Atomic clocks are utilised to provide a highly precise timing reference. Many types of clock use the generic term 'atomic'. Most atomic clocks are very expensive systems generally only found in National Physics Laboratories. Most commercial precise timing references are based on National time and frequency radio or GPS time. Utilising a radio or GPS time source makes available a highly accurate reference of time without the expense and complexity of a true atomic clock.



This article describes available atomic clock timing references and their use in computer network time synchronisation and NTP time servers.

National Radio time and frequency broadcasts transmit accurate time and date information from a radio transmitter. A number of radio time and frequency broadcasts are available, such as: WWVB, DCF-77 and MSF-60. The time and frequency radio transmissions are referenced to a precise atomic clock time source. By utilising a relatively low-cost radio receiver PC and NTP server systems have access to precise timing information. Additionally, radio antennas can generally obtain a good signal indoors close to the host time server making installation very straight-forward. However, radio broadcasts have a finite range and are generally only available to a regional audience.

The DCF77 time broadcast is generated from from Frankfurt, Central Germany. The transmission is a long-wave signal broadcast at 77.5 kHz. The transmitters are maintained by T-Systems, a division of Deuche Telecom. The time and frequency broadcast is referenced to precise atomic clocks located at the German National Standards Laboratory (PTB) in Brunswick. The DCF-77 signal can be received using a low-cost radio receiver and when decoded provides a precise timing reference for computer time servers.

The MSF radio time and frequency transmission is broadcast from Anthorn, Cumbria in the United Kingdom. The signal is broadcast as a long-wave radio transmission at 60 kHz. The transmitted time and date information is referenced to atomic clocks sited at the UK National Physics Laboratory. The transmitters are maintained by VT Communications.

WWVB is the US National Time and Frequency radio broadcast. It is a 60 kHz transmission broadcast from Fort Collins, Colorado. The transmission is referenced to atomic clocks installed at the US National Institute of Standards and Technology (NIST). WWVB provides a time and frequency reference to within 100 microseconds of the correct time and has been in operation for 45 years.

The GPS system is a Global Positioning System intended for worldwide navigation. The GPS system consists of 24 satellites in high orbit. By utilising triangulation, the GPS system can provide highly accurate positioning information anywhere on Earth. In order to calculate position, each orbiting satellite has an on-board atomic clock timing reference. Atomic time is constantly transmitted from each satellite and is available to a GPS receiver. The GPS signal is available subscription-free anywhere on the face of the Earth. By utilising a low-cost GPS antenna and receiver accurate timing information can be made available to NTP server and computer time synchronisation systems. A GPS antenna does however need to be installed ideally on a rooftop with a good 360-degree view of the sky.

The Network Time Protocol (NTP) is the standard means of achieving network time synchronisation. NTP is used to distribute accurate time around the Internet and other computer networks. The standard NTP server distribution for LINUXUNIX is available free-of-charge under the GNU public licence and provides reference clock drivers for many radio and GPS receivers. Radio and GPS receivers are available with serial or USB ports that can interface to a PC or NTP time server to provide an accurate external timing reference. Depending on time source, NTP servers can synchronise to within a few microseconds of the correct time. Depending on network traffic they can synchronise network time clients to within a few milliseconds.

To summarise, there are a number of national and global time and frequency references available for synchronising computers and computer networks. Many time sources provide a highly accurate time reference that can provide synchronisation to within a fraction of the correct time. Utilising accurate radio or GPS timing references, precise time is freely available without the expense of installing an atomic time clock.