[eli5] How does an altimeter work? How does it know how high you are off the ground?
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There are two different types: barometric altimeter and radar altimeter. Barometric altimeters can read your altitude relative to sea level by using the fact that ambient air pressure reduces as altitude increases. The higher your elevation, the lower the air pressure.
Radar altimeters use radar aimed down at the ground to measure the distance between you and the ground. Imagine an aircraft flying straight and level over a hill or mountain. As the aircraft is flying at the same altitude relative to sea level, the barometric altimeter would not change. However, the mountain means the distance between the aircraft and the ground is decreasing, so the pilot’s radar altimeter would show an altitude drop and then rise again as he flies over the mountain.
Source: I’m an Aviation Electrician. I don’t fly ‘em, so there are probably some pilots here who could tell you a few things from their perspective.
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