"Running Your Own Stratum-1 Servers
Stratum-1 time servers are available in a single 1U rack-mountable form factor that can easily be installed in your server room or data center and connected to your network, and most have a way of connecting to a stratum-0 reference clock built in. The most commonly used ways to connect to a stratum-0 device are by terrestrial radio or GPS signals.
"Terrestrial radio based connections use radio signals such as WWVB out of Fort Collins, Colorado, MSF from Anthorn, UK, or DCF77 from Frankfurt, Germany. This is similar to the way consumer devices such as watches and alarm clocks update themselves with signals from reference clocks to keep accurate time.
"Statum-1 time servers that sync with GPS satellite signals are more accurate, but are less convenient to install as they need to be connected to an antenna fitted in a suitable position on the roof of the building. Using time data from a number of satellites, and by calculating the distance of each satellite from the antenna, a stratum-1 time server that uses GPS reference clock signals is able to get the precise time to within 50 or so nanoseconds."
