It's a very thorough and well reasoned response and well worth reading in full.
It
is a thorough and well reasoned response, but it does contain some misconceptions:
...you discuss the need to operate the UK ground environment in “mixed mode”, that is to say, all radars interrogate in both Mode S and Mode A/C because of safety issues. So all Mode S transponders reply in both modes and we understand this is likely to continue for some years, certainly until 2012 and beyond if equipment programmes are not delivered by then.
Until the day that Mode A/C interrogation is turned off, all Mode S transponders will also respond in Mode A/C. This has not been a problem so far as virtually all aircraft that have fitted Mode S, previously had Mode A/C so the number of units has remained broadly constant.
If all 15,000 aircraft that are now proposed for Mode S fit were to do so by March 2009, they too would respond to Mode A/C interrogation and UK ATC systems would be overwhelmed.
Mixed mode operations are considerably more sophisticated than the author assumes. "All-call" periods, during which Mode A/C interogations are sent, occupy only a small part of the overall cycle. The rest is "roll call", during which the interrogation is selective. Additionally, Mode A/C interrogations sent during that all-call period can be modified (a "short P4") in such a way that a Mode S transponder does not respond.
Thus having 15,000 Mode S transpoder equipped aircraft poses no problem to mixed mode operations -- having 15,000 Mode A/C only transponder equipped aircraft certainly would be more of an issue.