Altimeter altitude vs GPS altitude

[Fair.Pilot]

Flyers,

Cruising at 39,000 (altimeter set to STD) but GPS altitude reads 41,000 ft.

That's 2,000 ft above actual a/c altitude.

To a certain extend the GPS accuracy world-wide is controlled by the US Defence dept.

But why 2,000 ft difference?

Thanks.

[galaxy flyer]

Couple of possibilities:

1/ Your altimeter is measuring a pressure level, not necessarily altitude above mean sea level. IOW, 29.92 pressure level is not altitude.

2/ GPS errors are more apparent in altitude than laterally. This is a factor for GPS, but solved by WAAS or LASS.

GF

[SomeGuyOnTheDeck]

Cruising at 39,000 (altimeter set to STD) but GPS altitude reads 41,000 ft.

I think you've answered your own question...

[ampclamp]

Alt set to STD gives everyone the same alt reference number but actual could well be quite different.
GPS is far far more accurate in lat long than altitude.

[PBL]

There are many possible sources of discrepancy, one of which has been mentioned.

A barometric altimeter is a device based on pressure differences within the International Standard Atmosphere. The only means you have of adjusting it is to set the MSL baseline pressure. There are two ways you can do this: first, let ATC or weather services tell you what to dial in; second, set your altimeter to airport elevation when on the ground at an airport. But you cannot adjust for rate-of-change of pressure with altitude. The ISA norm is hard-wired in.

Differences between barometric-measured altitude and your true altitude would stem from an inaccurate MSL baseline (in this case, you are in the Flight Levels, so you are setting it to ISA anway, 1013 HPa); or from a different rate of change of pressure with altitude as that in the ISA. You can affect the former, but do nothing about the latter. Both deviations are likely, most of the time.

A GPS altimeter is a device for figuring where you are in space, based on comparing and computing with radio reference signals from a number of GPS satellites. Differences between GPS altitude and true altitude would come from the difference between GPS coordinates (based on a specific idealised ellipsoid model of the geoid, known as WGS84) and the geoid itself under your notional feet wherever you happen to be. The computation errors involved in receiving and processing the signals can for these purposes be ignored.

None of this has to do with the owners of GPS playing around with users. They have never done it and they won't, after having spent a number of decades proclaiming to the world that GPS is trustworthy and guaranteed to remain so. The signal quality is broadcast along with the signal.

PBL

[Groundloop]

To expand on PBL's answer, if you are displaying WGS84 coordinates on your GPS the GPS height will be the height above the WGS84 reference ellipsoid, which is a smooth mathematical surface.

The surface of the earth is a geoid (earth shaped!) which is anything but smooth. Altimeters will read height relative to the geoid allowing for any regional/international settings. The variations across the world between the surface of the geoid and the ellipsoid can be up to nearly 200 metres.

[ATCast]

Well explained by PBL & Groundloop.

The most significant cause of the discrepancy is probably the temperature difference from the International Standard Atmosphere. When the temperature rises the air expands and the Flight Levels will rise higher above the earth. Although 2000 ft seems a lot, it's not uncommon.

It has nothing to do with GPS accuracy. GPS is approx. 3 times less accurate vertically then horizontally. The horizontal position accuracy is in the order of 5 meters, so expect to be vertically accurate to about 50 ft.

Differences between GPS height and pressure altitude have been discussed before: see this (http://www.pprune.org/tech-log/398245-gps-altitude.html) thread on GPS altitude as well.

Slightly off-topic:
To safe a few bits in the data transmission, ADS-B sends out
the difference between GPS Height Above Ellipse (HAE) & pressure altitude instead of full GPS HAE. Together with the pressure altitude, which is encoded fully, the GPS HAE can be recalculated. The maximum difference that can be encoded is +/- 3125 feet. This little trick saved 4 bits in the message.

ATCast

[bookworm]

The most significant cause of the discrepancy is probably the temperature difference from the International Standard Atmosphere. When the temperature rises the air expands and the Flight Levels will rise higher above the earth. Although 2000 ft seems a lot, it's not uncommon.

