Why so expensive (flight recorders)

Chip Bearden wrote:
On Feb 24, 7:07 pm, Marc Ramsey wrote:

The easiest way to see this is in an IGC file from an approved flight
recorder. During summer months, note the relative difference between
the pressure and GPS altitude at the lower and higher altitudes. As
altitude increases, GPS altitude will increase relative to pressure
altitude, as pressure altitude is reading too low at higher altitudes.
Here in the western US, we can see this clearly near mountain peaks, as
a correctly set altimeter will be reading as much as 1000 feet too low,
whereas GPS altitude matches the known elevations of the peaks.

OK, I'm a layman, late middle age, and little slow. What am I missing?
The pressure altitude (per the altimeter, at least) is less accurate
than the GPS altitude? By up to 1000 feet at Western USA soaring
altitudes? In the old days, we used a start gate that evaluated
optically how high we were above the ground. Assuming no one tripped
over the guy wires, that actual altitude stayed the same during a
contest. Now we're evaluated using a pressure-altitude-recording
device that may or may not reflect how high we really are?

Once again, pressure altitude and GPS altitude measure two different
things using the same units. Because we like to fly when there is a
non-standard temperature lapse rate, there is almost always noticeable
pressure altitude error above a few thousand feet AGL. If an optical
start gate is showing the actual heights, most gliders will appear to be
starting high, since the altimeters are reading low, and the validity of
the start is determined from the recorded pressure altitude. Those
pilots who are recording only GPS altitude (GPS handhelds, etc.) need to
be careful, as they start will be scored based on actual height, which
means they have to start lower. They need to be watching the GPS
display, rather then the altimeter, when they are flirting with the top
of the start cylinder.

Do the experts maintain that GPS altitude is bad because (a) it
DOESN'T have the errors inherent in pressure altitude or (b) because
its precision isn't good enough? It seems like I've seen both
positions on this forum.

As you know, experts generally maintain whatever favors their position.
Look at it this way, GPS altitude is more accurate at measuring actual
height, pressure altitude is more accurate at measuring, well, pressure
altitude. Since one function of the flight recorder is to detect and
penalize airspace incursions, pressure altitude will continue to be a
consideration, no matter what else happens. Some of the other air
sports, like ballooning, have already switched over to using actual
height, they use sounding data and software to convert to/from pressure
altitude as needed.

Marc

Chip Bearden wrote:
OK, I'm a layman, late middle age, and little slow. What am I missing?
The pressure altitude (per the altimeter, at least) is less accurate
than the GPS altitude? By up to 1000 feet at Western USA soaring
altitudes? In the old days, we used a start gate that evaluated
optically how high we were above the ground. Assuming no one tripped
over the guy wires, that actual altitude stayed the same during a
contest. Now we're evaluated using a pressure-altitude-recording
device that may or may not reflect how high we really are?

As an aside, while I flew a few contests using optical start gates, I
can't remember how one used to avoid bad starts. Did we dive through
with enough of a buffer beneath the stated maximum start altitude to
allow for the pressure altimetry error, or did we generally start lower
(so the wing numbers could be read through binoculars), thus keeping
error pretty small?

Marc

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

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

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!)

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?

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

thanks!
Paolo



"Marc Ramsey" ha scritto nel messaggio
et...

Once again, pressure altitude and GPS altitude measure two different
things using the same units. Because we like to fly when there is a
non-standard temperature lapse rate, there is almost always noticeable
pressure altitude error above a few thousand feet AGL. If an optical
start gate is showing the actual heights, most gliders will appear to be
starting high, since the altimeters are reading low, and the validity of
the start is determined from the recorded pressure altitude. Those pilots
who are recording only GPS altitude (GPS handhelds, etc.) need to be
careful, as they start will be scored based on actual height, which means
they have to start lower. They need to be watching the GPS display,
rather then the altimeter, when they are flirting with the top of the
start cylinder.

Do the experts maintain that GPS altitude is bad because (a) it
DOESN'T have the errors inherent in pressure altitude or (b) because
its precision isn't good enough? It seems like I've seen both
positions on this forum.

As you know, experts generally maintain whatever favors their position.
Look at it this way, GPS altitude is more accurate at measuring actual
height, pressure altitude is more accurate at measuring, well, pressure
altitude. Since one function of the flight recorder is to detect and
penalize airspace incursions, pressure altitude will continue to be a
consideration, no matter what else happens. Some of the other air sports,
like ballooning, have already switched over to using actual height, they
use sounding data and software to convert to/from pressure altitude as
needed.

