gps altitude accuracy

On Thu, 10 Jul 2003 08:52:52 +1000, Mike Borgelt
wrote:

On Wed, 09 Jul 2003 11:00:12 +0100, Martin Gregorie
wrote:

On Wed, 09 Jul 2003 10:27:00 +1000, Mike Borgelt
wrote:

...snippage...

For badges and records ballooning and I think the rest of aviation
converts pressure readings from barographs etc to geometric altitude.
It is long past time we did this in gliding. The Ballooning people
have a nice worksheet to do this. FR's would get considerably cheaper
if the pressure altitude requirement was dropped.

Mike, do you mean that non-gliding FRs and barographs also record
ambient air temperature?

If that's not the case then surely they can only do what we do and use
the calibration chart to correct the FR altitude to the standard
pressure altitude without temperature corrections.

- Curious of Essex


No, the worksheet asks for the mean temperatures in the layer in
question by interpolation from met office temperature soundings at two
or three nearby stations. Also QNH values at the stations at the time
in question.
The whole process is done properly with error bands etc and you get
credited with the minimum after the errors are accounted for. All very
proper and obviously designed by someone who knew what he or she was
doing unlike anything official I've seen in soaring.

Thanks for your explanation.

Pressure altitude in soaring barographs and FR's is a joke. The
calibration chart was done in the lab at room temperature and we
expect all this to be the same at -40 degrees at 30.000 feet.
Lotsaluck!

Thanks for that, too. I had wondered if that might be the case.

I have to ask, though, does that matter? Unlike the situation in the
real atmospheric column the temperature in the chamber can't affect
the pressure unless there's a temperature dependency in either the
chamber's pressure measurement or (more likely) in the FR's pressure
sensor.

I'd appreciate your thoughts on this too.

GPS altitude will give this directly, the only matter for discussion
is what error band we put on it. I'd suggest add 100 feet to the low
point and subtract 100 feet from the high point. This is probably
conservative in the direction of crediting you with smaller altitude
gains.

I've no argument with that!

BTW, there's been a lot of discussion of the effect of EPE error on
height measurements, but are there any systematic GPS errors that
don't show up in the EPE figure? What about satellite clock drift and
ephemeris errors? I've been looking at

http://vancouver-webpages.com/peter/

but it doesn't appear to answer this question although it does give
all the error sources and their magnitude. Judging by the EPE figure I
usually see, my GPS II+ may only be calculating the EPE from
ionospheric and P-code error estimates.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

I have seen GPS altitude at 19,200 when the panel altimeter set to local
pressure said 17,990. I figure that gives me about 1200 more feet of
"headroom" to play in below Class A airspace in the USA. In the high
mountain country of the western US, GPS altitude gives much better final
glide calculations than pressure altitude.

Hopefully, the feds won't take away this extra useable attitude by switching
to GPS altitude for ATC.

Bill Daniels

"Mike Borgelt" wrote in message
...
On Tue, 08 Jul 2003 02:45:18 GMT, "Peter Kovari"
wrote:

Last season I find some great discrepancy between my GPS altitude and
indicated altitude by my standard mechanical altimeter. The differences
were
minimal at ground level, 2-300ft at 10,000' and 7-800ft at 17,000msl.
I had the altimeter checked by a certified repair station, who certified
it
within acceptable tolerances, and it is still off. The question I have
therefore, how accurate is the GPS altitude?
Peter K.



Peter, The pressure altimeter measures the difference between some
reference(the setting in the subscale window) and the ambient pressure
where you are. If you take that layer of atmosphere and heat it the
two pressure levels move apart, hence for the same altimeter reading
you are actually higher above the reference level. The GPS altitude
and pressure altimeter will read the same within instrument and GPS
system errors in an ISA standard atmosphere. In soaring we mostly fly
in thermals in warmer than standard atmospheres hence the GPS will
show a higher than pressure altimeter number.

On a really hot day at 10,000 feet you could get an error of 800 feet
GPS vs pressure altimeter. i.e.pressure alt 10,000 GPS 10,800 feet.

As to why flight computers don't use GPS altitude - the B2000 does.
I was about to build the pressure altitude module for it when SA got
turned off and the GPS altitude accuracy got to be at least as good
and mostly much better than pressure altitude for glider performance
purposes.( there are pads for a socket for that module on the main
circuit board)
Using a Garmin 35 GPS source set for 3D nav only with no averaging
and no dead reckoning I get the very isolated single reading GPS
altitude glitch on examination of the flight record. Never noticed in
flight. A simple software patch could take these out as they are
always totally weird and nothing like the readings either side in
time.

