GPS Altitude with WAAS

Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

John Bell
www.cockpitgps.com

"John Bell" writes:

Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

Thanks -- those are good articles. The Nav Canada paper on non-WAAS
GPS VNAV (the Graham paper) is especially interesting.


All the best,


David

I would say that we are exceedingly fortunate in having Nav Canada as an
information source to supplement (complement?) the FAA.

Bob Gardner

"David Megginson" wrote in message
...
"John Bell" writes:

Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

Thanks -- those are good articles. The Nav Canada paper on non-WAAS
GPS VNAV (the Graham paper) is especially interesting.


All the best,


David

John Bell wrote:
Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

John Bell
www.cockpitgps.com


This one's for the Canadians on this thread. A notice on the new
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):

"BARO-VNAV NA below -15 deg C (5 deg F)"

And this is for a decision altitude of only 294 ft AGL. Seems like the
FAA is moving towards taking into account temperature errors in
barometric alitmetry. And, by implication, this supports the premise
that WAAS altitude figures are more accurate than the trusty old
"sensitive altimeter." (In the legend they specifically state that
WAAS-based VNAV can be used when BARO-VNAV is not approved due to
temperature.)

"Fred E. Pate" writes:

And this is for a decision altitude of only 294 ft AGL. Seems like
the FAA is moving towards taking into account temperature errors in
barometric alitmetry.

Thanks -- that's an interesting note.

And, by implication, this supports the premise that WAAS altitude
figures are more accurate than the trusty old "sensitive altimeter."
(In the legend they specifically state that WAAS-based VNAV can be
used when BARO-VNAV is not approved due to temperature.)

The altimeter becomes increasingly accurate near the ground (assuming
you have the correct altimeter setting) and increasingly inaccurate
away from the ground; WAAS, I'll guess, has about the same accuracy
all the way down (or up). I don't know at what point they typically
cross over, but it would vary depending on the temperature gradiant.


All the best,


David

I've never lived in Oakland, but I thought it was exceedingly rare for =
the
temperature there to dip below -15C.
---JRC---

"Fred E. Pate" wrote in message =
...
=20
This one's for the Canadians on this thread. A notice on the new=20
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach=20
=
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):
=20
"BARO-VNAV NA below -15 deg C (5 deg F)"
=20
And this is for a decision altitude of only 294 ft AGL. Seems like =
the=20
FAA is moving towards taking into account temperature errors in=20
barometric alitmetry. And, by implication, this supports the premise=20
that WAAS altitude figures are more accurate than the trusty old=20
"sensitive altimeter." (In the legend they specifically state that=20
WAAS-based VNAV can be used when BARO-VNAV is not approved due to=20
temperature.)

On Mon, 29 Sep 2003 00:16:00 -0700, "Fred E. Pate"
wrote:

John Bell wrote:
Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

John Bell
www.cockpitgps.com


This one's for the Canadians on this thread. A notice on the new
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):

"BARO-VNAV NA below -15 deg C (5 deg F)"

And this is for a decision altitude of only 294 ft AGL. Seems like the
FAA is moving towards taking into account temperature errors in
barometric alitmetry. And, by implication, this supports the premise
that WAAS altitude figures are more accurate than the trusty old
"sensitive altimeter." (In the legend they specifically state that
WAAS-based VNAV can be used when BARO-VNAV is not approved due to
temperature.)

It seems a bit strange...
..
DH on the ILS at MBS is only 200 feet AGL and that is with no
temperature correction. In the winter we regularly see below zero
F and it's not rare to see it at minus 20 at night

OTOH with those temperatures we either have severe clear, or blowing
snow. It's rather uncommon to see clouds near the ground when it's
that cold here in the flat lands.

That and you can be "on top" of a raging blizzard at 4,000.



Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

Roger Halstead writes:

OTOH with those temperatures we either have severe clear, or blowing
snow. It's rather uncommon to see clouds near the ground when it's
that cold here in the flat lands.

Maybe it's just not cold enough. Once you get past -30 degC or so,
you can get a dense freezing mist (like smoke) rising off any open
water, like a lake or wide river -- the parts that are frozen over
start to make a creepy, moaning sound.

I agree that truly cold temperatures ( -20 degC) tend to mean VMC, at
least where I live -- it's one of the fantastic things about winter
flying (clear skies, excellent visibility, good climb performance,
minimal turbulence, high ground visibility at night, and early sunsets
that make it easy to stay night-current). The downsides are having to
plug in the engine heater overnight and dealing with the @#$%#@ wing
covers.


All the best,


David

On Mon, 29 Sep 2003 17:17:49 GMT, David Megginson
wrote:

Roger Halstead writes:

OTOH with those temperatures we either have severe clear, or blowing
snow. It's rather uncommon to see clouds near the ground when it's
that cold here in the flat lands.

Maybe it's just not cold enough. Once you get past -30 degC or so,
you can get a dense freezing mist (like smoke) rising off any open
water, like a lake or wide river -- the parts that are frozen over
start to make a creepy, moaning sound.

I agree that truly cold temperatures ( -20 degC) tend to mean VMC, at
least where I live -- it's one of the fantastic things about winter
flying (clear skies, excellent visibility, good climb performance,
minimal turbulence, high ground visibility at night, and early sunsets
that make it easy to stay night-current). The downsides are having to
plug in the engine heater overnight and dealing with the @#$%#@ wing

I have a couple layers of the new blankets sewn together to fit around
the entire front of the plane from mid windshield forward around the
engine compartment and underneath where it completely covers the nose
gear doors, and around the front to wrap the first quarter of the
prop. The back of the spinner is even warm.

Problem is...the Deb doesn't warm up till I'm at cruise, or duri8ng a
long climb to altitude.

You can actually stay comfortable in heavy slacks and a long sleeve
shirt, but that is *after* climb out. :-))

Getting the Cub Cadet with snow blower out, cleaning the ramp and
maybe even part of the taxiway does not make the inside of the plane
feel any warmersigh It's kinda like working in an unfinished house
in the winter. Man, but it gets cold in there.

I've flown from Michigan to Gainsville Ga (LGM) without seeing a
cloud. Then again, I've flown from the middle of Tennessee and not
seen the ground till some where around Jackson Michigan.

Unlike the summer storms, you can fly over a blizzard at some where
between 3000 and 5000 AGL. The winter storms seldom get very high.
But I have been sitting "up there" and after hearing some of the
conversations on UNICOM thinking it'd be real nice to have one more
fan. That was when I realized it's a good idea to not only have a way
out, but have a way down when single engine. If it's really cold you
don't have to worry about ice in the clouds either, but since then I
make it a point to try to stay near the edges of those big storms when
there is no safe way down.
covers.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

All the best,


David

Roger Halstead wrote:

It seems a bit strange...
.
DH on the ILS at MBS is only 200 feet AGL and that is with no
temperature correction. In the winter we regularly see below zero
F and it's not rare to see it at minus 20 at night


Yup, but temperature doesn't effect the glideslope, just where you put
the MAP along that glideslope. On the so-called "Baro-VNAV" approaches,
the glideslope is defined using barometric altimeter data, so the entire
glideslope will be shallower and closer to the ground on a cold day.
And if there is a low-level temperature inversion the glideslope won't
even be a straight line.