My rule of thumb is 0.35% per degC mean deviation from ISA. So for a 2000 ft difference at 40000 ft, that's about ISA+15 degC if it's all temperature error. If the QNH is 1033 hPa, that's worth 600 ft feet on top of an altimeter set to 1013, so it could be a little of each.

if you are displaying WGS84 coordinates on your GPS the GPS height will be the height above the WGS84 reference ellipsoid,

But do common aviation GPSs display HAE for "altitude"? Given that other elevations (e.g. terrain and obstacles) are referenced to mean sea level, it seems strange that GPS manufacturer would choose to ignore a relatively simple correction like geoid height.

The GNS430 manual for example says:

"The altitude calculated by GNS 430 GPS receivers is geometric height above Mean Sea Level and could vary significantly from the altitude displayed by pressure altimeters in aircraft." (my italics) It doesn't say "above the WGS-84 reference ellipsoid".

[auh_to_auh]

The Altimeter is not a very accurate instrument at altitude as many variables come into play, ISA temp v actual, ISA density v actual, rate at which pressure reduces is not constant, Working out a "true alt" with your wizz wheel will get you closer. This is why we have high ALT correction tables in our manuals. All the Altimeter does is gives us the opportunity not to run into each other as it measures atmospheric pressure, not Altitude

Hope this makes sense

AUH

[Fair.Pilot]

Thanks everyone!

[f4212]

Nice to see someone in the desert with intellectual input.

[ATCast]

But do common aviation GPSs display HAE for "altitude"? Given that other elevations (e.g. terrain and obstacles) are referenced to mean sea level, it seems strange that GPS manufacturer would choose to ignore a relatively simple correction like geoid height. I don't know what the standard is at display level, but at output level there are two different standards unfortunately. Internally, GPS usually works in WGS84, and calculates HAE. The output (through Arinc 743, label 076) can be either HAE or above MSL (743A). In the latter case the GPS unit must obtain the difference between MSL and HAE (geoid undulation), for example by looking up values in a database and interpolating between them.
If the GPS gives MSL on label 076, then it gives HAE on label 370, because some systems require HAE (for example GLS).
MSL is used for example by the EGPWS.
Latitude and longitude is always in WGS84 for aeronautical GPS units.

The EGM96 (http://en.wikipedia.org/wiki/EGM96) model accurately defines the difference between HAE and MSL. The range is approx. +/- 100 meters.

[Microburst2002]

With the altimeter, we are flying along a constant pressure surface, which can actually go up and down several thousand feet.

In a long flight from the subtropical troposphere to the polar one, whith large changes in ISA dev and surface pressure, you can go from 2,000 above to 2,000 below geometric or GPS altitude while maintaining FL 390.

[Old Smokey]

Even if the GPS Altitude was exactly correct, and ignoring the difference between QNH and Standard (1013.25/29.92), at F/L 390 a temperature difference of 12.77 degrees from ISA (upon which your Altimeter is calibrated) would make a 2000 foot difference.

Can the original poster recall what the temperature was?

A very good discussion regarding GPS et al, sometimes thread creep can become very interesting.

Regards,

Old Smokey

[bookworm]

Can the original poster recall what the temperature was?

The problem is that the error is related to the temperature profile through the entire column of air below the aircraft. And it will be weighted to the levels of higher density. So even if it's ISA temperature at 39,000 ft, an ISA +20 level band lower down could make for a 2000 ft difference between indicated and true altitude.

[Slasher]

Even if the GPS Altitude was exactly correct, and ignoring the difference between QNH and Standard (1013.25/29.92), at F/L 390 a temperature difference of 12.77 degrees from ISA (upon which your Altimeter is calibrated) would make a 2000 foot difference.

True but who cares unless youve lost all your ADCs? Main thing is to be on 1013 above the Transish so you dont bang into anyone else.

[galaxy flyer]

Because not many have seen them, the C-5 has a all-altitiude radar altimeter, presumably for pressure pattern flying. It was interesting to see your height vary, even over the ocean, while maintaining a flight level. A bit off thread, but related.

GF