Not one that might be linked to GPS, the good old fashioned analog kind.
Air pressure. The higher you go, the less pressure there is at a pretty predictable pace. Just to make sure you’re in reason, though, the airports will give you an altimeter setting on the ground so that you can tune it.
barometric pressure. They need to be calibrated quite often. Not real accurate, but it’ll get you in the ballpark.
There are two types: pressure and radar. A pressure altimeter is calibrated to local elevation above sea level before each flight and uses changes in air pressure to determine altitude.
Operation of a radar altimeter should be obvious from the name.
Nope, barometric pressure, strictly speaking is ambiant pressure (which is what you meant) corrected to a sea level equivilent Thus “normal” barometric pressure is ~29.92"Hg at the top of Pikes Peak, same as it is in death valley, same as it is on Mt. Everest. Even though the atmospheric pressure in those locations will differ by around 12"Hg for equivilent weather conditions.
a Kollsman altimeter (what we are talking about) does not need frequent calibration… Perhaps a check every few years. What they do need is offset adjustment to reflect weather conditions. This is done on the ground (known altitude) as part of the pre-flight. A second indicator (the “Kollsman window”) then indicates the local barometric pressure.
If the aircraft exceeds 18,000’ MSL, the pilot will re-adjust the altimeter for a baro setting of
29.92"Hg. This is because:
1)There are no mountains to run into at that altitude.
2) The aircraft at these high altitudes may be crossing through several weather systems,
or indeed crossing a continent.
3)It doesn’t matter the true altitude two airplanes are flying at, as long as the difference in a given location can be established.
Altitudes based on this convention are refered to as “flight levels” FL 28.5 means 28,500’ indicated on an altimeter set to 29.92" Hg.
When the aircraft desends through FL-18, The air traffic controller will usually supply a baro setting. Alternativly, large airports have automated radio anouncements (ATIS) which supplies winds, baro setting, info on closed and active runways, various warnings, etc. This would be stated as “Winds light and variable, Altimeter 27.3…” for example.
Oh, As for accuracy, properly set for baro pressure, they repeat to around +/- 25’, the markings are typically 20’ intervals.
The Baro adjustment is only an offset. Stricctly speaking, the slope of the function should actually change, but this would be tricky to impliment mechanically (a continously variable, non slip geartrain would be required. A Kollsman altimeter tends to understate altituce changes when the barometric pressure is low, and overstate changes when the pressure is high.
Electronic altimeters can easilly be made to more accuratly corrilate pressure and altitude, but then they would not match the long established standard. Since having the altimeters track each other is important to keeping aircraft from running into each other, the “offset only” correction is kept.
They can be found that are very accurate - to +/- 1 to 5 feet. The American Paulin barometer has increments of 1 foot.
Yes, but when the question was asked I think it was safe to assume that the Kollsman altimeter was the one being referred to.
Kevbo covered things pretty thoroughly - this may herald the arrival of yet another Doper Pilot in our midst. Or at the very least a knowledgable aviation enthusiast
Although I would like to point out that while 18,000 carries you safely over mountains over North America (unless Denali exceeds that - can’t recall right now) there are some places on Earth, such as the Himalayas, where mountains exceed 20,000 feet. So please, if you are crossing one of the higher mountain ranges please remember to consult your sectionals, WACs, and instrument charts for minimum safe altitudes, just to be on the safe side. Those cumulo-granite clouds can really ruin your day.
“But how do they work?”
Inside, there’s a flattened tube, wound into a spiral shape, with a needle attached to one end - sort of like the bimetal strip in a bullseye thermostat, except it’s a sealed tube (called a Bourdon tube). The housing around the tube is vented to the outside of the airplane via the static port, and changes in the outside pressure create a pressure differential across the tube walls. The spiral tube winds up tighter or opens up wider in response to those pressure differences, rotating the needle. Actually, it’s usually 3 needles in a geared arrangement like on a clock - a 10,000 ft, 1000 ft, and 100 ft needle. The Kollsman knob essentially re-baselines the needles by rotating the end of the tube opposite the needle-supporting end.
