A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.
Frequency standards in a network or facility are sometimes administratively designated as primary or secondary. The terms primary and secondary, as used in this context, should not be confused with the respective technical meanings of these words in the discipline of precise time and frequency.
A frequency reference is an instrument used for providing a stable frequency of some kind. There are different sorts of frequency references, acoustic ones such as tuning forks but also electrical ones that emit a signal of a certain frequency (a frequency standard).
Among the most stable frequency references in the world are caesium-based atomic clocks (caesium standards) and hydrogen masers.
The carrier of time signal transmitters, LORAN-C transmitters and of several longwave and mediumwave broadcasting stations is derived from an atomic clock and can be therefore used as frequency standard.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188). Hydrogen masers offer the most precise measurements that are reproducible. For a long while there was a definition, based on cesium standards by those supporting cesium standard research. It was proven that the definition, did not consider the research that showed that hydrogen masers actually do measure out better, meaning to a much smaller increment of time. Being reproducible means that when researchers use atomic hydrogen maser clocks, the results of their research occur over and over, again, where as some research happens in a rare instance, but cannot be confirmed by additional research around the world. This is very important for highly precise measurements. Where as we do not need such precision on clocks as during the day as we go about common tasks, we don't even think about what takes a second, much less a nano second, however, if we're measuring quantities in the deep oceans, the distances of stars, or the relationship that a space ship has as it travels out into deep space, such differencs, can make all the difference. For example, if a spaceship travelling to another galaxy was off two seconds of a degree, it may very well pass completely by its target.
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