The technologies normally incorporated into vehicles today to achieve autonomy include radar, cameras, a variety of different sensors, and GPS. For example, most (ACC) Adaptive Cruise Control systems are either laser or radar based and mounted to the front of the car. Laser based ACC systems must be exposed, whereas radar based sensors can be hidden behind plastic because they make use of infrared or microwave sensors, wavelengths that can simply pass through plastic. Radar systems are more common given that laser systems cannot track vehicles in poor weather conditions. An additional advantage of radar is that it not only returns obstacle distance, it can also determine obstacle speed using the Doppler Effect. Most systems are single radar, but some automakers opt for two radars, one each for close (100 feet) and long (600 feet) range. Lane keeping systems typically use a camera mounted in or around the rear view mirror to watch the lane markings, which they use to guide the vehicle. In addition to lane keeping, some vehicles also support lane changing. Tesla uses 12 long-range ultrasonic sensors that are positioned such that they vehicle can sense 16 feet around it in all directions, at all speeds. Mercedes-Benz also offers automated lane changing using an army of 12 ultrasonic sensors, six radar sensors, and up to 8 cameras to monitor all 360 degrees around the car.