I^IIC^r ^"'^^^ states Department of Commerce

I XI %mm^ I National Institute of Standards and Technology

Ai:

N!ST

NIST Special Publication 432 (Revised 1990)

NIST

TIME AND FREQUENCY SERVICES

RADIO STATIONS WWV, WWVH, WWVB

GOES SATELLITE TIME CODE

AUTOMATED COMPUTER TIME SERVICE (ACTS)

NIST Special Publication 432 (Revised 1990)

NIST

TIME AND FREQUENCY SERVICES

Roger E. Beehler

Michael A. Lombard!

Time and Frequency Division

Physics Laboratory

National Institute of Standards and Teclinology

Boulder, CO 80303-3328

(Supersedes NBS Spec. Publ. 432 dated September 1979)

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director

June 1991
National Institute of Standards and Technology Special Publication 432 (Revised 1990)

(Supersedes NBS Special Publication 432 dated September 1979)

Natl. Inst. Stand. Technol., Spec. Publ. 432, 30 pages (June 1991)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON, D.C. 1991

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
CONTENTS

Introduction 1

Summary of Radio Broadcast Services 1

Shortwave Services—WWV and WWVH 4 Accuracy and Stability 4

Radiated Power, Antennas, and Modulation 4 Time Announcements 4

Standard Time Intervals 6 Standard Audio Frequencies 6

Silent Periods 6 BCD Time Code 6 UTl Time Corrections 6

Official Announcements 7 OMEGA Navigation System Status Reports 7 Geophysical Alerts 7 Marine Storm Warnings 8 Global PosmoNiNG System (GPS) Status Announcements 9 WWV and WWVH Audio Signals by Telephone 9

Low-Frequency Services—WWVB 9 Accuracy and Stability 9 Station Identification 9 Radiated Power, Antenna, and Coverage 9 WWVB Time Code 10 How NIST Controls the Transmitted Frequencies 10 GOES Satellite Time Services 10 GOES Satellites 12 GOES Time Code 12 Coverage Area of the GOES Time Code 12

Signal Characteristics 12 Antenna Pointing 12 Performance Levels 13

Special Considerations 13 User Support 15

MIST Frequency Measurement Service 15

Automated Computer Time Service (ACTS) 17

NIST Time and Frequency Bulletin 17

Appendix A: Dating of Events in the Vicinity of Leap Seconds 19

Appendix B: WWV/ WWVH Time Code 19

Appendix C: WWVB Time Code 22

Appendix D: GOES Satellite Time Code 24

Appendix E: Time-code Format for Automated Computer Time Service (ACTS) 24

Inquiries and Tour Information 27

lii
Telephone Time-of-Doy Services

WWV: (303) A99-71 1 1 (Boulder, Colo.)

WWVH: (808) 335-4363 (Kauai. Hawaii)

(Regular long-distance charges apply when called from outside the local celling area.)

Questions? scc top of [?agc 27
ABSTRACT

NIST Time and Frequency Services [Special Publication 432 (Revised 1 990)] is a revision of SP 432, last published in 1 979. It describes services available, as of December 1 990, from NIST radio stations WWV, WWVH,

and WWVB; from GOES satellites; from Loran-C; by telephone (voice and modem); and from the NIST Frequency

Measurement Service.

Key v^ords: broadcast of standard frequencies; computer time setting; frequency calibrations; GOES satellite; high frequency; low frequency; satellite time code; shortwave; standard frequencies; time calibrations; time signals.

Introduction

Precise time and frequency information is needed by electric power comparues, radio and

television stations, telephone companies, air traffic control systems, participants in space exploration,

computer networks, scientists monitoring data of

all kinds, and navigators of ships and planes.

These users need to compare their own timing

equipment to a reliable, internationally recognized

standard. The National Institute of Standards and

Technology (NIST), formerly the National Bureau

of Standards, provides this standard for most users

in the Uruted States. NIST began broadcasting time and frequency

information from radio station WWV in 1923,

Since then, NIST has expanded its time and

frequency services to meet the needs of a growing

number of users. NIST time and frequency services

are convenient, accurate, and easy to use. They

contribute greatly to the nation's space and defense

programs, to manufacturers, and to transportation

and communications. In addition, NIST services are widely used by the general public.

Broadcast services include radio signals from

NIST radio stations WWV, WWVH, and WWVB;

the GOES satellites, and Loran-C. Services are also

available using telephone voice and data lines. This

booklet is a guide to these services.

Summary of Radio Broadcast Services

Characteristics

& Services: WWV WWVH WWVB

Date Service Began March 1923 November 1948 July 1956

Geographical

Coordinates

40" 40' 49.0' N

105° 02' 27.0" W

21° 59' 26.0' N

159° 46' 00.0' W

40° 40' 28.3' N

105° 02' 39.5' W

Standard Carrier Frequencies

2.5 & 20 MHz 5, 10, & 15 MHz 2.5 MHz 5, 10, & 15 MHz 60 kHz

Power 2500 W 10,000 W 5000 W 10,000 W 13,000 W

Standard Audio

Frequencies 440 (A above middle C), 500, & 600 Hz

Time Inten/als 1 pulse/s; minute mark; hour mark s; min.

Time Signals: Voice Once per minute

Time Signals: Code BCD code on 100-Hz subcarrier, 1 pulse/s BCD code

UTl Corrections UTl corrections are broadcast with an accuracy of ±0.1 s

Special Announcements : Omega Reports, Geoalerts, Marine Storm Warnings,

Global Positioning System Status Reports

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Shortwave Services—WWV and WWVH

NIST operates two high-frequency (shortwave) radio

stations, WWV and WWVH. WWV is in Ft. Collins,

Colorado, and WWVH is in Kauai, Hawaii. Both stations broadcast continuous time and frequency signals on 2.5,

5, 10, and 15 MHz. WWV also broadcasts on 20 MHz. All frequencies provide the same information. Although

radio reception conditions in the high-frequency band

vary greatly with factors such as location, time of year, time of day, the particular frequency being used, atmospheric and ionospheric propagation conditions, and the type of receiving equipment used, at least one frequency

should be usable at all times. As a general rule, frequencies above 10 MHz work best in the daytime, and the lower frequencies work best at night.

Services provided by WWV and WWVH include: Time announcements

Standard time intervals Standard frequencies

UTl time corrections

BCD time code

Geophysical alerts Marine storm warnings

OMEGA Navigation System status reports

Global Positioning System (GPS) status reports

Figures 1 and 2 show the hourly broadcast schedules

of these services along with station location, radiated

power, and details of the modulation.

ACCURACY AND STABILITY

WWV and WWVH are referred to the primary NIST

Frequency Standard and related NIST atomic time scales

in Boulder, Colorado. The frequencies as transmitted are accurate to about 1 part in 100 billion (1x10'") for frequency and about 0.01 ms for timing. The day-to-day

deviations are normally less than 1 part in 1,000 billion (1x10'^). However, the received accuracy is far less due

to various propagation effects. The usable received accuracy is about 1 part in 10 million for frequency

(1x10"^) and about 1 ms for timing.

Radiated Power, Antennas,

AND Modulation

WWV and WWVH radiate 10,000 W on 5, 10, and 15 MHz. The radiated power is lower on the other frequencies: WWV radiates 2500 W on 2.5 and 20 MHz while

WWVH radiates 5000 W on 2.5 MHz and does not

broadcast on 20 MHz.

The WWV antennas are half-wave dipoles that radiate omnidirectional patterns. The 2.5-MHz antenna

at WWVH is also of this type. The other antennas at WWVH are phased vertical half-wave dipole arrays. They

radiate a cardioid pattern with the maximum gain

pointed toward the west. Both stations use double sideband amplitude modulation. The modulation level is 50 percent for the steady

tones, 25 percent for the BCD time code, 100 percent for the seconds pulses and the minute and hour markers,

and 75 percent for the voice announcements.

Time Announcements

Voice announcements are made from WWV and

WWVH once every minute. Since both stations can be

heard in some locations, a man's voice is used on WWV,

and a woman's voice is used on WWVH to reduce

confusion. The WWVH announcement occurs first, at about 15 s before the minute. The WWV announcement

follows at about 7.5 s before the minute. Though the announcements occur at different times, the tone markers are transmitted at the exact same time from both

stations. However, they may not be received at exactly the same instant due to differences in the propagation delays

from the two station sites. The announced time is "Coordinated Universal

Time" (UTC). UTC was established by international agreement in 1972, and is governed by the International

Bureau of Weights and Measures (BIPM) in Paris, France. Coordination with the international UTC time scale keeps

NIST time signals in close agreement with signals from

other time and frequency stations throughout the world.

