The AN/FRC-117 Survivable Low Frequency Communications System (SLFCS) was a communications system designed to be able to operate, albeit at low data transfer rates, during and after a nuclear attack.
Single channel, receive only capability was provided at ICBM launch control centers.
The single channel operated between 14 kHz and 60 kHz to receive commands from remotely located Combat Operations Center – Transmit/Receive (T/R) sites; this low frequency range is only slightly affected by nuclear blasts.
SLFCS' primary advantage was that it would experience minimal radio signal degradation as a result of nuclear detonations.
There were a handful of GREEN PINE stations in the northern portions of Alaska and Canada.