Robert Ehrlich wrote:
An experiment in the french Alps made with a group of tow planes
mimicking glider flight, i.e. circling together from time to time has
shown that transponders, except in mode S, may not be very useful in
gliders. As soon as 2 or more gliders are close together, e.g. circling
in the same thermal of working together the same ridge, they are hit
simultaneaously by the radar beam and generate simultaneaously their
responses, which results in both interfering and nothing useful
received at ATC. I had the chance of having one of the engineers
involved in the experiment as a passenger last September and he confirmed
this. In mode S, as each transponder is specifically adressable,
this mess will probably not occur, a new experiment using them is
planned.

This study is sometimes cited as an excuse to put off installation of
transponders until inexpensive mode S transponders are available. My
take on it is that it addressed a fairly narrow concern, the possible
inability of ATC to properly discern a group of thermalling mode C
equipped gliders. It did not examine whether airborne collision
avoidance systems would continue to provide warnings when confronted by
such situations.

The times when I've been surprised by the close approach of larger
aircraft have been while cruising between thermals, when I'm generally
alone or at a fair distance from other gliders. While thermalling, I
have a view of pretty much the entire sky, and I have a much better
chance of seeing approaching traffic in plenty of time to avoid it.

Marc