Marc

On Feb 24, 11:45 pm, Marc Ramsey wrote:
Chip Bearden wrote:
OK, I'm a layman, late middle age, and little slow. What am I missing?
The pressure altitude (per the altimeter, at least) is less accurate
than the GPS altitude? By up to 1000 feet at Western USA soaring
altitudes? In the old days, we used a start gate that evaluated
optically how high we were above the ground. Assuming no one tripped
over the guy wires, that actual altitude stayed the same during a
contest. Now we're evaluated using a pressure-altitude-recording
device that may or may not reflect how high we really are?

As an aside, while I flew a few contests using optical start gates, I
can't remember how one used to avoid bad starts. Did we dive through
with enough of a buffer beneath the stated maximum start altitude to
allow for the pressure altimetry error, or did we generally start lower
(so the wing numbers could be read through binoculars), thus keeping
error pretty small?

Marc

Marc,

It was like Tennis. You went for two serves, the first was redline
and right at altitude. If Charlie said good start you had scored an
ace and were on your way. If you got a fault (bad start) you went
back and added a hundred or two hundred feet as a safety margin.

Ah, the fun of multiple ships diving at a gate at redline at the same
time and aggressive prestart gaggles to get that extra 1000 feet so
you could dive.

Thanks BB for the new rules!

Tim

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

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

On Feb 25, 12:53*pm, "PCool" wrote:
Everyone agree on the fact that
the GPS Altitude is not accurate.

Actually, what I took away from this discussion is that the GPS
altitude is MORE accurate with regard to actual height above the
ground but not necessarily equal to pressure altitude in the real
world. So what I hear now is the Certified Flight Recorder crowd
saying we shouldn't use GPS altitude even if it's more accurate, for
the reason that it's not comparable with the way we've always
evaluated badge and record claims in the past. This seems precisely
the opposite argument used to justify GPS flight recorders in the
first place: i.e., that their 2D positional accuracy was better so we
simply *must* use it.

I agree airspace incursions are a different issue. But in the old
days, we could only measure incursions on the Y axis (i.e. altitude)
anyway. Would it be so bad if now we could only measure them on the X
and Z axes (i.e., lat/long)? At most contests where I've flown
recently, including US Nationals, that's been the case: i.e., we're
not allowed to fly over or under most airspace that is restricted to
gliders. Worst case, users of COTS receivers might have to self impose
that condition or leave, say, a 1,000 ft. buffer

I'm not trying to make trouble but I'm genuinely baffled as to what is
the problem. If GPS altitude is more accurate and COTS receivers are
no more vulnerable to a determined hacker than, say, my Cambridge
Model 20 (which I've had open several times), then why not allow them?
Saying that the casual pilot can easily borrow an expensive Certified
Flight Recorder from a more serious, more affluent club member on
occasion is sort of like prohibiting the sale of affordable cars to
the average citizen on the rationale that he/she can borrow an Audi or
Lexus or Mercedes from a generous neighbor anytime they need to drive
somewhere.

Chip Bearden
ASW 24 "JB"
USA

On Feb 23, 6:28 am, Ian Strachan wrote:
And what is so complicated in adding GPS altitiude as an IGC
recognized measurement ? Especially since GPS lat,lon is a
recognized. I am serious, please tell me.

Todd Smith
3S- Hide quoted text -


.... snip usefull explanation of accuracy ...

There is more, but the above summarises the reasons why IGC has, so
far, not added GPS altitude to the Sporting Code where accurate
measurements are required.

Ian Strachan
Lasham Gliding Centre, UK
Chairman IGC GNSS Flight Recorder Approval Committee (GFAC)

Ian, thanks for that explanation.

My desire, even after hearing your clear explanation, to be able to
use GPS altitude comes from a willingness to allow "inaccurate"
measurements. Or more specifically to question, what level of
accuracy is truly needed for a Silver height gain claim ?

I have to read the report you referenced and learn what kind of
errors, but I would propose that even if the expected accuracy was as
large as 100m-200m, I would be comfortable allowing the Silver badge
to be claimed in the GPS altitude gain was greater that 1000m +
"expected max error".

It's my willingness to accept reduced accuracy for reduced cost that
would have me allow GPS altitude.

Again, this high tolerance for error would be for the lower level of
badges, not records.


