he National Institute of Standards and Technology was established in 1988 by Congress to " assist industry in the development of technology ... needed to improve product quality, to modernize manufacturing processes, to ensure product reliability ... and to facilitate rapid commercialization ... of products based on new scientific discoveries. " NIST, originally founded as the National Institute of Standards in 1901, works to strengthen U.S. industry's competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce, industry, and education with the standards adopted or recognized by the Federal Government. As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and applied research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related services. The Institute does generic and precompetitive work on new and advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80305. Major technical operating units and their principal activities are listed below. For more information visit the NIST Website at http://www.nist.gov, or contact the Publications and Program Inquiries Desk, 301-975-3058.

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Handbook of frequency stability analysis

The Network Time Protocol (NTP) is widely deployed in the Internet to synchronize computer clocks to each other and to international standards via telephone modem, radio and satellite. The protocols and algorithms have evolved over more than a decade to produce the present NTP Version 3 specification and implementations. Most of the estimated deployment of 100000 NTP servers and clients enjoy synchronization to within a few tens of milliseconds in the Internet of today. This paper describes specific improvements developed for NTP Version 3 which have resulted in increased accuracy, stability and reliability in both local-area and wide-area networks. These include engineered refinements of several algorithms used to measure time differences between a local clock and a number of peer clocks in the network, as well as to select the best subset from among an ensemble of peer clocks and combine their differences to produce a local dock accuracy better than any in the ensemble. This paper also describes engineered refinements of the algorithms used to adjust the time and frequency of the local clock, which functions as a disciplined oscillator. The refinements provide automatic adjustment of algorithm parameters in response to prevailing network conditions, in order to minimize network traffic between clients and busy servers while maintaining the best accuracy. Finally, this paper describes certain enhancements to the Unix operating system kernel software in order to realize submillisecond accuracies with fast workstations and networks. >

/pdf/improved-algorithms-for-synchronizing-computer-network-5f2cq3iak4.pdf

Improved algorithms for synchronizing computer network clocks

The authors describe the use of GPS (Global Positioning System) for time coordination. When GPS first became available, it provided a quantum leap in time coordination with little or no effort on the part of the user. However, workers realized that with more sophisticated analysis techniques even greater accuracy could be wrung from GPS. The authors describe the level of time coordination these more sophisticated techniques have affected. The principles of GPS time transfer are described with emphasis on the so-called common view method. The sources of error during GPS time transfer are discussed and the various possibilities of reducing them are investigated. Some possibilities for overcoming SA (selective availability of GPS) are also discussed. GPS is additionally shown to be an outstanding tool for the dissemination of Coordinated Universal Time (UTC). >

GPS time transfer

I will discuss the three general methods that are commonly used to transmit time and frequency information: one-way methods, which measure or model the path delay using ancillary data, two-way methods, which depend on the symmetry of the delays in opposite directions along the same path, and common view, in which several stations receive data from a common source over paths whose delays are approximately equal. I will describe the advantages and limitations of the different methods including uncertainty estimates for systems that are based on them.

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A review of time and frequency transfer methods

In this article, I will review the definition of time and time interval, and I will describe some of the devices that are used to realize these definitions. I will then introduce the principles of time and frequency metrology, including a discussion of some of the types of measurement hardware in common use and the statistical machinery that is used to analyze these data. I will also introduce various techniques of distributing time and frequency information, with special emphasis on the global positioning system satellites. I will then discuss the advantages of clock ensembles and a prototype time-scale algorithm. I will conclude with a discussion of how clocks are synchronized to remote servers using noisy and poorly characterized transmission channels.

Introduction to time and frequency metrology

Frequency differences between major national timing centers are being resolved with uncertainty of less than 1 part in 1014, using satellites of the Global Positioning System (GPS) in common-view. Portable clock and GPS time differences are in excellent agreement. Around the world GPS measurement between three laboratories had a time residual of 5.1 ns.

Accuracy of International Time and Frequency Comparisons via Global Positioning System Satellites in Common-View

The NIST Automated Computer Time Service (ACTS) is a telephone time service designed to provide computers with telephone access to time generated by the National Institute of Standards and Technology at accuracies approaching 1 ms. Features of the service include automated estimation by the transmitter of the telephone-line delay, advanced alert for changes to and from daylight saving time, and advanced notice of insertion of leap seconds. The ASCII-character time code operates with most standard modems and computer systems. The system can be used to set computer clocks and simple hardware can also be developed to set non-computer clock systems.

https://nvlpubs.nist.gov/nistpubs/jres/094/jresv94n5p311_A1b.pdf

The NIST automated computer time service

An extremely cost-effective, modest-accuracy method of remotely obtaining time and frequency via a telephone model is demonstrated. This method uses the operation of the National Institute of Standards and Technology's (NIST) Automated Computer Time System (ACTS), which was begun in 1988. Time and frequency dissemination by this service depends on the reciprocity of the telephone system. The round-trip delay is measured by the NIST equipment. The advance of an on-time marker is adjusted so as to arrive at the user's site on time. A frequency calibration method taking advantage of this service, and preliminary tests are discussed. A computer is not required to access this service. All that is required is a telephone modem, a simple peripheral circuit to generator an on time marker and standard time and frequency measurement and data processing equipment. >

New inexpensive frequency calibration service from NIST

A comparison among three precise timing centers in the United States has been conducted for more than 1 year using three different synchronization methods The timing centers involved were the United States Naval Observatory (USNO) in Washington, DC, Newark Air Force Station (NAFS) in Newark, Ohio, and the National Bureau of Standards (NBS) in Boulder, Colorado The three methods were cesium beam portable clocks; Loran-C transmissions from Cape Fear, North Carolina, and Dana, Indiana; and ABC, CBS, and NBC network television broadcasts commonly received by the three timing centers Cesium beam portable clocks have the capability of accurately and precisely synchronizing remote clocks to within 01 ?s The Loran-C data involved a 3500 km (2180 miles) ground wave path ? the longest Loran-C ground wave path that has been studied with the precision and accuracy reported herein The long-term precision achieved was about 1 ?s over 1 year The accuracy is limited on occasion by inability to resolve the 10 ?s ambiguity of the 100-kHz pulse train The precision capability of maintaining remote clock synchronization within the majority of the continental United States using network television broadcasts was inferred to be about 5 ns??1/3 s-1/3 over the range of ? from 86 400 s (1 day) to about 107 s (324 days) but with definite accuracy limitations caused by such factors as occasional network re-routing of the television signals Some estimates of the long-term frequency stabilities among the references used at the three timing centers were measured or inferred

Precision and Accuracy of Remote Synchronization via Network Television Broadcasts, Loran-C, and Portable Clocks

A tutorial description is given of the smart clock, which makes it possible for any clock to be automatically synchronized to an external standard with a minimum of measurements. The concept covers a range of applications from wrist watches and household clocks to the specialized world of high-accuracy clocks. The smart clock enhances the accuracy or stability of a clock or oscillator by characterizing it against an external standard. The smart clock algorithm uses optimal estimation and prediction to apply a correction to the output of the oscillator, maintaining any combination of time or frequency accuracy or stability within specified limits. The algorithm decides when external measurements are necessary to maintain the desired accuracy or stability. An example of how the smart clock could be used to maintain a simple clock to 1 s using measurements over the telephone lines is given. >

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D. D. Davis

Papers

Accuracy of International Time and Frequency Comparisons via Global Positioning System Satellites in Common-View

The NIST automated computer time service

New inexpensive frequency calibration service from NIST

Precision and Accuracy of Remote Synchronization via Network Television Broadcasts, Loran-C, and Portable Clocks

Smart clock: a new time