UTC differs from your local time by a specific

number of hours. The number of hours depends on the number of time zones between your location and the

location of the zero meridian (which passes through

Greenwich, England). When local time changes from

Daylight Saving to Standard Time, or vice versa, tTTC

does not change. However, the difference between UTC

and local time does change—by 1 hour. Use the chart of world time zones (figure 3) to find out how many hours

to add to or subtract from UTC to obtain your local standard time. If DST is in effect at your location, subtract 1 hour less in the U.S. than shown on the chart. Thus, Eastern Daylight Time (EDT) is only 4 hours

behind UTC, not 5 as shown on the chart for EST.

UTC is a 24-hour clock system. The hours are numbered beginning with 00 hours at midnight through

12 hours at noon to 23 hours and 59 minutes just before

the next midnight.

The international agreement that established UTC in 1972 also specified that occasional adjustments of exactly

4

1 s will be made to UTC so that UTC should never differ from a particular astronomical time scale, UTl, by more

than 0.9 s. This was done as a convenience for some

time-broadcast users, such as boaters using celestial navigation, who need to know time that is based on the

rotation of the Earth. These occasional 1-s adjustments

are known as "leap seconds." When deemed necessary by the International Earth Rotation Service in Paris,

France, the leap seconds are inserted into UTC, usually

at the end of June or at the end of December, making

that month 1 s longer than usual. Typically, a leap

second has been inserted at intervals of 1 to 2 years. (See

also: "Un Time Corrections," page 6, and appendix A).

Standard Time Intervals

The most frequent sounds heard on WWV and

WWVH are the seconds pulses. These pulses are heard

every second except on the 29th and 59th seconds of each minute. The first pulse of each hour is an 800-ms

pulse of 1500 Hz. The first pulse of each minute is an

800-ms pulse of 1000 Hz at WWV and 1200 Hz at WWVH. The remaining seconds pulses are short audio

bursts (5-ms pulses of 1000 Hz at WWV and 1200 Hz at WWVH) that sound like the ticking of a clock. Each seconds pulse is preceded by 10 ms of silence and followed by 25 ms of silence. The silence makes it easier to pick out the pulse. The total 40-ms protected zone around each seconds pulse is shown in figure 4.

Figure 4. Format of WWV and WWVH seconds pulses.

Standard Audio Frequencies

In alternate minutes during most of each hour, 500- Hz or 600-Hz audio tones are broadcast. A 440-Hz tone

(the musical note A above middle C) is broadcast once

each hour. In addition to being a musical standard, the 440-Hz tone provides an hourly marker for chart recorders and other automated devices. The 440-Hz tone is

omitted, however, during the first hour of each UTC

day. See figures 1 and 2 for further details.

Silent Periods

The silent periods are without tone modulation.

However, the carrier frequency, seconds pulses, time

announcements, and the 100-Hz BCD time code continue

during the silent periods. In general, one station will not

broadcast an audio tone while the other station is broadcasting a voice message.

On WWV, the silent period extends from 43 to 46 and from 47 to 52 minutes after the hour, WWVH has

two silent periods; from 8 to 11 minutes after the hour,

and from 14 to 20 minutes after the hour. Minutes 29 and

59 on WWV and minutes 00 and 30 on WWVH are also

silent. BCD Time Code

WWV and WWVH continuously broadcast a binary

coded decimal (BCD) time code on a 100-Hz subcarrier. The time code presents UTC information in serial fashion

at a rate of 1 pulse per second. The information carried by the time code includes the current minute, hour, and

day of year. The time code also contains the 100-Hz

frequency from the subcarrier. The 100-Hz frequency

may be used as a standard with the same accuracy as the audio frequencies. More information about the time code

format is given in appendix B. At the time of publication of this revision of Special

Publication 432 (late 1990), further changes to the content

of the WWV and WWVH time codes are in the planning

stage. The proposed changes will not affect currentiy

encoded information, but will add information in the form of the last two digits of the current year, improved

indicators for when Daylight Saving Time is in effect, and a warning for the insertion of a leap second at the end of the current month. Details of the affected code

bits are given in appendix B.

UTl Time Corrections

The UTC time scale broadcast by WWV and WWVH

meets the needs of most users. UTC runs at an almost

perfectly constant rate, since its rate is based on cesium

atomic frequency standards. Somewhat surprisingly, some users need time less stable than UTC but related to the rotation of the Earth. Applications such as celestial navigation, satellite observations of the Earth, and some

types of surveying require time referenced to the rotational position of the Earth. These users rely on the UTl

time scale. LTTl is derived by astronomers who monitor

the speed of the Earth's rotation.
You can obtain UTl time by applying a correction to the UTC time signals broadcast from WWV and WWVH.

Un time corrections are included in the WWV and

WWVH broadcasts at two levels of accuracy. First, for those users only needing UTl to within 1 s, occasional

corrections of exactly 1 s are inserted into the UTC time

scale. These corrections, called leap seconds, keep UTC

within ±0.9 s of UTl. Leap seconds are coordinated

under international agreement by the International Earth

Rotation Service in Paris, France. Leap seconds can be

either positive or negative, but so far, only positive leap

seconds have been needed. A positive leap second is normally added every 1 or 2 years, usually on June 30 or December 31. More information about leap seconds is given in appendix A. The second level of correction is for the small

number of users needing LTTl accurate to within 0.1 s. These corrections are encoded into the broadcasts by

using doubled ticks during the first 16 s of each minute.

The amount of correction (in tenths of 1 s) is determined

by counting the number of successive doubled ticks heard each minute. The sign of the correction depends on whether the doubled ticks are in the first 8 s of the minute or in the second 8 s. K the doubled ticks are in the first 8 s (1-8) the sign is positive, and if they are in the second 8 s (9-16) the sign is negative. For example, if ticks 1, 2, and 3 are doubled, the correction is "plus"

0.3 s. This means that UTl equals UTC + 0.3 s. If UTC

is 8:45:17, then UTl is 8:45:17.3. If ticks 9, 10, 11, and 12

are doubled, the correction is "minus" 0.4 s. If UTC is 8:45:17, then UTl is 8:45:16.6. An absence of doubled

ticks indicates that the current correction is 0.

Official Announcements

Announcement segments 45 s long are available by

subscription to other Federal agencies (see figures 1 & 2). These segments are used for public service messages.

The accuracy and content of these messages is the

resiponsibility of the originating agency. For information about the availability of these segments, contact the NISTTime and FrequencyDivision

(see inside back cover).

The segments currentiy in use (late 1990) are described below. Since these are subject to change from

time to time, contact NIST for more current status information.

OMEGA Navigation System Status Reports—The

OMEGA Navigation System status reports are voice

announcements broadcast on WWV at 16 minutes after the hour, and on WWVH at 47 minutes after the hour.

The OMEGA Navigation System consists of eight radio

stations transmitting in the 10- to 14-kHz frequency

band. These stations serve as international aids to navigation. The status reports are updated as necessary by the U.S. Coast Guard.

For more information about the OMEGA Navigation

System or these announcements, contact: Commanding

Officer, U.S. Coast Guard OMEGA Navigation System

Center, 7323 Telegraph Road, Alexandria, VA 22310-3998; telephone (703) 866-3800.

Geophysical Alerts—Current geophysical alerts (Geoalerts) are broadcast in voice from WWV at 18 minutes

after the hour and from WWVH at 45 minutes after the hour. The messages are less than 45 s in length and are updated every 3 hours (typically at 0000, 0300, 0600,

0900, 1200, 1500, 1800, and 2100 UTC). Hourly updates

are made when necessary.

Part A of the message gives the solar-terrestrial indices for the day: specifically the 1700 UTC solar flux from Ottawa, Canada, at 2800 MHz, the estimated Aindex for Boulder, Colo., and the current Boulder

K-index.

Part B gives the solar-terrestrial conditions for the previous 24 hours.

Part C gives optional information on current conditions that may exist (that is, major flares, proton or polar

cap absorption [PCA] events, or stratwarm conditions).

