Why so expensive (flight recorders)

This http://www.gpsinformation.net/main/altitude.htm guy (7 years ago) says
that accuracy was +-40 meters, but with DGPS should improve to +-10m (what
we should have now.).

This http://www.scottkurowski.com/flying.htm#accuracy made accurate
calculation on precisions, coming to +-7 feet precision in 2006 (?) with a
garmin 296.

These http://docs.controlvision.com/pages/gps_altimetry.php people say in
2004 that there is a bug in most of cheap gps chips that make altitude
calculation wrong by +-200 feet, and why it happens.

Most trekking gps have a baro sensor to compensate and calibrate gps
altitude nowadays (garmin for all S serie).





"toad" ha scritto nel messaggio
...
On Feb 21, 6:25 pm, Marc Ramsey wrote:
Ian wrote:
On Thu, 21 Feb 2008 18:42:11 +0000, Marc Ramsey wrote:

At the next IGC Plenary Meeting (29 February through 1 March), there
will be a specific proposal from the IGC Sporting Code Committee to
change SC3 to allow use of COTS GPS in conjunction with barographs for
Silver and Gold badges. May I, once again, suggest that you contact
your IGC delegate to make your viewpoint known?

Why bother with the barographs?

It's rather simple, GPS altitude is not currently an IGC recognized
means of measuring altitude in soaring performances, and changing that
is far more complex than adding COTS GPS as an alternate means of
documenting position evidence.

Marc

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

toad wrote:
On Feb 21, 6:25 pm, Marc Ramsey wrote:
It's rather simple, GPS altitude is not currently an IGC recognized
means of measuring altitude in soaring performances, and changing that
is far more complex than adding COTS GPS as an alternate means of
documenting position evidence.

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.


GPS altitude and pressure altitude measure two distinct concepts which
happen to use the same units. Conversion between the two is an
approximation requiring both local sounding data and agreement on use of
a specific set of formulas. The IGC has been using pressure altitude
almost exclusively for quite some time, and will continue to do so for a
variety of reasons. These ease of comparability with past flight
performances and records, the need to detect incursions into airspace
defined by pressure altitude limits, collection of redundant altitude
data for security and reliability, and general bureaucratic inertia.
While there is provision in the Sporting Code for optical and radar
altitude measurements (which are more closely related to GPS altitude),
other requirements pretty much eliminate the possibility of actual use,
and if someone managed to find a legitimate way, the conversion would
have to be handled on a case specific basis. I'm aware of two efforts
in recent years to amend the Sporting Code to allow for use of geometric
altitude (GPS, radar, optical) in certain circumstances, neither
proposal got very far.

Latitude and longitude do not suffer from these problems. The only real
issue is the datum used, IGC approved flight recorders always use a
specific datum, and conversion to/from other datums used to locate photo
turnpoints, ground observers, etc., can be done using well understood
transformations often backed by international standards.

Tying the COTS GPS rules to the use of GPS altitude would almost
certainly result in failure to pass the proposal. Requiring use of a
pressure altitude recording device (i.e. barograph) increases the
probability that a majority of the IGC delegates will vote in favor of
the proposal. That's politics for you...

Marc

On Fri, 22 Feb 2008 09:39:06 -0800, Marc Ramsey wrote:

GPS altitude and pressure altitude measure two distinct concepts which
happen to use the same units. Conversion between the two is an
approximation requiring both local sounding data and agreement on use of
a specific set of formulas. The IGC has been using pressure altitude
almost exclusively for quite some time, and will continue to do so for a
variety of reasons. These ease of comparability with past flight
performances and records, the need to detect incursions into airspace
defined by pressure altitude limits, collection of redundant altitude
data for security and reliability, and general bureaucratic inertia.
While there is provision in the Sporting Code for optical and radar
altitude measurements (which are more closely related to GPS altitude),
other requirements pretty much eliminate the possibility of actual use,
and if someone managed to find a legitimate way, the conversion would
have to be handled on a case specific basis.

