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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.

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    $\begingroup$ What do you want to use a 3A/C mode interrogator for? $\endgroup$
    – Jan Hudec
    May 21 '20 at 18:39
  • $\begingroup$ Why would one be interested in putting money in that old technology that causes an extraordinary amount of extra transmissions on an already busy radio frequency? $\endgroup$
    – DeltaLima
    May 22 '20 at 6:27
  • $\begingroup$ I'd love to, but I can't go into detail due to privacy concerns, sorry. $\endgroup$
    – MichaelF
    May 22 '20 at 20:02
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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.

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