A fair question Bids!
As far as the Mode S ground station is concerned, it transmits a background All Call interrogation at regular intervals. Any Mode S equipped aircraft flying into coverage will respond with an All Call reply that contains its Mode S address. Once the ground station has detected the new target it will commence selective interrogations and lock-out the All Call - tell the aircraft to stop responding to All Call interrogations. If the aircraft stops receiving scheduled interrogations the lockout will time-out after 18 seconds.
A TCAS equipped aircraft continuosly transmits a Mode S 'squitter' reply once per second that also contains its Mode S address.
RR