Hi all,
I have done some testing - involving nearly 1000 IGC files - which
suggests that the mean difference between recorded GPS and pressure
altitude is about 70 metres. These flights came from all over
Australia, plus some from Canada, USA and Argentina. There were several
million B (position) records in the sample.
This data was obtained from flight recorders of all types including
Flarms. The Flarms don't appear to be better or worse in this regard
than other types. The worst performers are the older recorder types
which sample less satellites and have less well developed algorithms.
In a large percentage of files, across the entire sample, the GPS
altitude reading is lower than the pressure altitude.
There was no obvious indication of a change of results with height.
Please note that this is merely a comparison of two values recorded in
the position records of IGC files. There is nothing to suggest that one
is more accurate than the other, nothing to suggest that one is more
reliable than the other, and in the figure quoted (~70m) I did not
attempt any correction.
When I applied a simple correction (by finding the difference between
GPS and pressure altitude at takeoff and subtracting that from all other
readings) the average error reduced to about 46 metres.
I do not know definitely which value is used by Flarm in its
calculations, but I'd be inclined to think it is GPS. I think it is
more likely to be consistent between instruments since GPS readings will
be calculated from similar satellite data in similar instruments using
similar algorithms. Pressure transducers may be less consistent because
they will not necessarily be similarly calibrated and may live in
different environmental conditions - remember that pressure transducers
in Flarms and loggers are reading cockpit pressure, not outside
pressure, which will vary from glider to glider.
Cheers
/Tim/
/tra dire e fare c'รจ mezzo il mare/
On 28/08/2011 10:32, Geoff Vincent wrote:
Hi Chris,
Your GPS hint is a tad misleading given that the unit has a native
internal GPS that seems to satisfy the needs of a great many Flarm
owners. While we are on the subject of Flarm can anyone definitively
confirm whether the Flarm vertical separation algorithms rely on GPS
or pressure altitude data. I have some very interesting empirical
results that suggest Flarm GPS altitude data can be seriously out of
step with other GPS derived data - although not necessarily out of
step with other Flarm units.
Regards,
Geoff V
At 09:54 PM 28/08/2011, you wrote:
Hi Pam,
The OZ Flarm I have, which is one of the early ones, stops working
after a software upgrade.
The fix is to set the Radio Settings Area to Australia (921 Mhz).
The other thing to be aware of is that GPS information only comes
from the RJ45 port labelled with GPS.
Regards
Chris
------------------------------------------------------------------------
*From:* [email protected]
[mailto:[email protected]] *On Behalf Of
*Pam Kurstjens
*Sent:* Sunday, 28 August 2011 6:12 AM
*To:* 'Discussion of issues relating to Soaring in Australia.'
*Subject:* [Aus-soaring] Oz Flarm and Altair Pro
I am keen to hear from anyone using an OzFlarm manufactured by RF
Developments, which has been updated to the latest software, in
combination with Altair or Altair Pro. We have one such combination
which is giving a lot of trouble and would like to know if other
people are finding that it works, or if they are also having trouble
since the recent upgrade to Flarm software.
Thanks
Pam
Suggest you contact me offline
[email protected]
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