NIST time and frequency services
Michael A. Lombardi, Michael A. Lombardi · 2002
How NIST Provides Time and Frequency Standards for the United States The National Institute of Standards and Technology (NIST) maintains the standards for time and frequency for most users in the United States.NIST provides a variety of services designed to deliver time and frequency signals to the people who need them.The signals are broadcast via several mediums, including high and low frequency radio, the Internet, and telephone lines.These signals are used to synchronize millions of clocks everyday, throughout the United States and around the world.This booklet is a guide to NIST Time and Frequency Services.It describes the signals and services offered by NIST, how they work, and how you can use them.Beginning with Chapter 2, we'll take a detailed look at each of the time and frequency services that NIST provides.However, let's begin by discussing why time and frequency services are needed in the first place, and how NIST provides and controls them. Who Needs Time and Frequency Standards?Everybody needs time and frequency standards.If we stop and think about it, time and frequency standards are involved in one way or another in just about everything we do.Time and frequency standards supply us with three basic types of information.The first type, date and time-of-day, tell us when something happened.Date and time-of-day can be used to record events, or to make sure that multiple events are synchronized, or happen at the same time.It's easy to think of ways we use date and time-of-day in our everyday lives.For example, we use date information to remind us when birthdays, anniver- saries, and other holidays are scheduled to occur We use time-of-day information to set who needs them.If a measurement is made using a NIST reference, and if the uncertainty of the measurement is known and documented, the measurement is said to be traceable.Establishing tmceability is important to many organizations, because it helps them prove that their measurements are being made correctly.In some cases, traceability is even a legal or contractual requirement.NIST strives to develop in-house measurement capabilities that exceed the highest requirements of users in the United States.Since these requirements become more demanding every year, NIST scientists and researchers are continually developing new standards and measurement techniques to keep up with this demand.Wliile these new standards are being developed, other NIST personnel are busy distributing the existing standards and measurement techniques, so that everyone can make traceable measure- ments that are nearly as good as those made inside the national laboratory.Although most of NIST is located in Gaithersburg, Maryland, the Time and Frequency divi- sion is located in Boulder, Colorado (Figure 1.1).The time and frequency services con- trolled from Boulder are excellent examples of how NIST is able to distribute its standards and measurement capability to a wide variety of users throughout the United States.Atomic Time and tlie Definition of tiie Second We mentioned earlier that 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.Tliis means that the second and the other base units are defined the same way all over the world.As a result, the timekeeping standards maintained by the major countries tend to closely agree with each othertypically to witliin 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-Fl, is a custom device that was designed and built at NIST (Figure 1.2).The primary standard is used to help calibrate the ensemble.