Photo: Tim Dees
A "police science" textbook in use during the 1970s listed among the necessary tools for a surveillance assignment a pair of binoculars, a thermos of coffee, and a mayonnaise jar for the "used coffee." In that era, the author assumed his only readers would be men. The game has become a little more sophisticated since then. Cops still sit in cars watching houses and businesses, but they are just as likely to be using clandestine tactics to see inside buildings and over fences, scan faces in a crowd, and review security camera footage.
Sometimes it's not possible to conduct surveillance without being seen, and sometimes it doesn't matter. Organized crime "wiseguys" have been known to be on a first-name basis with their watchers, especially when a surveillance goes on for months or years. Like the cop on the street corner or a parked patrol car in a school zone, the perception of being watched can deter crime. Even so, most surveillance is covert, so that the watched will let down their guard and leave an opening for discovery of their misdeeds or a breach of their sanctuary.
The newest and most innovative surveillance tools work without the bad guys ever knowing they are being watched until it's too late.
Seeing Through Walls
Recent research at the Massachusetts Institute of Technology (MIT) developed a radio-based system that detects movement through walls as much as eight inches thick and 20 feet away. S-band radio waves, similar to those used in wireless Internet networks, move through an array of 21 antennae (13 transmitting and eight receiving) aimed at the barrier the watcher wants to see through. The reflected radio energy still displays the barrier as the brightest point, but the waveform of that return is a function of the distance between the antenna array and the wall. A wall 20 feet away might show up as a 20KHz sine wave, while objects on the other side of the wall show as a 30KHz sine wave. By suppressing the lower frequency return with an analog crystal filter, objects on the far side of the wall show up on the detector's display.
This technology is new and the discrimination between objects is fairly crude. Output consists of video at 10.8 frames per second, and the system is able to detect only movement by comparing one frame to another. Experimenters can show two humans moving behind concrete and cinder-block walls, and a person swinging a metal pole, both from about 20 feet away. The biggest downside thus far is the size of the device. The antenna array is 8.5 feet long, so it requires a good-sized truck to move it. Refinement will almost certainly reduce the device to a more manageable size.
A different approach by researchers at the University of Utah uses radio transmitters deployed around, on top of, and even within a building to detect movement within the building. Tomography is the technique for producing focused images of structures at specific depths within an enclosure by the use of reflected energy. It's the "T" in a CAT scan (for Computerized Axial Tomography) used to detect injuries or growths inside the body. This passive detection system is called variance-based radio tomographic imaging (VRTI). The scientists who developed this technology visualize a process of throwing, launching, or dropping small radio sensors throughout a structure, which would form a mesh network and report their location to a central monitor against a database from Google Maps or a building diagram.
As people move through the network, they disrupt the radio signals between the sensors, and a measureable value called the residual sum-of-squares (RSS) variance (don't ask-the math is deadly) produces an image of the motion path. The effect is to show movement as if the people moving were carrying tracking devices, only without the tracking devices.
This system can be pre-deployed in high-security buildings or where there is a special need for life safety in a fire situation, or deployed on the go when operators needed to detect movement in a building that hadn't been prewired. The sensors would establish a coarse network by placement around the perimeter and on the roof, and searchers would drop more sensors as they moved through the building, refining the network and giving greater precision to location monitoring outside. Proof-of-concept was demonstrated at the home of one of the researchers, using 34 nodes distributed around the perimeter of a 30-by-30-foot enclosure.
If you want to work a little closer to the action, the Prism 200 through-wall radar unit from Cambridge Consultants is about the size of a small backpack, with rigid handles where the shoulder straps would be. The operator holds the flat side against a wall and sees a display with blips indicating any moving objects on the other side of the wall. The manufacturer says the device will detect anyone up to 20 meters away from the base wall, with a field of view 120 degrees horizontally and 90 degrees vertically. A similar line of through-wall imaging systems is the Xaver series from Camero.