According to Wikipedia: "The RCA 474L Ballistic Missile Early Warning System (BMEWS, "474L System", Project 474L) was a USAF "Big L" Cold War system of radar, computer, and communications systems that included the first operational ballistic missile detection radar. The network of 12 radars for detecting "a mass ballistic missile attack launched on northern approaches for 15 to 25 minutes warning time" also provided Project Space Track satellite data (e.g., about 1/4 of SPADATS observations)."