How come GPS reads different

[Aluckyguess]

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

[Matt Whiting]

Aluckyguess wrote:
> How come the GPS reads a different altitude than the Altimeter?
>
>

For many reasons. The main reason is that they derive altitude by two
completely different methods and this inevitably introduces errors.
Then each method has its own inherent errors.

Even two altimeters side by side won't read the same altitude other than
by accident.


Matt

[Aluckyguess]

I am getting over 500ft. difference and more.
"Matt Whiting" > wrote in message
...
> Aluckyguess wrote:
>> How come the GPS reads a different altitude than the Altimeter?
>
> For many reasons. The main reason is that they derive altitude by two
> completely different methods and this inevitably introduces errors. Then
> each method has its own inherent errors.
>
> Even two altimeters side by side won't read the same altitude other than
> by accident.
>
>
> Matt

[Casey Wilson]

"Aluckyguess" > wrote in message
...
>I am getting over 500ft. difference and more.
> "Matt Whiting" > wrote in message
> ...
>> Aluckyguess wrote:
>>> How come the GPS reads a different altitude than the Altimeter?
>>
>> For many reasons. The main reason is that they derive altitude by two
>> completely different methods and this inevitably introduces errors. Then
>> each method has its own inherent errors.
>>
>> Even two altimeters side by side won't read the same altitude other than
>> by accident.
>>
>>
>> Matt

The number of sattelites being received is one of the variables -- how many
are you registering on the GPS? What brand/model is the GPS. Is the GPS
certified for navigation? Does the GPS have a Kohlsman equivalent input?
What are the altitude specifications of the GPS? Are you comparing it to an
altimeter in an airplane? Is the calibration data on the altimeter valid?
Have you compared the altimeter to the GPS while parked at a known reference
(i.e., the benchmark of your local airport)? My Magellan handheld takes
about five mintues of averaging to get a reasonably close altitude
correlation -- how much time are you giving yours?

Bottom line, forget the GPS altitude reading, dial in the local pressure
data into the airplane's altimeter and trust it.

[Cub Driver]

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)



-- 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

[Matt Whiting]

Aluckyguess wrote:

> I am getting over 500ft. difference and more.
> "Matt Whiting" > wrote in message
> ...
>
>>Aluckyguess wrote:
>>
>>>How come the GPS reads a different altitude than the Altimeter?
>>
>>For many reasons. The main reason is that they derive altitude by two
>>completely different methods and this inevitably introduces errors. Then
>>each method has its own inherent errors.
>>
>>Even two altimeters side by side won't read the same altitude other than
>>by accident.

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


Matt

[Blueskies]

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

Unless the GPS is WAAS capable?

[Dan Luke]

"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.
--
Dan
C172RG at BFM

[GeorgeB]

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.

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.

[Thomas Borchert]

Aluckyguess,

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

Main reason: Altimeter indication is corrected for barometric pressure.
First order of business before comparing is to set the altimeter to
29.92. Even then, 500 feet off is not unusual IF the GPS is not WAAS
capable.

--
Thomas Borchert (EDDH)

[Aluckyguess]

Its a 296. It seems the like the higher I go the farther off it is.
I am one to think the GPS would be more accurate than the altimeter.
I was thinking that we get incorrect altimeter settings
"Casey Wilson" <N2310D @ gmail.com> wrote in message
news:x1Pqe.2896$9a1.185@trnddc01...
>
> "Aluckyguess" > wrote in message
> ...
>>I am getting over 500ft. difference and more.
>> "Matt Whiting" > wrote in message
>> ...
>>> Aluckyguess wrote:
>>>> How come the GPS reads a different altitude than the Altimeter?
>>>
>>> For many reasons. The main reason is that they derive altitude by two
>>> completely different methods and this inevitably introduces errors. Then
>>> each method has its own inherent errors.
>>>
>>> Even two altimeters side by side won't read the same altitude other than
>>> by accident.
>>>
>>>
>>> Matt
>
> The number of sattelites being received is one of the variables -- how
> many are you registering on the GPS? What brand/model is the GPS. Is the
> GPS certified for navigation? Does the GPS have a Kohlsman equivalent
> input? What are the altitude specifications of the GPS? Are you comparing
> it to an altimeter in an airplane? Is the calibration data on the
> altimeter valid? Have you compared the altimeter to the GPS while parked
> at a known reference (i.e., the benchmark of your local airport)? My
> Magellan handheld takes about five mintues of averaging to get a
> reasonably close altitude correlation -- how much time are you giving
> yours?
>
> Bottom line, forget the GPS altitude reading, dial in the local pressure
> data into the airplane's altimeter and trust it.
>

[Jim Fisher]

"Aluckyguess" > wrote in message

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

Altimeters are calibrated for local barometric pressure. GPS altitude based
on a computed, simulated sphere that approximates sea level.

