timing & time code reference

Timing & Time Code Reference

APPLICATION NOTE

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Time Scales of MeasurementIntroduction.....................................................................................................................1Definition of Time ...........................................................................................................1Universal Time (UT0) ......................................................................................................1Time and Navigation.......................................................................................................1UT1 ..................................................................................................................................2UT2 ..................................................................................................................................2Ephemeris Time .............................................................................................................2Atomic Time....................................................................................................................2Coordinated Universal Time...........................................................................................2Local Time ......................................................................................................................3Formal Definitions of Time Interval...............................................................................3Accurate Time: A Summary ...........................................................................................3

Digital Clock Accuracy and SynchronizationIntroduction.....................................................................................................................4Time Base Standards in Clocks.....................................................................................4Cesium Beam Standards (Cs)........................................................................................4Hydrogen Maser .............................................................................................................4Rubidium Standard (Rb) .................................................................................................4Quartz Oscillators...........................................................................................................5Sources of Oscillator Error ............................................................................................5Accumulated Time Error versus Elapsed Time ............................................................6Setting the Clock ............................................................................................................7WWV, WWVH, WWVB Time Synchronization..................................................................7Loran ...............................................................................................................................7Geostationary Operational Environmental Satellite (GOES) .........................................7Transit (Navy Navigation Satellite System, NNSS) .......................................................7Global Positioning System (GPS) ...................................................................................7Selection and Use of Time Code Formats.....................................................................8Inter Range Instrumentation Group (IRIG) Time Code Formats ..................................8Description of IRIG Time Code Formats........................................................................9

IRIG Time Code FormatsIRIG Standard Format A 1000 PPS Code.....................................................................10IRIG Standard Format B 100 PPS Code ......................................................................11IRIG Standard Format D 1 PPM Code .........................................................................12IRIG Standard Format E 10 PPS Code.........................................................................13IRIG Standard Format G 10 KPPS Code ......................................................................14IRIG Standard Format H 1 PPS Code ..........................................................................15

Other Time Code FormatsNASA 36-Bit One-Second Time Code..........................................................................16NASA 28-Bit One-Minute Time Code ..........................................................................17NASA 20-Bit One-Hour Time Code..............................................................................18XR3/2137 25 PPS One Second Time Code...................................................................19Symmetricom Bi-Level 5-Rate Slow Code..................................................................20WWV/WWVH Time Code ...............................................................................................21WWVB Time Code .........................................................................................................22

IEEE 1344 Compliance ......................................................................................23

TABLE OF CONTENTS

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IntroductionMany centuries have been spent devisingmethods for the determination and meas-urement of time. In the beginning, timerequirements were quite crude, mostlylimited to the prediction of such phenome-na as seasonal changes and, the ability todivide a day into smaller, more under-standable and predictable segments.

Ultimately, the determination of time wasreduced to scales of measurement. Theearliest references for use in the calcula-tion of time scales were the sun and moon.By observation of the moon’s variousstages, a reliable and repeatable scalecould be devised for dividing a seasonalyear into a number of fairly equal seg-ments.

As the ability to make astronomical meas-urements was further refined, our ances-tors turned to the sun for a more precisereference. Like the moon, the sun provideda constant and repeatable reference fromwhich to devise more precise scales oftime measurement. The sundial is themost familiar of these early attempts.

Until fairly recently, the sun’s positiondefined the time of day. When the sun wasnot visible it was impossible to know theexact time, so clocks were developed tomeasure out the hours between checkswith the sun. The study of different clocksis an interesting branch of history, but thishistory will be reviewed only to the extentneeded to understand the meaning of“accurate time.”

All clocks measure time, but differentclocks can have different status or impor-tance. For example, a clock can be a pri-mary reference, like the sun’s position, ora secondary reference, which only interpo-lates between checks with the primaryclock or time standard, thus providing anapproximation of time.

