Can you use a mode 3AC interrogator in EU airspace? I'm investigating the possible application of mode 3AC interrogators in EU airspace.

My experience is unfortunately limited to North America, and I'm unfamiliar with Mode S systems / function. 

All the information I've managed to find describes mode S trabsponders as "backwards compatible" but fail to go into detail.

Is the backwards compatibility seamless and simultaneous?, Able to reply to both mode S and mode 3 interrogators in real time? Or a toggle switch between modes, in which when mode S is selected mode 3 interrogations will be ignored?

Do European standards allow mode 3A/C interrogators to operate within their airspace? Could this cause interference with local mode S infrastructure?

Are there any other potential concerns with operating a mode 3 interrogator in EU airspace?

To clarify I am speaking from the perspective of a ground station. As in using a mode 3AC SSR on an airport in EU airspace.

Any information would be greatly appreciated, thank you.
Partial answer:


  Is the backwards compatibility seamless and simultaneous?


Yes.


  Able to reply to both mode S and mode 3 interrogators in real time? 


Yes.


  Or a toggle switch between modes, in which when mode S is selected mode 3 interrogations will be ignored?


No - a Mode S transponder will respond to interrogations by a "pure" mode 3 interrogator. Mode S interrogators have a special interrogation format that allows them to interrogate "pure" mode 3 transponders without triggering replies from mode S transponders even though those transponders do support mode 3 interrogations.


  Do European standards allow mode 3A/C interrogators to operate within their airspace?


Don't know.


  Could this cause interference with local mode S infrastructure?


Yes, in principle, for the same reason (FRUIT) that mode S was introduced in the first place - "overcrowding" of the airwaves. It won't be catastrophic. There are already plenty of airborne mode 3 interrogators operating (TCAS and AWACS).


  Are there any other potential concerns with operating a mode 3 interrogator in EU airspace?


Consider both radio communication regulations and aviation regulations.