If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the
Galaxy) and you happened to be flying at that moment, your GPS would
continue to happily display an altitude even though there was no actual
Earth underneath you.

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.

I've always been curious about something: The Earth is not a perfect
sphere. It bulges at the equator due to forces of tidal gravitation and
centrifugal force. My question is, do GPS altitude computations take this
into effect when you're, for example, flying over the equator instead of
over the north pole?

--
Jim Fisher

[Tauno Voipio]

Jim Fisher wrote:
> "Aluckyguess" > wrote in message
>
>
>>How come the GPS reads a different altitude than the Altimeter?
>
>
> Altimeters are calibrated for local barometric pressure. GPS altitude based
> on a computed, simulated sphere that approximates sea level.
>
> If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the
> Galaxy) and you happened to be flying at that moment, your GPS would
> continue to happily display an altitude even though there was no actual
> Earth underneath you.
>
> 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.
>
> I've always been curious about something: The Earth is not a perfect
> sphere. It bulges at the equator due to forces of tidal gravitation and
> centrifugal force. My question is, do GPS altitude computations take this
> into effect when you're, for example, flying over the equator instead of
> over the north pole?
>
> --
> Jim Fisher
>
>

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

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

--

Tauno Voipio
tauno voipio (at) iki fi

[Fred G. Black]

Aluckyguess wrote:
> Its a 296. It seems the like the higher I go the farther off it is.
> I am one to think the GPS would be more accurate than the altimeter.
> I was thinking that we get incorrect altimeter settings

The altimeter is only "accurate" when the temperature profile of the
airmass is equal to the ISA standard atmosphere. The rest of the time,
there will be some error which increases with altitude (wrt wherever the
source for the altimeter setting is located). This isn't a concern for
en-route operations since everybody's altimeter has the same "error".
It can be a concern for terrain clearance in the mountains or in some
cases instrument approaches in cold weather since the altimeter reads
higher than the plane actually is.

[Stefan]

Fred G. Black wrote:

> This isn't a concern for
> en-route operations since everybody's altimeter has the same "error". It
> can be a concern for terrain clearance in the mountains or in some cases
> instrument approaches in cold weather since the altimeter reads higher
> than the plane actually is.

No concern, either. When IMC, there is a safety margin built into the
approaches, and when VMC, there are windows in the copckpit.

Pneumatic altimeters are reliable, independant of any infrastructure and
work without electricity.

Stefan

[Bob Gardner]

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."

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

Bob Gardner

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

[Peter Duniho]

"Thomas Borchert" > wrote in message
...
> Main reason: Altimeter indication is corrected for barometric pressure.
> First order of business before comparing is to set the altimeter to
> 29.92.

No. As Dan points out elsewhere, the main error in the barometer is not
correcting for non-standard temperature. There are other errors as well,
but none of them involve setting the altimeter to 29.92 in an attempt to
make the altimeter more accurate. Unless the correct altimeter setting is
actually 29.92, setting it to 29.92 will simply make the error worse.

Pete

[Fred G. Black]

Stefan wrote:
> Fred G. Black wrote:
>
>> This isn't a concern for en-route operations since everybody's
>> altimeter has the same "error". It can be a concern for terrain
>> clearance in the mountains or in some cases instrument approaches in
>> cold weather since the altimeter reads higher than the plane actually is.
>
>
> No concern, either. When IMC, there is a safety margin built into the
> approaches, and when VMC, there are windows in the copckpit.
>
> Pneumatic altimeters are reliable, independant of any infrastructure and
> work without electricity.
>
> Stefan

There is a buffer in the procedure design however given that cold
temperature corrections are published (at least in Canada, ref CAP GEN
p19), it's not unreasonable to assume that there are situations where
they should be used.

Fred.

[Thomas Borchert]

Peter,

> Unless the correct altimeter setting is
> actually 29.92, setting it to 29.92 will simply make the error worse.
>

Uh, yes. Brainfart on my side. Sorry

--
Thomas Borchert (EDDH)

[Stefan]

Fred G. Black wrote:

> There is a buffer in the procedure design however given that cold
> temperature corrections are published (at least in Canada, ref CAP GEN
> p19), it's not unreasonable to assume that there are situations where
> they should be used.

