Travel Technology - Why are my GPS and Airshow diff for altitude?

My GPS says we are at (say) 37,412 ft. Accuracy within 30 ft.

However, on channel 9 (united), and on the airshow map, it says 36,000 ft (which is our assigned flight level).

I understand that flight-level is all standardized above 18,000 ft in the US, but this is much too large a descrepancy.

Anyone know why?

Not sure if this is a valid explination, but google found this: http://docs.controlvision.com/pages/gps_altimetry.php

Doesn't explain that much of a difference though. How many sat's did you have acquired? Do you have a DGPS signal (I wouldn't imagine so at that alt and with something you could carry on an airplane)? What type of GPS? Is the 30ft figure you are quoting for alt or just horizontal? Is it based on unobstructed sky view?

Just some ideas that came up in my head, no idea if they are valid reasons or not.

Not sure if this is a valid explination, but google found this: http://docs.controlvision.com/pages/gps_altimetry.php

I think you found his answer. Reading that page it's apparent there could be a considerable deviation between true altitude and indicated altitude. All aircraft in the vicinity will be subject to the same error, though, so it isn't dangerous.

Since many planes won't have GPS available air traffic control will have to rely on what they do have--an altimeter. Since they are based on air pressure they are inaccurate but that's irrelevant so long as it's the same inaccuracy for everyone.

Above 18,000 (in the U.S.) all aircraft fly at a standard 29.92" Hg regardless of the actual air pressure. Your GPS is showing the true altitude while the aircraft is actually flying the true "flight level" (i.e. FL360 - Flight Level 360 or 36,000 feet at 29.92"). Not all airplanes have GPS but they all have pressure sensitive altimeters!

Above 18,000 (in the U.S.) all aircraft fly at a standard 29.92" Hg regardless of the actual air pressure. Your GPS is showing the true altitude while the aircraft is actually flying the true "flight level" (i.e. FL360 - Flight Level 360 or 36,000 feet at 29.92"). Not all airplanes have GPS but they all have pressure sensitive altimeters!

So, I mentioned this last night to the pilot where (once again) there was a 1,500 foot descrepancy. I told him that I understood the standard 29.92" of Mercury setting for all aircraft above 18,000', and that local altimeters were used below that level. But that I was getting 1,500' higher readings on my GPS.

He said he didn't think it could be 1,500' off from the standard, so my GPS must be wrong.

I had something like 9 satellites (though all on one side of the airplane, of course).

Not sure if this is a valid explination, but google found this: http://docs.controlvision.com/pages/gps_altimetry.php

Interesting... thanks!

The article says 500' off at 10,000' altitude. If it's linear (probably not) then that's 1,500' of deviation at 30,000', which would make perfect sense.

Actually, the higher you go, the worse the error for GPS, which isn't terribly accurate for showing altitude to begin with. However, as alt increases so does the error (and it's not linear AFAIK).

It's the difference between standardized pressure altitude and not. Not to sweat.

The real point here is there is no 'error' involved.

Aircraft fly at pressure altitudes. GPS measures geometric altitude. Pressure altitudes differ from geometric altitudes due to variations in local air pressure, differences in the gravitational field of the earth (pressure altitudes are a type of geopotential height), humidity, temperature, height of the tropopause etc. etc.

Neither is 'wrong', they're both perfectly valid measures - it's all about context.

If you want to know how long a ladder you'd need to drop down from the plane to reach the ground (or, more accurately, the surface of WGS-84 reference ellipsoid), you need the geometric altitude. If you want to follow a cleared flight level, you need the pressure altitude.

If you want to know how long a ladder you'd need to drop down from the plane to reach the ground (or, more accurately, the surface of WGS-84 reference ellipsoid), you need the geometric altitude. If you want to follow a cleared flight level, you need the pressure altitude.
Well, "altitude" is height above sea level. If we're "flying at an altitude of 36,000 feet" (as the pilot says) and we descend 35,000' only to find ourselves 500' underwater, I'd call that an error.

My 2 cents.

Well, "altitude" is height above sea level.
No: it isn't - that's pretty much the whole point.

That's one definition, in fact, not one that's even particularly commonly used in actual practice for aviation.

It's a mistake to think that 'altitude' without context means anything in particular. A modern aircraft might include three or more devices that are each capable of reporting different altitudes: an altimeter that reports a pressure altitude, a GPS that would report geometric altitude and a radar altimeter that reports altitude above the surface.

Each has its use: you wouldn't use the GPS altimeter for en-route navigation; and you wouldn't use the altimeter to know whether you were at fifty feet or only forty.

A pilot that says "we are cruising at 36,000 feet" is actually telling you "we are cruising at FL360".

If we're "flying at an altitude of 36,000 feet" (as the pilot says) and we descend 35,000' only to find ourselves 500' underwater, I'd call that an error.


And, if you were flying a plane from a blacked-out cockpit and relying on GPS for altitude, you'd crash at about 600 ft. where I live. Even a corrected barometric pressure altitude won't give you your height above the ground. That's why for precision instrument landings pilots rely on radio altimeters when they're within about 2500 ft. of the ground .