How come GPS reads different

I'll file that away for the day when most of the GA fleet is WAAS-capable.
I'm sure that the AIM will have changed by that time.

Bob Gardner

"Dan Luke" wrote in message
...

"Bob Gardner" wrote:
AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude
should not be relied upon to determine aircraft altitude since the
vertical error can be quite large."

Except when using WAAS, when it is quite small.

--
Dan
C172RG at BFM

Bob Gardner wrote:

AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude should
not be relied upon to determine aircraft altitude since the vertical error
can be quite large."

Main reason is that other planes will be using pressure-based altimeters
so maintaining separation can only be done when everyone uses the same
method with the same errors.

GPS altitude is measured above the GPS sphere, which is not sea level.

No, GPS altitude is measured above the ellipsoid defined by WGS-84, not
any spherical surface. Furthermore, the altitude is then corrected
using a lookup table to account for the difference between the geoid
(which represents the sea-level surface) and the ellipsoid. The
altitude figure displayed by the GPS receiver is therefore measured from
the hypothetical sea-level surface.

Bob Gardner

"Aluckyguess" wrote in message
news
How come the GPS reads a different altitude than the Altimeter?

As others have mentioned, the pressure-based altimeter assumes a
standard atmosphere model for the temperature lapse rate and can be
quite far off if the actual conditions don't match the model - even when
the altimeter is corrected for the current ground-level pressure.

Thanks
"Peter" wrote in message
...
Bob Gardner wrote:

AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude
should not be relied upon to determine aircraft altitude since the
vertical error can be quite large."

Main reason is that other planes will be using pressure-based altimeters
so maintaining separation can only be done when everyone uses the same
method with the same errors.

GPS altitude is measured above the GPS sphere, which is not sea level.

No, GPS altitude is measured above the ellipsoid defined by WGS-84, not
any spherical surface. Furthermore, the altitude is then corrected using
a lookup table to account for the difference between the geoid (which
represents the sea-level surface) and the ellipsoid. The altitude figure
displayed by the GPS receiver is therefore measured from the hypothetical
sea-level surface.

Bob Gardner

"Aluckyguess" wrote in message
news
How come the GPS reads a different altitude than the Altimeter?

As others have mentioned, the pressure-based altimeter assumes a standard
atmosphere model for the temperature lapse rate and can be quite far off
if the actual conditions don't match the model - even when the altimeter
is corrected for the current ground-level pressure.

On Sun, 12 Jun 2005 14:19:32 -0700, Peter
wrote:

Main reason is that other planes will be using pressure-based altimeters
so maintaining separation can only be done when everyone uses the same
method with the same errors.

Boaters run into this problem all the time (as it were). The GPS is
more accurate than the chart. Not a problem when it comes to buoys,
since NOAA uses GPS now when setting them in place, but a big problem
when it comes to rocks. They were charted by optical means.


-- all the best, Dan Ford

email (put Cubdriver in subject line)

Warbird's Forum: www.warbirdforum.com
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"Aluckyguess" wrote in message
news How come the GPS reads a different altitude than the Altimeter?

GPS altitudes are calculated in a way that is similar to how latitude and
longitude is calculated. Your GPS altitude is what is called a geopotential
altitude and it is based on a sphere. If you don't already know, the Earth
is an imperfect sphere. That is where wide area augmentation system
(WAAS) signals kick in with corrections to the latitude, longitude
and altitude. Even without WAAS, altitude is usually pretty close.
Don't forget the altimeter in the airplane can be off also.

CryptWolf wrote:

"Aluckyguess" wrote in message

news How come the GPS reads a different altitude than the Altimeter?


GPS altitudes are calculated in a way that is similar to how latitude and
longitude is calculated. Your GPS altitude is what is called a geopotential
altitude and it is based on a sphere. If you don't already know, the Earth
is an imperfect sphere. That is where wide area augmentation system
(WAAS) signals kick in with corrections to the latitude, longitude
and altitude. Even without WAAS, altitude is usually pretty close.

Yes it is, which makes it very clear that your previous statement about
GPS altitudes being based on a spherical earth is false. The earth's
polar and equatorial radii differ by about 13 miles, so altitudes based
on a spherical model would be off by miles. But my measurements with my
non-WAAS GPS show the altitude measurements to be within 40' well over
95% of the time.

In fact, the GPS measurements are based on the WGS-84 defined
ellipsoidal shape of the earth and then corrected for the difference
between the geoid (the hypothetical mean sea-level surface) and the
ellipsoid.

Don't forget the altimeter in the airplane can be off also.

If the GPS has a good skyview of the satellites then this is the most
likely source of the discrepancy. Pressure-based altitudes depend on an
assumed standard model of the atmosphere. The actual conditions can be
quite different and result in considerable variation of the reported
altitude.

GPS is most accurate when your position is surrounded by satellites. In
the horizontal plane, that is usually true. In the vertical direction,
you always have satellites above you, but none below you. This also
makes altitude slightly more difficult to resolve.

http://gpsinformation.net/main/altitude.htm
--
Gene Seibel
Gene & Sue's Aeroplanes - http://pad39a.com/gene/planes.html
Because we fly, we envy no one.

Gene Seibel wrote:

GPS is most accurate when your position is surrounded by satellites. In
the horizontal plane, that is usually true. In the vertical direction,
you always have satellites above you, but none below you. This also
makes altitude slightly more difficult to resolve.

http://gpsinformation.net/main/altitude.htm

It's true that GPS vertical accuracy is worse by about a factor of 1.5
to 2 than horizontal accuracy. But despite this, GPS vertical accuracy
is still better than using barometric pressure when you're at a
significantly different altitude than the reference point giving the
barometric calibration.

The continued use of pressure-based altimeters is for consistency and
reliability - not accuracy.