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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 transmitted and reflected signal.
The radio altimeter commences operation when the aircraft descends below 2500 ft above ground level, and this is indicated visually (e.g. by dissapearance of an 'OFF' flag and emergence of the altitude needle from behind a mask).
Radio altimeter calls may be either:
- Announced by the PNF; or,
- Generated automatically by a synthesized voice.
The rate of change of phase of the radio signal is proportional to the rate of climb or descent, and this is used along with the radio altitude as an essential input to ground proximity warning systems.
- IR-OPS SPA.LVO.110&120 deals with Low Visibility Operations
- See also Acceptable Means of Compliance and Guidance Material to IR-OPS SPA.LVO.110&120