GPS altitude vs altimeter altitude

Jim Macklin wrote:
The barometric altimeter is calibrated to zero at the wheel
contact point with the ground. The GPS measures at the
antenna. Like all things made by man, there are tolerances.

Reason the baro-altimeter is wheel height, think of a 747,
do you want the cockpit or the wheels to clear the trees at
the end of the runway?




Having been on an overloaded business Cessna that took off just clearing
the trees (he raised the gear in ground effect to get a little more
speed to make it) I'd hope that commercial aircraft piloting would be a
little less "bold". :-)

Richard

Good point. My GPS antenna is 5' 7" off the ground... The error is less
then I thought. I am VERY impressed now.!!!!!!!!!!

In article .com,
" wrote:

To respond to the original posters question. I have often asked the
same thing. My observations are this. My plane is sitting in the hangar
at 6412.37 msl. The airport was just recently surveyed so I am sure
this is a real close number. I set my altimeter to the current setting
and it shows within 3 feet. I fire up my King KMD150 MFD and it will
almost always show 8- 12 feet higher then that, never seen it lower.

where is the GPS antenna vs the altimeter?

--
Bob Noel
Looking for a sig the
lawyers will hate

In article .com,
says...


To respond to the original posters question. I have often asked the
same thing. My observations are this. My plane is sitting in the hangar
at 6412.37 msl. The airport was just recently surveyed so I am sure
this is a real close number.


They just surveyed the airport, and they put the datum point in your
hangar?

The airport elevation is the highest spot on a runway, not
the ramps or taxiways. Depending on the local terrain,
there can be more than 50 feet difference on the runways.


--
James H. Macklin
ATP,CFI,A&P

--
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"Greg Farris" wrote in message
...
| In article
.com,
| says...
|
|
| To respond to the original posters question. I have often
asked the
| same thing. My observations are this. My plane is sitting
in the hangar
| at 6412.37 msl. The airport was just recently surveyed so
I am sure
| this is a real close number.
|
|
| They just surveyed the airport, and they put the datum
point in your
| hangar?
|

DUH ......Any given survey uses multiple elevations to show contour
lines. It just happens the apron right infront of my hangar was a
reference spot for one of those data points. I will just lurk here and
watch the chaos run rampant....... G

wrote in message
oups.com...
DUH ......Any given survey uses multiple elevations to show contour
lines.

Irrelevant to the point.

It just happens the apron right infront of my hangar was a
reference spot for one of those data points.

ONE of those data points. Was the reference spot marked with the elevation
at that point?


--
Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO

DUH ......Any given survey uses multiple elevations to show contour
lines.


Irrelevant to the point.


It just happens the apron right infront of my hangar was a
reference spot for one of those data points.


ONE of those data points. Was the reference spot marked with the
elevation
at that point?

--
Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO


I sure hope you have nothing to do with building houses....... God help
the poor fool who buys one from ya...

It seems as though those who actually read what I was really asking
didn't think it was important to find or know the answer so let me get a
little more specific. I am going to launch a remote control airplane
that has an autopilot. The autopilot has an altitude hold function that
is based on barometric pressure sensor. I will also have a GPS used for
guidance. The data from that GPS will be transmitted using APRS on
144.39 mhz to any amateur station listening. Once the autopilot is
turned on it will hold the pressure altitude it is at, so as it flies
along it's route (maybe as many as a few hundred miles) and the
barometric pressure changes the plane will climb and descend to maintain
the same pressure altitude. However the only data I will be getting
back is the GPS altitude. I need a way to do a reality check so if I
see the plane is descending or climbing I will know it is because of
changes in the barometric pressure and not the something that has gone
wrong. The plan is to get the latest METAR data from the closest
observation point to the current position of the plane and then do the
math compared to what it was where and when it launched so I will know
about what the GPS altitude should be reading as that is all I will be
able to see. For those who want to know why I don't just have it
transmit the pressure altitude back, I have four good reasons; cost,
weight, size, complexity. My first flights will be only 20 miles or
so. For safety I will be sure it steers clear of any class B, C, and D
air space. I'm not sure what pressure altitude I will have it fly at
..... probably somewhere between 1500' and 6000' AGL depending on the
distance for it to cover.

--
Chris W
KE5GIX

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Chris W wrote:
It seems as though those who actually read what I was really asking
didn't think it was important to find or know the answer so let me get a
little more specific.

Hard to give a definitive response since there are quite a few
variables that aren't known to us. There's already been a discussion
of the various sources of discrepancy, so I'd look at each and decide
how large a value to assign, then add them up to get an overall
estimate. Those included:

1) Measurement uncertainty in the pressure sensor. Presumably the
manufacturer has a specification for this.

2) Measurement uncertainty in the GPS. Does it have WAAS? Without it
I've found that altitude is generally within about 35' 95% or more of
the time. With WAAS that should go down to about 20'. Both figures
assume a good skyview which you should have in your remote plane
provided the antenna is located properly. But note that there's still
that 5% of the time when the error could be greater.

3) Variations due to ground level pressure changes (i.e. Kollman window
settings). You indicate checking the value from the nearest measuring
points along the route so this depends on how far apart those are. But
more importantly it depends on how rapidly the weather is changing.
The uncertainly here could be quite large if you're flying even when a
front of thunderstorms/tornadoes is passing through - but I'd guess you
wouldn't fly in those conditions. Look at weather charts for a few
days with somewhat worse conditions than the ones you'll fly in to get
an idea of the expected variations.

4) Variations due to lapse/temperature deviations from the standard
atmosphere model. This will again depend on what range of conditions
you'll be willing to fly under, but also depends on the altitude - i.e.
not a big issue at 1000' AGL but would be more significant at the 6000'
level. But unless you fly in bad weather I wouldn't expect this to
change very much over the few hundred mile range that you mentioned -
especially at these relatively low altitudes.

5) The geoid vs. ellipsoid model effects. Do you know if your GPS
corrects for this (most consumer stand-alone models do, but you should
check on your particular one which I presume is a minimal board type)?
Early SiRF-based models generally didn't do the correction but the
later SiRFii and I think all SiRFiii models do have the correction
capability. Even if not, this varies pretty slowly over such a short
distance and you can use the website previously given to see how it
changes over the specific route in question.