| An atomic clock provides an extremely accurate | | | | Colorado, USA. The time reference is maintained by |
| source of time. There are various types of atomic | | | | the US National Institute of Standards and Technology. |
| clock, mostly found in laboratories: Caesium Clocks; | | | | WWVB has provided continuous time and frequency |
| Hydrogen Clocks; and Rubidium Clocks. Most | | | | broadcasts since 1962. The signal provides a timing |
| commercially available atomic clock time | | | | reference with an accuracy of less than 100 |
| synchronisation systems utilise a radio or GPS time | | | | microseconds. |
| signal that is linked to an accurate time reference. In | | | | GPS Atomic Clock Time Reference |
| this manner a highly accurate source of time is readily | | | | The Global Positioning System (GPS) is a US military |
| available to everyday users, without the expense of | | | | system for worldwide navigation. The system consists |
| installing highly expensive and complex equipment. | | | | of 24 orbiting satellites, each satellite has a highly |
| This article provides an overview of atomic clock | | | | accurate atomic clock on-board synchronised to UTC |
| systems with particular emphasis on their use with | | | | time. The satellites continuously broadcast time and |
| NTP server systems for PC and computer network | | | | position information. The time and position information |
| time synchronisation. | | | | can be obtained worldwide with a GPS receiver and |
| Radio Atomic Clock Time References | | | | antenna. GPS works continuously in any whether |
| Radio time transmissions such as the MSF-60 (UK), | | | | conditions, anywhere in the world. Additionally, there is |
| DCF-77 (Germany) and WWVB (US) time signals | | | | no set up fee or subscription charges to utilise the |
| broadcast highly accurate time information from a | | | | GPS systems. Many computer timing systems and |
| radio transmitter. The time transmissions are derived | | | | NTP Server systems utilise GPS as an accurate |
| from an atomic clock time reference, and can be | | | | external timing reference. GPS timing is generally much |
| received by timing equipment with a low-cost radio | | | | more accurate than radio based timing references. |
| receiver. The MSF-60 radio signal is transmitted from | | | | Computer Time Synchronisation |
| Rugby in the United Kingdom with coverage of the | | | | Accurate computer time synchronisation can be |
| whole of the British Isles and much of North-Western | | | | achieved by combining a GPS or Radio timing receiver |
| Europe. The DCF-77 time signal is transmitted from | | | | with a RS232 or USB interface. Software drivers can |
| Frankfurt, Germany, and covers most of Central | | | | then be installed on the host PC to obtain accurate |
| Europe. While the WWVB signal is transmitted from | | | | time and synchronise the host PC’s system time. |
| Fort Collins, Colorado, USA. | | | | Most PC operating systems can be synchronised, |
| MSF-60 Time Transmission | | | | including Microsoft Windows 2000, 2003 and XP, |
| The MSF-60 time signal is a long wave radio time | | | | LINUX, UNIX and Novell. In many cases, the host |
| signal broadcast at 60kHz from Rugby in the UK. The | | | | PC’s system time can be synchronised to within a |
| radio signal is maintained by BT Engineering Services. | | | | few microseconds of the correct time. |
| The MSF-60 time signal is generated from extremely | | | | Network Time Synchronisation |
| accurate atomic clocks located at the National Physics | | | | The standard protocol for achieving computer network |
| Laboratory (NPL). When decoded, it provides a highly | | | | time synchronisation is the Network Time Protocol |
| accurate timing reference for NTP servers, reference | | | | (NTP). NTP is the standard way of distributing time |
| clocks and other computer timing equipment. | | | | around the Internet and other networks. Stratum 1 NTP |
| DCF-77 Time Transmission | | | | servers obtain time from an external timing reference, |
| The DCF-77 time signal is a long-wave radio time | | | | such as GPS, MSF-60, DCF-77 or WWVB. The |
| signal broadcast at 77.5kHz from Mainflingen, near | | | | external time reference is then used to synchronise |
| Frankfurt, Germany. The radio signal is maintained by | | | | the NTP server system time. The synchronised |
| T-Systems, a sub-division of Deuche Telecom, and | | | | system time is then used by the NTP Server to |
| has been in operation since 1959. The DCF-77 signal is | | | | distribute accurate time to network time clients over an |
| generated from extremely accurate atomic clocks | | | | IP network. NTP operates in a hierarchical manner; |
| located at the German National Physics Laboratory. | | | | lower stratum NTP servers obtain time from higher |
| When decoded, it provides a highly accurate timing | | | | stratum devices. |
| reference for clocks and computer timing equipment. | | | | NTP server systems can synchronise to within a few |
| WWVB Time Transmission | | | | microseconds of the correct time. Depending on |
| The WWVB time signal continuously broadcasts time | | | | network traffic, NTP time clients can synchronise to |
| and frequency signals at 60 kHz from Fort Collins, | | | | within a few milliseconds of a NTP server. |