A radio altimeter is an airborne electronic device capable of measuring the height of the aircraft above terrain immediately below the aircraft.
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.
Many operators have an SOPs which requires a pilot call of "Rad Alt Live" to be made during descent as soon as practicable after height indications reappear at 2500 feet agl in order to enhance crew awareness of proximity to terrain.
Use of the radio altimeter is integral to both the function of ground proximity warning systems and to the operation of aircraft during Cat 2/3 approaches where it is used to determine the position of the aircraft in relation to the applicable decision height.