Definition

A radio altimeter is an airborne electronic device capable of measuring the height of the aircraft above terrain immediately below the aircraft.

Description

Early radio altimeters determined altitude by measuring the time between transmission of a radio signal from the aircraft and reception of the reflected signal. Modern systems use other means, for example, measurement of the change of phase between the transmitted and reflected signal.

In almost all cases, the display of radio height ceases when an aircraft climbs through 2500' above ground level (agl) and recommences when it descends through 2500' agl. This is confirmed visually by the appearance/disappearance of an 'OFF' flag and emergence of a pointer from behind a mask or activation of a digital display.

Radio altimeter calls may be either: