GPS altitude vs altimeter altitude

[Chris W]

It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the
2 values will not be the same. Does anyone know about how far off they
are and if there is a way to calculate what the difference should be?
My guess is that at different points around the earth, the correction
will be significantly different, making a generic formula difficult if
not impossible to write.

--
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

[Jim Macklin]

If you have an perfectly calibrated barometric altimeter and
have a perfect altimeter setting taken just a few seconds
before, there are mechanical errors. Altimeter settings on
the ground will vary in just a few miles. The lapse rate is
not constant. But the GPS errors are very small and are
calculated and corrected in real time. But aircraft
separation is based on barometrics, so GPS is not used to
establish cruise altitudes.

GPS could be used for instrument approaches since the ground
and obstructions are fixed and generally known quantities.
GPS is used for photography and surveying.

GPS and terrain mapping will keep you safe from the ground
and the barometric altimeter is still needed to keep the
cruise altitudes set.


--
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:q2x0g.917$9c6.179@dukeread11...
| It is my understanding that even if a GPS is reading
perfectly and your
| standard altimeter is reading perfectly even on a standard
day that the
| 2 values will not be the same. Does anyone know about how
far off they
| are and if there is a way to calculate what the difference
should be?
| My guess is that at different points around the earth, the
correction
| will be significantly different, making a generic formula
difficult if
| not impossible to write.
|
| --
| 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

[Grumman 236]

The altimeter is a barometer and the GPS is a computerized stopwatch.
Considering that they measure such different things, amazing that
there's any realationship at all, huh?

The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point
(points, actually!). So, at any particular location you're over, your
actual distance from the ground can be different from what the GPS's
software thinks it is. So point A is that the GPS altitude can be
inaccurate.

Point B is that your altimeter is only as accurate as the setting you
have in the Kollsman window. If it's based on old data or data for a
distant location or if rapid weather change is occurring, it won't
exactly reflect the pressure corrected for MSL for the location you're
above.

If your GPS has WAAS enabled, you're in an area where the model of the
earth closely matches reality and you have a current altimeter setting
for the exact area, they should come very close to matching. If any of
the above isn't true, you'll see some degree of difference between
them.

Chris W wrote:
> It is my understanding that even if a GPS is reading perfectly and your
> standard altimeter is reading perfectly even on a standard day that the
> 2 values will not be the same. Does anyone know about how far off they
> are and if there is a way to calculate what the difference should be?
> My guess is that at different points around the earth, the correction
> will be significantly different, making a generic formula difficult if
> not impossible to write.
>
> --

[peter]

Grumman 236 wrote:
> The altimeter is a barometer and the GPS is a computerized stopwatch.
> Considering that they measure such different things, amazing that
> there's any realationship at all, huh?
>
> The mathematical model of the earth the the GPS software uses is
> idealized somewhat and provides only an approximation of the bulging of
> the surface of the earth relative to its geometric center point
> (points, actually!). So, at any particular location you're over, your
> actual distance from the ground can be different from what the GPS's
> software thinks it is. So point A is that the GPS altitude can be
> inaccurate.

But note that most consumer GPS receivers do not report altitudes based
only on the idealized mathematical model of the earth's shape
(generally the WGS-84 ellipsoid). They also include correction tables
that reflect the difference between the geoid shape and the ellipsoid
as a function of your position over the earth's surface. So the
reported altitude is with respect to sealevel, not relative to the
idealized ellipsoid. Naturally there will still be measurement
uncertainties and the correction table is also limited in precision,
but in terms of aircraft altitudes the errors are pretty small.
>
> Point B is that your altimeter is only as accurate as the setting you
> have in the Kollsman window. If it's based on old data or data for a
> distant location or if rapid weather change is occurring, it won't
> exactly reflect the pressure corrected for MSL for the location you're
> above.

True, but then there's also Point C. Even if you've entered the
correct current value into the Kollsman window for your location, it
still just reflects the sealevel-adjusted pressure at ground level
below you and your altimeter then uses an idealized standard atmosphere
model to determine your altitude. If the temperature and lapse rate of
the actual atmosphere doesn't match the model then the reported
altitude can be significantly different from reality; hundreds or even
a thousand feet or more of variation.

But, as mentioned, aircraft vertical separations are based on
barometric altitudes and as long as everyone in a given area is off by
the same amount the separations will still be maintained.

[[email protected]]

On 16 Apr 2006 13:52:58 -0700, "peter" > wrote:

>Grumman 236 wrote:
>> The altimeter is a barometer and the GPS is a computerized stopwatch.
>> Considering that they measure such different things, amazing that
>> there's any realationship at all, huh?
>>
>> The mathematical model of the earth the the GPS software uses is
>> idealized somewhat and provides only an approximation of the bulging of
>> the surface of the earth relative to its geometric center point
>> (points, actually!). So, at any particular location you're over, your
>> actual distance from the ground can be different from what the GPS's
>> software thinks it is. So point A is that the GPS altitude can be
>> inaccurate.
>
>But note that most consumer GPS receivers do not report altitudes based
>only on the idealized mathematical model of the earth's shape
>(generally the WGS-84 ellipsoid). They also include correction tables
>that reflect the difference between the geoid shape and the ellipsoid
>as a function of your position over the earth's surface. So the
>reported altitude is with respect to sealevel, not relative to the
>idealized ellipsoid. Naturally there will still be measurement
>uncertainties and the correction table is also limited in precision,
>but in terms of aircraft altitudes the errors are pretty small.
>>
>> Point B is that your altimeter is only as accurate as the setting you
>> have in the Kollsman window. If it's based on old data or data for a
>> distant location or if rapid weather change is occurring, it won't
>> exactly reflect the pressure corrected for MSL for the location you're
>> above.
>
>True, but then there's also Point C. Even if you've entered the
>correct current value into the Kollsman window for your location, it
>still just reflects the sealevel-adjusted pressure at ground level
>below you and your altimeter then uses an idealized standard atmosphere
>model to determine your altitude. If the temperature and lapse rate of
>the actual atmosphere doesn't match the model then the reported
>altitude can be significantly different from reality; hundreds or even
>a thousand feet or more of variation.
>
>But, as mentioned, aircraft vertical separations are based on
>barometric altitudes and as long as everyone in a given area is off by
>the same amount the separations will still be maintained.

Have a look at the following url:
http://sps.unavco.org/geoid/

David

[peter]

wrote:
> On 16 Apr 2006 13:52:58 -0700, "peter" > wrote:
>
> >Grumman 236 wrote:
> >> The mathematical model of the earth the the GPS software uses is
> >> idealized somewhat and provides only an approximation of the bulging of
> >> the surface of the earth relative to its geometric center point

> >But note that most consumer GPS receivers do not report altitudes based
> >only on the idealized mathematical model of the earth's shape
> >(generally the WGS-84 ellipsoid). They also include correction tables
> >that reflect the difference between the geoid shape and the ellipsoid
> >as a function of your position over the earth's surface.

> Have a look at the following url:
> http://sps.unavco.org/geoid/

Yes, that URL gives you the correction between geoid and ellipsoidal
heights. But as I already said, similar correction tables are already
built into most consumer GPS receivers (incl. all Garmins and
Magellans) so the altitudes they report are not what the above website
refers to as "GPS Elevation" but rather the elevation with respect to
MSL.

If you look at the NMEA data output of a GPS receiver, the altitude is
given in the $GPGGA sentence and one of the data fields specifies the
value being used by the receiver for the 'Height of geoid above WGS84
ellipsoid' when reporting the elevation.
See http://aprs.gids.nl/nmea/#gga for details.

[Bob Gardner]

AIM 1-1-19a(8) tells you to not to rely upon GPS altitude. Until the AIM is
changed, I'm not concerned with the details.

Bob Gardner

"Chris W" > wrote in message
news:q2x0g.917$9c6.179@dukeread11...
> It is my understanding that even if a GPS is reading perfectly and your
> standard altimeter is reading perfectly even on a standard day that the 2
> values will not be the same. Does anyone know about how far off they are
> and if there is a way to calculate what the difference should be? My
> guess is that at different points around the earth, the correction will be
> significantly different, making a generic formula difficult if not
> impossible to write.
>
> --
> 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

[Bob Gardner]

OOps. AIM 1-1-19 (a) (4). Doggone feds changed the numbers on me!

Bob Gardner

"Chris W" > wrote in message
news:q2x0g.917$9c6.179@dukeread11...
> It is my understanding that even if a GPS is reading perfectly and your
> standard altimeter is reading perfectly even on a standard day that the 2
> values will not be the same. Does anyone know about how far off they are
> and if there is a way to calculate what the difference should be? My
> guess is that at different points around the earth, the correction will be
> significantly different, making a generic formula difficult if not
> impossible to write.
>
> --
> 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

[[email protected]]

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. It
must be that mathmatical modeling thing. Then I fire up my Garmin 196
and it varies between 15 feet higher and 10 feet lower. I attribute
that to a less sensitive circuitry in the handheld unit altho I might
be wrong surmising that. I would be curious to see what others see in
their respective areas.?

Ben
www.haaspowerair.com

[Jim Macklin]

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?



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

> wrote in message
oups.com...
| 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. It
| must be that mathmatical modeling thing. Then I fire up my
Garmin 196
| and it varies between 15 feet higher and 10 feet lower. I
attribute
| that to a less sensitive circuitry in the handheld unit
altho I might
| be wrong surmising that. I would be curious to see what
others see in
| their respective areas.?
|
| Ben
| www.haaspowerair.com
|

[Richard]

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

[Chris W]

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

[Bob Noel]

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

[peter]

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.