Actually, altitude is the one thing GPS receivers can’t fix as well as location. Your ground position can be fixed to about +/- fifteen meters, but altitude it’s more like 75. There’s a reason aircraft still carry altimeters.
DD
OK, here I go with a big simplified how-ya-doing:
A barometric altimeter is nothing more than a pressure-sensing device. Just like a computer: garbage in; garbage out. If you give your altimeter bad data (ie the pressure), you will get bad data on the gauge. But if you give it good data, it will give you VERY accurate reading of your barometric (aka above sea level) altitude.
As an aside, many countries have a “transition altitude” (the point at which to you change to a standard pressure: 2992 in Hg or 1013 Mb) much lower than 18,000 feet. Many countries in Europe have a TA of 6,000 feet and the Netherlands has a TA of FL 30 (That’s 3,000 feet). It makes for some interesting climb clearances.
In any case, BAROMETRIC altitude is measured by your altimeter based on some pressure setting - hopefully a correctly set altimeter.
A RADAR altimeter is something completely different - it is a unique instrument. It shoots a radar beam directly down and uses the time delay to determine the aircraft’s height above the ground. These are very accurate and are often used as decision points in very low visibility approaches.
For separation of air traffic BAROMETRIC altimeters are used, and this is why it is important for even the most casual pilot to check in with ATC and get a current altimeter setting. Flying around with an incorrect altimeter setting could result in you being MUCH closer to another aircraft than you planned; in fact you could cuase a near-miss if you were off enough.
Also, ElvisL1ves covered the physical operation of an altimeter pretty thoroughly!
Privliges as a guest are fairly crippling, so forgive my not attributing the quotes…have’t worked out how to do that exactly.
Possible, but I’m pretty sure only the crudest altimeter (sold to hikers) would be based on a Bourden tube. Aircraft altimeters are driven by a sort of bellows, pancake shaped, with concentric convolulited rings on each face. Evacuated, this is called an aneroid. I’ve been inside a couple repairing crash damage. (Hang glider) Externally they looked completly different, inside they were nearly identical.
This is the ** resolution** not the accuracy. A very cheap mechanical watch has sub-second resolution…doesn’t mean it keeps time very well…it’s probably not accurate at all. Any pressure based altimeter is at the mercy of weather, and how well here and today’s atmosphere matches the ICAO* “standard” atmosphere model which is the corrilation between altitude and pressure.
Hello. Glider rating ~300 hrs. few hours of power dual. ~600 hrs in hang gliders when I had time for that.
How I learned this stuff:
When I started flying hang gliders a good variometer (sensitive, fast rate of climb w/audio output) and altimeter went for more than half the price of a starter glider…so I stretched to buy the glider and had no instruments. Being an electrical engineer, I took it as a challenge, and built my own…somewhat bulkier, but much better performing than commercial equivilents. I was employed by Sperry (actually Unisys, or maybe even Honeywell, by that time ??k) so I had access to the test rig used on F-15 air data computers. My altimeter tracked ICAO atmosphere within a few feet, and has 1 foot resolution.
I expect the equations are a lot easier to track down today (internet) than they were in 1987.
Also did a little bit of work calibrating barographs (recording altimeter used in sailplane competitions, for records, awards, etc) before they went out of fashion. GPS tracks are the standard now.
*International Civil Aeronautics Organization. Standard atmosphere is defined in terms of three functions for the ranges h < ~36,000’ ~36,000<h<~70,000’ and h>~70,000’.
Temperature is taken to be constantly decreasing with altide in the lower range, Steady across the middle range, and rapidly increasing with altitude in the upper range. I don’t have reference book handy, so the breakpoints are only as good as my memory.
The equation for the lower segment is approximatly a 5th root function with some scaling and offsets. A 4 segment piecewise-approximation will track it to within a few feet.