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. As the name implies, radar (radio detection and ranging) is the underpinning principle of the system. Radiowaves are transmitted towards the ground and the time it takes them to be reflected back and return to the aircraft is timed. Because speed, distance and time are all related to each other, the distance from the surface providing the reflection can be calculated as the speed of the radiowave and the time it takes to travel are known quantities.

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