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

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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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

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.

Sounds like a really cool plane. I've always thought doing something
like this would be neat. What speed will it cruise at?

I suggest that you contact the FAA before you launch a robot
airplane and kill somebody.



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

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Chris W" wrote in message
news:TKB0g.933$9c6.344@dukeread11...
| 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
|
| Gift Giving Made Easy
| Get the gifts you want &
| give the gifts they want
| One stop wish list for any gift,
| from anywhere, for any occasion!
| http://thewishzone.com

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

I do hope this UAV flight has been cleared with the local aviation
authorities, or is contained within a "restricted" airspace to keep other
VFR and IFR aircraft safe.

BT

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.

"BTIZ" wrote

I do hope this UAV flight has been cleared with the local aviation
authorities, or is contained within a "restricted" airspace to keep other
VFR and IFR aircraft safe.

I've got to agree with this one.

To the OP: When an RC airplane leaves your sight line, it is now a UAV, and
under another set of rules.

This is a rather hot topic, and in the news, as of late. Don't be the one
that gets used as an example. It could cost you a bunch of money, jail
time, or a lifetime of regret. (over the people that your plane killed)

Most likely, nothing bad would happen. Is that something you want to
gamble your like on? Is it worth it?
--
Jim in NC

Morgans wrote:

Most likely, nothing bad would happen. Is that something you want to
gamble your like on? Is it worth it?


I'm guessing that you either live east of the Mississippi or on/near the
west coast, and not in Oklahoma. RC airplanes can and have killed
people. However, generally this happens from a much larger plane than I
plan on using, and the chances of someone getting injured is reduced
significantly when the engine isn't running. With this in mind these
are the situations I foresee potential for serious injury. The only way
I see this coming in contact with someone on the ground is if the engine
is not running. Therefore, the biggest danger to someone or something
on the ground is gone. So for the plane to impact a person while the
engine was running it would have to be an impact with a real plane. If
the difference in heading of the real plane and the model were between
greater than 0 and less than 90, then the prop wold be history before it
penetrated the cabin and would the now dead engine would loose most of
it's energy before in impacted someone in the plane, most of the rest of
the plane would never enter the cabin. If the difference between the
heading of the real plane and the model were between 90 and 180, that's
big a problem. I'm not sure how feasible it would be to have some kind
of sensor to detect such an impending collision and do something about
it. Other than avoiding busy airspace, I'm not sure what to do about
it. Keep in mind that the class C around KOKC which is with in 3 miles
of my house and I plan on staying away from, really isn't very busy at
all. There is still the possibility that something goes wrong with the
autopilot and sends the plane down the wrong side of the highway at 3
feet off the ground. To virtually eliminate this type of situation, I
plan on having a completely independent system that will kill the engine
and put the plane in a slow flight configuration should the altitude
drop below a given amount. Granted that could also fail but the chances
of both the autopilot and that system failing is very remote, that "fail
safe" system would be pretty small and light and could be made redundant
and possibly include a the deployment of a parachute. All this said I
am open to other ideas to make it safer.

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com