I cannot agree that “FLARM, ADS-B, or any other GPS based technology
is useless in collision avoidance at close quarters”. It/they are not
panaceas that will always avert collisions, but it/they are much
better than not having them.

I nearly had a head-on collision that I believe Flarm helped avert.
Two closer gliders at say 1 and 2 o’clock momentarily distracted me
from a third, just behind, in my 12 o’clock. His and my Flarms went
off, and he was already turning to avoid me by the time I picked him
up.

If somebody approaches you from behind, you CANNOT see them. If Flarm
alerts, you as well as the other now have the chance to do something
about it – e.g. dive to accelerate away when you get say 6 seconds
warning of a probably collision from behind. If the pilot behind was
misjudging the proximity, or has his head down, that becomes your only
chance. The pilot in front of the two, in a collision I referenced
above, certainly wished that both their Flarms had been working. The
Flarm company analysis of their logger traces showed exactly how much
notice they would have had.

Chris N.