Monday, June 27, 2011

Innovative Radar System Sees People Behind Concrete Walls

MIT Lincoln Laboratory researchers developed a new radar system that looks through walls. This ultrawideband (UWB) multiple-input, multiple-output phased-array sensor has real-time acquisition and processing capability and provides video-like synthetic aperture radar (SAR) images of people moving behind a 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. 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."

One of the through-wall experimental setups is shown here. The radar system is located to the right and the cinder block wall is to the left. An absorber is mounted on the vertical reinforcement sections of the wall and clutter fences using a pyramidal absorber are mounted on the front of the wall to approximate the conditions of two-dimensional air-wall-air models. Plywood sheets on the ground allow the radar system to be easily moved around the target area. A Styrofoam "A-frame" holds the calibration target in front of the wall. When measurements are acquired, one or two humans walk around about 10 feet behind the wall.

The technology will be useful for providing soldiers or emergency responders in urban environments with increased awareness of activity inside or behind structures. "Such a system could reduce mission risk for soldiers in the urban environment and help to save lives," says Charvat. The system, which decreases through-the-wall data collection from a prior 1.9-second process to a less-than-100-millisecond one, will enable faster recognition of the presence of people and their locations inside a walled structure.

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.

Prototype of the UWB antenna element used. This antenna element is an efficient and directional radiator from about 900 MHz to 18 GHz even though it is only used from 2 to 4 GHz in this application.

At the core of the system is a range-gated continuous-wave radar architecture that provides dynamic range and sensitivity to acquire weak signals scattered from targets behind the wall. The radar set is connected to an array of UWB antenna elements consisting of two subarrays made up of 8 receive elements and 13 transmit elements. Microwave switches guide the transmitter port from the radar to one transmit element at a time. Similarly, microwave switches guide the receive port to one receive antenna at a time.

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.

The radar system as seen from the back. Antenna feed lines are the blue coaxial cables. Each receive element has a low-noise amplifier attached directly to it. Radar equipment and processing are mounted in the two racks under the antenna array. Attached on the far left of the lower rack is the commercial off-the-shelf PC used to process radar data in real time. The first rack includes radar transmitter, monitor scope, digital interface box, and power supply. The second rack holds the receiver front end, the intermediate frequency stage, and the beat frequency oscillator.

A subset of transmit and receive antenna pairs is used to acquire a SAR image of the target scene in less than 100 milliseconds. "In order to achieve a data-acquisition time of less than 100 milliseconds, a gated sample clock was used to acquire a complete image data set, also providing a mechanism for synchronous beam multiplexing, in conjunction with direct memory access transfers over a high bandwidth data bus, PCI-Express," says Peabody, who developed the data-acquisition pipeline.

These data are 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. According to Ralston, who developed the real-time imaging algorithm, "When optimizing a computational pipeline for image reconstruction, one must go through the processing chain with scrutiny to identify where to precalculate values, avoid redundant calculations by keeping reused computations persistent in memory, make use of hardware-accelerated algorithms and libraries, utilize additional CPUs for parallelized computations, and avoid memory latencies by performing computations on the same memory within a single thread."

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).

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 the 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."

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