What James may be referring to is the fact that Flarm indications are in
relation to your ground track not heading. The extreme example is if
flying in very strong winds, say in wave, and actually going backwards.
Another glider coming from your 6 o'clock will actually show as head on.
This effect is still there in any cross wind.
This is a fundamental flaw in Flarm, which surely could be much improved by
building in a wind algorithm to correct the indication nearer to heading.
All navigation programs have them.
Dave




At 03:22 27 November 2015, jfitch wrote:
On Thursday, November 26, 2015 at 2:45:07 PM UTC-8, James Metcalfe wrote:
At 21:29 25 November 2015, jfitch wrote:
I am curious about your experience with the interface. Did you just
have
the BF display, or was the Flarm also displaying targets on a
moving=20
map?

I have not had any Flarm warnings that would have saved a midair (3=20
years
with it now). But I have always had Flarm targets displayed on the
glide
computer moving map, and so nearly always see them long before they=20
could be
considered a threat. Two times in those three years I have gotten an
unexpected warning, though not close enough to require action to
avoid=
=20
collision.
I use those events as a learning experience to see how I need to
change=
=20
my
scans and operations, so that they do not happen again.

From this I conclude that the situational awareness, far away from an
potential conflict, is more valuable for preventing potential
conflicts
=
in
=20
the
first place than the actual warning facility.=3D20

Countless times that Flarm has identified and displayed glider within
a=
=20
km
of me that I had not seen and might never have seen.
=20
I have used only the 'butterfly' display. To be clear, of course I
am=20
seeing target alerts all the time. That is not a problem, and even=20
occasionally interesting (such as when someone starts following).
The problem is with false collision warnings. Many pilots appear to
(and=
=20
some of my pupils certainly do) find it almost impossible to resist
turni=
ng
=20
away from the Flarm direction of the 'threat', before they have seen
the=
=20
target. That is dangerous, as I wrote in a post yesterday. And note
that=
=20
the Flarm direction is often significantly different from the true
direction=20
(occasionally diametrically opposite), as it is track-based, not
heading-
based.
=20
I can see that my experience of Flarm in a very busy environment
(the=20
French Alps) will be very different from that of those flying largely
in=
=20
isolation (such as flatlands, particularly with what I would regard as
hi=
gh
=20
cloudbases (I'm a Brit!))
J.

James, if you are plagued by false alarms coming even from the wrong
headin=
g, 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
th=
at happening? It would be interesting to put it into SeeYou or other
softw=
are (or even look at it in a text editor) to see what the accuracy of fix
w=
as. 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
t=
he precision of fix, but it seems like that is the most likely source of
th=
e errors you describe. For example the Flarm IGC file from my glider will
s=
how a typical precision of fix of around 3 - 4 meters, but sometimes it
wil=
l go up to 30-40 for unknown reasons. The Air Avionics gps will show a
prec=
ision of 1 - 2 meters on the same flight and might also go up in the same
a=
reas but not as much. Ridge flying in the Alps you might have the antenna
s=
haded 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.