The Guardian device from Sense Technologies was developed to warn a driver visually and audibly of objects up to 3.6 m behind the vehicle (7.3 m for the industrial version). The FCC-approved device, which fits into a license plate holder, uses microwaves to detect the object behind and indicate its distance from the vehicle. The unit is activated when the car is placed in reverse and automatically shuts down when the car is in neutral or a forward gear.