None of this has provides any good reason why a badge for silver or gold
height gain should not be awarded on the basis of height gain measured by
means of GPS altitude instead of barometric pressure altitude.

Both GPS and pressure altitude are subject to errors and pressure
altitude is not necessarily the more accurate. Two performances by
different pilots on different days in different weather, both providing
equal height gain measured by pressure altitude does not imply that the
two gliders actually achieved the same height gain if could be measured
with a tape measure. Just the same if one performance had been measured
with GPS altitude. But both performances would be an excellent
demonstration of pilot skills and worthy of recognition with the
appropriate badge!

This may not be 100% "fair". But a distance performance navigated with a
compass and map and measured with turn point photographs is a more
difficult achievement than the same task navigated with GPS and measured
with a Flight Recorder. You have to fly further to get a decent photo and
the cockpit workload is significantly higher. Yet we award the same badge
for both performances - thus some "unfairness" already exists in the code.

Tying the COTS GPS rules to the use of GPS altitude would almost
certainly result in failure to pass the proposal. Requiring use of a
pressure altitude recording device (i.e. barograph) increases the
probability that a majority of the IGC delegates will vote in favor of
the proposal. That's politics for you...

This political argument sounds far more plausible than any technical one.
I would like to see a list of IGC delegates (identified by country) who
are apposed to the use of GPS altitude to measure height gain. Then we
could explain to them that their attitude is quietly killing our sport...

To put it another way, if we accept COTS GPS together with COTS altitude,
then overnight there would be 10000 Flarm equipped gliders which would
each have their own dedicated Flight Recorder at no additional cost at
all... Now that would be a powerful boost for our sport.


Ian

Ian wrote:
On Fri, 22 Feb 2008 09:39:06 -0800, Marc Ramsey wrote:

GPS altitude and pressure altitude measure two distinct concepts which
happen to use the same units. Conversion between the two is an
approximation requiring both local sounding data and agreement on use of
a specific set of formulas. The IGC has been using pressure altitude
almost exclusively for quite some time, and will continue to do so for a
variety of reasons. These ease of comparability with past flight
performances and records, the need to detect incursions into airspace
defined by pressure altitude limits, collection of redundant altitude
data for security and reliability, and general bureaucratic inertia.
While there is provision in the Sporting Code for optical and radar
altitude measurements (which are more closely related to GPS altitude),
other requirements pretty much eliminate the possibility of actual use,
and if someone managed to find a legitimate way, the conversion would
have to be handled on a case specific basis.

None of this has provides any good reason why a badge for silver or gold
height gain should not be awarded on the basis of height gain measured by
means of GPS altitude instead of barometric pressure altitude.

You may not like the answer, but the IGC measures height gains, loss of
height, etc., using pressure altitude. You are welcome to contact your
IGC delegate and indicate you would like this to change.

Both GPS and pressure altitude are subject to errors and pressure
altitude is not necessarily the more accurate. Two performances by
different pilots on different days in different weather, both providing
equal height gain measured by pressure altitude does not imply that the
two gliders actually achieved the same height gain if could be measured
with a tape measure. Just the same if one performance had been measured
with GPS altitude. But both performances would be an excellent
demonstration of pilot skills and worthy of recognition with the
appropriate badge!

The altitudes measured by the IGC are not the same as heights measured
by tape measure, GPS, etc. You are effectively comparing apples and,
uh, pears.

This may not be 100% "fair". But a distance performance navigated with a
compass and map and measured with turn point photographs is a more
difficult achievement than the same task navigated with GPS and measured
with a Flight Recorder. You have to fly further to get a decent photo and
the cockpit workload is significantly higher. Yet we award the same badge
for both performances - thus some "unfairness" already exists in the code.

Positions are positions. Pressure altitudes are not heights or elevations.

Tying the COTS GPS rules to the use of GPS altitude would almost
certainly result in failure to pass the proposal. Requiring use of a
pressure altitude recording device (i.e. barograph) increases the
probability that a majority of the IGC delegates will vote in favor of
the proposal. That's politics for you...

