Xandem Technology LLC has developed a capability while entirely hidden to detect motion through the walls of a building using a network of little radios that cost only a few dollars each. Xandem's Tomographic Motion Detection (TMD) is system covers large areas, remains hidden, detects through walls and furniture, and is easily integrated into standard security and lighting panels. The system works by recording radio waves that have been reflected from the object under observation. The 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 a moving object such as a person temporarily blocks the signal, 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. This process is called radio tomographic imaging, due to the fact that constructing an image through measuring the strength of radio signals is similar to computer tomographic body scanning. The radios used are low-cost types designed for transmitting data between devices such as thermostats, fire detectors and some automated factory equipment. They are not even as powerful as the radios used in Wi-Fi networks to link computers together. Each radio is constantly in contact with all of the others. Besides military, police and private-security uses, radio networks might be employed to locate people trapped by fire or earthquake. Commercially, they might also be used to measure âfootfallâârecording how people use stores and shopping centres. This is done with cameras or by triangulating the position of signals given off by mobile phones that customers are carrying. Radio tomography could be simpler, more accurate and less intrus