WAAS question -- altitude accuracy?

Jay Honeck wrote in message ...
How accurate is GPS for determining altitude, now that WAAS is active?

I was in the air over five hours yesterday, and while droning along found
that our new AvMap's altitude readout was just about dead-nuts accurate --
it was maybe 50 feet off, at times.

In fact, due to the constantly changing barometric pressure along our route
of flight, there were times when I might have trusted it before my
altimeter.

Anyone know how precise it is now?
--

Jay, The FAA seems to say 3 meters vertical throughout the majority of the
continental U.S. and portions of Alaska:

http://gps.faa.gov/Programs/WAAS/waas.htm


But the site below says 6 meters (95 percent of the time) with a worst case
of 15.7 meters.

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



Here are some more interesting sites:

http://waas.stanford.edu/old_metrics.html

http://waas.stanford.edu/

http://www.edu-observatory.org/gps/dgps.html#WAAS

Jay Honeck wrote in message ...
How accurate is GPS for determining altitude, now that WAAS is active?

I was in the air over five hours yesterday, and while droning along found
that our new AvMap's altitude readout was just about dead-nuts
accurate --
it was maybe 50 feet off, at times.

In fact, due to the constantly changing barometric pressure along our
route
of flight, there were times when I might have trusted it before my
altimeter.

Anyone know how precise it is now?
--

Jay, The FAA seems to say 3 meters vertical throughout the majority of the
continental U.S. and portions of Alaska:

http://gps.faa.gov/Programs/WAAS/waas.htm


But the site below says 6 meters (95 percent of the time) with a worst
case
of 15.7 meters.

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

Beat me to replying with my link. Note the FAA site does not say if it is
quoting 95%, 50% or what. As my measurements are now a little old, I will
be making new ones soon. But today I will be cleaning up after not so nice
a lady (Isabelle).

I was on top of Lugnaqilla mountain in Ireland today and the GPS (Meridiam
Plat) put it at 925 - 924 metres. The topo puts it at 925. Just lucky
maybe...

"Craig Davidson" wrote in message
.net...

Jay Honeck wrote in message ...
How accurate is GPS for determining altitude, now that WAAS is active?

I was in the air over five hours yesterday, and while droning along found
that our new AvMap's altitude readout was just about dead-nuts
accurate --
it was maybe 50 feet off, at times.

In fact, due to the constantly changing barometric pressure along our
route
of flight, there were times when I might have trusted it before my
altimeter.

Anyone know how precise it is now?
--

Jay, The FAA seems to say 3 meters vertical throughout the majority of the
continental U.S. and portions of Alaska:

http://gps.faa.gov/Programs/WAAS/waas.htm


But the site below says 6 meters (95 percent of the time) with a worst
case
of 15.7 meters.

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



Here are some more interesting sites:

http://waas.stanford.edu/old_metrics.html

http://waas.stanford.edu/

http://www.edu-observatory.org/gps/dgps.html#WAAS

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if particular
GPS receivers perform this adjustment.

(I would hope that an aeronautical GPS receiver would perform this
adjustment!) As for my decidedly non-aeronautical-non-WAAS-enabled
Magellan 410 GPS, it rarely correlates very closely to known altitude
benchmarks when I try it, even with many satellites 'locked.' God help
anyone who attempts a precision approach with my GPS!

Steve


bob mologna wrote:
I was on top of Lugnaqilla mountain in Ireland today and the GPS (Meridiam
Plat) put it at 925 - 924 metres. The topo puts it at 925. Just lucky
maybe...

"Craig Davidson" wrote in message
.net...

Jay Honeck wrote in message ...

How accurate is GPS for determining altitude, now that WAAS is active?

I was in the air over five hours yesterday, and while droning along found
that our new AvMap's altitude readout was just about dead-nuts

accurate --

it was maybe 50 feet off, at times.

In fact, due to the constantly changing barometric pressure along our

route

of flight, there were times when I might have trusted it before my
altimeter.

Anyone know how precise it is now?
--

Jay, The FAA seems to say 3 meters vertical throughout the majority of the
continental U.S. and portions of Alaska:

http://gps.faa.gov/Programs/WAAS/waas.htm


But the site below says 6 meters (95 percent of the time) with a worst

case

of 15.7 meters.

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



Here are some more interesting sites:

http://waas.stanford.edu/old_metrics.html

http://waas.stanford.edu/

http://www.edu-observatory.org/gps/dgps.html#WAAS

steve writes:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if
particular GPS receivers perform this adjustment.

I think that most of them use WGS84, which is a little more complex
than a simple ellipsoid.


All the best,


David

"David Megginson" wrote in message
...
steve writes:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if
particular GPS receivers perform this adjustment.

I think that most of them use WGS84, which is a little more complex
than a simple ellipsoid.

Some models (Garmin's included) display orthometric (above the geoide or
roughly above mean sea level) by using a look up table for the geode height,
and some other models display the ellipsoid altitude.

"David L. Wilson" wrote in message
t...
"David Megginson" wrote in message
...
steve writes:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if
particular GPS receivers perform this adjustment.

I think that most of them use WGS84, which is a little more complex
than a simple ellipsoid.

Some models (Garmin's included) display orthometric (above the geoide or
roughly above mean sea level) by using a look up table for the geode
height,
and some other models display the ellipsoid altitude.

And it should also be injected into this thread that Delta won't be doing
blind approaches with SP IIIs. As has been pointed out in many, many
threads, there's GPSRs, and there's GPSRs, and that second set has 2 or
three more zeros on the end.

Frank

steve wrote:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if particular
GPS receivers perform this adjustment.

Most do but some do not. The computation required to derive true
altitude is not complex but requires a table of data that can be very
large. The math is simply to interprolate between table locations and
provide a simple addition or subtraction to the ellipsoid altitude. You
can determine if your receiver does this by looking at the NMEA data.
Check my web site for an article on how to decode this information. You
can compare the computed value with the accurate data that is avaialbe
on government web sites to see how good the table is.

Dale

--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

steve wrote:

Doesn't the GPS give you 'altitude' -- computed as height above the
surface of an ellipsoid? Or does it correct for mean sea level? There
seem to be several computations required to derive true altitude from
the raw GPS altitude coordinate and it is not clear to me if particular
GPS receivers perform this adjustment.


See: http://edu-observatory.org/gps/height.html