This article discusses a number of aspects associated with Atomic Clock time references and specifically their use in computer systems and network time synchronization. Topics covered:
- What is an Atomic Clock?
- National Time and Frequency Radio Broadcasts
- Time and the Global Positioning System
- The Galileo GNSS System
- Synchronizing Computer Systems and Networks
What is an Atomic Clock?
Atomic clocks are extremely complex and astronomically expensive pieces of equipment. They are used to provide an extraordinarily precise time reference. Typically, they will only lose one second in one million years ! Due to their exorbitant cost however, most Atomic Clocks are only found in Government backed National Physics Laboratories, such as NPL in the UK and NIST in the US. You may ask, what use are these ultra-accurate clocks if they are hidden away in national laboratories. Fortunately, they are often used as the basis of national time and frequency broadcasts that anyone can receive completely free-of-charge. Radio time broadcasts such as MSF, DCF-77 and WWVB can be received free-of-charge with low-cost equipment to provide an accurate time reference. Additionally, the Global Positioning System (GPS) is a satellite based source of very precise time.
Atomic clocks are currently the most accurate clocks in the world. They are used as the primary reference clock for all national time standards. The technology behind atomic clocks is based on atomic physics, by detecting the minuscule microwave signal that an electron emits when it changes energy levels.
New developments using quantum clocks are providing the most accurate clocks ever constructed. Clocks that are accurate to one second in one billion years are now a reality. Improving the accuracy of any measurement, including time, can provide any number of advances in many other often unrelated areas.
National Time and Frequency Radio Broadcasts
There are national time and frequency radio broadcasts available in a number of countries across the world. Each time signal is referred to by it’s handle, DCF for the German signal, MSF for the UK signal, WWVB for the US time code signal.
Handle Location Frequency Power Output Range
DCF Meinflingen, Germany 77.5 KHZ 50 KW 1500 Km
MSF Anthorne, UK 60 KHz 17 KW 1000 Km
WWVB Colorado, USA 60 KHz 70 KW
HBG Switzerland 75 KHz Service Terminated
JJY Japan 40 KHz 50 KW 1500 Km
TDF France 162 KHz 2 MW 3500 Km
The transmissions are reference to atomic clock standards and provide an accurate source of time to anyone with the equipment to receive and decode the radio signal. The signals transmit an encoded time and date stamp along with a marker at the start of each minute. The most popular signals in Europe are the MSF and DCF signals, which require only simple low-cost receiving equipment. The French TDF signal is a very powerful long-range signal that has a range of 3500 Km, however, it requires much more complex equipment to receive and decode timing information.
Low-cost radio receivers can generally be used to receive these signals. A simple indoor-located antenna can often be used making installation very straight-forward. However, radio signals have a limited range and often signal reception can be challenging if located at the extremes of reception range.
The DCF-77 transmitter is located in Mainflingen, near Frankfurt, Germany. The transmission is a long-wave signal broadcast at 77.5 KHz. The system comprises of two transmitters which are maintained by T-Systems, a division of Deuche Telecom. The broadcast is referenced to atomic clocks located at the German National Standards Laboratory (PTB) in Brunswick.
The MSF transmitter is located in Anthorn, Cumbria in the United Kingdom. The transmission is a long-wave radio signal broadcast at 60 KHz. The transmitters are maintained by a private company, VT Communications. The broadcast is referenced to clocks sited at the UK National Physics Laboratory, Teddington, London.
WWVB is the handle of the United States of America National Time and Frequency radio broadcast. It is a 60 kHz transmission broadcast from Fort Collins, Colorado. The transmission is referenced to clocks installed at the US National Institute of Standards and Technology (NIST). WWVB has been in continuous operation since 1962 and provides a timing reference with accuracy better than 100 microseconds.
Atomic Time and the Global Positioning System
The Global Positioning System, or GPS as it is widely known comprises of a constellation of 24 orbiting satellites. Primarily intended as a global positioning and navigation system, very precise timing information is also transmitted. Each orbiting satellite has an integral Atomic Clock that is periodically synchronized to clocks located at NIST as each satellite orbits overhead. By calculating the time signals take from three or more satellites to reach a receiver on Earth and using triangulation, very precise positioning information can be obtained. The timing information provided by GPS can also be used by other applications requiring very precise time.
The GPS signals can be received anywhere on the face of the Earth by utilising a low-cost GPS antenna and receiver. Additionally, the system is provided as a free-to-air, subscription-free service. However, a GPS antenna ideally needs to have line-of-sight location in order to receive the weak radio signals transmitted from each satellite. Therefore, the antenna generally needs to be located outdoors or even on a roof-top to provide good signal reception. This can increase installation costs and require surge-suppression devices to protect against lightning strikes. The GPS system is a US military system. Some users are distrustful of the fact that the system is maintained by the US military, mainly because of the potential of service disruption in times of war.
The Galileo GNSS System
Galileo is the new civilian European global satellite-based navigation system (GNSS) that is currently under construction by the European Union and European Space Agency (ESA). When fully operational in 2019, it will provide a civilian alternative to the current military maintained GPS system.
Synchronizing Computer Systems and Networks
To summarize, there are many national radio time and frequency broadcasts that are referenced to atomic clocks and can provide synchronization of computer clocks and networks. The GPS systems can provide sub-microsecond timing anywhere in the world. New GNSS systems coming on-line in the next few years will also add to the choice of systems. The free-to-air radio and GPS timing broadcasts provide the accuracy of atomic clock time synchronization without the cost and complexity.
Andrew Shinton has worked in the Computer Time and Frequency sector for almost his entire career. He now leads TimeTools development department. Andrew has written many articles that help IT professionals make informed decisions about network and computer systems timing solutions.