Part D gives the expected conditions for the next 24

hours. For example:

A) "Solar-terresbrial indices for 26 October follow: Solar flux 173 and estimated Boulder A-index

20; repeat: Solar flux one-seven-three and est- imated Boulder A-index two-zero. The Boulder K-index at 1800 UTC on 26 October was four; Repeat: four."

B) "Solar-terrestrial conditions for the last 24 hours

follow: Solar activity was high.

Geomagnetic field was unsettled to active."

C) "A major flare occurred at 1648 UTC on 26

October. A satellite proton event and PCA are in progress."

D) "The forecast for the next 24 hours follows: Solar activity will be moderate to high. The

geomagnetic field will be active."

7
Definitions

1 . Solar Activity is defined as transient perturbations

of the solar atmosphere as measured by enhanced x-ray emission, typically associated with flares. Five standard

terms are used to describe solar activity:

Very low:

Low:

Moderate:

High:

Very High:

X-ray events less than C-class. C-class x-ray events,

isolated (1 to 4) M-class x-ray events, several (5 or more) M-class x-ray

events, or isolated (I to 4) M5 or greater x-ray events. several M5 or greater x-ray events.

2. The Geomagnetic Field experiences natural variations classified quantitatively into six standard categories

depending upon the amplitude of the disturbance. The

Boulder K- and estimated A-indices determine the category according to the following table:

Condition

Range of A-index

Quiet 0 < A

Unsettled 8 < A

Active 16 < A

Minor storm 30 < A

Major storm 50 < A

Severe storm 100 < A

81630

50

<100

Typical K-indices

usually no K-indices > 2 usually no K-indices > 3

a few K-indices of 4 K-indices mostly 4 and 5 some K-indices 6 or greater

some K-indices 7 or greater.

3. Solar Flares are classified by their x-ray emission as:

Peak Flux Range (1-8 Angstroms)

Class SI system (W m"^) cgs system (erg cm"^ s"^) A

B

C

M

X

4> 10-' < <t> 10^ < 4> 10^ :S <l> 10^ < <t>

10-^

10-*

10-^ 10^

10^ < <l>

<t> 10-' < 4> 10-^ < <i>

10^

10-"

10-^

10-^

The letter designates the order of magnitude of the peak value. Following the letter the measured peak value

is given. For descriptive purposes, a number from 1.0 to 9.9 is appended to the letter designation. The number

acts as a multiplier. For example, a C3.2 event indicates an x-ray burst with peak flux of 3.2 x 10"* Wm'^.

Forecasts are usually issued only in terms of the broad C, M, and X categories. Since x-ray bursts are observed as a full-Sun value, bursts below the x-ray background level are not discernible. The background

drops to class A level during solar minimum; only bursts

that exceed Bl.O are classified as x-ray events. During

solar maximum the background is often at the class M

level, and therefore class A, B, and C x-ray bursts cannot be seen. Data are from the NOAA GOES satellites, monitored in real time at the NOAA Space Environment

Services Center. Bursts greater than 1.2 x 10"^ Wm"^ may

saturate the GOES detectors. If saturation occurs estimat- ed peak flux values are reported.

4. The remainder of the report is as follows:

Major Solar = flare which produces some geoFlare physical effect, usually flares that have x-rays ^ M5 class.

Proton Flare = protons by satellite detectors (or polar

cap absorption by riometer) have

been observed in time association with H-alpha flare.

Satellite Level = proton enhancement detected by

Proton Event Earth-orbiting satellites with measured particle flux of at least 10 pro- tons cm-'s'^sr^ at > 10 MeV.

Polar Cap = proton-induced absorption > 2 dB

Absorption daytime, 0.5 dB night, as measured

by a 20-MHz riometer located within

the polar cap.

Stratwarm = reports of stratospheric warmings in the high latitude regions of the winter

hemisphere of the Earth associated

with gross distortions of the normal

circulation associated with the winter

season.

To hear these Geophysical Alert messages by

telephone (at any minute of the hour, but without time

information), dial (303) 497-3235.

Inquiries regarding these messages should be

addressed to:

Space Environment Services Center

NOAA R/E/SE2

325 Broadway or caU (303) 497-5127

Boulder, CO 80303-3328 FTS: 320-5127

Marine Storm Warnings—^Marine storm warnings are broadcast for the marine areas that the United States has

warning responsibility for under international agreement.

The storm warning information is provided by the National Weather Service. Storm warnings for the

Atlantic and eastern North Pacific are broadcast by voice on WWV at 8, 9, and 10 minutes after the hour. Storm

warnings for the western, eastern, southern, and north

Pacific are broadcast by WWVH at 48, 49, 50, and 51 nunutes after the hour. An additional segment (at 11

8
minutes after the hour on WWV and at 52 minutes after the hour on WWVH) is used occasionally if there are unusually widespread storm conditions. The brief voice

messages warn mariners of storm threats present in their areas.The storm warnings are based on the most recent

forecasts. Updated forecasts are issued by the National

Weather Service at 0500, 1100, 1700, and 2300 UTC for WWV; and at 0000, 0600, 1200, and 1800 UTC for WWVH.

A typical storm warning announcement text is as

follows:

*North Atlantic weather West of 35

West at 1700 UTC; Hurricane Donna,

intensifying, 24 North, 60 West, moving northwest, 20 knots, winds 75

knots; storm, 65 North, 35 West, moving east, 10 knots; winds 50 knots, seas 15 feet.'

For more information about marine storm warnings,

write to: The Director, National Weather Service, Silver Spring, MD 20910.

Global Positioning System (GPS) Status Announcements—Since March 1990 the U.S. Coast Guard has

sponsored two voice announcements per hour on WWV

and WWVH, giving current status information about the GPS satellites and related operations. The 45-s announce- ments begin at 14 and 15 minutes after each hour on WWV and at 43 and 44 minutes after each hour on WWVH. For further information, contact the Commanding Officer, U.S. Coast Guard Center, 7323 Telegraph

Road, Alexandria, VA 22310-3998.

WWV AND WWVH Audio Signals

BY Telephone

The audio portions of the WWV and WWVH broadcasts can also be heard by telephone. The accuracy of the telephone time signals is normally 30 ms or better in the continental United States. In rare instances when the telephone connection is made by satellite, there is an

additional delay of 0.25 to 0.5 s. To hear these broadcasts, dial (303) 499-7111 for WWV, and (808) 335-4363 for WWVH. CaUers are disconnected after 3 minutes. These are not toll-free numbers; callers outside the local calling area are charged

for the call at regular long-distance rates. Low-Frequency Services—WWVB

Radio station WWVB is located on the WWV site near Ft. Collins, Colorado. WWVB continuously broadcasts time and frequency signals at 60 kHz, primarily for the continental United States. WWVB does not broadcast

voice announcements, but provides standard time

information, including the year; time intervals; Daylight

Saving Time, leap second, and leap-year indicators; and

Un corrections by means of a BCD time code. In

addition, the 60-kHz carrier frequency provides an

accurate frequency standard which is referenced to the NIST Frequency Standard.

AS

/

\

\

\ 100

Figure 5. Measured field intensity contours of WWVB

at 1 3-lcW effective radiated power.

ACCURACY AND STABILITY

The transmitted accuracy of WWVB is normally

better than 1 part in 100 billion (1x10 "). Day-to-day

deviations are less than 5 parts in 1000 billion (5x10'*^).

The BCD time code can be received and used with an

accuracy of approximately 0.1 ms. Propagation effects are minor compared to those of WWV and WWVH. When

proper receiving and averaging techniques are used, the received accuracy of WWVB should be nearly as good as the transmitted accuracy.

Station Identification

WWVB identifies itself by advancing its carrier phase

45° at 10 minutes after the hour and returning to normal

phase at 15 minutes after the hour. WWVB is also identified by its unique time code.

Radiated Power, Antenna,

and Coverage

The effective radiated power from WWVB is 13,000

watts. The antenna is a 122-m, top-loaded vertical, installed over a radial ground screen. Some measured

field intensity contours are shown in the coverage map

in figure 5.

9
WWVB Time Code

The WWVB time code is synchronized with the 60- kHz carrier and is broadcast continuously at a rate of 1 pulse per second using pulse-width modulation. Each

pulse is generated by reducing the carrier power 10 dB at the start of the second, so that the leading edge of every

negative-going pulse is on time. Full power is restored

either 0.2, 0.5, or 0.8 s later to convey either a binary "0", or a position marker, respectively. Details of the time code are in appendix C.

