Atomic Clocks have been around for over fifty years or so. They are clocks that use an atomic resonance frequency as its timekeeping element rather than conventional oscillating crystals such as quartz.
Most atomic clocks use the resonance of the atom caesium-133 which resonates at an exact frequency of 9,192,631,770 every second. Since 1967 the International System of Units (SI) has defined the second as that number of cycles from caesium -133 which makes atomic clocks (sometimes called caesium oscillators) the standard for time measurements.
Because the resonance of the caesium-133 atom is so precise, this makes atomic clocks accurate to less than 2 nanoseconds per day, which equates to about one second in 1.4million years.
As atomic clocks are so accurate and can maintain a continuous and stable time scale, a universal time, UTC (Coordinated Universal Time or Temps Universel Coordonné), has been developed and supports such features as leap seconds – added to compensate for the slowing of the Earth’s rotation.
However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP (Network Time Protocol), the standard means for achieving time synchronisation on computer networks, can synchronise to an atomic clock by using either the Global Positioning System (GPS) network or specialist radio transmissions.
The most widely used is the GPS (Global Positioning System), developed by the United States military. GPS incorporates at least 24 communication satellites in high orbit providing accurate positioning and location information. Each GPS satellite can only do this by utilising an atomic clock which in turn can be can be used as a timing reference.
A GPS time server is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components. Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at 1575 MHz, Low-cost GPS antennas and receivers are now widely available.
There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.
Using a GPS NTP server or a radio based NTP time server, network time clients, can be synchronized to within a few milliseconds of UTC depending on network traffic.