Some good discussion, although some responders apparently did not really read my original post. As I said in the original post, I am aware that cell phones don't talk directly to each other (goTenna and similar notwithstanding). I am aware that cell phones systems aren't meant to work with phones at altitude although they seem to do well enough to track our gliders in contests. I am aware that cell coverage is highly discontinuous. I did say that I would not expect such a system to do what Flarm does, hence my "outside of racing gaggles" remark". That's why I originally titled my post "situational awareness" with "collision avoidance" in parentheses.

We have a bad habit of letting better be the enemy of good enough. Flarm is wonderful thing. I have a big 'ol gold PowerFlarm with too many antennas installed in my glider. I like it and I use it every flight. Unfortunately, in the US PowerFlarm is quite expensive and still requires a lot fussing with to get it to work properly. Cost and hassle have limited it's adoption. If we had a $15 app on cell phones that just told us where to look for traffic, and everyone had it, the gain in safety would be well beyond what we currently get from Flarm (gaggles excepted).

I see the purpose of such an app as letting everyone in the air know where to look for traffic. Not really a collision avoidance app, but really a situational awareness aid. So cheap and easy that everyone from skydivers to big iron drivers would have it. It's not so much to keep us from running our gliders into each other, it's to make us visible so that we can use the 100% installed base of Mark I Human Eyeballs and Human Brains to do the collision avoidance part.

WB