[Dan]

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

[Jim Macklin]

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

[[email protected]]

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

[BTIZ]

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.

[Morgans]

"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

[Chris W]

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

[peter]

Chris W wrote:
> 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.

What is the purpose of the project? Why couldn't it be done in a way
where the model remains within sight at all times - say by flying a
pre-established figure-8 course or something similar, and having a
manual override capability if it's observed to stray from the planned
course? Your current plan certainly appears to violate the model
airplane safety guidelines which state that "10. The operator of a
radio-controlled model aircraft
shall control it during the entire flight, maintaining visual contact
without enhancement
other than by corrective lenses that are prescribed for the pilot. No
model aircraft shall be
equipped with devices which allow it to be flown to a selected location
which is beyond
the visual range of the pilot."
http://www.modelaircraft.org/PDF-files/105.PDF

[Jim Macklin]

I lived in Tulsa and OKC and have now been in Kansas over 25
years. Your proposed altitudes are above the MEA on
airways. Done properly, with reserved airspace [you can get
approval to use the MOAs] what you want to do is safe. But
if you just want to launch and depend on luck to avoid
killing somebody you're being reckless.
A quick Google returned this...
Drone aircraft may prowl US skies | CNET News.com Drone
aircraft may prowl US skies | Can unmanned aerial vehicles
doing ... the FAA says it's created a UAV "program office"
to come up with new rules of the ...
news.com.com/Drone+aircraft+may+prowl+U.
S.+skies/2100-11746_3-6055658.html - 54k - Cached - Similar
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flown under the AMA Experimental Aircraft Rules; ...
www.house.gov/transportation/
aviation/03-29-06/03-29-06memo.html - 16k - Cached - Similar
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www.washingtonpost.com/wp-dyn/
content/article/2006/03/29/AR2006032901814.html - Similar
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(UAVs) Unmanned Aerial Vehicles (UAVs) – sometimes called
“unmanned aircraft systems,” ... The FAA’s main concern
about UAV operations in civil airspace is safety. ...
www.faa.gov/news/news_story.cfm?type=fact_
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www.faa.gov/ATpubs/AIM/Chap7/aim0705.html - 57k -
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[ More results from www.faa.gov ]




I see on your website that your name is Woodhouse and you
sell wishes. Does your planned UAV that will fly up to
6,000 feet for many miles [you said you early test would be
20 miles] make up a "wish" for a customer?
Since you're in OKC, why not visit the Feds and learn what
is legal and how to do it safely?

--
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:TpF0g.942$9c6.755@dukeread11...
| 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

[Greg Farris]

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?

[Jim Macklin]

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

--
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.


"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?
|

[Cub Driver]

On Sun, 16 Apr 2006 14:37:53 -0500, Chris W > wrote:

>It is my understanding that even if a GPS is reading perfectly and your
>standard altimeter is reading perfectly even on a standard day that the
>2 values will not be the same

Mine generally read within 50-100 feet of one another. However, I
rarely fly above 3,500 feet. (Altimeter is 60 years old, assuming same
age as airplane.)


-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
In Search of Lost Time: www.readingproust.com

[Jim Macklin]

Does your old Cub have a sensitive altimeter or just a
standard?



--
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.


"Cub Driver" <usenet AT danford DOT net> wrote in message
...
| On Sun, 16 Apr 2006 14:37:53 -0500, Chris W
> wrote:
|
| >It is my understanding that even if a GPS is reading
perfectly and your
| >standard altimeter is reading perfectly even on a
standard day that the
| >2 values will not be the same
|
| Mine generally read within 50-100 feet of one another.
However, I
| rarely fly above 3,500 feet. (Altimeter is 60 years old,
assuming same
| age as airplane.)
|
|
| -- all the best, Dan Ford
|
| email: usenet AT danford DOT net
|
| Warbird's Forum: www.warbirdforum.com
| Piper Cub Forum: www.pipercubforum.com
| In Search of Lost Time: www.readingproust.com

[Cub Driver]

On 16 Apr 2006 22:42:05 -0700, "peter" > wrote:

>"10. The operator of a
>radio-controlled model aircraft
>shall control it during the entire flight, maintaining visual contact
>without enhancement
>other than by corrective lenses that are prescribed for the pilot. No
>model aircraft shall be
>equipped with devices which allow it to be flown to a selected location
>which is beyond
>the visual range of the pilot."

What about the guys who flew the RC plane to Ireland?

(Gotta love that bit about corrective lenses!... "My glasses! Where
did I put my glasses?")



-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
In Search of Lost Time: www.readingproust.com

[Jim Macklin]

It wasn't radio controlled, but a robot. It was also out of
US airspace over the ocean and well below any Atlantic
aircraft traffic and above the ships. It may have had a
chase airplane until it was off-shore and it may have been
met at the other end. It wasn't a secret, they had to have
all the paperwork in order to get the record.

