How do altimeters measure an aircraft's height?

Altimeters measure an aircraft's height using two primary methods, depending on whether the altitude is referenced to sea level or the ground surface.1

Pressure Altimeters

The pressure altimeter, or aneroid barometer, estimates altitude above sea level by measuring atmospheric pressure. Because air pressure decreases as altitude increases, the instrument uses thin, evacuated metallic bellows called aneroid capsules. As the aircraft climbs or descends, these capsules expand or contract, and this mechanical movement is converted through gears to display the altitude on a dial. These instruments must be calibrated for local barometric variations to ensure accuracy.12

Radio Altimeters

Radio altimeters provide the absolute altitude—the actual distance above the terrain—by timing radio wave signals. There are two common approaches:1
* Pulse-based: The system measures the time it takes for a radio pulse to travel to the ground and back, calculating distance based on the speed of light.1
* Continuous-wave: The system transmits waves continuously and determines height by measuring the phase difference between the transmitted and reflected signals.3

Radio altimeters are essential for automatic navigation and landing systems.13