Bookworm, to avoid too much repetition, can I refer you back to this thread:
You can, but it doesn't address the issue, which I failed to identify clearly in my brief response.
You wrote:
What is then received by the interrogator (and displayed to the controller, etc) is the usual squawk code, altitude and so on. So, from an ATC operational point of view, it's still the squawk they track, not the 24 bit ID.
It's not the
usual sqauwk. The Mode S reply includes the 24-bit address, and a flight identification code. Those, which are unique, are tracked, not the Mode A code, which is just there for backward compatability.