The passive radar process is actually quite simple. In any space that has WiFi, you are constantly being bombarded by 2.4GHz and 5GHz radio waves. When these waves hit a moving object, their frequency is altered (the Doppler effect). By carefully “sniffing” the WiFi signals, Woodbridge and Chetty are able to reconstruct an image any objects or humans that are moving on the other side of the wall.