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Photo of instruments in an airplane cockpit.

Altimeters

by Chris Woodford. Last updated: November 17, 2013.

Ladies and gentlemen, we are now cruising at 10,000 meters. Next time you hear your pilot say words like these, stop to ask yourself one question: how on Earth do they know? It's not as if a plane is like a car, easily able to keep track of how far its wheels have rolled along the ground. Indeed, there's no simple way to know how far you've flown at all (unless you know your equations of motion). How, then, does a pilot measure a plane's height in the air? Simple! Using a handy gadget called an altimeter. Let's take a closer look at what these things are and how they work!

Photo: Flying an airplane is a tricky operation. The altimeter, which measures your height above sea-level, is only one of dozens of instruments you have to keep an eye on. Photo by Kevin J. Gruenwald courtesy of US Air Force.

Why does altitude matter?

Hercules airplane flying low over forest.

Photo: How low is too low? This plane is getting ready to drop supplies over a forest in Japan—and accurate altitude measurement is absolutely critical. Photo by MSGT Val Gimpis courtesy of US Air Force.

You might not think it matters very much whether pilots know how high in the air they're flying; after all, they can always peer through the windows! But measuring altitude (your height above sea level) is much more important to a pilot than you might think. Some of Earth's mountain ranges are surprisingly high and harder to miss in bad weather than you might suppose. Mount Everest, for example, is an amazing 8.8 km (5.5 miles) above sea level, so flying at 10,000 meters (6.2 miles) doesn't give you that much room to maneuver. Then there are other planes to avoid. And it's actually more efficient to fly at higher altitudes where the air is thinner and overcoming air resistance uses less fuel. All told, there are plenty of good reasons for flying high. But how high, exactly?

There are two main ways to measure altitude accurately. One is to measure the air pressure and figure altitude out from that. The other is to bounce a radio beam down from your plane and time how long it takes to reflect back up again.

How does a pressure altimeter work?

Close up of an aneroid barometer showing the dial and the spring inside.

Why do we have air pressure? Earth's gravity pulls everything toward it—and everything includes even the smallest things you can imagine, like molecules of air. If you're an air molecule near the ground, you've got lots of other air molecules above you pushing and squashing you down; higher up, there are fewer molecules and there's less pushing. That's why air pressure is highest at Earth's surface and gradually falls, in a systematic way, the higher up you go. Measuring the air pressure is therefore (in theory, at least) a simple and effective way of measuring altitude.

The altimeters onboard most planes are actually aneroid barometers (pressure-measuring instruments) that have been calibrated (marked with a scale) so they show height instead of pressure. Like normal aneroid barometers, they consist of a hollow, sealed box that expands (as the pressure falls) or contracts (as the pressure rises). As the box changes size, by very tiny amounts, an intricate system of levers and gears magnifies its movements and makes a pointer rotate on a dial marked with height measurements. Hey presto, tiny changes in air pressure become accurate measurements of altitude.

Altimeters that work this way measure height by measuring the pressure compared to sea-level, but that's not the only thing that causes pressure variations on Earth. Air pressure is constantly fluctuating across Earth's surface due to changes in the weather—so that has to be corrected for if pressure-based altimeters are to work accurately. Another drawback of pressure-based altimeters is that they don't take account of dangerous obstacles such as buildings and power lines.

Patent diagram fo a typical pressure altimeter showing the main parts inside.

Photo: Left: A pressure altimeter is a more sophisticated version of an aneroid barometer, like this. Instead of showing the pressure on the dial, it shows height—but that's okay because there is an exact correlation between the two.

Photo: Right: How a pressure altimeter works: This is a typical pressure altimeter, invented in the 1930s by Victor Carbonara of the Bendix Aviation Corporation. Looking from the side, you can see a sealed, air-filled box (red) that expands or contracts according to changes in pressure. Its movements are amplified by a system of linkages and levers (blue) and a gear mechanism (green), which operates the altimeter pointers. Now looking at the diagram on the right, we can see how the altitude is displayed by moving pointers on a dial (brown), calibrated in hundreds and thousands of feet. This diagram is from US Patent #2,099,466: Altimeter, courtesy of US Patent and Trademark Office.

How does a radio altimeter work?

Radio altimeters don't suffer this problem. They're simpler and work in a similar way to radar (the system planes, ships, and other vehicles use to navigate): they just fire a beam of radio waves down from the plane and wait for the reflections to return. Since radio waves travel at the speed of light (300,000 km or 186,000 miles each second), it takes only a few hundredths of a second for a radio beam to make the 20,000-meter or so round trip to Earth's surface and back. The plane times the beam and calculates its altitude in kilometers by multiplying the time in seconds by 150,000 (that's 300,000 divided by two: don't forget the beam has traveled twice as far as its own altitude going to the ground and back again). Radar altimeters are much quicker and more precise than pressure instruments and are widely used in high-speed airplanes or ones that need to fly at particularly low altitudes, such as jet fighters.

Other ways of measuring altitude

There are at least two more ways of measuring altitude, but they're not widely used on airplanes. One method is to use GPS (global positioning system) signals from navigation satellites in space. In much the same way that GPS signals from three satellites can be used to pinpoint your position on Earth's surface (as explained in our main article on how GPS works), using signals from four or more satellites lets you calculate your height above Earth as well. Unfortunately, GPS altitude measurements are not as accurate as those made with conventional altimeters, so they're not likely to replace existing technologies on airplanes anytime soon.

Topographical maps of Mars made by the Mars Orbiter Laser Altimeter (MOLA) in 2001.

Another way of measuring altitude involves shining laser beams of infrared light down from a plane, helicopter, or satellite and calculating the time to return, much like using radio and radar. The reflected beam is collected by mirrors and lenses and focused on a photocell detector sensitive to infrared light. As the aircraft flies along, it systematically measures its altitude and plots what's called a topographical map of the surface contours beneath it. This technique is called laser altimetry and it has been widely used by space probes to map surface features on other planets. NASA's Mars Orbiter Laser Altimeter (MOLA) works this way.

Photo: These topographical maps of the surface of Mars were made in 2001 by NASA's MOLA space probe, using laser altimetry. Red, orange, and yellow areas are mountains; green, blue, and violet areas are craters. Photo by courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).

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If you're interested in more technical details of how altimeters work, the following patents are worth a look. This is just a quick, representative selection to give you an idea of pressure, radio, and laser altimeters and how they compare:

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Text copyright © Chris Woodford 2009, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Altimeters. Retrieved from http://www.explainthatstuff.com/how-altimeters-work.html. [Accessed (Insert date here)]

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