Definition of Time In order to understand time, the conceptsof date, interval and synchronization mustbe first understood. “Time” can mean

either date or time interval (i.e., duration).An example of date is November 15, 1996,15:35:14 PST (Pacific Standard Time),where 15:35:14 indicates time of day inhours, minutes and seconds. An exampleof time interval is the amount of timerequired to fly between two cities, say3h:51m:12s. This latter example gives noindication of when (i.e., the date) the flightoccurred, only that it lasted 3 hours, 51minutes, 12 seconds. Note that a notationof hours, minutes and seconds can indi-cate either time of day or duration.

Synchronization is the third important timeconcept. For example, it is not normallycrucial for an orchestra to begin its con-cert at a precise hour, minute or second ofthe day, but it is essential that all mem-bers of the orchestra begin at the sameinstant and that they stay at the sametempo. A gasoline engine’s timing must becorrect within a thousandth of a second orso, otherwise the sparks will not fire thefuel at just the right time to provide powerto the pistons. Beyond the casual user,many electronic navigation systems, com-puter networks and even televisionreceivers require synchronization to trans-mitted signals with an accuracy of a mil-lionth of a second or better.

A time scale is a system of assigning datesto events. The sun’s apparent motion in thesky provides one of the most familiar timescales, but it is certainly not the only one.In order to completely specify a solar date,you must count days (i.e., make a calendar)from some beginning date to which allparties have agreed. In addition (dependingon accuracy needs), you must measure thefractions of days (commonly, hours, min-utes and seconds). That is, you must countcycles (and fractions of cycles) of the sun’sdaily apparent motion around the earth.Time derived from the sun’s apparent posi-tion is called apparent solar time. A sundi-al indicates the fractions of cycles (i.e.,time of day) directly. Calendars, like theGregorian Calendar which we commonlyuse, aid in counting the days and namingthem. Another system, used byastronomers, is called Julian Day. It num-

bers the days that have occurred sincenoon January 1, 4713 B.C. In this system(which is not related to the JulianCalendar) noon January 1, 2000 began day2,451,910. This time scale is useful for cal-culating the number of days between twoevents.

Universal Time (UT0)Since the earth’s orbit around the sun isnot a perfect circle, apparent solar timecannot be a uniform time scale. That is,the time interval between successive(apparent) noons changes throughout theyear. The length of this solar day is alsoaffected by the inclination of the earth’sspin axis to the plane of the earth’s orbit.

To correct for these non-uniformities,astronomers calculated the effects of theearth’s noncircular orbit and the polarinclination on apparent solar time.Universal Time (UT0) is apparent solartime corrected for these two effects. Thecorrection used to obtain UT0 is called theEquation of Time. It is often engraved onsundials, a correction which adds or sub-tracts up to 15 minutes to or from appar-ent solar time, depending on the season.

Astronomers actually measure UniversalTime using the stars rather than the sun. Ifyou count cycles (and fractions) of a distantstar’s apparent position, you get a differenttime scale, called sidereal (pronounced“sy-deer-e-al”) time. Since the earth cir-cles the sun once each year but does notcircle the distant star, sidereal time accu-mulates one more “day” each year thanUniversal Time (and our calendar). The“clock” for both Universal Time and side-real time is the spinning earth; only thecounting methods differ. In actual practice,astronomers observe sidereal time andcorrect it to get Universal Time.

Time and NavigationTime is essential to navigation. In effect, anavigator, using a sextant, determines thelocal time based on the sun’s apparentposition. The difference between his localtime and Universal Time (or Greenwich

TIME SCALES OF MEASUREMENT

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time) is equivalent to the local longitude,since zero longitude passes throughGreenwich, England.

Even though we express longitude indegrees, not hours, minutes and seconds,the difference in time is proportional to thedifference in longitude. Since the earthmakes about one revolution (360°) relativeto the sun in 24 hours, the translation todegrees is simple: 360°/24h = 15° perhour. The navigator’s sextant is the meansof determining local apparent solar timeand the navigator’s clock (chronometer)and the means of determining UniversalTime. As a result of using Universal Timefor navigation, scientists developed tworefinements of UT0: UT1 and UT2.