A question for the group, what level of error (for badges) are you
comfortable with ?

Thanks again
Todd Smith

Chip Bearden wrote:
On Feb 25, 12:53 pm, "PCool" wrote:
Everyone agree on the fact that
the GPS Altitude is not accurate.

Actually, what I took away from this discussion is that the GPS
altitude is MORE accurate with regard to actual height above the
ground but not necessarily equal to pressure altitude in the real
world. So what I hear now is the Certified Flight Recorder crowd
saying we shouldn't use GPS altitude even if it's more accurate, for
the reason that it's not comparable with the way we've always
evaluated badge and record claims in the past. This seems precisely
the opposite argument used to justify GPS flight recorders in the
first place: i.e., that their 2D positional accuracy was better so we
simply *must* use it.

There are certainly some in this Certified Flight Recorder Crowd (like
myself, I guess) who think that GPS altitude, properly recorded and
evaluated, should be quite adequate for demonstrating that one has met
the requirements for at least a Gold badge.

I agree airspace incursions are a different issue. But in the old
days, we could only measure incursions on the Y axis (i.e. altitude)
anyway. Would it be so bad if now we could only measure them on the X
and Z axes (i.e., lat/long)? At most contests where I've flown
recently, including US Nationals, that's been the case: i.e., we're
not allowed to fly over or under most airspace that is restricted to
gliders. Worst case, users of COTS receivers might have to self impose
that condition or leave, say, a 1,000 ft. buffer

You're looking at this from a US perspective, in Europe there are lots
of places where if you don't fly under or over proscribed airspace, you
won't be going very far. Plus, I believe the floor of Class A is
somewhat lower over there.

I'm not trying to make trouble but I'm genuinely baffled as to what is
the problem. If GPS altitude is more accurate and COTS receivers are
no more vulnerable to a determined hacker than, say, my Cambridge
Model 20 (which I've had open several times), then why not allow them?
Saying that the casual pilot can easily borrow an expensive Certified
Flight Recorder from a more serious, more affluent club member on
occasion is sort of like prohibiting the sale of affordable cars to
the average citizen on the rationale that he/she can borrow an Audi or
Lexus or Mercedes from a generous neighbor anytime they need to drive
somewhere.

That is why it is so necessary to work with the IGC delegates. They are
the only ones who can change the rules...

Marc

PCool wrote:
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.

You sort of lost me here 8^)

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.

Actually, GPS altitude is quite accurate at measuring actual height (to
within +/- 10 meters or so) *most* of the time. That last part is
important. Depending on lots of things, like bad satellite positions, a
wing or a rock blocking view of a critical satellite, the phase of the
moon, etc., GPS altitude can occasionally be hundreds of meters off. If
you take a longer term average, accuracy will normally be to within less
than a meter.

By contrast, pressure sensors are quite accurate and reliable at
measuring ISA pressure height (to within +/- a few meters or better,
below the tropopause), but can't accurately measure actual height. They
do not suffer from short term fluctuations and occasional wild
excursions like GPS altitude does (with the exception that some flight
recorder sensors will show large errors at low battery voltages).

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.

Yes, to be more precise, when a Garmin is in auto-calibrate mode, the
pressure sensor is used to compensate for short term fluctuations in GPS
altitude, so you get the long term accuracy of GPS altitude, with the
short term stability and resolution of a pressure sensor.

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.

Correct, though some also argue that the short term accuracy of GPS
altitude is not good enough to allow verification of height gains and
loss of height. I believe there are ways to work around this, others don't.

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.

This is true, but the pressure sensors need to have rather good
temperature compensation and long term stability, which may not be the
case with the sensors in consumer grade GPS receivers.

BUT, but, the manufacturer should also provide a calibrated sensor at the
factory.

They could, but remember, the market for glider pilots is insignificant
in comparison to the number of these units sold. It may simply not be
worth the added expense to the manufacturer.

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.

No quite, if the Garmin is auto-calibrating, it is reading actual
altitude (actually height above an ellipsoidal Earth model, but that is
another issue), just like the GPS-only unit (without the fluctuations).
So, the Garmin with sensor might read 5250m, GPS only might be 5230,
and IGC might read 4820m. They are all more or less correct, the IGC
unit is measuring something different. If the Garmin pressure sensor is
set to a proper fixed calibration, it will read the same as the IGC unit
(assuming adequate temperature compensation).

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.

At the moment, that is correct. The rest is up to the IGC.

Marc