GPS altitude
GPS altitude shown in MCDU of A320 - is it measured from ground or MSL?
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Surely if it is showing "Altitude" it must be AMSL. If it was AGL it would read "Height"
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Altitude above the WGS84 sea level datum.
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Slight clarification-- no such thing as WGS84 sea level. WGS84 uses a spheroid that approximates the sea level surface; here in California the spheroid is 30+ meters above sea level.
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So Tim, are you saying that if your altimeter in CA is set to the correct local Altimeter Setting (QNH to the rest of the world) the altimeter will read approimately 100 feet different from the Altitude reading on your GPS?
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For all I know maybe GPS's nowadays include a worldwide map that gives the altitude difference between spheroid and sea level, wherever you are. If so, they could give a good approximation to height above sea level. I'm just saying such an addition to WGS84 (based on thousands? millions? of additional measurements) would not be WGS84 itself.
Or put it this way-- once you've determined sea level, it's the same sea level whether your GPS is set to WGS84 or NAD27 or OSGB36. The WGS84 spheroid won't be at the same height as the NAD27 or the OSGB36, so the map of altitude corrections will be different for each-- but they're all aiming at the same result. |
The GPS system uses WGS84 internally. WGS84 is an ellipsoid earth model. The GPS height with respect to that ellipsoid is called Height Above Ellipse (HAE). Unfortunately the Mean Sea Level does not coincide with this ellipsoid. In fact there are quite a few bumps and dents in the sea caused by local deviations in Earth's gravity field. The difference between mean sea level and WGS84 is modeled by another model : EGM96. When this model is available, it is easy to transform the HAE in height above MSL. I am not sure whether this is done in the Airbus, but I would expect so.
Now if you are on the ground, and your altimeter setting is local (QNH) of course your altimeter should read the local elevation, and so should your GPS if it's using the EGM96. However, in the air it's a different story. Your altimeter derives altitude from ambient pressure using formula's from the ICAO Standard Atmosphere. When the atmosphere is different from the standard, and it usually is, then the altitude shown by your altimeter is different from the GPS height above MSL. For example, on a hot day air expands, and so the "barometric foot" is longer than the "GPS foot", in other words on a hot day the GPS will read a higher number than your QNH tuned altimeter. I hope this explains the different readings. ATCast |
GPS altitude measurement is also 1.5 times less accurate than horizontal, so if you have 5m horizontal accuracy it could be out by 7.5m. Altogether not good for RVSM.
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I'm not au fait with the aeronautical GPS world, but wasn't WAAS designed with aviation, and particularly approaches, in mind? I believe over the US, WAAS-equipped GPS can be accurate to within 2 meters vertically and 1 meter horizontally.
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Greetings
GPS Altitude is true altitude, however it needs to be corrected from Geodic error. |
ATCast
Very interesting post. Never heard of that model before. I wonder if EGPWS systems use these models. And I guess it will be used in GPS precision approaches. |
Greetings,
Scientists have redefined level Zero as being the point where Gravity is equal, at sea level it works, but it gets complicated on mountaneous area, because gravity is measured perpendicularly to the surface, and unfortunately a mountain has a slope....So it offsets the level by sometimes 700ft or more. Geodic correction software are available on the net just google it:ok: |
So why not replace Barometric altimeters with GPS data derived ones ?
I'm probably missing something but if this could be done you would eliminate pressure data derived errors and there would be no need for transition altitudes / levels. You could retain the standby pneumatic altimeter as a back up. |
Stilton:
I'm probably missing something but if this could be done you would eliminate pressure data derived errors and there would be no need for transition altitudes / levels. You could retain the standby pneumatic altimeter as a back up. Consider the following:
ATCast |
what temperature limits are built into procedure design?
is this info on 8168? thanks. |
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