Having calibrated quite a few IGC approved FR's of various makes the
pressure sensor accuracy in them all can be unimpressive and I
wouldn't use it for final glides.

For badges and records ballooning and I think the rest of aviation
converts pressure readings from barographs etc to geometric altitude.
It is long past time we did this in gliding. The Ballooning people
have a nice worksheet to do this. FR's would get considerably cheaper
if the pressure altitude requirement was dropped.

Mike Borgelt

Borgelt Instruments

Bill,

GPS altitude gives you absolutely NO more headroom below Class A
airspace, since FL180 is a PRESSURE altitude (referenced to 29.92),
not an absolute altitude above sea level (which is approximately what
GPS altitude indicates). At 17,990 ft you have 9 feet of headroom,
you still can't go above 17,999' without an IFR clearance or wave
window) regardless of what the GPS is telling you.

GPS is probably more accurate for final glides.

There is practically no chance that ATC will switch to GPS altitude
for airspace control, since it would require ALL aircraft to have WAAS
GPS with RAIM and all that kind of fancy "stuff". Whereas a simple
pressure altimeter, good for +-75'when set to the local altimeter
setting, works fine for traffic separation - and doesn't require an
electrical system.

Kirk
66

"Bill Daniels" wrote in message ...
I have seen GPS altitude at 19,200 when the panel altimeter set to local
pressure said 17,990. I figure that gives me about 1200 more feet of
"headroom" to play in below Class A airspace in the USA. In the high
mountain country of the western US, GPS altitude gives much better final
glide calculations than pressure altitude.

Hopefully, the feds won't take away this extra useable attitude by switching
to GPS altitude for ATC.

Bill Daniels

Kirk Stant wrote:

Bill,

GPS altitude gives you absolutely NO more headroom below Class A
airspace, since FL180 is a PRESSURE altitude (referenced to 29.92),
not an absolute altitude above sea level (which is approximately what
GPS altitude indicates). At 17,990 ft you have 9 feet of headroom,
you still can't go above 17,999' without an IFR clearance or wave
window) regardless of what the GPS is telling you.


I disagree. If the flight level at which class A begins has a true
higher altitude that in standart atmosphere, you have mode room. Here
in the vicinity of Paris we are very concerned with this, since in
some places class A begins at FL045. As usually soaring is done in
good weather associated with high pressures (higher than in the standart
atmosphere), FL045 is usually significantly higher than 4500ft AMSL.

All this talk about computing pressure altitude from GPS reading is
irrelevant to the task of determining flight level or altitude (defined as
height above MSL).

It seems pretty relevant if I can save money and weight and
not have to calibrate an altimeter/transponder encoder. If I can get a
certified GPS with a transponder that is able to simulate exactly what an
altimeter would display, I have no need for the weight or
expense or certification for the altimeter, I wouldn't ever
need to update a Kohlsman window, I wouldn't have the power
consumption of a traditional transponder, yet I could calculate
final glides very well and have excellent and accurate information
about height above terrain even in places where the weather
forecasters had no pressure information.

The pressure altimeter is a fine device, but it's only advantage
(since June 10) in the United States over GPS is that it requires no
electricity. I have no doubt that if WAAS capable GPS had
preceded the invention of the pressure sensitive altimeter,
that WAAS GPS would be the altitude standard in the US for
ATC. WAAS GPS is cheaper, insensitive to temperature and
pressure gradients, passive (no reseting Kohlsman windows),
gives accurate altitude with respect to the ground, and
uses less power than a traditional transponder (since the air
doesn't need to be heated to 55 degrees C).

I think the only thing missing to make this system work
with the old standard is a pressure data signal,
perhaps added to the current GPS signals. This would allow
the GPS to simulate the altimeter, yet also provide final
glide and terrain separation information.

Does the current system do a great job of separating traffic?
As pointed out it is fine. Does it do a great job of
avoiding terrain? Ask the families of those killed when
airliners crash because the Kohlsman windows were set incorrectly.

UPSAT is banking on 250 ft 3/4 mile vis precision WAAS GPS
approaches being published over the next two years. It isn't
such a stretch to imagine U.S. ATC using GPS altitude for
IFR traffic separation at some point. The altimeter and encoder
may go the way of the 90 and 720 channel radio in the next
10 years...