Questions on high altitude pressures

es330td writes:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.

GPS is too inaccurate for most purposes in vertical positioning, anyway. It
is not designed to determine altitude with a high degree of accuracy, and can
easily be hundreds of feet off.

In article ,
Wolfgang Schwanke wrote:

In theory you could use GPS altitudes when flying MSL/QNH setting,
because both systems measure absolute altitued, so you would expect
them to be the same. If GPS could be relied upon that is. Unfortunately
it cannot, GPS altitued measurements are unrealiable and can drift
wildly (as opposed to GPS 2D positioning which is quite accurate).
Traditional altimeters are much better.

as a rule of thumb, regular SPS GPS altitude error is roughly
50% greater than the horizontal error.

The advantage of traditional altimeters is realized when everyone
is using them (and the same setting).

Note that above FL290 traditional altimeters errors increase to
the point where separations were increased to 2000' vertical
separations (except for RVSM airspace)

On Nov 24, 8:38*am, Robert Moore wrote:
es330td wrote

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.


Thanks for the reminder. I hope someday to have to care about that. :-)

es330td wrote

On Nov 24, 8:38*am, Robert Moore wrote:
es330td wrote

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.


Thanks for the reminder. I hope someday to have to care about that. :-)

Just remember.... on the way up, it's Transition Altitude, on the way
down, it's Transition Level. :-)

Bob Moore
ATP CFI
PanAm(retired)

In article , re says...
Bob Noel wrote in
:

as a rule of thumb, regular SPS GPS altitude error is roughly
50% greater than the horizontal error.

That would be minimal. But IME, while most of the time it's fairly
accurate, it can sometimes go wild. For example I was cross-countrying
at 3,500ish ft, when suddenly the GPS went through 3,000 2,500 ...
4,500 4,000 and then settled down at 3,500 once again as if nothing had
happend. It all lasted just a couple of seconds. It had not lost
coverage, that would be indicated by an error screen and the map
animation stopping which it did not; model GPS Pilot III. I understand
this is normal behaviour. If so, GPS altitude measurements are
completely unreliable.

No way. The screen you shoulda been looking at is the satellite screen
(I bet if you'd been watching that, you'd have noticed some had dropped
out or gone weak).

I've taken my GPS on a number of commercial flights and it's interesting
to watch the differences between FL and GPS indicated alt (as one moves
through high and low pressures).

--
Duncan

In article ,
Wolfgang Schwanke wrote:

Bob Noel wrote in
:

as a rule of thumb, regular SPS GPS altitude error is roughly
50% greater than the horizontal error.

That would be minimal. But IME, while most of the time it's fairly
accurate, it can sometimes go wild. For example I was cross-countrying
at 3,500ish ft, when suddenly the GPS went through 3,000 2,500 ...
4,500 4,000 and then settled down at 3,500 once again as if nothing had
happend. It all lasted just a couple of seconds. It had not lost
coverage, that would be indicated by an error screen and the map
animation stopping which it did not; model GPS Pilot III. I understand
this is normal behaviour. If so, GPS altitude measurements are
completely unreliable.

A properly operating GPS receiver, with 4 SVs in view in an
appropriate geometry, should not report such extreme altitude
variations.

What is unknown: exactly how the GPS Pilot III selects SVs,
whether or not it will require 4 in view or let altitude "drift"
in order to maintain a 2D solution by using just 3 SVs, whether
or not it will report outages/drops within a few seconds, whether
or not it performs reasonableness checks on solutions.

I assume the GPS Pilot III doesn't have RAIM or FDE

Bob Noel writes:

as a rule of thumb, regular SPS GPS altitude error is roughly
50% greater than the horizontal error.

GPS altitude error is enormous, sometimes two orders of magnitude greater than
lateral error. It's completely unreliable.

Mxsmanic wrote:
Bob Noel writes:

as a rule of thumb, regular SPS GPS altitude error is roughly
50% greater than the horizontal error.

GPS altitude error is enormous, sometimes two orders of magnitude greater than
lateral error. It's completely unreliable.

Wrong yet again.

Two orders of magnitude means 100 times.

The GPS specification with SA on is 100 m lateral, 150 m vertical, which
is 1.5 times.

GPS measured accuracy with SA off is 2.5 m lateral, 4.7 m vertical, which
is 1.9 times.

WASS specification is 7.6 m lateral, 7.6 m vertical, which is 1 times.

WASS measured accuracy is 0.9 m lateral, 1.3 m vertical, which is
1.4 times.

In all cases, altitude error is less than 2 times lateral error.

It looks as though we can add GPS and simple arithmetic to the list of
things which you pontificate about yet know nothing about.


--
Jim Pennino

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writes:

Two orders of magnitude means 100 times.

Yes, I know.

The GPS specification with SA on is 100 m lateral, 150 m vertical, which
is 1.5 times.

SA has been off for years.

GPS measured accuracy with SA off is 2.5 m lateral, 4.7 m vertical, which
is 1.9 times.

The vertical accuracy is far worse than 4.7 metres.

It looks as though we can add GPS and simple arithmetic to the list of
things which you pontificate about yet know nothing about.

Be sure to fly IFR sometime in IMC with GPS as your altimeter, and you'll see
what I mean.

Mxsmanic wrote:
writes:

Two orders of magnitude means 100 times.

Yes, I know.

The GPS specification with SA on is 100 m lateral, 150 m vertical, which
is 1.5 times.

SA has been off for years.

Yes, that at least is correct.

The point is that even with SA, GPS accuracy is pretty good and has nowhere
near 2 orders of magnitude altitude error compared to lateral as you stated.

GPS measured accuracy with SA off is 2.5 m lateral, 4.7 m vertical, which
is 1.9 times.

The vertical accuracy is far worse than 4.7 metres.

Absolutely wrong.

See the the data:

http://users.erols.com/dlwilson/gpswaas.htm

GPS with SA off, vertical error:

No WAAS

50% 3.0 m
Mean 3.6 m
RMS 4.5 m

With WAAS:

50% 2.2 m
Mean 2.6 m
RMS 3.2 m

A quick Google (source of all your knowledge) finds numerous sites
with similar data.

You are wrong, wrong, wrong once again.

It looks as though we can add GPS and simple arithmetic to the list of
things which you pontificate about yet know nothing about.

Be sure to fly IFR sometime in IMC with GPS as your altimeter, and you'll see
what I mean.

If you knew anything about real flying, you would know that you NEVER
use GPS as your altimeter under ANY circumstances for reasons entirely
unlrelated to GPS accuracy.

But how would you know as you have never been in a real airplane with
a real altimeter and a real GPS?


--
Jim Pennino

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