Re: WAAS question -- altitude accuracy?

[Craig Davidson]

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

[David L. Wilson]

> 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).

[bob mologna]

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]

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
>>
>>
>>
>>
>>
>>
>
>
>

[David Megginson]

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 L. Wilson]

"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.

[Frank Looper]

"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

[Mike Rapoport]

"Frank Looper" > wrote in message
...
>
> >
> 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
>

The accuracy is the same though.

Mike
MU-2

[Dale DePriest]

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

[Frank Looper]

"Mike Rapoport" > wrote in message
k.net...
>
> "Frank Looper" > wrote in message
> ...
> >
> > >
> > 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
> >
>
> The accuracy is the same though.
>
> Mike
> MU-2
>
There are a number of people who would disagree with you on that, and most
likely not just folks who sell those five figures units, but their customers
and others as well.

Frank

[Sam Wormley]

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