Through-the-wall vision

A few weeks ago The Economist wrote Through-the-wall vision. I'm not sure if non-subscribers can follow the link but this is the gist:

"Radar works by recording radio waves that have been reflected from the object under observation. Dr Patwari’s and Mr Wilson’s insight was to look not for reflections but for shadows. Their device broadcasts a radio signal through a building and, when that signal comes out the other side, monitors variations in its strength. The need for variation means the system cannot see things that are stationary. When the signal is temporarily blocked by a moving object such as a person, however, it shows up loud and clear.

Using a network of small transmitters and receivers, the researchers have found it is possible to plot a person’s position quite accurately and display it on the screen of a laptop. They call the process radio tomographic imaging, because constructing an image by measuring the strengths of radio signals along several pathways is similar to the computerised tomographic body-scanning used by hospitals—though medical machines employ X-rays, not radio waves, to do the scanning."