The NTP implementations anticipate the use of three major kinds of external clock sources:
External NTP servers
Many ISPs and government agencies offer access to NTP servers that are in turn synchronized with the international standard clocks. This access is usually offered on a restricted basis.
External clock broadcasts
The US, Canada, Germany, and others offer radio broadcasts of time signals that may be used by local receivers attached to an NTP server. The US and Russia broadcast time signals from satellites, e.g. GPS. Some mobile telephone services broadcast time signals. These signals are synchronized with the international standard clocks. GPS time signals are popular worldwide time sources. Their primary problem is difficulties with proper antenna location and receiver cost. Most of the popular low cost consumer GPS systems save money by sacrificing the clock accuracy.
External pulse sources
For extremely high accuracy synchronization, atomic clocks can be attached to NTP servers. These clocks do not provide a time estimate, but they provide a pulse signal that is known to be extremely accurate. The optimal estimation logic can use this in combination with other external sources to achieve sub microsecond synchronization to a reference clock even when the devices are separated by the earth’s diameter.
The details regarding selecting an external clock source and appropriate use of the clock source are outside the scope of the NTP protocol. They are often discussed and documented in conjunction with the NTP protocol and many such interfaces are included in the reference implementation of NTP.