Seeing through walls

[Eugen Leitl]

http://web.mit.edu/newsoffice/2011/ll-seeing-through-walls-1018.html

Seeing through walls

Researchers at MITbs Lincoln Lab have developed new radar technology that
provides real-time video of whatbs going on behind solid walls.

Emily Finn, MIT News Office

Lincoln Laboratory researchers John Peabody and Gregory Charvat explains how
their teambs system can see through walls.

Video: Melanie Gonick

The ability to see through walls is no longer the stuff of science fiction,
thanks to new radar technology developed at MITbs Lincoln Laboratory.

Much as humans and other animals see via waves of visible light that bounce
off objects and then strike our eyesb retinas, radar bseesb by sending out
radio waves that bounce off targets and return to the radarbs receivers. But
just as light canbt pass through solid objects in quantities large enough for
the eye to detect, itbs hard to build radar that can penetrate walls well
enough to show whatbs happening behind. Now, Lincoln Lab researchers 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 researchersb device is an unassuming array of antenna arranged into two
rows b eight receiving elements on top, 13 transmitting ones below b and some
computing equipment, all mounted onto a movable cart. But it has powerful
implications for military operations, especially burban combat situations,b
says Gregory Charvat, technical staff at Lincoln Lab and the leader of the
project.

Waves through walls

Walls, by definition, are solid, and thatbs certainly true of the four- and
eight-inch-thick concrete walls on which the researchers tested their system.

At first, their radar functions as any other: 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 thatbs only half the battle: Once the waves bounce off
any targets, they must pass back through the wall to reach the radarbs
receivers b and again, 99 percent donbt 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 even the main
challenge. b[Signal] amplifiers are cheap,b 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. bIf youbre in a high-risk combat
situation, you donbt want one image every 20 minutes, and you donbt want to
have to stand right next to a potentially dangerous building,b Charvat says.

The Lincoln Lab teambs system may be used at a range of up to 60 feet away
from the wall. (Demos were done at 20 feet, which Charvat says is realistic
for an urban combat situation.) And, it gives a real-time picture of movement
behind the wall in the form of a video at the rate of 10.8 frames per second.

Filtering for frequencies

One consideration for through-wall radar, Charvat says, is what radio
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 b that is, fairly short. That means more signal loss b
hence the need for amplifiers b but the actual radar device can be kept to
about eight and a half feet long. bThis, we believe, was a sweet spot because
we think it would be mounted on a vehicle of some kind,b Charvat says.

Even when the signal-strength problem is addressed with amplifiers, the wall
b whether itbs concrete, adobe or any other solid substance b will always
show 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. bSo if the wall is 20 feet away, letbs say, it shows
up as a 20-kilohertz sine wave. If you, behind the wall, are 30 feet away,
maybe youbll show up as a 30-kilohertz sine wave,b 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
doesnbt overpower the receiver.

bItbs a very capable system mainly because of its real-time imaging
capability,b says Robert Burkholder, a research professor in Ohio State
Universitybs Department of Electrical and Computer Engineering who was not
involved with this work. bIt also gives very good resolution, due to digital
processing and advanced algorithms for image processing. Itbs a little bit
large and bulky for someone to take out in the field,b he says, but agrees
that mounting it on a truck would be appropriate and useful.

Monitoring movement

In a recent demonstration, Charvat and his colleagues, Lincoln Lab assistant
staff John Peabody and former Lincoln Lab technical staff Tyler Ralston,
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 b comparing each new picture
to the last, and seeing whatbs changed b 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 humanbs location.

The system digitizes the signals it receives into video. Currently, humans
show up as bblobsb that move about the screen in a birdbs-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. bTo understand the blobs requires a lot of extra
training,b Charvat says.

With further refinement, the radar could be used domestically by
emergency-response teams and others, but the researchers say they developed
the technology primarily with military applications in mind. Charvat says,
bThis is meant for the urban war fighter b& those situations where itbs very
stressful and itbd be great to know whatbs behind that wall.b