altimetro

An altimeter is a device that measures altitude, the vertical distance above a reference level such as mean sea level. Most commonly it uses barometric pressure, which decreases with altitude. The instrument converts ambient pressure to an altitude reading using a model of the standard atmosphere. In aviation, the indicated altitude is adjusted with a pressure reference called the QNH so the readout reflects height above sea level; setting QFE yields height above the airfield.

Types include barometric (aneroid) altimeters, radar (radio) altimeters, and GNSS/GPS-based altimeters. Barometric altimeters use a sealed aneroid cell that expands or contracts with pressure changes, translating movement to a dial or digital readout. Radar altimeters emit radio waves and measure distance to the ground, providing height above terrain for low-altitude flight and landing. GNSS/GPS altimeters compute altitude from satellite data, but can be less accurate in built-up areas or during rapid altitude changes.

Applications: In aviation, altimeters are essential instruments for flight safety and are required under instrument flight rules. Barometric devices provide altitude information during cruise, while radar altimeters are used during approach and landing. Handheld altimeters are common in mountaineering and skydiving, and many drones carry barometric sensors to estimate altitude.

History: The development of the aneroid barometer in the 19th century enabled compact altimeters. Early aircraft adopted barometric altimeters in the first half of the 20th century; modern systems integrate digital processing, automatic pressure setting, and air data computer inputs to improve accuracy and safety.

Note: In Italian, the term altimetro is used for the same device.