Atomic Nixie Clock
The system works like this. WWVB is the call sign of the atomic clock in Colorado. It transmits at a very low frequency, 60kHz. This allows the signal to travel far, but requires a sensitive receiver to pick it up. Then the signal must be decoded. In most areas, one cannot get reception all the time.
   Atomic Nixie Clock
Full description at Atomic Nixie Clock website
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