[eli5] How does an altimeter work? How does it know how high you are off the ground?
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The altimeter measures the pressure of the atmosphere at the plane. The pressure is higher at ground level.
regular altimeter reads air pressure, and subtracts from it the air pressure at the ground level (which varies on weather). If you use this to land, somebody at the airport needs measure and tell you the air pressure there
radar altimeter indeed knows exactly how high you are, but only above a single spot on the ground directly below you. There is [at least one major plane crash](https://admiralcloudberg.medium.com/all-the-presidents-men-the-story-of-the-smolensk-air-disaster-and-the-death-of-lech-kaczyński-590a3977f) where pilots were misguided by a ravine right in front of the landing strip
finally, GPS-based systems such as [Terrain Avoidance](www.flyingmag.com/how-it-works-terrain-awareness-and-warning-system/) can tell you exactly where you are in all 3 dimensions. it also uses radar altimeter
It works based on detecting atmospheric pressure. You start by knowing the elevation of where you are. i.e. you need to know how high the land is where you are from sea level. Most mechanical altimeters have a small knob on them that allows you to make adjustments based on the current barometric pressure and this his how you calibrate it. You set the altimeter to the current barometric pressure and make sure it matches your current elevation.
The higher you go up in altitude, the lower the atmospheric pressure, this is why your ears pop when you go up in an elevator. Altimeters sense this difference in pressure through either an electronic pressure sensor or a device called a bourdon tube. It’s basically a small tube of metal that’s formed into an arc. As the pressure around the tube changes, the tube flexes and changes shape. The tube flexing pushes against a lever that turns some gears attached to a dial to show your current altitude.
The cheapest kind of altimeters use barometric pressure sensor readings to detect changes and map them as elevation. This only works well if the barometric region is somewhat static. For instance, locations along a coast like Southern California USA can have drastically different micro climates within just a few miles of the water, and the differences in water heavy coastal air and dry desert winds can get interpreted as elevation changes.
I think some older devices used a GPS location and lookup tables that contain the average elevation of the region/location.
I think newer devices use extra GPS signals from additional sats and then do the geometry calculation.
GPS sats are basically beacons that transmit/stream their location and the time. The most accurate GPS ground instruments also use the timing of the transmission signal to dial in changes. This is used by systems like the USGS earthquake monitoring sensors.
Additional question: planes are going at speed, so isn’t it hard to get a measure of the actual pressure?
*An altimeter is a device that measures altitude—a location’s distance above sea level. Most altimeters are barometric, meaning they measure altitude by calculating the location’s air pressure. Air pressure decreases as altitude increases.* [Source](https://education.nationalgeographic.org/resource/altimeter)
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.
Pretty much correct explanations so far, but missing a crucial detail.
When you are approaching a airport for landing, you tune in to the airport’s information broadcast. This includes local wind, traffic pattern and the current local barometric altimeter setting.
This info is updated periodically, and each update is given a sequential alphabetic name, e.g. Alpha, Bravo, Charlie, Delta, Foxtrot,…
So when you call the airport, you announce where you are, and that you have heard, e.g. “Bravo” information.
Side question, I skydive with a wrist altimeter. How can those be accurate with so many variables like temperature and wind
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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