This political argument sounds far more plausible than any technical one.
I would like to see a list of IGC delegates (identified by country) who
are apposed to the use of GPS altitude to measure height gain. Then we
could explain to them that their attitude is quietly killing our sport...

http://www.fai.org/directory/delegates.asp?id=6

The fact that you don't agree with the technical arguments doesn't
necessarily mean that they're wrong.

To put it another way, if we accept COTS GPS together with COTS altitude,
then overnight there would be 10000 Flarm equipped gliders which would
each have their own dedicated Flight Recorder at no additional cost at
all... Now that would be a powerful boost for our sport.

Flarm units have pressure sensors, there will be news fairly soon.

Marc

Marc wrote: Positions are positions. * Pressure altitudes are not
heights or elevations.

And yet the IGC calls them height gains for badges and absolute
altitude for records?

Bob (5 more weeks till flying season!)

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 -

I am afraid that GPS altitude as recorded in IGC flight data files,
has been shown to be unreliable for accurate measurement purposes,
compared to traditional pressure altitude calibrated to the ICAO ISA.

In the Sporting Code, GPS altitude has always been OK for evidence of
flight continuity, but not where accurate figures are required such as
for gain-of-height or comparison with airspace bases. Some more
detail follows.

Pressure altitude is extremely reliable in IGC files and has a
negligible anomaly rate. In comparison, a significant proportion of
IGC files have GPS altitude anomalies, some small but some large. One
example is where high points are high and low points are low (compared
to pressure altitude), making gains-of-height different. If you look
closely at IGC file data, there are often differences between pressure
and GPS altitude that cannot be explained by the known differences
between the two due to atmospheric pressure changes and the different
scales. GPS altitude is vertical distance above the Geodetic Datum
that is selected (WGS84 for most purposes) whereas pressure altitude
used in aviation worldwide uses the ICAO International Standard
Atmosphere. In addition there are quite a few examples of major GPS
altitude anomalies in IGC files, involving differences from pressure
altitude not just of tens or hundreds of feet, but occasionally
thousands.

Another factor is that, even in ideal conditions, due to the geometry
of SatNav position lines making up a fix, altitude accuracy will
always be poorer than Lat/Long accuracy by a factor of about two.

A report on this was made to IGC in the year after the Selective
Availability accuracy degradation was removed on 1 May 2000. This
analysed many IGC files and came to the conclusion above. It was
posted on the IGC web site and may still be accessible if you look.
Many thousands of IGC files have been analysed since then and the
conclusion is still the same, anomalies in GPS altitude in IGC files
continue to occur. I say "in IGC files" because that is where the
data is from, and more expensive GPS receivers with more sophisticated
processing probably would not show these anomalies.

Fortunately, lat/long accuracy is not affected, the anomalies are
confined to GPS altitude. You might think that GPS receivers would
process one 3D position and then extract Lat/Long and altitude from
the same process. However, this appears not to be the way it is done
in the low-cost receivers used in IGC-approved recorders. Lat/Long is
processed separately from altitude and perhaps the manufacturer uses
more sophisticated processing for Lat/Long because that is what the
majority of customers want.

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)

On Feb 23, 6:28*am, Ian Strachan wrote:

Many thousands of IGC files have been analysed since then and the
conclusion is still the same, anomalies in GPS altitude in IGC files
continue to occur. *I say "in IGC files" because that is where the
data is from, and more expensive GPS receivers with more sophisticated
processing probably would not show these anomalies.


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

Ian,

I'd be curious to see the results of the detailed analysis of IGC
files. Is there a place on the IGC site where we could see the data?
I'm not looking for the annecdotal sort (i.e. see this file here) but
the volumetric sort (i.e. based on an automated review of 10,000
files). I had started on this a year back but stopped when I was
told that the IGC was already doing this.

In my experience in writing the specs for running a large batch
analysis program on this topic, what happens is that INDIVIDUAL fixes
or SMALL GROUPS of fixes do display anomalous results. I think most
people recongnize this possibility. However, they can easily be ruled
out by post-flight analysis programs by predefined parameters (for
example, a 10,000 fpm climb rate over a rolling 4 or 6 fix
average).

