How come GPS reads different

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.

"Matt Whiting" wrote:
It's usually more accurate than the altimeter, which is not corrected
for non-standard temperature.

That's not what my KLN89B manual says...

It was probably written 1) before the military stopped deliberately
wonking the signal and 2) to please the lawyers.

On Sun, 12 Jun 2005 10:14:33 -0500, "Jim Fisher"
wrote:

So, for safety reasons, I think we should all use GPS altitude so that we
can all safely land if the Earth ever actually disappears.

Hmmm. Where would you land?


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

"Ron Rosenfeld" wrote in message
...
On Sun, 12 Jun 2005 10:14:33 -0500, "Jim Fisher"
wrote:

So, for safety reasons, I think we should all use GPS altitude so that we
can all safely land if the Earth ever actually disappears.

Hmmm. Where would you land?


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

In the parking lot for the Restaurant At The End Of the Galaxy, or was
that Universe?

"Casey Wilson" N2310D @ gmail.com wrote in message
news:6u6re.5344$2K4.62@trnddc08...

"Ron Rosenfeld" wrote in message
...
On Sun, 12 Jun 2005 10:14:33 -0500, "Jim Fisher"
wrote:

So, for safety reasons, I think we should all use GPS altitude so that
we
can all safely land if the Earth ever actually disappears.

Hmmm. Where would you land?


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

In the parking lot for the Restaurant At The End Of the Galaxy, or was
that Universe?

That was universe. Don't remember if hamburger prices were given.

"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.

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
Piper Cub Forum: www.pipercubforum.com
the blog: www.danford.net
In Search of Lost Time: www.readingproust.com

Hmmm, and what alti do you think to meter without an earth?
I would be more worried about avoiding the big yellow things that are
the cause of all this, if it happens.

-Kees

Does anybody know a quick rule of thumb about how much the altitude changes
with non-standard temperature?
I guess is also has to have in consideration that the airport altimeter
setting is also kinda "corrected".
I.E., you are flying at 4000 ft, then if you set up the altimeter setting
for an airport at 3800', you'll be far more accurate than if you set it up
for an airport at 1000' (assuming that the atmosphere is exactly the same).
or, in other words, the closer you are to the airport altitude, the more
accurate your altitude will be.


"Dan Luke" wrote in message
...

"Stefan" wrote:
Pneumatic altimeters are reliable,

Most of the time.

In very cold weather, you can run into something by relying on a
pneumatic altimeter.
--
Dan
C172RG at BFM