hypnosec writes: Researchers at University college of London have applied principles of radar used in defense and designed a detector using home based Wi-Fi router to spy on people across walls. Using the principles behind the Doppler effect, wherein radio wave reflects off a moving object, Karl Woodbridge and Kevin Chetty, at University College London, have built a prototype unit that uses Wi-Fi signals and recognizes frequency changes to detect moving objects. The size of the prototype unit is more or less the size of a suitcase. The unit contains a radio receiver comprising of two antennas and a signal-processing unit. The duo carried out test runs and the results were interesting. They managed to determine a person’s location, speed and direction and that too even through a one-foot-thick brick wall. The device can be used to spot intruders, monitor children or the elderly, and can even be used in military.