NTP is a standard Internet protocol for distributing accurate time to network time clients. There are a large number of public NTP time servers available on the Internet. However, it may often be the case, for security purposes, that an organisation may require an internal stratum 1 NTP time server. This article discusses a number of time sources that can be used as a reference clock for a stratum 1 time server.

Probably the most widely used NTP reference clock resource is currently the Global Positioning System (GPS). The GPS system consists of a number of orbiting satellites providing accurate positioning and location information. However, each GPS satellite also has an integral highly accurate atomic clock that can be used as a timing reference. Utilising the GPS system for time synchronisation has a number of advantages. The Global Positioning System is ideal to provide an accurate timing reference utilising low-cost components. A typical GPS receiver can provide timing information to within a few nanoseconds of UTC. Also, provided the antenna is shown a good view of the sky, the GPS timing signal can be received world-wide. The disadvantage of using the GPS system is the very fact that the antenna requires a view of the sky. Often sitting an antenna on a rooftop with a good view of the sky can be impractical and expensive.

There are also a number of national time and frequency radio transmissions that can be used to synchronise a stratum 1 NTP server. The advantage of using a radio time reference is that generally a good signal can be obtained indoors close to the host computer. This can considerably reduce system installation costs. However, radio time references are generally less accurate than GPS systems. Typically, national time and frequency transmissions are accurate to 1 - 20 milliseconds. The radio transmission is regional and has a finite range dependant on transmitter power. Additionally, local interference or environmental issues can cause problems with reception. Placing a radio antenna too close to electrically noisy equipment can result in signal loss. Also, locating an antenna inside a metal enclosure or underground in a basement can also be a problem.

A number of national time and frequency standards are available broadcasting to the local region.

The WWVB time and frequency signal is a 60kHz radio broadcast transmitted from Fort Collins, Colorado, USA. The WWVB signal continuously transmits time and frequency signals throughout the US and much of Northern America. The signal provides UTC time to an accuracy of 100 microseconds.

The DCF-77 time and frequency signal is transmitted from Frankfurt, Germany. DCF-77 broadcasts timing information continously at 77.5kHz. The transmission covers Germany and much of Central and North Western Europe. Time and frequency information is broadcast to an accuracy of <20msec.

The MSF radio time and frequency broadcast is transmitted at 60kHz from Anthorn, Cumbria, UK. The broadcast covers the British Isles and much of North Western Europe. Time and frequency information is broadcast to an accuracy of <1msec.

The TDF time and frequency radio broadcast is transmitted from Allouis, France. It continuously broadcasts time and frequency information at 162kHz. The transmitter has a power output of 2000kW, with coverage of France and much of Central and Western Europe.

The JJY long-wave time and frequency broadcast is transmitted from Tokyo, Japan. The broadcast is continuously transmitted at 60kHz and covers much of Japan and Far Eastern Asia.

To conclude, there are many sources of time and frequency reference broadcasts available. When selecting a timing reference for a time server, expense, geographic availablilty, accuracy and ease of installation should all be taken into account.