A radar altimeter, radio altimeter, low range radio altimeter (LRRA) or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level.
Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions and also automatic landings, allowing the autopilot to know when to begin the flare maneuver.
DIY Altimeter