Of course. That's one more reason why you must pass a test to get the
license.

Stefan

[Dan Luke]

"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

[Dan Luke]

"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]

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

[Bob Gardner]

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
>
>

[Dan Luke]

"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

[Matt Whiting]

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

[Matt Whiting]

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

[Peter]

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
> ...
>
>>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.

[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

[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

[Aluckyguess]

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
>> ...
>>
>>>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.
>

[Dan Luke]

"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.

[Ron Rosenfeld]

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)

[Casey Wilson]

"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?

[CryptWolf]

"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. ;)

[CryptWolf]

"Aluckyguess" > wrote in message
...
> 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.

[Peter]

CryptWolf wrote:

> "Aluckyguess" > wrote in message
>
...
>>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.

[Cub Driver]

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

[[email protected]]

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

[Guillermo]

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
>
>

[Thomas Borchert]

> I would be more worried about avoiding the big yellow things that are
> the cause of all this, if it happens.
>

and about the poetry...

--
Thomas Borchert (EDDH)

[Ross Richardson]

Aluckyguess wrote:

> I am getting over 500ft. difference and more.
> "Matt Whiting" > wrote in message
> ...
>
>>Aluckyguess wrote:
>>
>>>How come the GPS reads a different altitude than the Altimeter?
>>
>>For many reasons. The main reason is that they derive altitude by two
>>completely different methods and this inevitably introduces errors. Then
>>each method has its own inherent errors.
>>
>>Even two altimeters side by side won't read the same altitude other than
>>by accident.
>>
>>
>>Matt
>
>
>
GPS altitude is not accurate. That is why you need to have a baro sensor
included as input to a IFR approved GPS.

--
Regards, Ross
C-172F 180HP
KSWI

[Gene Seibel]

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.

[Frank Ch. Eigler]

Ross Richardson wrote:

> GPS altitude is not accurate.

.... except when it is, which is most of the time.

> That is why you need to have a baro sensor included as input to a
> IFR approved GPS.

That baro input is used as a cross-check for gross errors, to assist
RAIM. It does not "correct" inaccuracy, and thus is irrelevant to the
normal GPS altitude readout.


- FChE

[Peter]

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.

[John Galban]

Jim Fisher wrote:
> "Tauno Voipio" > wrote in message
>
>
> > 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.
>
Good idea! It's always smart to be prepared. I always carry a
towel with me whenever I fly. Just in case.

John Galban=====>N4BQ (PA28-180)

[[email protected]]

Thomas, I've a complete off-topic question.
I see you fly from Hamburg, is there any chance you ever been to
Borkum(EDWR)?
I'm send on a reconnaissance mission tomorrow to check out if the place
is oke for our vacaction.

Any info is helpfull,
Kees (EHSE)

[Thomas Borchert]

> I see you fly from Hamburg, is there any chance you ever been to
> Borkum(EDWR)?
>

Yes, I have been there. IMHO, while nice, it is not the nicest of the
East Frisian Islands. I would prefer Juist, one further down. Baltrum
is great, too, if your plane is REALLY short-field capable. Borkum
would be next on the list, with the others being much more touristy due
to a lot of one-day visitors.

But this all depends on what you want to do there. Borkum has car
traffic, the other two don't. On Juist, the field is far away from the
village, and you'll need to take the horse-drawn coach. Bicycle rental
in the village only.

--
Thomas Borchert (EDDH)

[[email protected]]

Thanks for the info.

The problem of visiting the other islands is not the short field
capability of my plane(it being a Rallye), but my lack of German.
According my Bottlang Borkum is the only one with English R/T.

The plan is to have a vacation for a week or so in August(when the sea
temp. is a bit up) and use Borkum as a base to fly from to other nearby
fields in Germany or maybe Denmark.
To be honest, Helgoland is high on my wish list.

Happy landings,
Kees.

[Thomas Borchert]

> According my Bottlang Borkum is the only one with English R/T.
>

I wouldn't care about that. Just fly in and talk to them in English.
Also, if you want, I can find out more by calling them. Just tell which
fields you are interested in. Use

thomas (the sign of signs) aeroversand.de

to send me an e-mail...

Our TB-10 won't do Helgoland - otherwise I'd have been there a long
time ago. I would have some recommendations for Denmark, too. Oh, and
if you're interested, we could meet at some point during your trip.

--
Thomas Borchert (EDDH)