finally, I don't see that the SSR will be overwhelmed if everyone in the air had a transponder; but it would be interesting to hear from a controller.
Actually, SSR can be overwhelmed. Both on a technical level, and on a human/interpretation level.
Mode A/C has the problem that every interrogation signal, both from any radar head and from any airborne TCAS unit, triggers a response from your transponder. This means that radar heads and transponders receive replies to interrogation signals that were not transmitted by them, and this leads to "ghost returns", or whatever they're called.
AFAIK these generally disappear in the next sweep but in huge numbers they may still cause enough clutter to confuse the software or the operator. I think I read a number of 300 somewhere, which is supposed to be the maximum number of transponders in a given area, before the technical limits of the system are reached. So two glider competitions launching might do the trick already. And remember that at this level there's no filtering possible yet. So a huge gaggle of gliders launching and holding at low level, may cause sufficient bandwidth saturation that the responses of high-level targets in a completely different bit of airspace, can also not get through.
One of the main features of Mode-S is "selective" interrogation, where the transponder will only respond to certain interrogation signals, but not to all, every time. This greatly reduces the overall bandwidth requirements, leading to a higher theoretical maximum of transponders in any given area without overwhelming the technical capabilities of the system. (Whether an operator is capable of dealing with those numbers is another matter.)
The second problem is information block density. When Mode-S was introduced in the Netherlands the radar operators did not have good filtering mechanisms. So all mode-S information - quite a block of text - was displayed for every target in the area regardless of its altitude or whether it was inside or outside the operators CAS. This lead to data blocks overlapping each other on a huge scale, making the information completely unreadable. Of course the operator had the ability to move text blocks around to unclutter them, but with 100+ targets in your area - which happened on the first day of beautiful weather after mode-S became mandatory in NL - it was still overwhelming. It took the software guys quite some time to implement filtering based on altitude and a few other things, so that radar operators can now filter out irrelevant targets. (And during that period there were NOTAMs in place that we were not allowed to use our shiny new mode-S transponders in a lot of places. In fact, for a while there was a huge area around Schiphol simply declared a no-go area for GA.)
So, yes, if someone decides to make transponders, or new transponder types, mandatory wholesale for a segment of GA, there are some lessons to be learned from the Dutch experience...
