High-Accuracy Timekeeping

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Who Needs High-Accuracy Timekeeping and Why?

High-accuracy timekeeping is critical to a number of important systems, including telecommunications systems that require synchronization to better than 100 billionths of a second and satellite navigation systems such as the Defense Department's Global Positioning System where billionths of a second are significant. Electrical power companies use synchronized systems to accurately determine the location of faults (for example, lightning damage) when they occur and to control the stability of their distribution systems. In the domain of space exploration, radio observations of distant objects in the universe, made by widely separated receivers in a process called long-baseline interferometry, require exceedingly good atomic reference clocks. And navigation of probes within our solar system depends critically on well-synchronized control stations on earth.

Time is also important in the ordering of many human activities including the activities of financial markets. Time/date stamps are used to identify transactions so that these can be placed in order, a process that is becoming increasingly important as commerce moves electronically at faster and faster speeds. The need for good timing in this area is emphasized by a recent Securities and Exchange Commission-approved rule of the National Association of Securities Dealers that requires all member brokers synchronize their time stamps to NIST.

The time-related quantity called frequency, basically the rate at which a clock runs, is needed by the radio and television broadcast industry to maintain proper control of transmissions and thus avoid interference between stations. Frequency is also the basis for many electronic systems such as event counters, and many physical quantities such as length, voltage, pressure, and so on are often transformed to frequency, since frequency is a quantity that can be measured with very high precision.

How Does NIST Support These High-Technology Requirements?

The most fundamental way in which NIST supports such commercial activities is through its traditional role in providing the very accurate measurements needed to assure that specifications of high-technology equipment can be trusted. In fact, most countries would not accept United States high-technology exports without a guarantee that the specifications for this equipment are based on measurements that are traceable to NIST, which must then be sure that these measurements are compatible with those of the rest of the world. So, one key use of this new standard is to serve as a reference for supporting specifications for equipment manufactured in this country.

Beyond this fundamental mission role, NIST works to distribute timing signals to a wide range of users through the Internet, by telephone, through radio broadcast signals, and by means of very precise signals distributed by satellite. The usage of NIST's Internet Time Service, now receiving more than 500 million calls per year, is growing extremely rapidly. Radio time broadcasts from stations WWV and WWVB are used to set a large number of clocks; in fact, there is a growing list of consumer products (desk clocks, wristwatches, etc.) that use these radio broadcasts to maintain accurate time and even handle daylight saving time changes automatically. At the highest level of accuracy, NIST supports scientific studies, the space program and the development of advanced timing equipment by delivering timing signals using satellite transfer methods.