On Saturday, November 22, 2014 9:03:18 PM UTC-6, son_of_flubber wrote:
On Saturday, November 22, 2014 5:15:04 AM UTC-5, Z Goudie wrote:

Remember, if you're flying too slowly into a strong headwind in wave and
thus proceeding backwards over the surface of the earth, when Flarm calls a
conflict at 12 o'clock it's behind you....

So more generally, 12 o'clock on the Flarm warning circle aligns with bearing rather than heading?

OK, time for some basic FLARM theory:

FLARM uses GPS and pressure altitude exclusively; it has no heading input. So the warnings it sends are based on pressure altitude, GPS position and GPS track - and it assumes you are moving forward. It has no way of knowing which way your fuselage is pointing.

So FLARM warnings are aligned to your TRACK, not your HEADING. And in most cases, this is sufficient, as you are not trying to nail down a bearing to another glider within a few degrees - each clock sector is 30 degrees, remember.

That's why a FLARM display on a moving map (like an Oudie) is nice - it will be based on the GPS position of the other FLARM equipped gliders, so even if you are backing up in wave, it will show their true position relative to you (although, you will appear to be flying backwards on your Oudie!).

So, to summarize, the displayed BEARING to a FLARM target is referenced to your TRACK, not your HEADING, so if there is a lot of drift due to wind, you will have to compensate for it.

(Any FLARM-smarter guys out there, please correct me if I have got it wrong!)

Kirk
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