FAQS - Frequently Asked Questions
What time does NIST distribute through its time and frequency
NIST provides UTC(NIST), a time scale referenced to atomic oscillators
located in Boulder, Colorado. At its source, UTC(NIST) is kept
in as close agreement as possible with other national and international
standards, typically within a few nanoseconds. The
current difference between UTC and UTC(NIST) is shown here.
The accuracy of the time sent to user depends upon the service
used to transfer time, and the receiving equipment that is used.
The simplest services may have uncertainties as large as 1 second,
but still meet the needs of most users. For example, to see the
current time displayed in your browser, see the http://nist.time.gov site.
How is Coordinated Universal Time (UTC) currently calculated?
There are two ways to think about Coordinated Universal Time (UTC).
The way it is usually thought of by most people is as an indicator
of time-of-day (hours, minutes, and seconds). For example, a wall
clock can display UTC in hours, minutes, and seconds. The second
way is to think of UTC as a stable frequency or rate which is used
to count seconds. These seconds are then accumulated to form minutes,
hours, days, and years. Let's take a brief look at UTC as a measure
of both time-of-day and frequency. When you use UTC for time-of-day,
keep in mind that it refers to local time at the zero meridian
which is near Greenwich, England. The UTC minutes and seconds are
exactly the same as your local time, but the hours are different.
The difference in hours between UTC and your local time depends
upon your time zone. For example, when Boulder, Colorado is on
Mountain Standard Time, the difference between Boulder time and
UTC is 7 hours (its 7 hours later in England than it is in Boulder).
However, UTC does not observe Daylight Saving Time, and never adds
or subtracts an hour. Therefore, when Boulder switches from Mountain
Standard Time to Mountain Daylight Time, the difference between
local time and UTC becomes just 6 hours. To get local time from
a UTC broadcast, both a time zone and daylight saving time correction
usually needs to be made. Fortunately, these corrections are made
automatically by the radio receivers and software packages that
access NIST services after you configure them for your time zone.
The frequency or rate of UTC is computed by the International Bureau of Weights
and Measures (BIPM) located near Paris, France. The BIPM uses a weighted
average from about 250 atomic clocks located in about 50 national laboratories
to construct a time scale called International Atomic Time (TAI). Once TAI
is corrected for leap
seconds, it becomes UTC, or the official world time scale. NIST distributes
a real time version of UTC called UTC(NIST) to the public through its time
and frequency services.
Why must time be measured so precisely?
Precise time synchronization has many uses in everyday life. Synchronization
between two or more locations is necessary for high speed communication
systems, synchronizing television feeds, calculating bank transfers,
and transmitting everything from email to sonar signals in a submarine.
Power companies use precise time to regulate power system grids
and reduce power losses. Radio and television stations require
both precise time-of-day and frequency in order to broadcast programs.Precise
time measurements are also essential for accurate navigation and
the support of communications on earth and in space. Scientific
organizations such as NASA depend on reliable and consistent time
measurement for projects such as interplanetary space travel. Fractional
disparities in times between a space probe and tracking stations
on Earth can dramatically affect the positions of spacecraft. Precise
time measurements are also essential to radio navigation systems
like the Global Positioning System (GPS). By synchronizing the
satellite clocks within nanoseconds of each other, it makes it
possible for a receiver to know its position on earth within a
Why are Cesium atomic clocks used?
Since 1967, the International System of Units (SI) has defined
the second as the period equal to 9,192,631,770 cycles of the radiation
which corresponds to the transition between two energy levels of
the ground state of the Cesium-133 atom. This definition makes
the cesium oscillator (sometimes refered to generically as an atomic
clock) the primary standard for time and frequency measurements.
Other physical quantities, like the volt and meter, also rely on
the definition of the second as part of their own definitions.
Atomic clocks are quite complex, but the basic theory is simple.
Like all clocks, they are intended to make the same event happen
over and over. The repetition of this event produces a frequency,
which is intended to be as stable as possible. For example, the
pendulum in a grandfather clock swings back and forth at the same
rate, over and over. The swings of the pendulum are counted to
keep time. In a cesium oscillator, the transitions of the cesium
atom as it moves back and forth between two energy levels are counted
to keep time. The best cesium oscillators (such as NIST-F1)
can produce frequency with an uncertainty of about 1 x 10-15,
which translates to a time error of about 0.1 nanoseconds per day.