Seeing through walls in real time

October 20, 2011
SAR Imagery

SAR imagery (in range vs. range) of two humans about 30 feet down range with the wall 20 feet down range. From left to right, the images as shown on the radar screen are for subjects moving in free-space, behind a 4-inch solid concrete wall, behind a cinder block wall, and behind an 8-inch solid concrete wall. Each target’s location is clearly identifiable in all scenarios, and the through-wall results are similar to those of the free-space (no wall obstruction). (Credit: MIT Lincoln Laboratory)

MIT Lincoln Laboratory researchers have developed a new radar system that looks through walls. This ultrawideband (UWB) phased-array sensor has real-time acquisition and processing capability and provides video-like synthetic aperture radar (SAR) images of people moving behind an 8-inch-thick cinder block or concrete wall.

The system demonstrated the ability to capture meaningful imagery at a 10.8 Hz frame rate through 4-inch- and 8-inch-thick, as well as cinder block, walls from a standoff distance of approximately 20 feet, but is estimated to work up to 60 feet away.

Dr. Gregory Charvat, who developed the system along with colleagues John Peabody and Tyler Ralston of Lincoln Laboratory’s Aerospace Sensor Technology Group, says, “We estimate the maximum range to be approximately 60 feet when looking through an 8-inch concrete wall.”

The technology will be useful for providing soldiers or emergency responders in urban environments with increased awareness of activity inside or behind structures, says Charvat. “Such a system could reduce mission risk for soldiers in the urban environment and help to save lives.”

The system exploits a well-known fact: while it is not possible to see through walls by using visible light, it is possible by using larger micro-wavelengths to radiate into a wall and receive a weak scattered signal that is representative of what is behind the wall.

Array Elements

In this front view, the radar system's array elements are shown. The 8 receive elements are in the top row and the 13 transmit elements are on the bottom row. The rear panels of the RF, analog, and digital hardware are shown at the bottom. The array measures 8.5 feet wide by 2 feet tall, and the entire system stands about 5 feet off the ground. Casters are used so that the system can be moved around easily during field testing (credit: MIT Lincoln Laboratory)

These data is processed using a SAR imaging algorithm and displayed on the radar screen in real time, providing a video-camera-like radar image (in range versus range) of what is behind the wall. Coherent processing techniques subtract the non-moving clutter (chairs, tables, furniture) from the signals of the humans moving behind the wall.

The image quality is sufficiently high to resolve multiple humans behind a wall. Humans were located through the three types of wall tested when they were moving or standing still. Humans were located through a 4-inch concrete and cinder block wall even if they were sitting still or holding their breath while standing. Because humans move slightly even while trying to remain still, the radar system detected those small movements by using coherent radar processing techniques.

Currently, the researchers are working on adding processing so that the radar screen displays crosses that are more easily interpreted than the “blobs” shown now. Charvat says that future work includes plans to mount the system on a vehicle and test it on a variety of random walls and representative urban structures. “Our objective is to determine the effectiveness of this technology in a representative environment and if successful, then to field a rapid prototype.”