I think the bigger question that still needs to be answered is "to
what level of precision" do we need all of this to work.
Considering the inherent issues in measuring pressure altitude during
the fluid conditions of a post-frontal day (for example), it feels as
if we are holding GPS to a higher standard.

Thanks in advance,

Erik Mann

Ian,

I and many others appreciate your long, dedicated service and
willingness to interact on this forum. Please take these questions as
genuine requests to understand better the issues involved, not as an
attempt to undermine your position:

I am afraid that GPS altitude as recorded in IGC flight data files,
has been shown to be unreliable for accurate measurement purposes,
compared to traditional pressure altitude calibrated to the ICAO ISA.

How are "reliable" and "unreliable" defined here? Given that there is
no independent verification of either method of determining altitude
(pressure and GPS)--at least in IGC files--I assume you use some
measure of the number and severity anomalies. Could you elaborate?

Pressure altitude is extremely reliable in IGC files and has a
negligible anomaly rate. *In comparison, a significant proportion of
IGC files have GPS altitude anomalies, some small but some large. *

Could you give be more specific? Does reliability involve any
comparison of the two methods to cross check each other? That is, if
there's a sudden change in pressure altitude, is that presumed to be
in error solely because of the severity of the change or also because
the GPS altitude didn't change in a corresponding manner (and vice
versa)?

In addition there are quite a few examples of major GPS
altitude anomalies in IGC files, involving differences from pressure
altitude not just of tens or hundreds of feet, but occasionally
thousands.

Same question: is an anomaloy defined as a discontinuity in the trace
of a GPS or pressure altitude record, or a difference between the two
traces, or both? The above statement seems to indicate that the
difference between the two is taken into account. If so, are you
comfortable with judging the accuracy of one method by assuming the
other is correct?

Another factor is that, even in ideal conditions, due to the geometry
of SatNav position lines making up a fix, altitude accuracy will
always be poorer than Lat/Long accuracy by a factor of about two.

What does this translate to in feet or meters, or in percentage terms?
And how does that compare with known accuracy of pressure altitudes?

A report on this was made to IGC in the year after the Selective
Availability accuracy degradation was removed on 1 May 2000. *This
analysed many IGC files and came to the conclusion above. *It was
posted on the IGC web site and may still be accessible if you look.
Many thousands of IGC files have been analysed since then and the
conclusion is still the same, anomalies in GPS altitude in IGC files
continue to occur. *I say "in IGC files" because that is where the
data is from, and more expensive GPS receivers with more sophisticated
processing probably would not show these anomalies.

Do these "more expensive GPS receivers" refer to some or all current
COTS receivers, or to something else?

If comparisons of pressure and GPS altitudes are made to determine
anomalies, are they based solely on the records in approved flight
recorders or also on comparisons of traces made with other GPS
receivers. For example, I carry a Garmin GPSMAP 76 as a backup and
have downloaded and compared its trace with that of my Cambridge Model
20 for a number of flights. There are differences, as you say, but as
a layman I have no way of knowing which is closer to being correct.
And I have not compared the GPS altitude from the Cambridge to the GPS
altitude from the Garmin. Do the studies to which you refer do this?

Fortunately, lat/long accuracy is not affected, the anomalies are
confined to GPS altitude. *You might think that GPS receivers would
process one 3D position and then extract Lat/Long and altitude from
the same process. *However, this appears not to be the way it is done
in the low-cost receivers used in IGC-approved recorders. *Lat/Long is
processed separately from altitude and perhaps the manufacturer uses
more sophisticated processing for Lat/Long because that is what the
majority of customers want.

This is an interesting speculation. How likely is it that if the
flight recorder manufacturers wished to reduce anomalies in their GPS
altitude records, they could do so? And (this is purely speculative on
my part) would you agree there has been a disincentive for them to do
so since the more accurate that GPS altitude is proven to be, the more
likely that they might lose their exclusivity in the gliding
market?

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.

