PC's have internal real time clocks designed to maintain system time. The internal clocks rely on relatively low-cost crystal oscillators, which are notoriously inaccurate. In many cases PC system time can drift by several minutes each day. This article describes various methods of maintaining a highly accurate time on your PC. It describes how to utilise Internet time references, radio and GPS clocks and dedicated NTP server systems.

Internet Time References

The Internet utilises the Network Time Protocol to distribute accurate time information to network time clients. NTP has been an important part of the Internet for over 25 years. The protocol was born through the need to provide synchronisation of time critical processes across the Internet.

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

time.nist.gov - US based time reference.
ptbtime1.ptb.de - German time reference.
ntp.my-inbox.co.uk - UK time reference.
canon.inria.fr - French time reference.
clock.isc.org - Canadian time reference.

Windows XP has pre-installed SNTP client software that can synchronise time with a NTP server. This is achieved by simply entering the domain name of an Internet 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 reference.

LINUX and UNIX systems have the NTP software distribution available from the NTP website, ntp.org. The NTP application runs as a daemon, constantly monitoring specified NTP servers. The application reads a list of NTP servers from a configuration file, 'ntp.conf', and periodically synchronises time. A list of NTP server references is specified with the 'server' configuration command thus:

server time-a.nist.gov # NIST, Gaithersburg, Maryland NTP server
server time-c.timefreq.bldrdoc.gov # NIST, Boulder, Colorado NTP server

Once configured, the NTP daemon can be started, stopped and restarted using the commands: 'ntpd start'; 'ntpd stop' and 'ntpd restart'.

Radio and GPS Clock Timing References

There are many commercially available radio and GPS clock timing references for PC's. Radio and GPS clocks obtain accurate time from national radio timing references or from the GPS system. The devices have serial or USB interfaces and provide an accurate timing reference to PC's. Driver software periodically synchronises the system time of the host computer to the received reference time. Often reference clock drivers are available that interface to the NTP application software to provide your own dedicated NTP server.

Radio time receivers often have the advantage of having the ability to obtain a good signal, indoors, close to the host computer. However, the signal can only be received locally to the transmitter. There are a number of radio time broadcasts available: MSF transmitted from Rugby, UK; DCF-77 transmitted from Frankfurt, Germany; WWVB transmitted from Colorado, USA; CHU, Canada and TDF, France. GPS has the advantage of providing a global time and frequency solution; however, an externally located antenna is required which can be impractical.

Dedicated NTP Servers

NTP servers are dedicated time servers that obtain time from an external source, such as radio or GPS, and provide a network with an accurate timing reference. NTP servers are generally rack-mountable devices with an external antenna and an Ethernet connection. The devices obtain time from a radio or GPS timing reference and maintain an accurate internal time. The accurate time is then distributed to network time clients over an IP network. Dedicated NTP servers often minimize the set-up and configuration effort required to get a NTP server installation up and running.