A radar altimeter measures altitude above the terrain beneath an aircraft as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level. Radar is the underpinning principle of the system. Radio waves that are reflected back from the ground are timed in order to calculate speed, distance, and time which are related to the each other and can be used to calculate the distance from one point to another.
Lloyd Espenschied invented the radar altimeter in 1924; however, Bell Labs put the device in a usable form for aircraft use 14 years later. Radar altimeters are used by commercial aircraft for approach and landing during low-visibility conditions and during automatic landings. The radar altimeter gives readings up to 2,500 feet above ground level. Most airliners are equipped with at least one, if not more, since they are essential to auto-land capabilities. They are also essential in ground proximity warning systems that warn the pilot if the aircraft is to low, however, they are limited to the terrain below and cannot see directly ahead of the aircraft.
The radar altimeter normally works in the E band or Ka band or S bands for more advanced sea-level measurement. Radar altimeters are used to avoid radar detection and targeting by anti-aircraft guns when flying in enemy territory. They allow the aircraft to fly low to the ground.