This matter does cause some confusion, I agree.
Regarding the setting transponder, assuming you are operating some form of air transport category aircraft, your transponder fit is likely to have a strut switch or some other means of determining whether your aircraft is on the ground or airborne. This state determination is used to control replies to Mode A/ Mode C (ATCRBS) / Mode S All-call interrogations, such that when on the ground you are not replying to these interrogations from airfield radars. This will be the case when operating in XPDR or AUTO depending on your aircraft configuration. This also determines whether you are ellicting airborne or ground squitters.
I believe that the usual SOP for transponder was to be in STNDBY or OFF then set to XPDR or AUTO or TA/RA just before you line up (not a pilot so don't quote me on that bit). However, if you're at EGLL say, if you were in STNDBY or OFF, your transponder would not be squittering and you would not be appearing on ASMGCS displays for GM. By setting your Mode A at the gate and enabling the XPNDR, you appear to ASMGCS and GM can track you round the airport. You also get your squawk validated at the gate.
Regarding TCAS, the replies elicited from Mode S equipped aircraft on the ground by the TCAS interrogations of overflying aircraft contain an indication of the airborne / ground state of the aircraft. The TCAS on the overflying aircraft therefore does not track the aircraft on the ground as it knows it is on the ground, hence should not generate TA or RA. As such I don't think in this situation it is necessary to select Mode A only. In fact, in today's environment, unless your altitude encoder went screwy, there is no reason to ever select Mode A only.
The only reason you would NOT select XPDR on the ground is if you aircraft does not have an automatic means of working out if it is on the ground or not, else you will be replying to the airfield or nearby SSR radar.
However, some transponder manufacturers (Garmin and Trig spring to mind) market a GA transponder that has an additional GROUND setting - this allows the aircraft to provide squitters on the ground to enable detection by ASMGCS yet at the same time not reply to Mode A / Mode C / Mode S All-calls from the airport radar.
All of the above is general and will depend on aircraft fit.
RS
Last edited by Radarspod; 14th November 2008 at 19:39.
Reason: ATCRBS added for our US cousins, even though the use of the acronym pains me!