I know that years ago there was a strong, justifiable sense of
appreciation for the work that Dave Ellis and Cambridge did to make
flight recorders possible and affordable for many pilots. Without
implying that GFAC or the IGC felt any moral obligation in the past to
"reward" this dedication by excluding other types of flight recorders
or verification methods, do you believe the flight recorder market is
well enough established now that new entrants, whether they be COTS
manufacturers or soaring equipment builders, are viewed without any
consideration for the early days of flight recorders or the difficulty
of making a profit in the relatively small world of gliding? To be
more provocative, is there any concern over the fact that opening up
the market to COTS equipment now would almost certainly harm flight
recorder manufacturers?

Chip Bearden
ASW 24 "JB"
USA

First, I wish to complement the IGC for it's work on GPS flight loggers.
They have made a huge and favorable impact on soaring performance and cross
country flying. The fact that the availible certified units aren't cheap
enough to satisfy all is just an artifact and not at all the intention of
the IGC. I'm sure everyone in the IGC hopes time will bring cheaper units.

For those pilot wishing to attempt a badge flight, just ask to borrow a
logger from a fellow pilot. My Volkslogger is available for the asking - as
long as I know you.

Now, some rambling thoughts.

Both pressure altitude and GPS altitude sensors suffer a "signal to noise"
ratio but of a different character.

In the case of pressure sensors, the main source of "noise" is the synoptic
distribution of highs and lows on the weather map. This noise signal
changes slowly over time scales of hours and distances of hundreds of miles.
There are errors due to differences in the temperature of the atmospheric
collumn and the Standard Atmosphere which change slowly on a daily cycle.
There may also be a shorter time scale noise source related to the static
source or cockpit pressure although this is small compared to synoptic
pressure changes.

A GPS receiver, by nature of being a radio, suffers noise of a different
type. 'Static' is short term noise on the scale of milliseconds. There is
also some noise from the slowly changing geometry of the Navstar satelites
but this is predictable, thus not really 'noise'. There's also the issue of
each system using a different sea level reference or "datum"

Both GPS altitude and pressure altitude are both "right" and "wrong" for
different reasons.

Sometimes, a clever engineer can fuse data from two different sources in a
way that uses the best of each and cancels out the worst. This is seen in
fusing data using a Kalman Filter from a GPS receiver and an Inertial
Measurement Unit. GPS, is a 'position finding system' and the IMU is a
'position keeping system'. GPS keeps the IMU honest and the IMU smooths out
the GPS signal. The result is basically a huge "signal to noise"
improvement.

Simularly, fusing pressure data with GPS altitude data could work the same
way. For example, a millisecond scale spike in GPS altitude, not confirmed
by presure data could be ignored with impunity. A slow drift of the
pressure signal from the smoothed GPS signal is almost certainly a synoptic
pressure change and can be canceled by the long term stability of the GPS
signal making the pressure signal more useful in detecting airspace
incursions. (Think automatic Kollsman settings.)

The differences between sea level references and temperatue differences from
the Standard Atmosphere can be subrtacted out producing an altitude signal
referenced to whatever datum you choose.

Of course, all this ignores the question of just what it is you want to
measure. If it is deemed desirable to have an altitude logging system for
future flights that is directly comparable to past flights that used a
barograph, then including GPS data may not be desirable.

Does any of this have something to do with reducing the cost of loggers?
Maybe. If two low cost sensors fused together can produce superior data to
one high cost sensor, the result might be a cheaper, more accurate logger.

Bill Daniels

On Feb 23, 1:08*pm, "Bill Daniels" bildan@comcast-dot-net wrote:



Simularly, fusing pressure data with GPS altitude data could work the same
way. *For example, a millisecond scale spike in GPS altitude, not confirmed
by presure data could be ignored with impunity. *A slow drift of the
pressure signal from the smoothed GPS signal is almost certainly a synoptic
pressure change and can be canceled by the long term stability of the GPS
signal making the pressure signal more useful in detecting airspace
incursions. *(Think automatic Kollsman settings.)


Bill Daniels

Which, interestingly, is exactly what the $200 Garmins with Pressure
Sensor do!

P3