Marc Ramsey wrote:
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.

Surely this situation occurs at Minden regularly. Does Reno ATC have
trouble "losing" gliders when they thermal together? Or are they still
aware that something is located there, even if Mode C info is lost? And
even if it is a problem, doesn't ATC still much prefer gliders to have a
transponder than not?

I'd expect at least ONE good signal to be received every 5-15 seconds,
as the gliders' positions change and one antenna is in a much better
position the other ones.

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Eric Greenwell
Washington State
USA