Mode S. Selective interrogation

[alexious85]

Hello
Can someone explain the mode S selective interrogation mode principle in simple words as initial search in internet tends to be quite complicated

[DaveReidUK]

Wikipedia's entry on Mode S is a good introduction: Aviation transponder interrogation modes - Wikipedia (https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes)

[Miles Magister]

You might find some more information here https://skybrary.aero/articles/mode-s

Skybrary is controlled by EuroControl and is a good source of reference