The working principle of this device can be explained from the picture. A simple Wi-Fi router fills the local area with radio waves of either 2.4 Gigahertz or 5 Gigahertz. When a person walks past these signals, the signals tend to bounce off them and the frequency of these radio waves increases or decreases depending on whether the person is walking
towards or away from the source. The first antenna present in the unit tracks the baseline of the radio signal while the other one monitors those frequencies that have bounced off a person. By comparing both the signals the computational unit present in the system calculates the position, speed and direction of the object. This technology has been tested to work across one foot thick walls.