Time and Frequency Measurements

Broadcast and Measurement Services

Technical Contacts:

Michael A. Lombardi (Frequency and Time)
Tel: 303/497–3212
E–mail: lombardi@boulder.nist.gov

Marc A. Weiss (Time)
Tel: 303/497–3261
E–mail: mweiss@boulder.nist.gov

John Lowe (Broadcast)
Tel: 303/497–5453
E–mail: lowe@boulder.nist.gov

Trudi Peppler
Administration and Logistics
Tel: 303/497–3338
Fax: 303/497–6461
E–mail: tpeppler@boulder.nist.gov

Do not ship instruments or standards to the mailing address listed below. contact the technical staff for the shipping address. 

Mailing Address:
National Institute of Standards and Technology
M.C. 847.40
325 Broadway
Boulder, CO 80305–3328

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NIST Broadcasts of Time and Frequency Signals

NIST time and frequency broadcast services are available free of charge to the general public. Services are provided via HF and LF radio, as well as via telephone lines and Internet connection. NIST broadcast services are coordinated with similar services in other countries. Commercial receivers (for radio signals) and software packages (for computer time services) are available from several manufacturers. NIST publications are available that explain how to use these services.

HF Signals—Broadcasts from WWV (Fort Collins, Colorado) and WWVH (Kauai, Hawaii) can be received on conventional shortwave receivers nearly anywhere in the world. Broadcast frequencies are 2.5 MHz, 5 MHz, 10 MHz, and 15 MHz for both stations and 20 MHz for WWV only. Standard uncertainties of 1 ms to 10 ms in time and a relative standard uncertainty in frequency of 1 10^–8 are typical from these broadcasts. The HF broadcasts provide standard frequencies, standard time intervals, time–of–day announcements, a binary–coded–decimal (BCD) time code, astronomical time corrections, and public service announcements for other government agencies. For individuals without receivers, the audio from both stations is simulcast by telephone. The telephone signal has a standard uncertainty of 30 ms or less due to delays in cross–country telephone line routings. The phone numbers (not toll free) are (303) 499-7111 for WWV and (808) 335-4363 for WWVH.

LF Signals—Radio station WWVB (Fort Collins, Colorado) provides standard uncertainties of 0.5 ms in time and a relative standard uncertainty in frequency of 1 10^–11 when averaged for one or more days. The station broadcasts a 60 kHz carrier and a BCD time code. The effective radiated power of the station is approximately 50 kW and the coverage area includes most of North America.

Automated Computer Time Service (ACTS)—This service allows computer users with modems to synchronize their clocks by telephone. The phone number for ACTS is 303/494–4774. Users can connect at speeds of up to 9600 baud and obtain time signals with standard uncertainty of 5 ms.

Internet Time Service (ITS) —This service allows users to synchronize computer clocks via the Internet. The service responds to time requests from any Internet client in several formats including the Daytime (RFC–867), Time (RFC–868), and Network Time Protocol (NTP, RFC–1305). The service uses multiple time servers.

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Frequency Measurement and Analysis Service (FMAS) (76100S)

NIST Frequency Measurement and Analysis Service (FMAS) provides frequency calibrations with a relative standard uncertainty of 2 10^–13, these can be obtained using signals from Global Positioning System (GPS) satellites as a reference frequency. Subscribers to the FMAS receive a complete frequency measurement system, which they install in their lab (all equipment remains the property of NIST). The system includes a GPS receiver and all of the hardware and software necessary to automate the calibration process. The system can simultaneously calibrate up to five frequency standards (quartz, rubidium or cesium), and graphically display the results. NIST provides all of the equipment, documentation, supplies, and technical support needed to operate the FMAS, and offers a free training seminar to teach the subscriber. The subscriber's only requirements are to supply the frequency standards to be measured and a dedicated phone line. The phone line allows NIST personnel to call each system to verify and analyze the data and quickly troubleshoot any problems that might arise. If any component fails, NIST replaces it immediately using an overnight delivery service. Each subscriber receives a monthly certificate of calibration that certifies that measurements of their primary frequency standard are traceable to NIST. This traceability can assist customers seeking accreditation through NVLAP (National Voluntary Laboratory Accreditation Program) or seeking compliance with ISO requirements.

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Time Measurement and Analysis Service (TMAS) (76101S)

NIST Time Measurement and Analysis Service (TMAS) provides time with a combined standard uncertainty of 15 ns with respect to UTC(NIST), and frequency with an uncertainty of approximately 1   10^–13 after 1 day of averaging. The service is based on the multi–channel GPS common–view technique. Subscribers to the TMAS receive a time measurement system, which they install in their lab (all equipment remains the property of NIST). The system and all associated cables are calibrated at NIST prior to shipment, and it includes a GPS receiver and all of the hardware and software necessary to automate the measurement process. The system accepts a 1 pulse per second (pps) timing output from the customer's primary time standard. The customer is required to supply an always–on Internet connection with a dedicated IP address, to install a GPS antenna in an outdoor area, and to provide a 5 or 10 MHz frequency source for use as a time base. All measurement data is sent via file transfer protocol (FTP) to a NIST server every 10 minutes, where it can be viewed immediately using any Java–enabled web browser. Therefore, customers can view their time and frequency offset and uncertainty with respect to the national standard at any given time, using data that is never more than 10 minutes old. If any measurement system component fails, NIST replaces it immediately using an overnight delivery service.

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Global Time Service (76110S)

This NIST service uses the Global Positioning System (GPS) satellites in a common–view mode and provides better time and frequency transfer than is afforded directly by GPS. Data from a receiver located at the user's facility are automatically downloaded (by phone) to a NIST computer. The computer stores the data, determines which data are suitable for time transfer calculations, and provides optimally filtered values for the time and frequency of the user's clock relative to UTC (NIST). Monthly reports are sent to the user, and users also receive an account on a NIST computer that allows them to access a daily, preliminary analysis. Tests between widely separated receivers have demonstrated standard uncertainties for time comparisons of less than 10 ns and relative standard uncertainties for frequency comparisons of less than 1   10^–13, both for averaging times of 1 d. The frequency uncertainty decreases as the averaging time increases. The frequency uncertainty is limited by the relative standard uncertainty of the NIST primary frequency standard which is 2   10^–15.

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Characterization of Global Positioning System (GPs) Satellite Receivers (76120S)

GPS disciplined clocks and GPS receivers that are designed to deliver accurate time and/or frequency outputs can be characterized using the UTC (NIST) time scale as a reference. The stability and accuracy of the signals delivered by the receiver can be determined. NIST has the capability to measure the accuracy of a one pulse per second (1 PPS) signal relative to UTC (NIST) at a level of at least ± 1 ns. The stability of the 1 PPS signal can be measured as either a time deviation at better than 0.1 n from 1 s to 10⁴ s, or as an Allan deviation at better than 1   10^–10/ level ( in seconds). The frequency accuracy of 5 MHz and 10 MHz frequency outputs from the receiver can be determined to 5   10^–15 for a one–day average, and the Allan deviation can be measured at the 1   10^–13/  level.

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References—Broadcast and Measurement Services

NIST Time and Frequency Services , M. A. Lombardi, Natl. Inst. Stand. Technol., Spec. Publ. 432 (2002).

NIST Frequency Measurement and Analysis System: Operator's Manual , M. A. Lombardi, Natl. Inst. Stand. Technol. IR 6610, (Aug. 2001).

Time and Frequency Measurements Using the Global Positioning System , M. A. Lombardi, L. M. Nelson A. N. Novick, and V. S. Zhang, Cal. Lab. Int. J. Metrology, pp. 26–33, (July–Sept. 2001).

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