How come GPS reads different

"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

"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

Dan Luke wrote:

In very cold weather, you can run into something by relying on a
pneumatic altimeter.

Pssst, I'll tell you a secret: All that stuff you had to learn to pass
the written was somehow linked to real life.

Stefan

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

"Stefan" wrote:
Pssst, I'll tell you a secret: All that stuff you had to learn to pass
the written was somehow linked to real life.

Really?

Gosh!

--
Dan

"How can an idiot be a policeman? Answer me that!"
- Chief Inspector Dreyfus

Dan Luke wrote:

"Matt Whiting" wrote:

Not that unusual. GPS isn't very accurate vertically. Most GPS
manuals tell you not to rely on the altitude in any serious way.


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

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


Matt

GeorgeB wrote:

On Sun, 12 Jun 2005 06:27:43 -0400, Cub Driver wrote:


On Sat, 11 Jun 2005 20:54:01 -0700, "Aluckyguess" wrote:


I am getting over 500ft. difference and more.

Oh,. good grief! I'm sure I've never had as much as 100 ft of
variance, and more often it's 20 feet or so. (Garmin 296)


I frequently drive by 2 signs marking the "eastern continental divide"
with the elevation. With my WAAS equipped, non-aviation GPSR
(Meridian Gold), I habitually flip to a screen with elevation
displayed and have yet to differ by more than 40 ft. As Dan says, I
am usually within 15 ft.

But sometime and someplace you may not be:
http://gpsinformation.net/main/altitude.htm


I took two trips across parallel bridges in eastern Virginia 3 days
apart; the reading was stable and within 5 ft of what I visually
estimated it should be. It differed by 2 ft betweent he 2 days; my
eyes told me the bridge heights did also ...

I would trust a WAAS equipped GPS, tracking 5 or more birds in a clear
environment, to be more ACCURATE than a barometric altimeter. I would
USE my barometric altimeter if it were working as other aircraft in
the area are, adjusted to local barometer per information gained by
radio from official sources.

Yes, augmented systems are a different matter.


Matt

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.

"Tauno Voipio" wrote in message

Yes - it's called the reference ellipsoid. There are actually
several of them and some GPS receivers allow selecting your
favorite one.

Interesting.

Actually, if Earth would disappear with all its mass, GPS would
get unusable, as the satellites would continue out of their
tracks due to the lack of gravity pull.

Yeah, but they'd work longer than a pressure altimeter so I think we should
still adopt GPS altimeters in case this ever actually happens.

--
Jim Fisher

"Tauno Voipio" wrote in message

Yes - it's called the reference ellipsoid. There are actually
several of them and some GPS receivers allow selecting your
favorite one.

Interesting.

Actually, if Earth would disappear with all its mass, GPS would
get unusable, as the satellites would continue out of their
tracks due to the lack of gravity pull.

Yeah, but they'd work longer than a pressure altimeter so I think we should
still adopt GPS altimeters in case this ever actually happens.

--
Jim Fisher