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<   No. 2729   2010-07-17   >

Comic #2729

1 Spanners: {into intercom} Relax. The autopilot is the leftmost green switch underneath the main altitude gauge.
2 Iki Piki: Er...
3 Spanners: {over intercom} The big dial in the middle. With the numbers. Counting down really, really fast...
4 Iki Piki: Got it!

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There are several methods with which you can measure your altitude above the surface of a planet.

The simplest is by measuring the air pressure, which drops in a predictable way as you go higher above the surface. This is subject to some variation because air pressure naturally changes slightly with different weather systems. It also only gives you your height above some fixed datum level, such as sea level. It's no good knowing you're flying at 4000 metres above sea level if you think there might be mountains 5000 metres high in the area.

So commercial and military aircraft usually use a more sophisticated system known as radar altimetry. This bounces a radio signal off the ground and times how long the echo takes to return. This gives a very accurate measure of how far you are above the ground, but not your altitude above sea level (unless you also know the altitude of the ground below you).

You can also triangulate your altitude from a GPS system, by figuring out your position relative to a set of satellites.

There are also some calculations you can make from geometry, if you know the size of the planet and can see the curvature of the surface.


Reader TJ writes:

All of this is technically true, and rather simplified, but sure. I just wanted to mention a few other things which you probably already know. In aviation, there are actually many different types of altitudes we use, including indicated, true, pressure, density, and absolute.

Radar altimetry in aircraft today is rather different than what you've described, though. Now, a lot of aircraft do have that basic radar-based system in them, but they are mostly used as backup devices these days. Primarily, though, they use something called TAWS... Terrain Awareness Warning System. Basically, it's all computerized. The avionics have a highly precise topographical digital maps built in them. The system will then use this map to tell how close you are to terrain, how long it will be before you fly into terrain, and how fast you're approaching it. It offers several advantages over basic radar altimetry, most notably that it can see ahead of the aircraft, rather than just straight down. So if you're flying straight at a cliff face, you'd get an aural warning, whereas with basic radar altimeters, you wouldn't know until it's too late. Also, since it's digital, it can be installed in any aircraft, not just large aircraft. If you're descending too fast into terrain, it'll start shouting "SINK RATE, SINK RATE, WHOOP WHOOP PULL UP, PULL UP" at you, which can be very annoying if you're doing it on purpose to land. It's also able to automatically change these warnings depending on where you're at and what phase of flight you're in. If you're closer to your destination, the tolerances are a lot lower, so it won't beep at you as you're coming in to land. If you're in cruise, the tolerances are a lot higher. Real radar altimeters are really used only as a backup device these days.

As far as GPS goes, yes, you can use it to derive your altitude, but it's really inaccurate. Too much so to use in flight. At my job, we use a system that is extremely precise. I've seen it triangulate our position with 12 GPS and 4 GLONASS satellites at once, and it's always accurate within a foot (usually centimeters) laterally. Even so, the GPS derived altitude is usually off by 100+ feet, sometimes as much as 300-400. That may sound pretty good, but when aircraft are being separated by only 500 feet, it's not nearly accurate enough. The FAA is really trying to pump out GPS WAAS instrument approaches, but they only provide lateral navigation. They can provide vertical navigation, but they use a trigonometry algorithm and the barometric altimeter to figure out what altitude you should be at.

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Last Modified: Saturday, 17 July 2010; 03:11:01 PST.
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