La Crosse Technology WT3126B 12" Aluminum Analog Atomic Wall Clock

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Features:

• Accurate to 1second per million years
• Sets automatically to WWVB radio signal
• Pushbutton selection of all four U.S. mainland time zones
• Automatically adjusts for daylight savings time
• Manual time-set capability for use anywhere outside the WWVB signal range
• White face, traditional numerals and hands, and a narrow surround of real aluminum that fits flush to the wall 
• Low battery indication via 2-second steps of second hand
• Powered by one AA alkaline battery, with normal life of over 1 year

Dimensions:

• Diameter:
  12 inches (30.5cm) overall
• Depth:
  1.5 inches (3.8cm)

The NIST (National Institute of Standards and Technology, Time and Frequency Division) maintains a radio station, WWVB, in Ft. Collins, Colorado. The WWVB radio station derives its signal from the NIST atomic clock in Boulder, Colorado. A team of atomic physicists is continually measuring every second of every day, to an accuracy of ten billionths of a second per day. These physicists have created an international standard, measuring a second as 9,192,631,770 vibrations of a Cesium-133 atom in a vacuum.

WWVB (the station’s identification just like any other radio station) continuously broadcasts time and frequency signals at 60 kHz. The carrier frequency provides a stable frequency reference traceable to the national standard. There are no voice announcements on the station, but a time code is synchronized with the 60 kHz carrier and is broadcast continuously at a rate of 1 bit per second using pulse width modulation. The time code contains the year, day of year, hour, minute, second, and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds.

The LaCrosse Technology WT3126B Analog Atomic Clock:

The La Crosse Technology Radio-Controlled Clock maintains its incredible accuracy by automatically tuning into the WWVB radio signal. The built-in antenna ensures reliable signal reception up to 2000 miles (3200km) from the WWVB transmitter. As a result, the La Crosse Technology clock will calibrate to the atomic clock not only throughout the mainland U.S. but also in much of Canada. You need only insert the battery, press the appropriate time zone button, and wait until the signal is received (overnight). In all other locations worldwide, the La Crosse clock can be manually set and will provide the high degree of accuracy of any quality quartz clock.

In a nutshell, here's how the the La Crosse clock works: The clock's antenna is specifically tuned for optimal reception of the 60 kHz time signal. The time signal received on the antenna is demodulated by an onboard receiver, which sends the information to the clock's CMOS microprocessor for decoding. Once your clock has automatically set its time for the first time, it ensures continuing accuracy by automatically tuning into the WWVB radio signal once per day after midnight. As long as your clock is kept within transmitter range, it will continue to display the absolutely precise time, even automatically adjusting for summer time/winter time changeovers. Should you move your clock out of transmitter range for an extended period of time, the clock will continue to operate as a highly accurate quartz-controlled clock. Upon re-entering the transmitter range, your clock will correct itself again to the precise WWVB time signal.