As Popular Science explains, Wi-Fi radio signals are found in 61 percent of households nationwide. Researchers Karl Woodbridge and Kevin Chetty developed a suitcase-size device that can use these signals along with the Doppler effect to detect movement. The Doppler effect happens when a radio wave reflects off a moving object. When it does, the wave's frequency changes.
The device reads these frequency changes and calculates not only when an object is moving, but also its speed, location and the direction it's going. For example, if a person is moving toward the Wi-Fi source, the frequency of waves increases; if a person is moving away from the source, the frequency of the waves decreases.
NEWS: Wi-Fi Fridge Suggests Recipes