the wall and amplifies the remaining 0.0025 percent of the signal that makes it through. This amplified signal is then used to generate a real-time, video "image" of targets on the other side, providing a potentially valuable tool for urban combat situations.

The 8.5 foot (2.6 m) long device uses an array of 21 antennae (13 transmitting and eight receiving), a National Instruments circuit board, a gaming PC and some clever algorithms to generate a bird's-eye-view of the space behind the wall. In deciding what radio wavelength to use, the MIT team had to choose between longer wavelengths that are better able to pass through the wall and back but require a correspondingly larger radar apparatus to resolve individual human targets, and shorter wavelengths that can suffer signal loss.

They settled on S-band waves, which have approximately the same wavelength as wireless Internet at about 10 cm (3.9 in). Overcoming the signal loss of these shorter wavelength waves required the use of amplifiers, which allowed the actual radar device to be kept to around 8.5 feet (2.6 m) long.

"This, we believe, was a sweet spot because we think it would be mounted on a vehicle of some kind," Charvat says.

While the use of amplifiers allowed the team to overcome the signal-strength problem, the wall the radar is attempting the see through will still show up as the brightest point by far. To address this, the team use an analog crystal filter that 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.

So setting the filter to only allow waves in the 30 kHz range to pass through to the receivers effectively deletes the wall from the image so it doesn't overpower the receiver. Once the signal back-scattered off inanimate objects is eliminated, only the moving objects are shown on the screen as blobs of color. Interpreting these requires some training, so the researchers are now working on a new detection algorithm that would allow replacing the blobs with more easily interpretable symbols, such as crosses or squares.

Because the system compares each new image to the last to see what's changed, it can only detect moving objects. However, the researchers say the system can detect the small movements a person makes when trying to stand completely still.

The MIT team has demonstrated the system generating video at 10.8 frames per second showing two humans moving behind solid concrete and cinder-block walls, as well as a human swinging a metal pole in free space from a distance of 20 feet (6 m). While Charvat and his colleagues from MIT's Lincoln Laboratory estimate that penetrating an 8-inch thick (10 cm) concrete wall is possible from a maximum distance of approximately 60 feet (18,3 m), the demonstrations were done at 20 feet because Charvat says that is a realistic distance for an urban combat situation.

It is now going to be tested in multiple urban scenarios to determine its effectiveness. Should the results be satisfactory, a field prototype will be made.

But, once past the research stage, will the radar be able to stand its ground against other contenders on the surprisingly crowded Sense Through The Wall (STTW) device market?

Portable STTW devices have been around for over a decade now, with Georgia Tech Research Institute's RADAR Flashlight (funded by the US National Institute of Justice) paving the way for a range of mobile devices that includes both handhelds, such as DARPA's Radar Scope, and backpack-sized contraptions such as the Prism 200 by Cambridge Consultants or the XAVER line of STTW devices by CAMERO.

However, convenient size comes at a price. It requires a serious trade-off, as all these devices need to be held to the wall in order to operate. So, while MIT's brainchild may not be able to compete on mobility, it can definitely compete on standoff distance and range, not unlike the Cougar20-H we covered earlier this year.

And that's not the only ace the MIT's baby has up its sleeve. The system dramatically speeds-up the through-the-wall data collection process, from the previous 1.9 seconds to under 100 milliseconds.

Of course, STTW radars are not the only possible approach to the X-ray vision problem. University of Utah engineers, for example, see a wireless network of radio transceivers as a viable alternative. And then there are huge surveillance schemes such as the UK government-funded Celldar project that turns a network of mobile phone masts into a gigantic radar potentially capable of tracking everything that moves within its range, be it outdoors or indoors.

MIT's Gregory Charvat and John Peabody detail their new radar technology in the video below.

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