The WWVB code contains information on the current year (since early 1990), day of year, hour, minute,

second, status of Daylight Saving Time, leap year, and a leap-second warning (planned for mid-1991 implementation). Since the WWVB code is undergoing some revision

as of the publication date of this booklet, users are encouraged to contact the NIST Time and Frequency

Division for the most current information.

How NIST Controls the Transmitted Frequencies

Figure 6 shows the relationship between the NIST

broadcasts, the primary NIST Frequency Standard, and

the NIST atomic time scale. The NIST Frequency Stan- dard and atomic time scale systems are located in Boulder, Colorado. They include:

• The primary NIST Frequency Standard. This stan- dard is a laboratory cesium beam device, built and

maintained by NIST. It provides a frequency and

time interval reference, based on the international

definition of the second.

• A group of commercial cesium standards, hydrogen

maser frequency standards, and possibly other

devices. These are kept in controlled environments,

and serve as the continuously operating "working"

standards.

• Sophisticated time comparison equipment, computer

systems, and computer software. This equipment

generates a composite time scale that is better than

any of the individual standards. The composite time

scale, TA(NIST), is based on (approximately) annual

calibrations of the working standards using the primary NIST Frequency Standard.

• Provisions for inserting small adjustments and leap

seconds into the time scale. These adjustments let NIST generate UTC(NIST), an internationally coordinated UTC time scale. UTC(NIST) is distributed to users of the services described in this booklet,

including WWV, WWVH, WWVB, the GOES satellites, the Frequency Measurement Service, and the telephone services.

The NIST radio station sites (Fort Collins, Colorado,

and Kauai, Hawaii) each have three commercial atomic

standards that emulate UTC(NIST). Each standard can

provide the frequency input to the station's time-code

generators and transimtters. The local standards at each station are compared to the UTC(NIST) time scale in Boulder, Colorado. These

comparisons are made using GPS (Global Positioning

System) satellites. GPS consists of orbiting navigation

satellites that broadcast precise timing signals. These

signals are received simultaneously at Boulder and at both radio station sites. The results obtained at the radio

stations are then compared to the results obtained in Boulder. The timing differences (accurate to less than

100 ns), are then used to calibrate the local standards.

Other time and frequency resources are also used to check the local standards. These include portable clocks

(physically carried to the radio station sites), Loran-C,

and GOES satellite broadcasts.

GOES Satellite Time Services

In 1974, NIST began broadcasting a time code from

the GOES (Geostationary Operational Environmental

Satellite) satellites of the National Oceanic and Atmospheric Administration (NOAA). This cooperative

arrangement between NIST and NOAA was formalized

by a renewable agreement, the latest version of which

extends until 1997. The primary purpose of the GOES

satellites is to collect environmental data from thousands

of sensing platforms located throughout the Western

Hemisphere and to relay this information to a central processing facility from where it is made available to

interested organizations, such as the World Meteorological Organization, radio and TV stations, and various

government agencies. The time code is referenced to UTC(NIST) and is broadcast continuously to the entire Western Hemisphere from two satellites (GOES/East and

GOES/West).

GOES time-code receivers are commercially avail- able. Some of them provide timing signals accurate to within 100 us over periods of hours, months, or years. Other versions are accurate to within 1-2 ms over the same time periods.

10
RATE DATA

INTERNATIONAL

ATOMIC TIME

SCALES: TAI, UTC

NIST PRIMARY

FREQUENCY STANDARD

PERIODIC iCALIBRATIONS

ATOMIC TIME SCALE

CLOCKS

COMPUTER ANALYSIS SAND AVERAGING

TA (NIST)

lERS, BIPM DATA LEAP SECONDS AND § COORDINATION ADJUSTMENTS

FREQUENCY

MEASUREMENT

SERVICE USERS

OTHER

INTERNATIONAL

TIME SCALES

WWV

FT. COLUNS,

CO

WWVH

KAUAI, HI

AUDIO VIA

TELEPHONE

WWVB

FT. COLUNS,

CO

GOES ACTS

SATELLITE COMPUTER

TIME CODE TIME SERVICE

EAST WEST

SATELLITE SATELLITE

2.5, 5, 10, 15, 20 MHz

2.5, 5, 10, 15 MHz

60 KHz 468.8375

MHz

468.825

MHz

TELEPHONE

SYSTEM

Figure 6. How NIST controls the various time and frequency services.

11
GOES Satellites

The GOES satellites are in geostationary orbit about

36,000 km (22,300 miles) above the equator; that is, they

stay above the same spot on the Earth's surface. Because

they are geostationary, the path delay for the time code

will remain relatively constant at all times. In the normal GOES satellite configuration there are always at least three GOES satellites in orbit. Two are operational, and one is a spare. GOES/West is normally

located at 135° West longitude and transmits at 468.825 MHz. GOES/East is normally located at 75° West longitude and transmits at 468.8375 MHz. The spare is located

at approximately 105° West longitude. The satellites do

not have clocks on board but rather serve to relay the time-code signals which originate on the ground from

NOAA's satellite control facility at Wallops Island, Va. During periods when satellite malfunctions occur or when a smaller-than-normal number of fully functional

satellites are available, NOAA may alter the normal

satellite configuration. As an example, NOAA plans to transmit the GOES/East time code from a satellite located

at 60° West longitude temporarily from mid-1990 until new replacement GOES satellites are launched sometime

during 1992. GOES Time Code

The GOES time code includes the current year, complete time-of-year information (day-of-year number,

hour, and minute), the UTl correction (the approximate

difference between the astronomical UTl time scale and

UTC), satellite position information, accuracy indicators. Daylight Saving Time and leap second indicators, and

system status information.

The satellite position data are included in the time

code to provide users with an option for correcting the received time signal for motion of the satellites in their

orbits. These relatively small motions cause variations in the signal path delay, which, in turn, produce variations

(usually less than 5 ms) in the received time signal. The

position data in the time code are predictions, updated

each minute, of each satellite position. They are computed from satellite tracking measurements, using a large

computer at NIST/Boulder, and then transferred

electronically to the time-code equipment at Wallops

Island. GOES time-code receivers can be equipped to decode the satellite position information, compute a path

delay correction each minute, and automatically correct the output reference signal to remain within 100 /iS of UTC(NIST). (Also see "Performance Levels," page 13.) The time code is interlaced with interrogation

messages that do not contain time and frequency infor- mation. The interrogation messages are used by NOAA

to communicate with systems gathering weather data. Once every half-second, a 4-bit time-code word is

transmitted at a data rate of 100 bits/s. A complete timecode message is transmitted every 30 s, beginning on

each minute and half-minute. The GOES time code is described in more detail in appendix D.

The time code is generated from a set of three atomic

clocks maintained by NIST at NOAA's facility at Wallops

Island, Va. The NIST time-code system also includes

triply redundant time-code generators, monitoring

equipment to receive Loran-C and GPS satellite signals

for comparisons with external time scales, automatic fault detection and alarm systems, and a dial-up telephone

link with NIST/Boulder for monitoring and controlling the Wallops Island system remotely.

The atonuc frequency standards at Wallops Island provide a stable and accurate reference for the time code.

They are compared to UTC(NIST) using Loran-C and

GPS. The transmitted accuracy from Wallops Island is kept within ±10 of UTC(NIST).

Coverage Area of the GOES Time Code

Figure 7 shows the coverage area of the GOES time

signals. Much of the Western Hemisphere is covered by

at least one satellite. The continental United States is cov-

•0" 60°

Figure 7. GOES/East and GOES/West coverage areas.

ered by both satellites. The heavy and light oval boundary lines in figure 7 show where the elevation angles to the GOES satellites are 7° and 3°, respectively.

Signal Characteristics

The satellites' signal characteristics are summarized

in table 1. Antenna Pointing

It is relatively simple to point an antenna to either

satellite. If the path to the satellite is clear, pointing in

12
the general direction of the satellite is usually sufficient,

particularly for relatively low-gain antennas (<10 dB).

If more detailed information about antenna pointing

is needed, figures 8 and 9 provide the elevation and

azimuth angles. For example, figure 8 shows that the pointing angles to GOES/East from San Francisco are about 119° azimuth and 24° elevation.

