Most computers have internal clocks dedicated to maintaining system time. The internal real-time clocks rely on relatively cheap crystal oscillators, which are very inaccurate. In many cases computer system time can drift by several minutes each day. This article describes various methods of keeping a highly accurate time on your computer. It describes how to utilise Internet time servers, radio and GPS clocks and dedicated NTP server systems.



Internet Time Servers

The Internet utilises the Network Time Protocol, NTP, to disseminate accurate time information to network time clients. NTP has been an important part of the Internet for more than 25 years. The protocol was born through the desire to provide synchronization of time critical procedures throughout the Internet.

Most modern operating systems, including Microsoft Windows 2003, XP, 2000, UNIX and LINUX have the ability to synchronize time with a NTP Time Server. There are a host of Internet based NTP Time Servers that the public can access that can be used to synchronise your computer. Below is a list of popular Internet based public access NTP Server time references:

ptbtime1.ptb.de - German NTP server time reference.

canon.inria.fr - French NTP server time reference.

time.nist.gov - US based NTP server time reference.

clock.isc.org - Canadian NTP server time reference.

ntp.my-inbox.co.uk - UK NTP server time reference.

Microsoft Windows XP has pre-installed SNTP client software that can synchronize time with a NTP server time reference. This is achieved by simply entering the domain name of a NTP Server in the time properties tab. The host computer will then periodically contact the NTP Server and synchronise the system time to the specified NTP reference.

LINUX and UNIX operating systems have the network time protocol software distribution available from the NTP website. The NTP application runs, in background, as a daemon, constantly monitoring specified NTP servers. The NTP daemon application reads a list of NTP servers from a configuration file, ‘ntp.conf’, and periodically synchronizes time. A list of NTP server references is specified with the ‘server’ configuration command thus:

server clock.isc.org # Canadian NTP server time reference

server ntp.my-inbox.co.uk # UK NTP server time reference.

Once configured, the network time protocol daemon can be started, stopped and restarted using the daemon commands: ‘ntpd start’; ‘ntpd stop’ and ‘ntpd restart’.

GPS Clock and Radio Time References

There a number of commercially available GPS clock and radio time references for computers. GPS and radio clocks obtain accurate time from national radio timing references or from the global positioning system. The devices have either serial or USB computer interfaces and provide an accurate timing reference to computers. Software drivers periodically synchronise the system time of the host computer to the received reference time. Often reference clock drivers are available that interface to the NTP daemon to provide your own dedicated NTP server resource.

LF radio time receivers often have the advantage of having the ability to obtain a signal, indoors. However, the radio signals can only be received locally to the reference transmitter. There are a number of world-wide radio time broadcasts available: WWVB transmitted from Colorado, USA; CHU, Canada; DCF-77 transmitted from Frankfurt, Germany; MSF transmitted from Rugby, UK and TDF, France. GPS has the advantage of providing a world-wide time and frequency system, however, an externally mounted antenna is required which can be impractical in many cases.

Dedicated NTP Servers

NTP servers are time servers that obtain time from an external timing reference, such as GPS or radio, and provide a network with an accurate time resource. NTP servers are usually rack-mounted devices with an antenna and an LAN connection. The devices obtain accurate time from a radio or GPS timing transmission and maintain an accurate internal time. The accurate time is distributed to network clients using UDP over an IP network. Dedicated NTP servers often minimize the set-up and configuration time required to get a NTP server up and running.