First, active radar units will bounce signals off the subject they want to track. Those radar units will be coupled with passive radar, which, as their name implies, don’t spew out signals. “Passive radar relies on existing signals that are out there,” says Gregory. Consider an HDTV broadcast signal. Since the location of the tower that transmits a signal like that is a known factor, passive radar can use disturbances in those waves to infer the presence of a drone.