Judah Levine

A review of time and frequency transfer methods

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.

Introduction to time and frequency metrology

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.

Carrier-phase time transfer

We have conducted several time-transfer experiments using the phase of the GPS carrier rather than the code, as is done in current GPS-based time-transfer systems. Atomic clocks were connected to geodetic GPS receivers; we then used the GPS carrier-phase observations to estimate relative clock behavior at 6-minute intervals. GPS carrier-phase time transfer is more than an order of magnitude more precise than GPS common view time transfer and agrees, within the experimental uncertainty, with two-way satellite time-transfer measurements for a 2400 km baseline. GPS carrier-phase time transfer has a stability of 100 ps, which translates into a frequency uncertainty of about two parts in 10/sup -15/ for an average time of 1 day.

Assessment of GPS carrier-phase stability for time-transfer applications

We have conducted global positioning system (GPS) carrier-phase time-transfer experiments between the master clock (MC) at the U.S. Naval Observatory (USNO) in Washington, DC and the alternate master clock (AMC) at Schriever Air Force Base near Colorado Springs, Colorado. These clocks are also monitored on an hourly basis with two-way satellite time-transfer (TWSTT) measurements. We compared the performance of the GPS carrier phase and TWSTT systems over a 236-d period. Because of power problems and data outages during the carrier-phase experiment, the longest continuous time span is 96 d. The data from this period show agreement with TWSTT within /spl plusmn/1 ns, apart from an overall constant time offset (caused by unknown delays in the GPS hardware at both ends). For averaging times of a day, the carrier-phase and TWSTT systems have a frequency uncertainty of 2.5 and 5.5 parts in 10/sup 15/, respectively.

An algorithm to synchronize the time of a computer to universal time

Describes an algorithm that synchronizes the time of a computer clock to UTC with an uncertainty due to all causes of about 1 ms RMS. The method uses periodic calibration data obtained via dial-up telephone access to the NIST automated computer time service. The interval between calibrations can be chosen to provide optimum time accuracy or reduced accuracy at reduced cost based on a preliminary evaluation of the statistical performance of the clock. The computer can serve as a primary network time server or can be used stand-alone whenever precise time-stamps are required. >

The leap second: its history and possible future

This paper reviews the theoretical motivation for the leap second in the context of the historical evolution of time measurement. The periodic insertion of a leap second step into the scale of Coordinated Universal Time (UTC) necessitates frequent changes in complex timekeeping systems and is currently the subject of discussion in working groups of various international scientie c organizations. UTC is an atomic time scale that agrees in rate with International Atomic Time (TAI), but differs by an integral number of seconds, and is the basis of civil time. In contrast, Universal Time (UT1) is an astronomical time scale deened by the Earths rotation and is used in celestial navigation. UTC is presently maintained to within 0.9 s of UT1. As the needs of celestial navigation that depend on UT1 can now be met by satellite systems, such as the Global Positioning System (GPS), options for revising the dee nition of UTC and the possible role of leap seconds in the future are considered.

Time transfer using the phase of the GPS carrier

We report on tests of time transfer using the phase of the GPS carrier. The first set of experiments used two clocks connected to independent GPS receivers with closely-spaced antennas. The second set of experiments compared a clock at NIST in Boulder with one at the US Naval Observatory in Washington, DC.

The NIST automated computer time service

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.

First comparison of remote cesium fountains

The frequencies of the cesium fountain primary frequency standards at the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt have been compared. Two-way satellite time and frequency transfer and GPS carrier-phase were the principal frequency-transfer techniques used to make the comparison. For the 15-day interval in which both fountains were in operation the frequencies were compared with an additional uncertainty due to the comparison process of only 6.2 /spl times/ 10/sup -16/. The two standards agree within their stated one-sigma uncertainties of /spl sim/ 1.7 /spl times/ 10/sup -15/.

Two-way time and frequency transfer using optical fibers

The National Institute of Standards and Technology (NIST) has built a two-way time-transfer device which uses any currently unused byte in the SONET (SDH) overhead to effect time transfer. The hardware shows stability which allows time transfer over short distances (km) with stabilities less than 10 ps. Time transfer over distances that require additional amplifiers in the fiber have not yet been investigated. Accuracy at the same level should also be possible.