Private Pilot Flight Training and Instruction Flight Training Article Library | Back To 4VFR.COM Absolute Altitude Absolute altitude is obtained by the use of radar. The first radar altimeter was the A/N 718. I worked with it during WWII. It consisted of two one-foot long antennae that were placed about 20 feet apart under the wings on each side of the aircraft fuselage. This was a frequency modulation system, which transmitted a burst of radio energy to the ground through one antenna and picked up the burst echo from the ground on the other antenna. During the transmission period the frequency was undergoing a constant rate of change in microseconds. The receiver measures the frequency change difference between the first burst and the frequency when it returns to the aircraft receiver. This amount of change is recorded and displayed in feet on a circular cathode ray-tube display. I would suspect that the ground proximity warning system (GPWS) system used on airliners works much the same. Doppler radar uses frequency modulation today but to my knowledge this was the first practical use of FM technology. Written by Gene Whitt Flight Training Article Library | Back To 4VFR.COM