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KF5JRV > TECH 01.04.16 04:23l 33 Lines 2558 Bytes #999 (0) @ USA
BID : 716_KF5JRV
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Subj: Atomic Time
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Sent: 160401/0122Z 716@KF5JRV.#NWAR.AR.USA.NA BPQ1.4.65
Atomic Time and the Definition of the Second
The standard unit for time interval is the second (s). Since 1967, the second
has been defined as the duration of 9,192,631,770 cycles of the
radiation associated with a specified transition of the cesium atom.
Frequency (expressed in hertz) is obtained by counting events over a 1 s interval.
The second is one of the seven base units of measurement in the International System
of Units (SI). These units are used to express the values of the seven physical quantities
that we mentioned earlier. The seven base units were defined by international agreement
and all other units of measurement can be derived from them. The International Bureau
of Weights and Measures (BIPM) located near Paris, France, is responsible for ensuring
that the major countries of the world use the SI units. This means that the second
and the other base units are defined the same way all over the world. As a result, the time-
keeping standards maintained by the major countries tend to closely agree with each
other—typically to within one microsecond, and often to within a few nanoseconds.
Since the second is defined based on a property of the cesium atom, it should come as
no surprise that the electronic device that produces the standard second is called a
cesium oscillator. Cesium oscillators (and other types of atomic oscillators) are called
intrinsic standards , because they produce frequency based on a natural phenomena,
in this case a property of an atom. NIST maintains an ensemble of atomic oscillators in
Boulder, Colorado. The outputs of these oscillators are averaged together to produce the
national standard for time and frequency. Most of the oscillators in the ensemble
are commercially available, but the primary standard, called NIST-F1, is a custom device
that was designed and built at NIST. The primary standard is used to help calibrate the ensemble.
NIST-F1 became operational in late 1999, and is the latest in a long line of NIST primary
time and frequency standards. NIST-F1 is a cesium fountain frequency standard, and has
many performance advantages over the earlier cesium beam standards.
NIST-F1 is one of the most accurate clocks in the world, and can keep time
to within about 0.1 nanoseconds per day. Along with the other atomic clocks in the
ensemble, NIST-F1 provides the reference for the NIST time and frequency services.
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