TECHNICAL ACTIVITIES 1998 - NISTIR 6268

CURRENT DIRECTIONS

• Time and Frequency Broadcast Services. The Division provides time and frequency broadcasts from stations WWV and WWVB in Fort Collins, Colorado and from WWVH in Hawaii and a time code broadcast from NOAA's GOES weather satellites. The Division is just completing an upgrade of the equipment and power level for WWVB. At a higher output power, these LF broadcasts will become substantially more useful for mobile and consumer applications because the antenna/receiver cost and size are very small. The Division also operates telephone and network time services, the Automated Computer Time Services (ACTS), designed for setting clocks in digital systems. The network (Internet) version of these services now receives more than 4,000,000 calls per day. These broadcasts serve applications in a broad range of systems in business, telecommunications, science, transportation, and radio/TV broadcasting. Industry calibration laboratories are served by the Division's Frequency Measurement Service, a system that provides these laboratories with continuous assurance of the accuracy of their frequency measurements.

• Time Scales. The NIST Time Scale is the flywheel clock system that provides accurate signals for services and applications and serves as a reference for research on new standards and measurement methods. The reliability and stability of this time scale is based on the use of an ensemble of commercial cesium-beam standards and hydrogen masers combined under the control of a computer-implemented algorithm. The Division is working to advance the performance of the time scale through acquisition of more-stable clocks and improvement of electronic systems that read the clock outputs. These improvements are critical to the successful evaluation and use of the next generation of primary standards now being developed by the Division.

• Frequency Standards. The accuracy of the time scale is derived from primary frequency standards that provide the practical realization of the definition of the second. To meet advancing needs, the Division built a new frequency standard, NIST-7, which went into operation in early 1993. This atomic-beam standard is based on optical pumping methods (using diode lasers) rather than the traditional, magnetic methods used for state selection and detection. The current uncertainty for this standard is 5 × 10^-15. The Division has also constructed and completed preliminary testing of a cesium-fountain frequency standard. Looking toward still higher accuracy, the Division is studying standards based on trapped, laser-cooled, atomic ions. Ion standards offer promise of accuracy improvements of many orders of magnitude. While the ion studies have involved demonstrations of prototype clocks, the work is treated as basic research providing the knowledge base for future, more-accurate standards.

• Methods of Time Transfer. Since the world operates on a unified time system, Coordinated Universal Time (UTC), highly accurate time transfer (to coordinate time internationally) is a critical ingredient in standards operations. The Division has long been a world leader in this field. The Division is working to further improve the NIST-developed, GPS common-view, time transfer method that is the standard for international time coordination. The Division continues to study the two-way time transfer and has completed preliminary tests of GPS carrier-phase time transfer, a method that could become very important for comparisons of the next generation of frequency standards. Both methods offer the promise of higher-accuracy time and frequency comparisons.

• Optical Frequency Standards. The Division develops improved optical frequency measurements important for primary frequency standards, secondary wavelength standards based on atomic-and-molecular transitions, advanced optical communication, analytical instrumentation, and laser systems for length measurement. There are several facets to this program. There is of course the interest in developing future primary frequency standards based on optical transitions, since, in general, higher frequency transitions yield a better fractional-frequency uncertainty. Another area of effort is on diode lasers, which can have very high spectral purity, tunability, simplicity, and low cost. The approach taken in this work is to prove concepts through demonstration of working systems. The Division develops accurate optical frequency and wavelength references such as the carbon dioxide laser and the calcium-stabilized diode laser. Such frequency references serve as standards in making accurate spectroscopic measurements for industrial and scientific applications. This program is also responsible for the development of advanced, optical-frequency standards needed to support improved length standards and length-measurement methods.

• Spectral-Purity Measurements. The Division's development of new spectral-purity measurements supports sound specifications for a range of aerospace systems. Systems capable of making highly accurate measurements of both phase-modulation (PM) and amplitude-modulation (AM) noise have been developed for carrier frequencies ranging from 5 MHz to 75 GHz. Portable systems covering this same range have also been developed and these are being used to validate measurements made in industrial and government laboratories. Further work will broaden the spectral coverage and simplify comparison of measurement accuracy among standards laboratories.

• Synchronization for Telecommunications. The Division has been engaged with the telecommunications industry in issues relating to synchronization of advanced generations of telecommunications networks. NIST has made useful contributions to emerging telecommunications systems, but with expansion of effort by the Division, it is clear that NIST could contribute even more significantly to this industry. The industry has requested such expansion.

• Application of Time and Frequency Technology. The Division is engaged in several activities applying time and frequency technology to important problems in high-resolution spectroscopy and quantum-limited measurements.

Mission  |  Organization  |  Current Directions  |  Technical Highlights  |  Future Directions

TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999