UT1Scientists discovered many years ago thatthe earth is not fixed on its axis. In effect,what one sees is a wandering of the polesrelative to the fixed astronomical observa-tories, which causes UT0 to vary. The logi-cal response to such a situation is to cal-culate a correction for polar motion andapply it to UT0. UT1 is the result of thiscorrection. The difference between UT0and UT1 is quite small; only about ±0.3second (±300 milliseconds).

UT2As the accuracy and consistency of pendu-lum and quartz crystal clocks improved,scientists discovered many years ago thatUT1 had periodic fluctuations of unknownorigin of periods of one year and one-halfyear. Since these periodic variations arepredictable, astronomers are able to cor-rect UT1 to get a still more uniform timescale, UT2. Again, the corrections aresmall, about ±0.3 second. Thus, thereexists a family of Universal Times based onthe earth’s spin and other refinements:

• UT0 is apparent solar time corrected for theearth’s noncircular orbit and inclined axis.

• UT1 is the true navigato r ’s time sca le re l a te dto the earth’s actual angular position re l a t i ve-to the sun.

• UT2 is a smoothed time sca le and does nota f fect the real periodic variations in thee a r t h ’s angular position. At le a st in principle ,if not in pra c t i ce, UT2 passed by the naviga-to r ’s needs. UT2 is not much used any more .

Ephemeris TimeNear the end of the 19th century, SimonNewcombe at the U.S. Naval Observatorycompiled a set of tables which predicted

the future positions of the sun, moon, andsome planets. He based these predictionson the best data and physical principalsavailable at that time. A table of this sort iscalled an ephemeris.

Newcombe discovered that the actualpositions gradually departed from the pre-dicted positions in a fashion too significantto be explained either by observationalerrors or approximations in the theory. Henoted that if the time were somehow inerror, all the tables agreed well with theobservations. At this point he correctlydetermined that in addition to all the varia-tions noted above, there are random fluc-tuations in the earth’s rotational rate.Later, quartz and atomic clocks confirmedthat the variations exist.

The astronomer’s natural response to thiswas, in effect, to use Newcombe’s tablesfor the sun in reverse to determine time, atime scale called Ephemeris Time (ET).The earth’s orbital (not rotational) positiondetermines Ephemeris Time, and ETshould be more uniform than UniversalTime because geometrical changes in theearth’s shape do not affect the orbitalmotion. ET is not very convenient to usebecause an accurate determination of ETrequires literally years of astronomicalobservations. In the early 1950’s, moreconvenient and precise clocks were devel-oped: atomic clocks. The atomic clocksprovide the uniformity of ET, but are farmore convenient to use.

Atomic TimeIt was mentioned earlier that counting thenumber (and fractions) of cycles of theapparent sun determines a date on theUniversal Time scale. Similarly, countingthe number of cycles of an electronic sig-nal whose frequency is controlled by anatomic or molecular resonance deter-mines date on an atomic time scale. Inmost atomic clocks, electronic circuitssteer a radio frequency into resonancewith a specific atomic or molecular transi-tion (vibration). The resonance is an atomicor molecular property, and its frequencycan be relatively insensitive to tempera-ture, magnetic fields, and other environ-mental conditions. Thus, these resonancesform natural standards of frequency.Atomic clocks are formed by counting thecycles of these atomically or molecularlycontrolled radio signals.

Today scientists and engineers have per-fected clocks based on a resonance in

cesium atoms to an accuracy of betterthan one part in 10-13 (one part in 10 tril-lion). Expressed another way, these clockskeep pace with each other to within two orthree millionths of a second over a year’stime. The earth, on the other hand, mightrandomly accumulate nearly a full sec-ond’s error during a year. Since there arenow literally thousands of atomic clocks inuse, and since they agree well with eachother, the variations in the earth’s rotationrate are easily measured.

