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Konstantinos

How an instrument calculates the altitude?

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How an instrument calculates the altitude? Can you explain? If it knows the pressure, then how altitude is calculated? And the altitude depends on pressure? Only on pressure?

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If you know the pressure at the surface, then the pressure will decrease at a logarithmic rate with height. So from the change in pressure between them and the surface, the height can be calculated. But aircraft need to regularly calibrate what the surface pressure is at that location, otherwise it could give them a false reading of being higher/lower than they actually are.

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Aircraft are fitted with aneroid altimeters which work on the pressure of the air which varies with height, however it is important that they are corrected during flight with the insertion of sea level pressure and this is known as the QNH. When a pilot is using this it is important also for him to know the height of the airfield where is is landing so he can make the necessary adjustment.

Another setting is known as the QFE and this is the setting at a particular airfield.

On the altimeter there is a little knob which is turned until the pressure given appears in the window - this is done manually.

What normally happens in practise is the pilot will get updates during his flight from air traffic control so he will be able to maintain an accurate height - this is very important because with the amount of traffic in the sky aircraft travelling in different directions have to maintain different flight levels to avoid chances of a collision in the air.

Nowadays most aircraft commercial aircaft also have altimeters which are based on reflecting a radar signal off the ground and this too can show the height of an aircaft but since it operates through electricity they is a chance of this failing whereas the old fashioned aneroid altimeter operates purely on air pressure and there is little that can go wrong with it, so these are always fitted as far as I am aware and a necessary backup.

The other method of finding one's height these days is through the use of a GPS (SatNav) - most of the time these are fairly accurate but there have been times when driving a car in coastal regions I have felt the need to have a periscope fitted because SatNav indicated I was driving below sea level!

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And what about if I am in a stable altitude, without moving, but the pressure is changing? The changes are not enough for changing my altitude?

For example now in my region I think we have 990 hPA but 12 hours ago we had 995 hPa. So?

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And what about if I am in a stable altitude, without moving, but the pressure is changing? The changes are not enough for changing my altitude?

For example now in my region I think we have 990 hPA but 12 hours ago we had 995 hPa. So?

I'm not quite sure what you are getting at Kon. Pressure decreases with height because as you get higher there is less atmospere above you, so the weight becomes less and it is the weight of the atmosphere which causes the pressure but it is not a straightforward linear graph but exponential.

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Another one for you, during my aviation training for meteorology the current rules where always good to apply due to the lower accuracy in Light Aircraft. 1mb = 30ft or thereabouts. 1013.2 hPa is the standard atmosphere.

According to the surface pressure, 1013.2 hPa the 850 hPa chart should be at 4890 ft. 500 hPa = 15390ft

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I have not understood yet. From this mathematical formula: http://en.wikipedia....ssure_variation

I can see that the altitude depends only on the pressure. Nothing else. So, if I know the pressure of my position I can find my altitude? Doesn't it depends on the pressure of my nearest sea? What is the mistake on this mathematical formula?

Or the "sea level standard atmospheric pressure" p0 is not constant?

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I will attempt to kick my meteorological brain into activity, its been a long time, and try and answer your query but not tonight!

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a quick answer is that aircraft fly on the ICAO standard atmosphere(International Civil Aviation Organisation).

This has the following

msl pressure=1013.25mb

msl temperature=15C(288.15K)

temperature lapse rate=6.5C(K) up to 11km where the standard temperature=216.65K=-56.5C.

Thus aircraft fly at standard heights in the atmosphere say 300mb 30,000ft, but that height in reality is hardly ever exactly that height but will vary depending on the actual temperature and pressure at the surface but an aircraft at 29,000ft is kept 1,000ft separate by the same rule.

Not sure if that is what you are after but your height will be dependent on what the actual temperature and pressure is at the surface and what the ACTUAL rate of fall of temperature is at the location you are at.

For the average person hiking in the hills of the UK then using the latest pressure chart before you leave is going to be reasonably accurate. To be exact then look at the latest skew-t nearest to where you are going to be to see what the actual environment curve is, then if you wish look at what is predicted 3, 6 or 12 hours later.

hope that helps?

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How an instrument calculates the altitude? Can you explain? If it knows the pressure, then how altitude is calculated? And the altitude depends on pressure? Only on pressure?

hi

I ask again was my reply any help to you please?

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Thank you johnholmes.

Hello again. A simple question: In the Barometric Formula, http://en.wikipedia.org/wiki/Atmospheric_pressure#Altitude_atmospheric_pressure_variation

if I replace the sea level standard atmospheric pressure = 101325 Pa, with the current sea level atmospheric pressure, near to my position, and the sea level standard temperature = 288.15 K with the current sea level temperature, then the accuracy will be better or badder?

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The accuracy will be better.This is simply because if the "actual" surface pressure is known then as others have mentioned the barometer responds logarithmically to give a defined pressure altitude at your location relative to the ground pressure BUT your reading would only be 100% accurate for a finite time depending on whether the atmospheric pressure is stable and the only way to keep it accurate when say hillwalking or flying is to get the actual updated ground pressure or as John Holmes stated to predict the changes using graphs.

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If I am in an altitude about 2000 m, after 5-6 hours hiking, then my multimeter will show me my position's current atmospheric pressure. So this time how can I know the current sea level temperature and atmospheric pressure? You meant that I can see it on internet (probably with a mobile internet 3G - 4G) or I can take with me a chart prediction of it, before I begin hiking?

flyer, I created a script on matlab (name altitude) using the barometric formula from wikipedia: http://en.wikipedia.org/wiki/Atmospheric_pressure#Altitude_atmospheric_pressure_variation which calculates the altitude. Please tell me your opinion.

Code:

fprintf ( 1,'Hello, I will calculate your altitude.n' );cslt=input('Please, give me the sea level temperature (degrees C):');fprintf ( 1, 'Thank you.n' );cslap=input('Please, give me the sea level atmospheric pressure (mb):');fprintf ( 1, 'Thank you.n' );cpsap=input('Please, give me the atmospheric pressure of your position (mb):');fprintf ( 1, 'Thank you.n' );alt=8.31447*(cslt+273.15)*log(cslap/cpsap)/(9.80665*0.0289644);altf= alt*3.2808399;fprintf('Your altitude is %d m or %d feetn', alt, altf);

And a running of my position now:

>> altitudeHello, I will calculate your altitude.Please, give me the sea level temperature (degrees C):17Thank you.Please, give me the sea level atmospheric pressure (mb):1017.9Thank you.Please, give me the atmospheric pressure of your position (mb):999.9Thank you.Your altitude is 1.515332e+002 m or 4.971562e+002 feet>> 

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The other method of finding one's height these days is through the use of a GPS (SatNav) - most of the time these are fairly accurate but there have been times when driving a car in coastal regions I have felt the need to have a periscope fitted because SatNav indicated I was driving below sea level!

 

Not as confusing as calculating QFE as a higher pressure than QNH in the Caspian Sea for landing choppers due to the local Baltic Datum being approximately 28m higher than the current water level of the Caspian.

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