Rader Altimeter (RA)

A radar altimeter (Rad Alt) that is also known as a radio altimeter (R ALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), that measures altitude above the terrain presently beneath an aircraft or spacecraft by measuring the time, how long it takes a beam of radio waves to travel to ground, reflect, and return to the aircraft. 

Rader  altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datum, usually reference pointlly I.e mean sea level.