Performance Levels

The GOES time code can be used at three perfor- mance levels: uncorrected for path delay, corrected for mean path delay only, and fully corrected.

Uncorrected: The total path delay from Wallops

Island, Va. (where the time code originates) to the

satellite and then back to Earth is about 260 ms. To

compensate for this path delay, the signals are advanced

by 260 ms before transirussion from Wallops Island. The

uncorrected signal arrives back on Earth nearly on time

(within 16 ms), depending upon the receiver's location. The received time signal will also vary by 1-2 ms during

a 24-hour period due to path delay variations produced

by satellite motion.

Corrected for Mean Path Delay: If the appropriate mean path delay correction is made, the signal arrival time is usually accurate within ±1 ms. (During some

periods, however, NOAA uses satellites with larger-thannormal orbit inclinations that may produce delay varia- tions of several ms during a 24-hour period.) For example, the mean path delay from San Francisco through

GOES/East is 255 ms. The mean path delay consists of

130.5 ms from satellite to Earth (see Figure 10), and

124.5 ms from Earth to satellite. Since the time is advanced by 260 ms before' leaving Wallops Island, it arrives at San Francisco 5 ms early. A correction for mean path delay in San Francisco can be made by

subtracting 5 ms from the received time signal. Use

Figure 11 to calculate mean delays for GOES/West.

Fully Corrected: The satellite's orbit is not perfectly

circular and is not precisely in the plane of the equator.

Therefore, the path delay at a fixed location on the Earth

typically varies by anywhere from a few hundred fis to several ms (depending mainly on the satellite's orbit inclination) throughout the day. Also, the satellites are sometimes moved to keep their orbital position within

assigned limits. This movement causes irregular changes

in the path delay. Obtaining a fully corrected time signal

requires correcting for these position changes. Since

satellite position data are included in the time code, users (and automatic receivers) can make these corrections. A fully corrected time signal is usually accurate to within ±100 /iS. The ultimate accuracy depends on

equipment delays and noise levels in addition to the path

delay.

Special Considerations

Interference from land-mobile radio services—The

land-mobile radio services can interfere with the GOES

time code, since they use the same, or nearly the same,

frequencies (468.8250 and 468.8375 MHz). This interfer- ence occurs mainly in urban areas with high land-mobile

activity. Since the land-mobile frequency allocations have

Table 1. Characteristics of GOES satellite signals.

Satellite Signal Characteristics: GOES/West GOES/East

Frequency 468.8250 MHz 468.8375 MHz

Polarization RHCP* RHCP*

Modulatio n CPSK (±60°)t CPSK (±60»)t

Data Rate 1 00 bits/s 1 00 bits/s

Satellite Location 135° West 75° West

s ignoi Stre ngth (output from iso tropic antenna) -131 dBm -131 dBm

Coding Manchester Manchester

Beandwidth 400 Hz 400 Hz

* right-hand circularly polarized t coherent phase shift keying

13
120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 2C 40

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 LONGITUDE

Figure 8. Pointing angles for GOES/East.

LONGITUDE

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 40

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 40 LONGITUDE

Figure 9. Pointing angles for GOES/West.

14
priority over the GOES allocations, complair\ing to the FCC will not help.

Because of the specific frequencies involved, the land-mobile services affect GOES/West more than

GOES/East. Therefore, users in urban areas should use GOES/East if land-mobile interference is a problem,

Larger-than-Nonnal Time Code Deviations—The

GOES time code is as accurate as stated more than 99

percent of the time. However, some situations can cause

larger-than-normal time code deviations.

One such situation occurs when NOAA uses a GOES/East and/or GOES/West satellite with larger-thannormal orbit inclinations. In this instance, users in "uncorrected" mode may receive time with variations of several ms. A more common problem (though of much shorter duration) occurs when stationkeeping maneuvers are performed on the satellites. These maneuvers are per- formed every 1 or 2 months to keep the satellites in their assigned orbit locations. Receivers that use the position

data can compensate for these maneuvers; however, the

position data are not updated until 0000 UTC on the day

after the maneuver. This means that for up to 24 hours

following a maneuver the time code may be off by more

than the normal 100 fis. NIST changes the accuracyindicator bit in the time code to indicate when this condition occurs.

Continuity—Since the GOES satellites are owned

and operated by NOAA rather than by NIST, NIST

cannot guarantee continuation of the service in the same

sense as it can for its own radio stations WWV, WWVB,

and WWVH. However, both NOAA and NIST have

stated their intentions of maintaining the time-code

service via a formal Memorandum-of-Agreement, the current version of which covers the period up until at least 1997. Availability of the service at any time, and the achievable time-code performance, is dependent on the

status of NOAA's current satellite configuration and

related support operations.

User Support

Information about the current status of the GOES

time code can be obtained from the U.S. Naval Observatory's Automated Data Service database in Washington,

D.C., and from the monthly NIST Time and Frequency

Bulletin. These sources inform users about outages, any

recent departures from normal performance, announce- ments of upcoming maneuvers, and other events. To

find out more about these information sources, contact

the NIST Time and Frequency Division.

NIST Frequency Measurement Service

The NIST Frequency Measurement Service (FMS)

has been offered since 1984. The FMS lets users make

accurate frequency calibrations at their site, rather than

sending their oscillators to NIST or elsewhere for calibration. The FMS calibrates oscillators with accuracy levels from 1 X 10"^ to 1 X 10'^. This includes nearly all quartz,

rubidium, and cesium oscillators. Users subscribe to the FMS by paying a one-time

subscription fee. They then pay a monthly fee for as long

as they use the service. Each subscriber receives and

installs a frequency measurement system. This system

remains the property of NIST, and NIST immediately

replaces any system parts that fail. Up to four oscillators can be connected to the measurement system. Each oscillator is measured

continuously for 24 hours, its performance is plotted and

printed out, and then the measurements begin again.

The system is controlled by computer software, and

almost no operator attention is required.

Each measurement system is linked by telephone to NIST, and the measurements are compared to measure- ments made at NIST and at other sites. By virtue of these

comparisons, NIST can certify that the calibrations made

by FMS users are traceable to NIST. Traceability to NIST

is often required of organizations in both the public and

private sectors. The FMS measures oscillator performance using

Loran-C radio signals as a reference frequency. Loran-C

is a radio navigation system (Loran stands for LOng

RAnge Navigation), consisting of stations located in many regions throughout the Northern Hemisphere. The

stations are maintained by the U.S. Coast Guard. All Loran-C stations broadcast on 100 kHz, but each station has a unique emission delay so that receivers can distin- guish among signals from different stations. Loran-C is ideal for calibrating oscillators. It is easy

to receive, more accurate (1 x 10'^ per day) than the other

services described in this booklet, and widely available. However, unlike WWV/ WWVH, WWVB, and GOES,

Loran-C does not have a time code and is not normally

used to obtain time. For more information about the FMS, write to the NIST Frequency Measurement Service, Time and Fre- quency Division (see page 27).

15
LONGITUDE

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 40

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 LONGITUDE

Figure 10. Mean delays for GOES/East; for total delay, add 124.5 ms to downlink delays shown.

LONGITUDE

120 140 160 E 180 W 160 140 120 100 80 60 40 20 0 20 40

120 140 150 E 180 W 160 140 120 100 80 60 40 20 0 20 LONGITUDE

Figure 1 1 . Mean delays for GOES/West; for total delay, add 133.5 ms to downlink delays shown.

16
Automated Computer Time Service

(ACTS)

In 1988, NTST began the Automated Computer Time

Service (ACTS). This service provides computers with

telephone access to NIST time at accuracies approaching

1 ms. Since the time code uses the standard ASQI

character set, it works with nearly all computer systems

and modems. Simple hardware can also be built to set noncomputer clock systems.

