Atomic clocks are the most accurate time keeping devices in existence, and use the oscillation of atoms stimulated by an electromagnetic field as a frequency standard for keeping time. There are various types of atomic clock, the most common being Caesium Clocks and Rubidium Clocks, and they are most commonly found in research facilities. Atomic Clocks are utilised as the primary standard for time distribution services worldwide. Most commercially available atomic clock time synchronization systems use a radio or GPS time broadcast which is connected to a precise time reference.
Here we discuss atomic clock applications in relation to time synchronization of PC’s and computer networks using NTP server systems, covering:
- GPS Atomic Clocks
- Radio Atomic Clocks
- MSF 60
- Time Synchronization of PC’s
- Time Synchronization of Computer Networks
GPS Atomic Clocks
The Global Positioning System, more commonly known as GPS, began as an US military application with the purpose of attaining highly accurate positioning information for global navigation. Operating via 24 satellites that orbit the Earth, each individual satellite is equip with a highly accurate atomic clock, synchronized to Coordinated Universal Time, UTC time. The satellites supply a constant stream of time and positioning information which can be received anywhere in the world using a GPS antenna and receiver, demonstrating its global application. Additionally, there are no set up or subscription fees in utilising the GPS system or the information it provides. GPS is considered to be one of the better external reference clocks available, providing a higher degree of accuracy than its radio alternative, resulting in the GPS system being referenced by computer timing systems and NTP Server systems worldwide.
Radio Atomic Clocks
Radio time broadcasts transmit precise time information from a radio transmitter. The broadcasts are copied from an atomic clock time reference and can be obtained with the use of a relatively low-cost radio receiver. There are numerous radio time broadcasts worldwide, however geographical location can influence signal strength and consistency.
MSF-60 Time Signal
The MSF 60 radio broadcast is a UK transmission which began in the 1950’s and was first transmitted from Rugby, Warwickshire. Due to its location it was more commonly known as the ‘Rugby Time Signal’ or ‘Rugby Clock’. In 2007 the transmission was moved to Anthorn, Cumbria, and serves the whole of the British Isles and parts of North-Western Europe. Operating on a frequency of 60kHz the MSF 60 signal is a long-wave radio time broadcast controlled by caesium atomic clocks located at the National Physics Laboratory (NPL). The signal can be decoded by a range of radio-controlled clocks and can act as a very accurate time reference for NTP time servers, reference clocks and other computer timing devices.
DCF-77 Time Signal
DCF-77 is a German long-wave radio time signal broadcast, transmitted at 77.5kHz. Unlike the UK which operates a lone transmitter, Germany runs two transmitters which operate as a primary and a back up transmitter and both broadcast from Mainflingen near Frankfurt. The DCF-77 signal began on 1 January 1959 and is generated from local atomic clocks that are linked to Master Clocks located at the Physikalisch Technische Bundesanstalt (PTB), Germany’s National Physics Laboratory. Maintained by Media Broadcast GmBH, the DCF-77 signal covers a large area of Europe and can be utilised as a precise time reference for timing equipment.
WWVB Time Signal
Transmitted from Fort Collins in Colorado, WWVB is the US radio time broadcast and is transmitted at 60kHz. The transmission began in 1962, and is maintained by the US National Institute of Standards and Technology, (NIST). While most broadcasts signal the local time of the broadcasting nation, the WWVB signal broadcasts time in UTC time, Coordinated Universal Time. The reason being the US broadcast covers multiple time zones, so offsets for time zones have to be applied as needed.
Time Synchronization of PC’s
Accurate computer time has become an increasingly important feature of commercial computer applications. The synchronization of computers to accurate time can be attained by combining a GPS or Radio timing receiver with a RS232 or USB interface. With the installation of software drivers on the host computer, it is possible for the PC to acquire accurate time to synchronization its internal system time. Generally the host computer’s system time can be synchronized to within a few microseconds of the correct time, and popular operating systems such as Microsoft Windows 2000, 2003, XP, LINUX, UNIX and Novell can be synchronized.
Time Synchronization of Computer Networks
Network Time Protocol, more commonly referred to as NTP, is a computer standard developed to distribute accurate time to computers and computer networks. NTP is a client-server based protocol utilised for computer time synchronization throughout the Internet and local networks. NTP was originally developed by Dr David Mills, of the University of Delaware, who acknowledged the need to provide a standard means of synchronizing time across the Internet. Stratum 1 NTP servers reference an external reference clock, such as GPS or radio time and frequency broadcasts in order to synchronize its system time. This accurate time stamp can then be distributed by the NTP server to network time clients over an IP network. NTP is organized in hierarchical form; primary servers known as Stratum 1, secondary servers (Stratum 2) and time clients. The accuracy of NTP server systems can be to within a few microseconds of precise time resulting in the NTP time clients being able to synchronize to within a few milliseconds of a NTP server.
Andrew Everett specialises in the continued development of accurate time synchronization equipment for computers and computer networks, encompassing GPS clocks and NTP Time Servers. Andrew contributes to the development on dedicated time servers, NTP synchronized clock applications and atomic clock synchronization.