Recall JAL 007, Sep 1984.
KAL 007, September 1983.
That was long before ADS-B or Mode-S. In the good old days, the sector air traffic control assigned a four digit octal code to each aircraft in their sector which the pilot would dial into his transponder. When aircraft passed between sectors the new sector would assign a new code. Russian ATC was not integrated with Alaska. There was an USAF RC-135 spy plane flying out of Shemya that was flying a figure eight in and out of Russian coverage. It was completely routine and the Russians were used to it. The RC-135 mission had ended and they headed back to Shemya. At that time KAL-007 showed up on Russian radar right where they expected the RC-135. KAL was flying on inertial navigation, before GPS, but apparently they had entered the wrong coordinates when they were in Alaska and were off course. KAL-007 was headed over Russia's Sakhalin Island. The Russians scrambled interceptors, who arrived below KAL-007 just before it was about to depart Russian airspace. The Russian pilot fired warning shots with its cannons at KAL-007, but the cannon fire probably did not get high enough to be seen by KAL-007. On orders from ground control, the pilot fired a missile, which hit KAL-007 before it exited Russian airspace, resulting the deaths of all aboard.
The Russians had no way of knowing or determining that the intruder was a civilian aircraft, and KAL-007 had inadvertently simulated the previous track of the USAF spy plane.