What the "S" is going on at LHR?
Guy I know said he was going into LHR the other day, and picked up a message on the ATIS for pilots to leave their Transponders on Mode S after landing with appropriate Mode 3A set.
Apparently, this confused all of them on the flight deck and he asked me as an 'old and bold' if I knew why, especially as he said they couldn't find a Mode S switch on any of the panels!!. Clearly I am too 'old and bold' as I didn't know either. Not wishing to appear too ignorant, I airily replied that I wasn't quite sure, but would find out.
So guys, can you illucidate? Why the Mode S?