An atomic clock provides an extremely accurate source of time. There are various types of atomic clock, mostly found in laboratories: Caesium Clocks; Hydrogen Clocks; and Rubidium Clocks. Most commercially available atomic clock time synchronisation systems utilise a radio or GPS time signal that is linked to an accurate time reference. In this manner a highly accurate source of time is readily available to everyday users, without the expense of installing highly expensive and complex equipment.

This article provides an overview of atomic clock systems with particular emphasis on their use with NTP server systems for PC and computer network time synchronisation.

Radio Atomic Clock

Time References Radio time transmissions such as the MSF-60 (UK), DCF-77 (Germany) and WWVB (US) time signals broadcast highly accurate time information from a radio transmitter. The time transmissions are derived from an atomic clock time reference, and can be received by timing equipment with a low-cost radio receiver. The MSF-60 radio signal is transmitted from Rugby in the United Kingdom with coverage of the whole of the British Isles and much of North-Western Europe. The DCF-77 time signal is transmitted from Frankfurt, Germany, and covers most of Central Europe. While the WWVB signal is transmitted from Fort Collins, Colorado, USA.

MSF-60 Time Transmission

The MSF-60 time signal is a long wave radio time signal broadcast at 60kHz from Rugby in the UK. The radio signal is maintained by BT Engineering Services. The MSF-60 time signal is generated from extremely accurate atomic clocks located at the National Physics Laboratory (NPL). When decoded, it provides a highly accurate timing reference for NTP servers, reference clocks and other computer timing equipment.

DCF-77 Time Transmission

The DCF-77 time signal is a long-wave radio time signal broadcast at 77.5kHz from Mainflingen, near Frankfurt, Germany. The radio signal is maintained by T-Systems, a sub-division of Deuche Telecom, and has been in operation since 1959. The DCF-77 signal is generated from extremely accurate atomic clocks located at the German National Physics Laboratory. When decoded, it provides a highly accurate timing reference for clocks and computer timing equipment.

WWVB Time Transmission

The WWVB time signal continuously broadcasts time and frequency signals at 60 kHz from Fort Collins, Colorado, USA. The time reference is maintained by the US National Institute of Standards and Technology. WWVB has provided continuous time and frequency broadcasts since 1962. The signal provides a timing reference with an accuracy of less than 100 microseconds.

GPS Atomic Clock Time Reference

The Global Positioning System (GPS) is a US military system for worldwide navigation. The system consists of 24 orbiting satellites, each satellite has a highly accurate atomic clock on-board synchronised to UTC time. The satellites continuously broadcast time and position information. The time and position information can be obtained worldwide with a GPS receiver and antenna. GPS works continuously in any whether conditions, anywhere in the world. Additionally, there is no set up fee or subscription charges to utilise the GPS systems. Many computer timing systems and NTP Server systems utilise GPS as an accurate external timing reference. GPS timing is generally much more accurate than radio based timing references.

Computer Time Synchronisation

Accurate computer time synchronisation can be achieved by combining a GPS or Radio timing receiver with a RS232 or USB interface. Software drivers can then be installed on the host PC to obtain accurate time and synchronise the host PC's system time. Most PC operating systems can be synchronised, including Microsoft Windows 2000, 2003 and XP, LINUX, UNIX and Novell. In many cases, the host PC's system time can be synchronised to within a few microseconds of the correct time.

Network Time Synchronisation

The standard protocol for achieving computer network time synchronisation is the Network Time Protocol (NTP). NTP is the standard way of distributing time around the Internet and other networks. Stratum 1 NTP servers obtain time from an external timing reference, such as GPS, MSF-60, DCF-77 or WWVB. The external time reference is then used to synchronise the NTP server system time. The synchronised system time is then used by the NTP Server to distribute accurate time to network time clients over an IP network. NTP operates in a hierarchical manner; lower stratum NTP servers obtain time from higher stratum devices.

NTP server systems can synchronise to within a few microseconds of the correct time. Depending on network traffic, NTP time clients can synchronise to within a few milliseconds of a NTP server.