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 routers to spy on people across walls. Using the principles behind the Doppler effect ... 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 ... they managed to determine a person's location, speed, and direction (even through a one foot thick brick wall). The device could be used to spot intruders, monitor children or the elderly, and could even be used in military applications."