On Sunday, November 29, 2015 at 4:45:58 PM UTC, jfitch wrote:
On Sunday, November 29, 2015 at 7:45:09 AM UTC-8, James Metcalfe wrote:
At 03:22 27 November 2015, jfitch wrote:
James, if you are plagued by false alarms coming even from
the wrong
heading, I am all the more curious. I have not had any false
alarms, perhaps
some false negatives (probably should had been an alarm).
Never from the wrong
direction. Do you have the IGC files from a flight in which you
remember
that happening? It would be interesting to put it into SeeYou or
other
software (or even look at it in a text editor) to see what the
accuracy of fix
was. I have noticed that the Flarm GPS is typically reporting a
larger
error, and in some cases quite large. I'm not sure what the
algorithms do with
the precision of fix, but it seems like that is the most likely
source of
the errors you describe. For example the Flarm IGC file from
my glider will
show a typical precision of fix of around 3 - 4 meters, but
sometimes it
will go up to 30-40 for unknown reasons. The Air Avionics gps
will show a
precision of 1 - 2 meters on the same flight and might also go
up in the same
areas but not as much. Ridge flying in the Alps you might have
the antenna
shaded on one or more sides, which will increase the HDOP.
The precision is
the normally the last three digits of the B record in the IGC
file. All of
my flying is high altitude and with a clear view of the sky.

Others have now posted in more detail on the track vs. heading
errors of Flarm. These are inevitable (until wind information is
available to Flarm), and not (I'm confident) a problem with any
of the 3 installations which I have used. I'm afraid I have not
kept a log of the various incidents to which I have referred, nor
do I have the IGC files.

It seems to me that you yourself have provided the explanation
of the differences in our experiences of Flarm:
- You fly largely alone, in wide open spaces, but occasionally
meeting other gliders on the same cloud street.
- I fly mainly in a busy Alpine setting, constantly close to (and
co-operating with) other gliders on ridges and in thermals. (If I
find myself alone I am reassured that it is not just me who is
finding the conditions difficult to soar in!)
J.

A diametrically opposed false alarm in a thermal suggests wind in the 30 - 60 knot range. I have not seen good thermal development in those conditions, even in extremely strong western desert conditions. I have flown in gaggles of 10 - 15 gliders near the same altitude in the same thermal and have never had such a false alarm.

Given that Flarm is comparing predicted flight paths of the two gliders and issuing alerts based on calculations of possible conflicts up to 18 seconds ahead you would have to be certain that both gliders were flying exactly concentric circles of the same radius and speed before you could be sure that an alert from a diametrically opposed glider was a "false alarm". It would be more likely that the Flarm units were performing as designed and within their fairly large buffers 18 seconds ahead. Granted, our personal buffer zones in thermals are tighter than the those of the Flarm algorithms. As I wrote previously I have demonstrated in flight to a copilot that what he thought was a false alarm from an almost diametrically opposed glider was in fact proven to be a valid alarm about 3/4 turn later. The other glider was diametrically opposed but not flying concentrically with us.