With atomic clocks calendar external NTP reference

Atomic clocks have been around for over fifty years or more. These are the atomic clocks that a resonant frequency to be used as a calendar item, rather than as a conventional oscillating quartz crystals.
Most atomic clocks use the resonance of cesium-133 in response to a specific frequency of vibration 9,192,631,770 every second. Since 1967, the international system of Units (SI) the second, the number of cycles of cesium atomic clocks -133, which means (sometimes called cesium oscillators) sets the standard for measuring time.
Since the resonance of cesium 133 is so precise that the atomic clock has an accuracy of less than 2 nanoseconds per day, about 1.4 million seconds in a year.

Developed as the atomic clocks are so accurate and can be a continuous and stable over time, universal time, I think the characteristics UTC (Coordinated Universal Time or Universal Time Coordinated), has created and maintains as the second jump he added - to slow the rotation compensation.
However, atomic clocks are extremely expensive and are usually found only in large physics laboratories. However, the NTP (Network Time Protocol), the standard for the implementation of time synchronization in computer networks, which can synchronize with an atomic clock, using the Global Positioning System (GPS) and radio network specialist.

The most commonly used is the GPS (Global Positioning System), developed by the U.S. military. Contains at least 24 GPS satellites in orbit high precision positioning and location information. Each GPS satellite can not do in turn is used as the reference time only by an atomic clock.
A GPS time server is the ideal time and frequency source because it can be that is extremely accurate in the world with relatively cheap components. Each GPS satellite transmits on two frequencies, L1 and L2 for military use to civilian use in 1575 MHz, GPS receivers and antennas at low cost are transferred now widely available.

There are also a number of national terms and radio frequency transmissions that are used that can synchronize with an NTP server. In Britain, the call sign (MSF) is supported by the National Physical Laboratory in Cumbria program, which serves the United Kingdom's time for the national reference, there are no similar systems in Colorado, USA (WWVB) and in Frankfurt am Main, Germany (DCF - 77).
These signals provides UTC time to an accuracy of 100 microseconds, but the radio signal has a limited scope and is vulnerable to disruption.
Using a GPS NTP server or a radio based NTP server, network clients can be synchronized with the time in milliseconds, depending on the traffic on the network clock.