Choosing an NTP server reference clock

NTP is the standard protocol for distributing accurate time around the Internet to time clients. There are many public stratum 1 NTP servers that reside on the Internet. However, often it may be necessary for an organisation to install a private local stratum 1 NTP server. This article describes a number of reference clock solutions available for synchronizing a stratum 1 NTP 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. Additionally, the GPS signal can be received anywhere in the world, where the antenna can be provided with a good view of the sky.

The disadvantage of using the GPS system is the very fact that the antenna requires a view of the sky. In many installations, locating an antenna on a rooftop can be impractical or expensive.

Radio reference standards

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:


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.

About the author

David Evans is a highly experienced technical author to the timing reference and telecommunications industry. David has provided a technical authoring resource to many Windows NTP server manufacturers and computer timing solutions providers. Click here for more detailed information about Windows NTP Server solutions.