The phone number for ACTS is (303) 494-4774. Six telephone lines are available on a rotary system and 6 redundant time-code generator systems are used to insure reliability. ACTS operates at 300 or 1200 Baud,

with 8 data bits, 1 stop bit, and no parity. Users of the 1200-Baud service receive the full time code once each

second. In addition to the UTC hours, minutes, and

seconds, the time code includes the date, the Modified

Julian Date (MJD), Daylight Saving Time and leap second

indicators, a leap second flag, a UTl correction, and

other information. Users at 300 Baud receive somewhat

less information over a 2-s interval due to the lower

information transfer rate. For more details about the time

code, see appendix E. With appropriate software, ACTS can set or check

computer time-of-day clocks in one of two different modes:

• Fixed Delay Mode—^In this mode, the user receives the time code and an on-time marker character. The

marker character has been advanced in time by a fixed amount to compensate for typical modem and

telephone line delays. Unless the connection is

routed through a satellite, the accuracy in this mode

should be better than 0.1 s. • Measured Delay Mode—^In this mode, the user's computer echoes all characters back to NIST where

the round trip line delay is measured. The on-time

marker character is then advanced to compensate for the line delay. The accuracy in this mode should be

less than 10 ms using a 1200-Baud modem, or about

1 ms using a 300-Baud modem. Accuracy at 1200

Baud is limited by the internal delays in 1200-Baud

modems. Repeatability at both 300 and 1200 Baud is about 1 ms. These accuracies assume that the telephone connection is reciprocal. Reciprocal connections follow the same

path, that is, the path delay is the same in both directions. Most phone connections are of this type.

NIST offers software (Research Material 8101) which

lets users use ACTS on several popular personal computers. The documentation for this software includes more

information about the features of the service, and a simple circuit diagram that can be used to obtain an ontime pulse. These materials cost $35 (late 1990). To order

RM 8101, contact: NIST Office of Standard Reference

Materials, B311 Chemistry Building, NIST, Gaithersburg,

MD 20899-0001; telephone (301) 975-6776. For further information about ACTS, write to NISTACTS, Time and Frequency Division (see page 27).

NIST Time and Frequency Bulletin

The NIST Time & Frequency Bulletin is published

monthly and distributed free to users of the NIST

services. The bulletin includes data related to WVW,

WWVH, WVWB, GOES, Loran-C, GPS, and the NIST

time scales.

To receive the bulletin, write to: Editor, Time and

Frequency Bulletin, Division 847, National Institute of Standards and Technology, 325 Broadway, Boulder, CO

80303-3328.

17

Appendices

Appendix A: Dating of Events in the ViONriY of Leap Seconds

Appendix B: WWV/ WWVH Time Code

Appendix C: WWVB Time Code

Appendix D: GOES Satellite Time Code

Appendix E: Time-Code Format of Automated Computer Time Service (ACTS)

Appendix A: Dating of Events in the Vicinity of Leap Seconds

Leap seconds are sometimes needed to keep UTC

within ±0.9 s of UTl. The addition or deletion of a leap

second always occurs at the end of a month. By international agreement, first preference is given to December 31

or June 30. Second preference is given to March 31 or September 30, and third preference is given to any other

month.

When Un is slow relative to UTC, a positive leap

second is needed. The second is inserted beginning at 23h 59m 60s of the last day of the month and ending at Oh Om Os of the first day of the following month. The

minute containing the leap second is 61 s long. Figure A

shows how to assign dates to events occurring near the leap second.

Since tlTC has historically run faster than UTl, only

positive leap seconds have been needed thus far. However, if the speed of the Earth's rotation were to increase to the point where UTl runs faster than UTC, a negative

leap second would be needed. In that case, exactiy 1 s would be deleted at the end of some UTC month. The

minute containing the negative leap second would be

only 59 s long.

Positive leap seconds were inserted in all NIST

broadcasts on June 30, 1972, on December 31, 1972-1979, on June 30, 1981-1983 and 1985, and on December 31 of

1987 and 1989. The dates for future leap seconds wUl be

determined by the International Earth Rotation Service in France. NORMAL MINUTE

(NO LEAP SECOND ADDED)EVENT

30 June, 23'59- M 1 July, 0"0"

Dating of event shown: 30 June, 23" 59" 59.5^ UTC

MINUTE WITH LEAP SECOND ADDED

EVENT 56 57 58 59 60 n 0 1 2 3

Dating of event shovirn: 30 June, 23" 59" 50.5^ UTC

Figure A. Dating of events near a leap second.

Appendix B: WWV/ WWVH Time Code

The WWV/ WWVH time code is a modified version

of the IRIG-H code. The code is transmitted on a 100-Hz

subcarrier at a rate of 1 pulse per second. The code is in binary coded decimal (BCD) format. Groups of binary

digits (bits) are used to represent decimal numbers. The

binary-to-decimal weighting scheme is 1-2-4-8. The least significant bit is always sent first. Table 2 shows the BCD

groups and the equivalent decimal number.

The decimal number is obtained by multiplying each

bit in the binary group by the weight of its respective

column and then adding the four products together. For

example, the table shows that the binary group 1010 is equal to 5. This is derived by:

(1x1) + (0x2) + (1x4) + (0x8) = 1 + 0 + 4 + 0= 5

In ihe standard IRIG-H code, a "0" bit consists of

exactly 20 cycles of 100-Hz amplitude modulation (200-ms

duration), and a "1" bit consists of 50 cycles of 100-Hz

(500-ms duration). The WWV/ WWVH code differs from

IRIG-H because all tones are suppressed briefly while the seconds pulses are transmitted (see Standard Time Intervals, page 6).

19
Table 2. 1-2-4-8 BCD-Decimal Equivalents

BINARY GROUP DECIMAL

EQUIVALENT

Weight: 124 8

0 0 0 0 0

10 0 0 1

0 100 2

1100 3

0010 4

1010 5

0 110 6

1110 7

0 0 0 1 8

100 1 9

Tone suppression also deletes the first 30 ms of each

binary pulse in the time code. This makes the WWV/-

WWVH bits 30 ms shorter than the IRIC-H bits. Therefore, 170-ms pulses are recognized as "0" bits, and 470- ms pulses are recognized as "1" bits. The leading edge of each pulse coincides with the positive-going crossing of the 100-Hz subcarrier, but due to the tone suppression,

it occurs 30 ms after the start of the second.

Within 1 minute, enough bits are sent to express the minute, hour, and day of year; two digits of the current year (to be implemented during the first half of 1991); a leap-second warning indicator (to be implemented during

the first half of 1991); the UTl correction, and a Daylight

Saving Time (DST) indicator. The coded time information

refers to the time at the start of the one-minute frame.

Seconds are determined by counting pulses within the frame. Two BCD groups are needed to express the hour

(00 to 23), minute (00 to 59), and year (00-99); and three groups are needed to express the day of year (001 to 366). Some bits in the BCD groups are unused, but may

provide additional information in the future. To repre- sent units, tens, or hundreds, the basic 1-2-4-8 weights

are multiplied by 1, 10, or 100 as appropriate.

Each frame begins with a unique spacing of pulses

that mark the start of a new minute. During the first second of the minute, no pulse is transmitted. This

creates a 1-s (1000-ms) hole. Since the pulses are already

delayed 30 ms by the tone suppression, the UTC minute

actually begins 1030 ms (1.03 s) earlier than the first pulse in the frame. For synchronization purposes, a position identifier pulse is transmitted every 10 s. The position identifier pulse lasts for 770 ms (77 cycles of 100 Hz).

UTl corrections are sent during the final 10 s of each

frame. These corrections are to the nearest 0.1 s. The

UTl correction is expressed with bits called control

functions. Control function #1 occurs at 50 s, and tells whether the UTl correction is negative or positive. If a "0" bit is sent the correction is negative, and if a

"1" bit

is sent the correction is positive. Control functions #7, #8,

and #9, tell the amount of the UTl correction. They occur

at 56, 57, and 58 s, respectively. Since the UTl corrections are in tenths of seconds, the binary-to-decimal

weights are multiplied by 0.1. Currentiy (late 1990), DST information is sent only

by conbrol function #6, at 55 s. If DST is in effect, a "l"

bit is sent. If Standard Time is in effect, a "0" is sent. The setting of this bit is changed a few hours prior to 0000 UTC on the date of change. This schedule notifies users in the continental United States of the time change

several hours before it occurs locally (usually at 2:00

a.m.). Receivers that display local time can read control function #6 and make the one-hour adjustment automatically when time changes occur. During the first half of 1991, an improved procedure

for alerting users of impending DST changes will be

implemented. Two bits, #2 and the currentiy used bit #55, will be used to indicate whether "Standard" or 'Daylight Saving Time" is in effect at any particular time.

