Tor,
As far as I know transponders 'talk' (callsign, altitude, track, speed,destination and even capability of A/c) on the 1030 and 1090MHz frequencies of the Secondary Surveillance radar - Mode S allows a significant rate of data transfer (5 time 10 to the 33 meassages) that TCAS systes can utilise. There are something like 16million A/C addresses available and
Ground stations are constantly interrogating transponders when over land and mode s systems have a squitter functoin to randomly reply over oceanic areas so other A/c can still 'see' each other.
(ps Tor do you like your new pick up or do I have the wrong guy?)
Reimers, I see your point about being academic and at the level its ended up maybe but as for selecting TA, not academic at all. OK it may be low onthe priority list but anyhting we can do to break the chain that leads to accidents helps...