Maybe the guy is wanting to build a cheap cruise missile?



--
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.


"Cub Driver" <usenet AT danford DOT net> wrote in message
...
| On 16 Apr 2006 22:42:05 -0700, "peter"
> wrote:
|
| >"10. The operator of a
| >radio-controlled model aircraft
| >shall control it during the entire flight, maintaining
visual contact
| >without enhancement
| >other than by corrective lenses that are prescribed for
the pilot. No
| >model aircraft shall be
| >equipped with devices which allow it to be flown to a
selected location
| >which is beyond
| >the visual range of the pilot."
|
| What about the guys who flew the RC plane to Ireland?
|
| (Gotta love that bit about corrective lenses!... "My
glasses! Where
| did I put my glasses?")
|
|
|
| -- all the best, Dan Ford
|
| email: usenet AT danford DOT net
|
| Warbird's Forum: www.warbirdforum.com
| Piper Cub Forum: www.pipercubforum.com
| In Search of Lost Time: www.readingproust.com

[Ron Rosenfeld]

On Sun, 16 Apr 2006 19:58:19 -0500, 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. 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.

I do not believe there is any formulaic method to convert from GPS altitude
to pressure altitude.

GPS altitude may be more akin to true altitude, with variations based on
the precise geographic location that could be placed into a table.

But if altitude is varying with pressure altitude, bearing in mind that the
pressure altitude sensor is also sensitive to temperature, I think you have
your work cut out for yourself.

Perhaps you could graph the METAR derived data and compare it with the GPS
derived altitude, and if the trend (direction of change) is the same, be
satisfied that the aircraft is performing as designed.

If you had the lookup table to derive geographic position vs GPS altitude
error; and also something like a SKEW-T plot to look at temperatures aloft;
and also the METAR data, perhaps you could develop something to convert
true altitude (from your corrected GPS output) to assumed pressure altitude
reading on your altimeter) and draw some conclusions that way.

It sounds like an interesting problem.


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

[[email protected]]

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>

[Matt Barrow]

> 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

[[email protected]]

> 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...

[Matt Barrow]

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

Another clueless punk with a nig mouth and a foot firmly embedded in it.

PLONK

[Cub Driver]

On Mon, 17 Apr 2006 04:58:42 -0500, "Jim Macklin"
> wrote:

>Does your old Cub have a sensitive altimeter or just a
>standard?

One of those that, when it feels like it, swings a hundred feet up and
down for ten or twenty minutes at a time. I think the best term for it
would be sub-standard.


-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
In Search of Lost Time: www.readingproust.com

[Stubby]

Cub Driver wrote:
> On Mon, 17 Apr 2006 04:58:42 -0500, "Jim Macklin"
> > wrote:
>
>> Does your old Cub have a sensitive altimeter or just a
>> standard?
>
> One of those that, when it feels like it, swings a hundred feet up and
> down for ten or twenty minutes at a time. I think the best term for it
> would be sub-standard.
>
Just tap on the face a bit.

[Jim Macklin]

sounds like it has water in the static line.



"Cub Driver" <usenet AT danford DOT net> wrote in message
...
| On Mon, 17 Apr 2006 04:58:42 -0500, "Jim Macklin"
| > wrote:
|
| >Does your old Cub have a sensitive altimeter or just a
| >standard?
|
| One of those that, when it feels like it, swings a hundred
feet up and
| down for ten or twenty minutes at a time. I think the best
term for it
| would be sub-standard.
|
|
| -- all the best, Dan Ford
|
| email: usenet AT danford DOT net
|
| Warbird's Forum: www.warbirdforum.com
| Piper Cub Forum: www.pipercubforum.com
| In Search of Lost Time: www.readingproust.com

[Jim Macklin]

that's what the engine does




"Stubby" > wrote in message
. ..
| Cub Driver wrote:
| > On Mon, 17 Apr 2006 04:58:42 -0500, "Jim Macklin"
| > > wrote:
| >
| >> Does your old Cub have a sensitive altimeter or just a
| >> standard?
| >
| > One of those that, when it feels like it, swings a
hundred feet up and
| > down for ten or twenty minutes at a time. I think the
best term for it
| > would be sub-standard.
| >
| Just tap on the face a bit.

[Cub Driver]

On Tue, 18 Apr 2006 10:17:55 -0400, Stubby
> wrote:

>> One of those that, when it feels like it, swings a hundred feet up and
>> down for ten or twenty minutes at a time. I think the best term for it
>> would be sub-standard.
>>
>Just tap on the face a bit.

I'd have to bring a broomstick!

(J-3s fly solo from the back seat.)



-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
In Search of Lost Time: www.readingproust.com