Thank you Mark, now it is all very clear to me, finally.

I wish to do a summary of what I have understood, very simplified.

What is Pressure Altitude for IGC standards
It is the altitude calculated with an ICAO-ISA formula . You need a baro
which has been calibrated at the factory, because the calibration is fixed
at the time the recorder leaves the manufacturer (just like mechanical
baro). The calibration is an important issue (exactly just like for the old
good mechanical barographs). Once calibrated, you read a pressure value.
You pass this value to a formula and get the ICAO-ISA altitude. In
practice, QNE and ICAO-ISA may differ by some tens of meters, once
calculated on the same value!
You can revert the formula: starting from an ICAO-ISA altitude you can get
the pressure.

What is Altitude for a Garmin and/or a GPS COTS
The altitude measured by a GPS could be the real altitude over ground, but
intrinsecally may contain geometric errors. Everyone agree on the fact that
the GPS Altitude is not accurate.
Some GPS like Garmin's use baro sensor to correct the GPS altitude, and
vice-versa, in order to achieve maximum precision and obtain possibly the
Real Altitude, above mean sea level. When we say "correct altitude" normally
we refer to this.

Why COTS' Altitude is not good for IGC badges
The answer has nothing to do with precision. IGC requires to read ICAO
Pressure-Altitude, not the real altitude.
It is exactly the same altitude you may read on a paper from an old
barograph.
There could be little difference among the two, but in principle we are not
talking about the same thing.
Of course a COTS could easily output an ICAO-ISA altitude, it's just a
matter of using the formula and unselect any other corrections. The
manufacturer could thus implement this feature, it is much easier than
correcting and auto-calibrating GPS altitude.
BUT, but, the manufacturer should also provide a calibrated sensor at the
factory.

In other words: if three devices are standing at the same height, they
should all read the same pressure value, say 747 mb.
The garmin with sensor may say you are at 4750m , another COTS basing only
on GPS may read 4680m, while the IGC may declare 4820m.

Conclusion: without a pressure sensor no COTS can be used today as an
alternative to IGC altitude loggers.
And in any case, calibration is an issue.



-- Mark did I get it right?
Paolo







"Marc Ramsey" ha scritto nel messaggio
et...
PCool wrote:
Thanks Marc,

is it correct to say that the Pressure Altitude is an altitude calculated
starting from a pressure value, following a sort-of a rule as for
ICAO-ISA ?
I guess official IGC loggers read the exact pressure as garmins and
suunto watches (!) and then they apply some calculations and name this
result as "altitude".

Yes

Do they do this without looking at what the GPS say?? Not even for an
hint?

Yes, the calibration is fixed at the time the recorder leaves the
manufacturer, and subsequent visits to the calibration lab simply provide
you with the data to manually correct the original calibration.

What is the formula used by all IGC loggers for doing this, then?

The details can be found here, in the section Standard Atmosphere and
Altimetry:

http://williams.best.vwh.net/avform.htm#Altimetry


It's beyond my comprehension why if we are talking about pressure which
is always measured in the same way (right?) then this value has different
meanings and cannot be simply converted like with QNE-QNH-QFE.
ICAO-ISA is sort of a more complicated QNE, right? (question!)

The ISA model assumes a standard lapse rate (0.0065°C/m) below the
tropopause (11.0 km), the real atmosphere is more complicated, which is
what causes the error. Altimeters are mechanical computers which do a
simple ISA to indicated altitude conversion, flight recorders do it in
software (but use the fixed 1013.2 hPa altimeter setting), in the end they
all have the same errors relative to actual height on days when the lapse
rate differs from the standard (which is every day). Calculating QNH
requires an inverted application of the ISA conversion, the weather guys
do it in the privacy of their offices.

On garmins you have a pressure sensor just like on a Colibrì, then this
sensor is used to compensate the gps and vice-versa, according to the
patent they have registered. By the way Marc could you understand
anything useful out of it?

Yes, they are describing in very mechanical terms (which is how you get a
patent on a software process) how to use GPS altitude to continuously
recalibrate the pressure sensor, such that you obtain actual height (but
not ISA/QNH indicated altitude) without the short term noise normally
present in GPS altitude. This is what was discussed earlier in the
thread.

After 4 years there are again the same questions on this matter so I
guess it's not very clear to everybody (me too).

I have to think about it myself every time it comes up, I can never
remember if altimeters read high or low on hot days. And, I still can't
keep my Q codes straight, which I no doubt demonstrated above...

Marc