This other page goes into a little more detail on the detection system: https://coldwar-ct.com/Blast_Detectors.html

“Most sites included Gamma detectors that were designed to detect the radiation wave as well. They were redundant systems, any detection, overpressure or Gamma would button-up the site at which point signals were sent to all Continental U.S. sites that a blast was detected, where it was, the size of the blast and wind speed and direction. Sites within 250 miles of any detection would go to Auto-Lock down.”