The US version of MLS, a joint development between the FAA, NASA, and the U.S. Department of Defense, was designed to provide precision navigation guidance for exact alignment and descent of aircraft on approach to a runway. It provides azimuth, elevation, and distance, as well as "back azimuth" for navigating away from an aborted landing or missed approach. MLS channels were also used for short-range communications with airport controllers, allowing long-distance frequencies to be handed over to other aircraft.