Atomic clock devices are expensive peices of equipment mainly confined to national laboratories. However, there are a number of atomic clock references available free to air for computer time synchronisation.
Firstly the MSF radio time signal is synchronised to the UK's National Physics Laboratory and broadcast as a long wave radio signal from Cumbria, UK. The radio signal is broadcast as a sequence of 60 pulses, one pulse per second, each pulse constitutes a data bit. A mintes worth of data bits encode the time and date information. The data string is terminated on the minute boundary by a marker to signify the start of a minute.
The MSF radio signal is broadcast from Cumbria UK and covers the whole of the British Isles and is available in much of North-Western Europe.
Galleon Systems NTP Servers and Radio based time servers have MSF reception options.