View Full Version : A320...GPS Altitude Vs Altimeter Altitude

18th Oct 2007, 18:00

i have noticed during flights that the GPS altitude is always higher than the maintained Flight Level...

i have seen it last time while we were at FL360 ...the GPS alt was 37650 feet.

i was trying to look for it the books but could not find the required info about it .

what is the base for this altitude...see level/ground level ?

i am confused about this issue...

in FCOM 4 it says GPS altitude displayed for information purposes !

appreciate any light shedding on this.

thanks in advance.

Hand Solo
18th Oct 2007, 20:33
Altitude is the least reliable indication from GPS and really is only given for information only. Unlike your aircraft altimeters which derive their readings from air pressure around your aircraft, and hence are pretty accurate as they are only reading the difference between your position and ground level, GPS altitude readings are derived by working out where all the distance spheres from each satellite are intersecting. Hence you are not measuring the difference between your position and ground level, but the distance between yourself and multiple satellites, which brings with it calculation and position errors.

18th Oct 2007, 20:34
Warmer than ISA in Kuwait by any chance?

bugg smasher
18th Oct 2007, 20:44
GPS altitude is an absolute altitude, in other words, it tells you your true height above sea level, as triangulated by the GPS satellite system. (Due to the satellite geometry needed for accurate height measurement, it cannot be relied upon for flight operations.)

Your altimeters indicate a pressure altitude. Therefore, when flying through a high-pressure area, GPS altitude should read higher than indicated on your altimeters, and lower when flying through a low-pressure weather system.

DGPS is promising to elevate accuracy levels in the vertical plane far beyond what they are at the moment. In my view, all systems should eventually be adapted to DGPS altitude (or itís successor), eliminating the need for transition altitudes and levels, altimetry errors induced by extreme temperatures, the effects of high wind in mountainous terrain, and rapidly moving pressure systems. It would also lend extreme accuracy to RVSM operations, and go a long way towards reducing dangerous 'altitude busts' in the approach/departure environment.

It would also save fuel. On long flights an aircraft may transit through several pressure systems, coasting downhill into a low (with regard to absolute altitude) and climbing uphill into a high. Eliminating constant 'hunting' for pressure altitude should result in a small, but justifiable savings.

18th Oct 2007, 22:22
If you allow me to comment on two of the common misconceptions mentioned earlier...

* GPS does not directly give you height above sea level. It gives you height above the WGS84 ellipsoid (http://en.wikipedia.org/wiki/Reference_ellipsoid). By applying a correction based on a stored geoid (http://en.wikipedia.org/wiki/Geoid) model, this can be translated to a mean sea level height, and most non-specialist GPS receivers have the ability to do that.

* While it is correct to say that the biggest uncertainty in a GPS position is in the height plane, such uncertainty is by no means worse than achievable by a barometric altimeter. IIRC, just the allowed instrument error of a primary reference altimeter is in the order of 100ft at altitude. You must add to that all the other sources of errors and your barometric height can easily be thousands of feet off, as you well know (which is how we all grew to hate the CRP-5). By comparison, an uncorrected single frequency GPS height (SA off) is usually not much worse than about 30m. Differential dual frequency GPS gets this down to usually not more than 0.5m 2-sigma (e.g., Starfire (http://en.wikipedia.org/wiki/StarFire_%28navigation_system%29) systems), and in the case of GPS the vertical error is independent of altitude.

I am not qualified to comment on the use of GPS in the field of aeronautics, but I would guess that the reason for your barometric altimeter being used as the primary altitude reference is not related to its precision or accuracy (of which it doesn't have much), but rather it owes more to its simplicity, to its working being independent from external factors other than having an atmosphere, and to its availability. At the end of the day, you could say it doesn't really matter how wrong your altitude is, as long as it's wrong by the same amount as everyone else's.

Hope this helps

mike wess
19th Oct 2007, 01:02
Please refer to the pdf it has a good explanation on how gps altitude is calculated


19th Oct 2007, 14:29
Hello Guys...

well , i really don't know how to thank you for this great inputs that every one is adding here.

Thanks and i really appreciate the help.

bookworm Kuwait is always Higher than ISA Temp in the summer but not in the winter .


19th Oct 2007, 14:52
Kuwait is always Higher than ISA Temp in the summer but not in the winter

As a rule of thumb, indicated altitude is 3.5% less than true altitude for every 10 degrees above ISA. Does that explain most of the discrepancy you observed?

19th Oct 2007, 15:58
bookworm yesterday our cruising altitude was FL320 ...

GPS altitude showing 33770 ( varying +/- 100 feet ) and the ISA DEV was +5c .

19th Oct 2007, 16:06
and what was the QNH? The point is that GPS altitude is the height above a defined ellipsoid using WGS84 coordinates, and FL is a height above a generic pressure datum (1013 HPa). When at a FL it is assumed that you are sufficiently above transition altitude that MSA considerations are less important, and the standard pressure setting is used primarily for aircraft vertical separation.

19th Oct 2007, 16:16
I bet the only deviation from the ISA atmosphere wasn't the temperature. Remember that the altimeter is calibrated for the ISA atmosphere and the reading will only match your true altitude if temperature and QNH are according to ISA.

If I my memory serves me right you can add 30' (will be more at high alt.) to your true altitude per 1 hPa above ISA.

19th Oct 2007, 17:36
a little deviation from the topic....

why is 1013.25 hpa was choosen for the ISA Standard.

why not 995,1000 or even 1005 hpa...

19th Oct 2007, 18:00
The International Standard Atmosphere is based on average conditions at the mid latitudes.

You can probably find all the details in document ISO 2533:1975 published by the International Organization for Standardization. :8

19th Oct 2007, 19:48
our cruising altitude was FL320 ...

GPS altitude showing 33770 ( varying +/- 100 feet ) and the ISA DEV was +5c .

So you'd expect an underread of about 2% or 600 ft. If the QNH were 1033 hPa or so (you don't say where you were), that would account for another 600 ft.

The correction for ISA deviation is based on the average deviation of the column of air below you -- it's possible that the average was more than +5 degC. A deviation of +10 degC would account for most of that.