Bit #55 will be used essentially as it is now—that is, it will be changed from "0" to "1" at 0000 UTC of the day

the time is to be changed from "Standard" time to DST

and from "1" to "0" for the change back to "Standard"

time in the Fall. The second DST bit, #2, will be changed

in the same way as bit #55 but 24 hours later, at 0000

UTC on the day following the time change. The use of two separate bits in this manner will allow timing

receivers that are powered on during the day of a time

change to be able to deterniine whether it is a day of change and, if so, how to adjust their current time

correctiy.

Also beginning during 1991, the last two digits of the current year (for example, "90" for 1990) will be encoded

using bits #4-7 (for units of year) in a 1-2-4-8 weighting

sequence and bits #51-54 (for tens of year) in a 10-20-40- 80 weighting sequence.

Bit #3 will be used, beginning in 1991, to indicate that a leap second is to be inserted at the end of the current month. A "1" will be transmitted beginning

early in the month at the end of which a leap second is to be inserted. Bit #3 will be returned to "0" at 0000 UTC

of the first day of the following month.

Figure B shows one frame of the time code. The six position identifiers are labeled PI, P2, P3, P4, P5, and PO. The minutes, hours, days, year, and UTl sets are marked

by brackets, with the weighting factors printed below the

bits. Wide pulses represent "1" bits and narrow pulses

represent "0" bits. Unused bits are set to 0. This diagram

includes all changes planned for implementation in 1991. In figure B, the decoded UTC at the start of the frame is 1990, 173 days, 21 hours, and 10 minutes. Since

ti\e Un correction is + 0.3 s, ti\e decoded UTl is 1990,

173 days, 21 hours, 10 minutes, and 0.3 s. Manufacturers, users, and others who may need to know more definitely when these changes in the code

formats are to be implemented should contact the NIST

Time and Frequency Division for the most current status.

20
LU(DQ

lUO

LU>

COO

a.Q

LU

I-Z

LU(O

ill CC

Q-lU

QCCO

LUCO

_l

a.

u_O

CD

LU

CQ

lij 5

COQZo

LUCO

CO

CD

\fj yjj LU LU

1— i_r —) —\ Z z o oE0 - CO Q> "O QC 0 D ZD U o o I I in CM CVI >

CO" 1 1 co

CO CO

21 o" o" D

.O) 0) u_

CJ) CJ> <

II <

1-Z z O o CL CL ^O1-3 3

21
Appendix C: WWVB Time Code

The WWVB time code is also sent in BCD format,

but the weighting is different from the WWV/WWVH

weighting. Bits are sent by shifting the power of the 60- kHz carrier. The carrier power is reduced 10 dB at the

start of each second. If full power is restored 200 ms

later, it represents a

"0" bit. If full power is restored

500 ms later, it represents a

"1" bit. Reference markers

and position identifiers are sent by restoring full power

800 ms later. The binary-to-decimal weighting scheme is 8-4-2-1. The most significant bit is sent first. This is the reverse of the WWV/ WWVH time code. The BCD groups and \he

equivalent decimal numbers are shown in table 3.

Table 3. 8-4-2-1 BCD-Decimal Equivalents

BINARY CROUP DECIMAL

EQUIVALENT

Weight: 8 42 1

0 0 00 0

0 0 0 1 1

0010 2

0011 3

0 100 4

0 10 1 5

0110 6

0111 7

1 00 0 8

100 1 9

The decimal number is obtained by multiplying each

bit in the binary group by the weight of its respective

column and then adding the four products together. For

example, the table shows that the binary group 0101 is equal to 5. This is derived by:

(0x8) + (1x4) + (0x2) + (1x1) = 0 + 4 + 0 + 1= 5

Every minute, the WWVB time code sends the current minute, hour, day of year, 2 digits of the current

year, a UTl correction, a leap-second warning bit, and

Daylight Saving Time (DST) and leap year indicators. Two BCD groups are needed to express the hour (00 to

23), minute (00 to 59), and year (00-99); and three groups

are needed to express the day of year (001 to 366). Some

bits in the BCD groups are unused, but may provide

additional information in the future. To represent units, tens, or hundreds, the basic 8-4-2-1 weights are simply

multiplied by 1, 10, or 100 as appropriate. The coded

information refers to the time at the start of the oneminute frame. Seconds are determined by counting

pulses within the frame.

Each minute begins with a frame reference pulse

lasting for 0.8 s. A position identifier pulse lasting for 0.8 s is transmitted every ten s. UTL corrections are broadcast at seconds 36 through

43 of each frame. These corrections are to the nearest

0.1 s. The bits transmitted at seconds 36, 37, and 38

show if Un is positive or negative with respect to UTC.

If "1" bits are sent at seconds 36 and 38, the UTl correction is positive. If a

"1" bit is sent at second 37, the UTl

correction is negative. The amount of the UTl correction

is sent in a four-bit BCD group at seconds 40, 41, 42, and

43. The binary-to-decimal weights are multiplied by 0.1, because the UTl corrections are expressed in tenths of seconds.

The WWVB time code also contains information

about leap years, DST, and leap seconds. The leap year

bit is transmitted at second 55. If it is set to "1", the current year is a leap year. The bit is set to "1" during

each leap year sometime after January 1 but before

February 29. It is set back to "0" shortly after January 1

of the year following the leap year. Receivers that read

this bit can automatically adjust themselves during leap

years.The two DST bits are sent at 57 and 58 s after each

minute. If "Standard" time is in effect, both bits (#57 and

#58) are set to 0. If DST is in effect, both bits are set to 1. On the day of change from "Standard" to DST bit #57 is changed from "0" to "1" at 0000 UTC. Exactiy 24 hours

later, bit #58 also changes from "0" to "1" at 0000 UTC.

On the day of change from DST back to "Standard" time

bit #57 goes from "1" to "0" at 0000 UTC, followed 24

hours later by bit #58. Receivers displaying local time can

read the DST bits and make the one-hour adjustment

automatically when time changes occur locally. Bit #56 is used to warn users that a leap second will be inserted into the UTC(NIST) time scale at the end of the current month. The bit is set to "1" near the beginning of the month in which a leap second is added. It is reset to "0" immediately following the leap second

insertion. Figure C shows one frame of the time code. The six position identifiers are labeled as PI, P2, P3, P4, P5, and

PO. The minutes, hours, days, year, and UTl sets are marked by brackets; with the weighting factors printed

below the bits. Wide pulses represent "1" bits and

narrow pulses represent "0" bits. Unused bits are set to "0". In Figure C, ihe decoded UTC at the start of ti\e frame is 1990, 258 days, 18 hours, and 42 minutes. Since

\he UTl correction is 0.7 s, the decoded UTl is 1990, 258

days, 18 hours, 41 minutes, 59.3 s.

22
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8? J

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23
Appendix D: GOES Satellite Time Code

The interrogation channel format and the timecode format are shown in

figure D.

The GOES time code is interlaced with interrogation

messages used for other

purposes. The interrogation

messages are broadcast at a

rate of 100 bits per second.

They are one-half second, or 50 bits in length. The first 4

bits of the message form a BCD time-code word. The

remaining 46 bits do not

contain timing information.

A time-code frame consists of 60 BCD time-code

words. It takes 60 interrogation messages, or 30 s, to complete a time-code frame.

The completed time-code

frame contains a synchronization word, the last two

digits of the current year, the time-of-year (day of year, hour, and minute),

time-code accuracy indicators. Daylight Saving Time

(DST) and leap second indicators, system status infor- mation, the Un correction,

and the satellite's position.

The position information is updated every minute. It includes the satellite's lati- tude, longitude, and height

above the Earth's surface.

1/2 S 1/2 S *r*- 1/2 8

TC INTERROGATION TC INTERROGATION TC INTERROGATION TC INTERROGATION WORD MESSAGE WORD MESSAGE WORD MESSAGE WORD MESSAGE

1/2 8

TIME CODE FRAME CONSISTING OF:

• SYNCHRONIZATION WORD

• DAYS, HOURS, MINUTES, SECONDS

• UT1 TIME CORRECTION; ACCURACY, DST,

AND LEAP SECOND INDICATORS

• SATELLITE POSITION

40

SYNC

WORD

32 8 20 16 16

TIME

OF

YEAR

LONG LAT RAD

t

UT1 SATELUTE

TIME POSITION

180

ACCURACY, DST,

AND LEAP SECOND

INFORMATION

CORRECTIONS 30 SECOND TIME FRAME

BEGINS ON

00 AND 30 SECONDS UTC

Figure D. (Top) GOES satellite interrogation channel format.

