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NIST WWVB
WWVB is a very interesting radio station. It has high transmitter power (50,000 watts), a very efficient antenna and an extremely low
frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of
high power and low frequency gives the radio waves from WWVB a lot of bounce, and this single station can therefore cover all of
the continental United States plus much of Canada and Central America as well (scroll about three-quarters of the way down the
page for a nice coverage map).
The time codes are sent from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second
(for comparison, a typical modem transmits over the phone lines at tens of thousands of bits per second -- imagine receiving a web
page at one bit per second!). The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a
period of 0.2, 0.5 or 0.8 seconds:
0.2 seconds of reduced power means a binary zero
0.5 seconds of reduced power is a binary one.
0.8 seconds of reduced power is a separator.
The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with
information about daylight savings time and leap years. The time is transmitted using 53 bits and 7 separators, and therefore takes
60 seconds to transmit.
A clock or watch can contain an extremely small and relatively simple antenna and receiver to decode the information in the signal
and set the clock's time accurately. All that you have to do is set the time zone, and the clock can display a very accurate time. The
only thing more accurate that you can carry around easily is a GPS receiver, which derives atomic clock accuracy in real time from
the atomic clocks in orbiting GPS satellites.
http://electronics.howstuffworks.com/gadgets/clocks-watches/question461.htm
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WWVB is a very interesting radio station. It has high transmitter power (50,000 watts), a very efficient antenna and an extremely low
frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of
high power and low frequency gives the radio waves from WWVB a lot of bounce, and this single station can therefore cover all of
the continental United States plus much of Canada and Central America as well (scroll about three-quarters of the way down the
page for a nice coverage map).
The time codes are sent from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second
(for comparison, a typical modem transmits over the phone lines at tens of thousands of bits per second -- imagine receiving a web
page at one bit per second!). The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a
period of 0.2, 0.5 or 0.8 seconds:
0.2 seconds of reduced power means a binary zero
0.5 seconds of reduced power is a binary one.
0.8 seconds of reduced power is a separator.
The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with
information about daylight savings time and leap years. The time is transmitted using 53 bits and 7 separators, and therefore takes
60 seconds to transmit.
A clock or watch can contain an extremely small and relatively simple antenna and receiver to decode the information in the signal
and set the clock's time accurately. All that you have to do is set the time zone, and the clock can display a very accurate time. The
only thing more accurate that you can carry around easily is a GPS receiver, which derives atomic clock accuracy in real time from
the atomic clocks in orbiting GPS satellites.
http://electronics.howstuffworks.com/gadgets/clocks-watches/question461.htm
Next