WWV transmits standard frequencies on 2.5 5, 10, 15, 20, and, experimentally, 25 MHz, derived from an ensemble of ceslium-beam stabilized clocks. Doppler shifts of the skywave results in received frequency error of up to 1 Hz or more, or 1 part in 10 to the 7th power. WWVB reduces this error several orders of magnitude by transmitting a 60 kHz signal, which propagates by ducting through the atmosphere, rather than by skywave.
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