(Bottom) GOES time code format.

Appendix E: Time-code Format for Automated Computer Time Service (ACTS)

The time code for ACTS is sent in ASCII at either 300 or 1200 Baud. Both Baud rates require 8 data bits, 1 stop bit, and no parity. The 1200-Baud format is shown

in figure E. The first part of the time code contains the Modified

Julian Date (MJD), the date (year, month, day), and the time (UTC hours, minutes, and seconds). This information is followed by the Daylight Saving Time (DST)

information. The DST code is always a two-digit number

(00 to 99). This code is normally a 00 when Standard

Time (ST) is in effect, or a 50 when DST is in effect. About 48 days prior to a time change, however, this code

starts counting the days until the change. When ST is in

effect, the DST code counts down from 99 to 51 in the 48

24
days prior to the time char\ge. Wherx DST is in effect, the DST code counts down from 49 to 01 in the 48 days prior

to the time change. In both cases, the code is updated at 0000 UTC.

The leap second (LS) flag is always a "O", a or

a "2". K no leap second is scheduled at the end of the current month, it is a '0'. It is a

"1" if a (positive) leap

second is scheduled to be added on the last day of the current month. The LS flag remains on for the entire month before the leap second is added. Once the leap

second is added, the LS flag is reset to "0". The leap

second flag is a

"2* only if a (negative) leap second is to be deleted on the last day of the current month. So far, a negative leap second has never been needed.

The UTl correction is shown as either a positive or negative number in steps of 0.1 s. The remainder of the time code shows the ms

advance (msADV) and the on-time marker (OTM). The

displayed time is valid at the OTM. The OTM is either a

**" or a character. When the connection is established, the OTM is displayed. This marker is trans- mitted 45 ms early with respect to UTC(NIST). The 45 ms

accounts for the 8 ms required to send 1 character at 1200

Baud, an additional 7 ms to compensate for delay from

NIST to the user, plus a 30-ms "scrambler" delay. The

"scrambler" delay approximately compensates for the

internal delay found in 1200-Baud modems.

If the user's equipment is set to echo all characters, or at least the OTM, NIST measures the round-trip delay

and advances the OTM so that the midpoint of the stop

bit arrives at the user's computer on time. When this happens, the msADV shows the actual required advance

in ms and the OTM becomes a "#". Four consecutive

stable measurements are needed before the OTM switches from "*" to "#". If the user's 1200-Baud modem has

the same internal delay used by NIST (30 ms), then the

*#" OTM should arrive at the user's location within

±2 ms of the correct time. Different brands of 1200 Baud

modems have different internal delays, and the actual

offset of the '#" OTM may be as large as ±10 ms. This

should still be more than adequate, however, since many

computer clocks can only be set within 20-50 ms. The 300-Baud time code includes less information

than the 1200-Baud code, but is more accurate. At 300

Baud, the MJD and DUTl values are deleted and the time is sent every 2 s with the OTM referring to the even-numbered second. Due to a simpler modulation

scheme, the OTM should arrive at the user's computer

within 1 ms of the correct time.

Users are allowed 56 s on ACTS unless all lines are busy. In that case, the first call that reaches 28 s is terminated.

? = HELP

National Instit ute of St andards and Techn ology

Telephone Time Service

(1 seco nd pau£je here)

D L D

MJD YR MO DA H M S ST S UT1 m sADV <OTM>

47222 88-03-02 2 1 :39:1£) 83 0 +.3 045,0 IITC (N [ST) * 47222 88-03-02 2 1 :39:ie5 83 0 +.3 045. 0 LJTC (N rST) * 47222 88-03-02 2 1 :39:17' 83 0 +.3 045.0 IJTC (NIST) * 47222 88-03-02 2 1 :39:1?J 83 0 +.3 045.0 I)TC(NIST) * 47222 88-03-02 2 1 :39:1£) 83 0 +.3 037.6 LJTC(NIST)

47222 88-03-02 2 1 :39:2C) 83 0 +.3 037.6 LJTC{NIST)

etc . etc , etc .

.

Figure E. Automated Computer Time Service (ACTS) screen display.

25

Inquiries about iUiST Time and Frequency Services

If you have specific questions about the operations of NIST radio stations, contact:

Engineer-in-Charge Engineer-in-Charge

NIST Radio Stations WWV and WWVB NIST Radio Station WWVH

2000 East County Road 58 OR: P.O. Box 417

Fort Collins, CO 80524 Kekaha, Kauai, HI 96752

(303) 484-2372 (808) 335-4361

If you have specific questions about the other tune and frequency services, contact:

NIST Time and Frequency Services, 847.40 National Institute of Standards and Technology

325 Broadway

Boulder, CO 80303-3328

(303) 497-3294

Tours of NIST Facilities:

Public guided tours of the NIST Laboratories in Boulder are held twice a week from Memorial Day to Labor Day, and once a week the rest of the year. They

offer a chance to see the NIST Atomic Clock that pro- vides the basis for the time and frequency services, as well as visiting other laboratories of NIST and the National Oceanic and Atmospheric Administration.

Contact the Tour Program Office, Division 360.06,

NIST, 325 Broadway, Boulder, CO 80303-3328; telephone

(303) 497-5507; for information about when tours are scheduled or to arrange special tours for groups of 15 to 30 people.

There are no public tours available at the radio

stations.

A Brief History of NIST Time and Frequency Services

March 1923 First scheduled broadcasts of WWV, Washington, D.C.

April 1933 WWV gets first 20-kW transmitter, Beltsville, Maryland.

January 1943 WWV relocated to Greenbelt, Maryland.

November 1948 WWVH commenced broadcasts, Maui, Hawaii.

January 1950 WWV added voice announcements.

July 1956 WWVB began 60-kHz broadcasts (as KK2XEI), Sunset, Colorado.

April 1960 WWVL began 20-kHz experimental broadcasts. Sunset, Colorado.

July 1963 WWVB began high power broadcasts, Ft. Collins, Colorado.

August 1963 WWVL began high power broadcasts. Ft. Collins, Colorado.

July 1964 WWVH added voice announcements.

December 1966 WWV relocated to Ft. Collins, Colorado.

July 1971 WWVH relocated to Kauai, Hawaii. June 1972 First leap second in history was added to UTC time scale. July 1972 WWVL went off the air. January 1974 Voice announcements changed from GMT to UTC (WWV/ WWVH).

July 1974 GOES satellite time code initiated. March 1975 Frequency calibration using network color TV became a nationwide service. August 1975 Line-10 time comparisons using TV synchronization pulses became a nationwide service. February 1977 20- and 25-MHz broadcasts from WWV and 20-MHz broadcasts from WWVH were discontinued.

December 1978 20-MHz broadcasts from WWV were reinstated. February 1984 Frequency Measurement Service began.

March 1988 Automated Computer Time Service (ACTS) began on experimental basis.

27

*U S GOVERNMENT PRINTING OFFICE: 1991-281-616/42755

BL-114A

(5-90) U.S. DEPARTMENT OF COMMERCENATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

1. PUBUCATIOK OR REPORT NUMBER

NIST/SP-432 (Revised 1990)

2. PERFORMING ORGANIZATION REPORT NUMBER

BIBLIOGRAPHIC DATA SHEET 3. PUBLICATION DATE June 1991

4. TITLE AND SUBTITLE

NIST Time and Frequency Services

5. AUTHOR(S)

Roger E. Beehler and Michael A. Lombardi

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 80303-3328

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Same as item 6 above,

10. SUPPLEMENTARY NOTES

Supersedes NBS Special Publication 432 dated September 1979

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

NIST Time and Frequency Services [Special Publication 432 (Revised 1990)] is a revision

of SP 432, last published in 1979. It describes services available, as of December 1990,

from NIST radio stations WWV, WWVH, and WWVB; from GOES satellites; from Loran-C; by

telephone (voice and modem); and from the NIST Frequency Measurement Service.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

broadcast of standard frequencies; computer time setting; frequency calibrations; GOES

satellite; high frequency; low frequency; satellite time code; shortwave; standard

frequencies; time calibrations; time signals

13. AVAILABIUTY 14. NUMBER OF PRINTED PAGES

X UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). 30

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402.

15. PRICE

X ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161. ELECTRONIC FORM



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