Pressure Altimeter Reading -vs- GPS Altimeter Readings

I was flying near KC the other day and wanted to make sure I did not bump the bottom of "B" space. So I made sure I had the baro reading of 30.43 set in both the glass and the standard round altimeters. Both read 2,300'MSL and life was good. Then I decided to check the reading on the hand heald 496 and it was 2130' MSL (approx). That is 170' difference. The difference seems to be the tempurature being colder than the standard 59F.

Here come the thinking part. The GPS WAAS is much more accurate than the altimeter reading adusted for pressure, but we use this technology in everyone's plane and we know it is not accurate. Why not compensate it for tempurature also and be accurate with the GPS?

When I landed all 3 sources were dead on the field elevation of 1,500'MSL.

Just an interesting observation.

Geico266 said:

Here come the thinking part. The GPS WAAS is much more accurate than the altimeter reading adusted for pressure, but we use this technology in everyone's plane and we know it is not accurate. Why not compensate it for tempurature also and be accurate with the GPS?

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Because everyone does not have GPS WAAS and we need to use a standard that everyone has.

Mel said:

Because everyone does not have GPS WAAS and we need to use a standard that everyone has.

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Maybe the government will buy us all WASS GPS units. They seems to be in a mood to spend money.

Someday we'll be switching to GPS altitude readings. No worries about tempurature, baro pressure, ect. Just an accurate reading of where you are.

Geico266 said:

...The difference seems to be the tempurature being colder than the standard 59F...

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It isn't a simple case of the surface temperature. It is the temperature at all the different altitudes on the way up. The altimeter uses standard conditions which seldom, if ever, actually occur.

One important point to note here is that altitude is generally the worst dimension in which GPS is able to calculate a position fix. This is the natural result of having all the satellites overhead. If we could get a signal from a satellite on the far side of the earth (essentially below us) we'd be able to obtain far better altitude resolution, but since GPS signals have a hard time going through the molten core of Mother Earth we kinda have to live with what we get! ;-)

The calculation used to obtain the altitude portion of the position fix is actually very complex. The crude measurements provided by the GPS receiver are actually based on something called ECEF - earth centered, earth fixed coordinates. In essence, the GPS is measuring aircraft position along a line running from the satellite constellation to the centre of the earth. Converting this ECEF altitude into altitude above mean sea level is quite complex because it relies on mathematical models which define the shape of the earth and thus the distance from the centre of the earth to mean sea level. A miniscule error in the mathematical model will result in many feet of difference between a pressure altimeter and GPS readings. Since the earth is a rather imperfectly shaped orb, some errors in the models used to define its shape are inevitable, thus creating inevitable errors in GPS altitude fixes.

Canadian_JOY said:

One important point to note here is that altitude is generally the worst dimension in which GPS is able to calculate a position fix. This is the natural result of having all the satellites overhead. If we could get a signal from a satellite on the far side of the earth (essentially below us) we'd be able to obtain far better altitude resolution, but since GPS signals have a hard time going through the molten core of Mother Earth we kinda have to live with what we get! ;-)

The calculation used to obtain the altitude portion of the position fix is actually very complex. The crude measurements provided by the GPS receiver are actually based on something called ECEF - earth centered, earth fixed coordinates. In essence, the GPS is measuring aircraft position along a line running from the satellite constellation to the centre of the earth. Converting this ECEF altitude into altitude above mean sea level is quite complex because it relies on mathematical models which define the shape of the earth and thus the distance from the centre of the earth to mean sea level. A miniscule error in the mathematical model will result in many feet of difference between a pressure altimeter and GPS readings. Since the earth is a rather imperfectly shaped orb, some errors in the models used to define its shape are inevitable, thus creating inevitable errors in GPS altitude fixes.

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At one point some years ago the airlines were testing GPS for CAT2 and 3 approaches but that never came to anything probably for lack of money for necessary ground and airborne equipment. Seems to me I read that such a system had sensors around the applicable runway which transmitted surface data to the aircraft so the on board system knew exactly where the flight was on a vertical and horizontal plain. This is called LAAS but is not yet usable.

WAAS has a similar function but it is more general in that it incorporates about 25 ground based sensors around the country connected to 2 primary units, one at the east coast and the other out west. This system does not provide adequate sensing to perform CAT2 or CAT3 approaches but is the basis for currently authorized GPS precision approaches to about 250 DH which is about the usefulness of a pressure altimeter on an approach. When I was working 10 years ago, precision approach minimums were determined by a radar altimeter, never the pressure altimeter. As I recall, the minimum DH without radar alt was about 300'.

The reason Larry noticed the altitude disparity is, as others have said, the pressure instruments are not corrected for non standard temperature but the GPS WAAS signal is. Everyone should be flying their pressure instruments in cruise and in fact this is the data the transponder reports on mode C to ATC and other aircraft equipped with TCAS.

Who cares if we are off by x number of feet just so long as everyone is doing the same thing so don't go flyin' around on the GPS altitude.

When I was working 10 years ago, precision approach minimums were determined by a radar altimeter, never the pressure altimeter. As I recall, the minimum DH without radar alt was about 300'.

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Close, the minimum Cat 1 approach is 200' and 1800 RVR. Of course only the RVR is needed to shoot the approach (civilian), in the military we were required to have both ceiling and visibility. And as David points out, Cat 2 and Cat 3 approaches require the use of a radar altimeter. Cat 3 requires the use of autoland.

The reason Larry noticed the altitude disparity is, as others have said, the pressure instruments are not corrected for non standard temperature but the GPS WAAS signal is. Everyone should be flying their pressure instruments in cruise and in fact this is the data the transponder reports on mode C to ATC and other aircraft equipped with TCAS.

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This is exactly correct. In cruise it doesn't matter what true altitude someone is flying at, only where they are relative to everyone else. This is not the case during approaches when we start caring where we are relative to the ground. In very cold temperatures during Cat 1 or non precision approaches, a temperature correction should be applied to approach minimums to guarantee adequate terrain clearance.

Mark

So far most of the things stated above match with my knowledge of GPS and navigation in general. But, as I understand it, the GPS altitude doesn't care about temperature, on the surface or at altitude.

Biggest issue is the one pointed out above about the translation from ECEF to geodetic (sea level based).

I live near the coast in FL, the GPS altitude is -17m (ECEF) but our location's surveyed altitude is +10m (above sea level). That would appear as about a 75 ft delta. Most GPSs (for the masses) output position and altitude info relative to WGS-84 (one of the sea level based references - there are many).

The most accurate constellation for both position and altitude would include 4 satellites; 3 120 degrees apart, about 10 degrees above the horizon, and 1 directly overhead.

Dave