Questions on high altitude pressures

Mxsmanic wrote in
:

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.



You#re aan idiot.


Bertie

"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

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.

Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

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.

--
I'd like to jump right on the floor


If GPS altitude is unreliable how do you shot a GPS approach or is this why
WAAS was implemented?

"Darkwing" theducksmail"AT"yahoo.com wrote in message
news

"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

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.

Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

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.

--
I'd like to jump right on the floor


If GPS altitude is unreliable how do you shot a GPS approach or is this
why WAAS was implemented?

Exactly. WAAS corrects GPS errors and makes the receiver accurate enough to
perform the equivalent of a Cat I ILS approach. If LAAS is ever
implemented, it will allow the GPS equivalent of Cat III approaches.

Darkwing theducksmail"AT"yahoo.com wrote:

"Wolfgang Schwanke" wrote in message
...
es330td wrote in
:

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.

Actually there are two reasons not to use GPS as altimeter.

The first is the one you stated: When flying in an airspace where
Flight Levels are used, everyone is suppoed to use an agreed-upon
altimeter setting. The result is that everyone flies along planes of
equal air pressure, the purpose being to ensure separation. The
aircraft's real distance from the ground or sea level is unknown,
irrelevant and can actually fluctuate with weather. But since all
aircraft measure the same "error" that is OK. If some of the airctrafts
would use a different measurement method (e.g. GPS or QNH altimeter
setting) that would defeat the whole system.

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.

--
I'd like to jump right on the floor


If GPS altitude is unreliable how do you shot a GPS approach or is this why
WAAS was implemented?

GPS specified altitude accuracy with Selective Availility (SA) turned
off is +/- 150 m.

WASS specified altitude accuracy is +/- 7.6 m.

Typical actuals are usually +/- 4.7 m and 1.3 m respectively, but not
guaranteed at any particular place and time.

The goal of LAAS is to provide a guaranteed accuracy of less than 1 m.


--
Jim Pennino

Remove .spam.sux to reply.

"Darkwing" theducksmail"AT"yahoo.com wrote in message
news

"Wolfgang Schwanke" wrote in message
...
es330td wrote in


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.

--
I'd like to jump right on the floor


If GPS altitude is unreliable how do you shot a GPS approach or is this
why WAAS was implemented?

Standard GPS approaches a flown with the altitudes from the altimeter. WAAS
approaches rely on GPS position correction from a local transmitter.


--

*H. Allen Smith*
WACO - We are all here, because we are not all there.

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)

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

Remove .spam.sux to reply.

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