• What time does NIST distribute through its time and frequency services?
• How is Coordinated Universal Time (UTC) currently calculated?
• How does Global Positioning System (GPS) time differ from Coordinated Universal Time (UTC)?
• Why must time be measured so precisely?
• Why are Cesium atomic clocks used?

What time does NIST distribute through its time and frequency services?

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.

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 more than 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.

How does Global Positioning System (GPS) time differ from Coordinated Universal Time (UTC)?

GPS time differs from UTC by the integer number of leap seconds that have occurred since the GPS time scale begam on January 6, 1980. This difference equaled 13 seconds at the end of 2004.The integer-second difference is included in the GPS broadcast message, and is usually applied automatically so that GPS clocks display the same hours, minutes, and seconds as UTC clocks.

GPS time also differs from UTC by a small number of nanoseconds (nearly always < 25 ns) that continuously changes. The small number of nanoseconds represents the difference between the GPS time scale on-time marker (OTM) and an estimation of the OTM for the UTC time scale maintained by the United States Naval Observatory, called UTC(USNO). The current difference between the UTC(USNO) estimate and GPS time is also part of the GPS broadcast message. GPS timing receivers generally apply this correction to their 1 pulse per second (pps) timing signals, so that the received 1 pps signal represents a real-time estimation of UTC(USNO). UTC(NIST) and UTC(USNO) are kept in very close agreement, and can be considered equivalent for nearly all purposes.

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 few meters.

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.