To those of us involved in air defense half a century ago, when IFF "Identifier: Friend or Foe"?), developed and first utilized during WWII, and transponders installed mostly only in military and commercial a/c (and radar only in its second generation, still "raw" image displayed on a round scope ["Plan Position Indicator"?], absent any "computer enhancement"), the topic brings back memories....late nights with those little dotted lines of coding appearing behind the radar "target" on the PPI, and that all too common radio transmission,,,"Bozo 309, Squawk Flash and tun right 90 degrees for ID".

Even now, 50 years later, I still note that those mattress spring radar antennae tracing their endless circles in the sky all still come with an "addendum" on top, the rectangular box housing the IFF/Transponder Interrogator.