What is Really the Best Source of UTC Time?

Coordinated Universal Time, or UTC as it is more commonly known, is now the world’s official global timescale. It replaced the old Greenwich Meantime (GMT) back in the seventies. Greenwich Meantime was based around the movements of the Earth’s sun; UTC is based upon the time as shown by atomic clocks. It is kept “in line” with GMT with the addition of so called “leap seconds” that are used to compensate for such changes as the slowing of the Earth’s rotation at certain times of the year.
In the world of computers UTC is used by computer networks across the globe to synchronize to the same exact time, making time sensitive transactions across the globe efficient and reliable. Many computer networks utilize a dedicated network time server (NTP) to carry the precise time across the entire network and ensure there is no drift or disagreement between the machines.
While most computer experts agree that the use of a dedicated NTP server is the best course of action for any computer network, they do tend to disagree about where the time source should come from. There are three alternatives that are commonly in use.
There is of course the Internet. Sites such as time.nist.gov and time.windows.com means that a dedicated NTP server is not required. However even Microsoft themselves do not advise the use of an internet based time source if security is in any way an issue, which for the majority of computer networks it certainly is. It is not possible to authenticate an internet time source and allowing any program to operate continually outside a firewall can be very dangerous, as it leaves the entire network open to security breaches.
There are NTP servers that utilize GPS. These devices make use of the same technology as the GPS navigational system in many cars today. GPS is in fact simply a time code generated by an atomic clock located on the satellite itself. This signal is available almost anywhere in the world, but in order to be reliable the receiving antenna has to have a clear view of the sky at all times.
The alternative to GPS for dedicated NTP time server is to use the long range radio transmissions sent out by many national physics laboratories. The downside to this method is that the signals are rather vulnerable to local interference and weather patterns, and may be blocked by tall buildings and the local topography in general.