Avioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11 Reply 3, posted (9 years 6 months 3 weeks 5 days 23 hours ago) and read 3403 times:
Actually Mode S is so named because it Selectively interrogates other nearby aircraft transponders in coordination with TCAS to determine proximity and threat. It also transmits the registration or international identification code of the aircraft so it may be specifically identified for interrogation by other Mode S transponders in the area.
The name isn't accurate in that a Transponder responds to interrogation and an Interrogator initiates interrogation or communication (i.e. DME Interrogator vs. ATC Transponder) The name stays because the primary purpose is still to respond to ATC queries for control.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533