And the civilian SSR system was built on the released frequencies and protocols of Mode 3 of the military Mark X IFF system, which was adopted by civilian agenc

And the civilian SSR system was built on the released frequencies and protocols of Mode 3 of the military Mark X IFF system, which was adopted by civilian agencies in the mid-1950s after a particularly nasty airborne collision. Sixty years later and we're still bounded by legacy. As a result of shortage the general-use squawk codes are namespaced into each national ATC region, so aircraft have to constantly change squawks even in supposedly contiguous regions such as Eurocontrol. Squawk 4463 means a different thing in UK airspace than in French. Ironically, military aircraft still support Mode 3 in order to integrate with civilian ATC, who call it Mode-A, but all their special don't-shoot-me-I'm-friendly stuff is handled by more modern encrypted protocols.