Questions on high altitude pressures

"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.