วันจันทร์ที่ 29 กันยายน พ.ศ. 2551

Accuracy in Timekeeping - Atomic Clocks and Time Servers

The development of atomic clocks throughout the twentieth century has been fundamental to many of the technologies we employ everyday. Without atomic clocks many of the innovations of the twentieth century would simply not exist.

Satellite communication, global positioning, computer networks and even the Internet would not be able to function in the way we are used to if it wasn't for atomic clocks and their ultra-precision in timekeeping.

Atomic clocks are incredibly accurate chronometers not losing a second in millions of years. In comparison digital clocks may lose a second every week and the most intricately accurate mechanical clocks lose even more time.

The reason for an atomic clock's incredible precision is that it is based on an oscillation of a single atom. An oscillation is merely a vibration at a particular energy level in the case of most atomic clocks they are based on the resonance of the caesium atom which oscillates at exactly 9,192,631,770 times every second.

Many technologies now rely on atomic clocks for their unbridled accuracy. The global positing system is a prime example. GPS satellites all have onboard an atomic clock and it is this timing information that is used to work out positioning. Because GPS satellites communicate using radio waves and they travel at the speed of light (180,000 miles a second in a vacuum), tiny inaccuracies in the time could make positioning inaccurate by hundreds of miles.

Another application that requires the use of atomic clocks is in computer networks. When computers talk to each other across the globe it is imperative that they all use the same timing source. If they didn't, time sensitive transactions such as Internet shopping, online reservations, the stock exchange and even sending an email would be near to impossible. Emails would arrive before they were sent and the same item on an Internet shopping site could be sold to more than one person.

For this reason a global timescale called UTC (Coordinated Universal Time) based on the time told by atomic clocks has been developed. UTC is delivered to computer networks via times servers. Most time servers utilise NTP (network time protocol) to distribute and synchronize the networks.

NTP time servers can receive UTC time from a number of sources most commonly the onboard atomic clocks of the GPS system can be used as a UTC source by a time server connected to a GPS antenna.

Another method that is quite commonly used by NTP time servers is to utilise the long wave radio transmission broadcast by several countries' national physics laboratories. Whilst not available everywhere and quite susceptible to local topography the broadcasts do provide a secure method of receiving timing source.

If neither of these methods is available then a UTC timing source can be received from the Internet although accuracy and security are not guaranteed.

Richard N Williams is a technical author and specialist in atomic clocks, telecommunications, NTP and network time synchronisation helping to develop dedicated NTP clocks. Please visit us for more information about a network time server or other ntp server solutions.

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