Re: gps altitude accuracy

[Martin Gregorie]

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 :

[Bill Daniels]

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

[Mike Borgelt]

On Fri, 11 Jul 2003 01:18:39 GMT, "Peter Kovari"
> wrote:

>.I merle find that maybe these tolerances I experienced were
>excessive, apparently some of you have experienced the same.
>Over and out. Peter K

They weren't. They are about what you would expect and the difference
between Pressure altitude and GPS altitude readings is completely
explained by the physics of what you are measuring in each case.

GPS altitude is accurate to 50 feet most of the time. Allow +/- 100
feet and you have it nearly all the time.

Pressure altimeters are of similar order of accuracy but measure
PRESSURE ALTITUDE which is a different physical entity from GPS
altitude and the numbers you get may or may not be the same as GPS
altitude.

Mike Borgelt

[Kirk Stant]

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

[Mark James Boyd]

It strikes me that with GPS altitude and weather soundings and
measurements, with just the GPS WAAS altitude it should
be possible to calculate pressure altitude.

If so, and this were reliable, there would be no need for
a power-hungry mode C transponder, one could use a GPS based
transponder and the ATC computer could simply spit out
altitude. This isn't so farfetched since that same computer
already compensates for pressure differences.

On the other hand, with a reliable automated way to get
these soundings to the transponder, the transponder
could make these calculations BEFORE spitting out the
calculated pressure altitude.

I wonder how close the altitudes calculated this way would
match the altimeter? I think this would be a matter of how
good the soundings are at helping this process.

[Robert Ehrlich]

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.

[Kirk Stant]

No, what he said was he would have more headroom "under the Class A
airspace." Since the Class A would start when his altimeter reads
18'000ft (when set to 2992) regardless of what his GPS says, he would
not have any more room. He could very well be higher than 18,000ft
above MSL, which is a different thing altogether, and is really what
we care about for final glides, etc.

It's been a long time since I've done any IFR aviating, guess I need
to get back into the AIM again!

Kirk

"Marc Ramsey" > wrote in message news:<LcEPa.231> What he is saying that when the pressure altitude is reading lower than the GPS
> altitude (which it does on typical summer soaring days), you get some extra
> headroom on things like final glide, as you are that much higher above the
> terrain. I've seen quite a few days when the altimeter, set to a nearby
> reporting station, reads just under 18,000 feet, while the GPS altitude is
> closer to 19,000 feet.
>
> BTW, the floor of Class A airspace is 18,000 feet MSL, not FL180...
>
> Marc

[Marc Ramsey]

"Kirk Stant" > wrote...
> No, what he said was he would have more headroom "under the Class A
> airspace." Since the Class A would start when his altimeter reads
> 18'000ft (when set to 2992) regardless of what his GPS says, he would
> not have any more room. He could very well be higher than 18,000ft
> above MSL, which is a different thing altogether, and is really what
> we care about for final glides, etc.
>
> It's been a long time since I've done any IFR aviating, guess I need
> to get back into the AIM again!

Yes, since the AIM is quite clear that Class A starts when the altimeter reads
18,000 feet MSL, set to the nearest reporting station, *not* set to 29.92 (which
would be FL180). ATC does not assign FLs to aircraft in Class A which would be
low enough to conflict with aircraft flying at 18,000 feet MSL, given the
ambient pressure.

The "headroom" argument is based on the fact that 18,000 feet MSL pressure
altitude (again. set to the nearest reporting station) is almost always more
than 18,000 geometric feet above mean sea level on hot summer days, due to the
fact that the ambient pressure gradient does not match the International
Standard Atmosphere, to which altimeters are calibrated. I often start my final
glides into Truckee with the altimeter (set to local pressure) reading 17,900
feet, while the GPS (more closely approximating actual elevation above mean sea
level) is reading in the range of 19,000 feet. Since my final glide range is
determined by my actual elevation, rather than by what the altimeter happens to
be reading, that extra 1000 feet or so is often what makes it possible to get
home. That meets the definition of "more headroom below Class A" as far as I'm
concerned...

Marc

[Bill Daniels]

"Kirk Stant" > wrote in message
om...
> OK, I'll buy that. My interpretation of the original post was that
> since the GPS altitude showed him lower than the altimeter, he could
> go up higher before entering Class A airspace (18000ft - I stand
> corrected). That interpretation would be incorrect, for all the
> reasons we both have stated.
>
I didn't write that I saw GPS altitude less than the altimeter. I wrote
that it was higher.

GPS altitude is NEVER less than the barometric altimeter set to a local
altimeter setting - at least in my experience.

Bill Daniels

[Kirk Stant]

You are absolutely correct, I went back and reread the original post
and realize my mistake. So we are all in agreement after all.

Bottom line - Altimeters for airspace, GPS for glides!

Kirk
66


"Bill Daniels" > wrote in message news

> I didn't write that I saw GPS altitude less than the altimeter. I wrote
> that it was higher.
>
> GPS altitude is NEVER less than the barometric altimeter set to a local
> altimeter setting - at least in my experience.
>
> Bill Daniels

[Robert Ehrlich]

Mark James Boyd wrote:
>
> It strikes me that with GPS altitude and weather soundings and
> measurements, with just the GPS WAAS altitude it should
> be possible to calculate pressure altitude.
>

When restricted to the conditions of usual soaring, no weather sounding
is necessary, we need only a reference point with known pressure and true
altitude. As we soar in constantly rising or sinking air and out of
clouds, i.e. no saturation, we can easily predict the result of the sounding
maybe with a constant offset that the reference point would drop : the
vertical temperature gradient in this conditions is the dry adiabatic lapse
(1°C per 100m) and everything can be computed from this.

[Mark James Boyd]

>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...

[Stefan]

Mark James Boyd wrote:
>
> If so, and this were reliable, there would be no need for
> a power-hungry mode C transponder, one could use a GPS based
> transponder and the ATC computer could simply spit out
> altitude. This isn't so farfetched since that same computer
> already compensates for pressure differences.

etc.

You're completely missing the point. We're talking gliders here, and
glider flights usually are uncontrolled. A pressure alitmeter works
without controller and without electricity, is fairly robust and
provides the pilot with everything he needs for airspace issues. If the
pilot is too dumb to compensate for wheather changes and flies into
terrain because he believes his altimeter more than the outside view, so
be it.

Stefan