[eli5] How does an altimeter work? How does it know how high you are off the ground?
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Old-school altimeters are not telling you how high you are off the ground; they’re telling you how high you are from sea level. An altimeter in a plane sitting on the tarmac at Denver’s Stapleton International Airport should read 5434 feet when properly calibrated.
**They use air pressure.** Air pressure increases as you go lower, and decreases as you go higher, in ways that we understand and can calculate. Altimeters that use this are sometimes called ‘barometric altimeters’ because they’re using a barometer—an air pressure measuring device—as a way to determine the altitude. Of course, air pressure changes with the weather, so then what?
Aviation weather services will calculate the reference sea level pressure for their location in inches of mercury, and report that to pilots in the area, so that they can adjust their altimeters accordingly. And if you’re on the tarmac at a remote airport that doesn’t have a weather service but does have a known altitude, you can also just adjust the calibration knob until you get the right reading. (For example, if Denver’s weather service was out of, uh, service, you could just adjust your altimeter until it read 5434.)
The only system that actually tells you how high you are off the ground is the *radar* altimeter. It bounces a radio signal off the ground and times how long it takes to get back. Radar altimeters are most often used when landing, when it’s more important to know exactly where the ground is so you don’t slam into it too early or overshoot the runway.
Other folks have talked about GPS technology used in altimeters… but those are not telling you how high you are off the ground. They’re telling you where you are in three-dimensional space.
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