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Low Frequency Time Signals
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This frequency band is used by most "atomic-controlled" clocks and watches that have become increasingly popular among consumers and businesses over the past 20 years. Such timepieces incorporate special-purpose radio receivers that tune into radio signals in this band. The signals contain precise time and date information, including daylight saving time adjustments and "leap-second" adjustments that are introduced into the worldwide time standard on an irregular basis.
In the United States, atomic-controlled clocks tune into a signal broadcast at a frequency of 60 kHz from the National Institute of Standards and Technology (NIST) station WWVB, located in Ft. Collins, Colorado. The time information broadcast from WWVB is controlled by the nation's civilian atomic clock standard, which is maintained at NIST's facility in Boulder, Colorado. The accuracy of NIST's atomic clock, compared to the average of many atomic clocks around the world, is about 20 nanoseconds (0.00000002 s), while the propagation delay from WWVB to a particular point in the United States is less than about 0.02 s. The atomic standard is also used to control the frequency of the WWVB signal, which has a stated accuracy of 1 part in 10^12, or about 0.00000006 Hz.
Propagation within this band is quite stable, with the signal typically being "trapped" in a pseudo-waveguide created by reflections off the Earth and water at the bottom and the ionosphere at the top. Signal strengths tend to be best at night when attenuation due to the lower-level ionosphere is less, so many atomic-controlled clocks and watches wait until the middle of the night to check for updated time information.
Although these clocks are often marketed as "atomic clocks," they are actually controlled by signals that are controlled by an atomic clock. The atomic clock itself is located in an elaborate government laboratory, and could never be incorporated into a consumer-level device.
There are several other standard frequency and time signal stations in or near this band around the world, including JJY (Japan) at 40 and 60 kHz, MSF (United Kingdom) at 60 kHz, BPC (China) at 68.5 kHz, and DCF77 (Germany) at 77.5 kHz.
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Frequencies |
| Frequency | Bandwidth | Use | Service | Table |
| 40 kHz | - | JJY (Fukushima, Japan; 50 kW) | Standard Frequency And Time Signal | - |
| 50 kHz | - | RTZ (Irkutsk, Russia; 10 kW) | Standard Frequency And Time Signal | - |
| 60 kHz | - | WWVB (Ft. Collins, Colorado, USA; 50 kW) | Standard Frequency And Time Signal | F |
| 60 kHz | - | JJY (Fukuoka, Japan; 50 kW) | Standard Frequency And Time Signal | - |
| 60 kHz | - | MSF (Anthorn, UK; 15 kW) | Standard Frequency And Time Signal | - |
| 66.66 kHz | - | RBU (Moscow, Russia; 10 kW) | Standard Frequency And Time Signal | - |
| 68.5 kHz | - | BPC (China) | Standard Frequency And Time Signal | - |
| 75 kHz | - | HBG (Prangins, Switzerland; 20 kW; compatible with DCF77) | Standard Frequency And Time Signal | - |
| 77.5 kHz | - | DCF77 (Mainflingen, Germany; 50 kW) | Standard Frequency And Time Signal | - |
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