1983 Soviet nuclear false alarm incident

Background to the incident
They knew there early launch radar was unreliable as they found out in The 1983 Soviet nuclear false alarm incident and so was of little if any use to them.

The incident
article: 1983 Soviet nuclear false alarm incident On the night of September 26, 1983, the Soviet orbital missile early warning system (SPRN), code-named Oko, reported a single intercontinental ballistic missile launch from the territory of the United States. Lieutenant Colonel Stanislav Petrov, who was on duty during the incident, correctly dismissed the warning as a computer error when ground early warning radars did not detect any launches. Part of his reasoning was that the system was new and known to have malfunctioned previously; also, a full-scale nuclear attack from the United States would involve thousands of simultaneous launches, not a single missile.