International Atomic Time (TAI) is an atom-ic time scale maintained by the BureauInternational de l’Heure (BIH) in Paris,France. The BIH forms TAI from an aver-age of more than one hundred atomicclocks located in many countries. The BIHinitially synchronized TAI with UT2 at zerohours (midnight) January 1, 1958. Sincethat time TAI and UT2 have accumulated adifference of about 35 seconds (July 1,1995). The difference is partly due to varia-tions in the earth’s spin, but mostly to thefact that atomic time was simply defined torun slightly faster than UT2. Even if TAIhad been defined to have exactly the samerate as UT2 in the beginning (1958), itwould soon begin to diverge, because TAIis very constant, while UT2 is always vary-ing with the erratic earth’s rotation.

During the past 37 years, two conflictingdemands on standard time have devel-oped. On one hand, science, communica-tions systems, and electronic navigationsystems have needed and exploited theextreme stabilities offered by atomicclocks. On the other hand, astronomy andcelestial navigation still need time relatedto earth position, no matter how erratic itmight be relative to atomic clocks.

Coordinated Universal Time(UTC)To achieve a workable compromisebetween these two opposing demands, theInternal Radio Consultative Committee(CCIR) created a compromise time scalecalled Coordinated Universal Time (UTC),which became effective January 1, 1972.On that date, the difference between TAIand UTC was 10 seconds. The rate of UTCis exactly the same as TAI. In fact, the“ticks” that mark the beginning of eachsecond of TAI and UTC are precisely syn-chronous. However, the date of any givenevent on the UTC time scale must agreewith its date on UT1 (not TAI or UT2) scaleto within 0.9 seconds. Offsetting UTC from

TAI by a precise, whole number of secondsaccomplishes both requirements. In orderto maintain the defined relationshipbetween UT1 and UTC, a one-second cor-rection, a leap second, is added to UTC asis required. As of January 1, 2000, UTC was32 seconds behind TAI (the original 10 sec-ond difference plus the addition of 22 leapseconds in the intervening 18 years.However, since the earth continuouslychanges its rate of rotation, this time offsetcannot be permanent. In order to keepUTC within 0.9 seconds of UT1, the BIHoccasionally adds (or deletes) a second to(or from) the UTC scale, and every stan-dard time system in the world follows suit.The CCIR recommended that these “leapseconds” should occur on the last day ofJune or the last day of December in anyyear in which a leap second is needed.Since the earth’s rotation rate is not per-fectly predictable, scientists cannot fore-cast the need for a leap second more thana few months in advance. Leap secondswill be needed as long as the UTC timescale is used to keep UTC approximately instep with the sun. Otherwise, our clockswould gradually show the sun rising laterand later until, after thousands of years,our clocks would indicate the sun was ris-ing at noon.

Local TimeIn most places local time differs from UTCby a whole number of hours, depending onthe local time zone. The UTC time zone isGMT (or Zulu time in military terms). Forexample, you subtract five hours from UTCto get Eastern Standard Time (EST). From2:00 a.m. on the first Sunday in April until2:00 a.m. on the last Sunday in October,local time is advanced one hour (daylightsaving time). Hence, only four hours aresubtracted from UTC to get EDT.

NoteUTC is never advanced or retarded inobservance of daylight saving time.

Formal Definitions of TimeInterval The Treaty of the Meter (which the Unite dS ta tes signed in 1875) established an inte r-national organization to ove rsee and admin-i ster the International System of Units (i.e.,the metric system). This inte r n a t i o n a lo rganization determines the definitions ofthe various units of measure, including the

unit of time inte r val, the seco n d .

Prior to 1956, the second was defined to be1/86,400 of a mean solar day (there are86,400 seconds on one 24-hour day). From1956 to 1967, the second was defined interms of Ephemeris Time:1/31,556,925.9747 of the tropical year 1900.In 1967 atomic clocks took over the role ofdefining time interval. The new definitionreads:

“The second is the duration of9,192,631,770 periods of the radiationcorresponding to the transitionbetween the two hyperfine levels of theground state of the cesium-133 atom.”

(13th General Conference of Weights andMeasures (CGPM) (1967), Resolution 1).

The definition of the International AtomicTime scale (TAI) incorporates the definitionof the second.

The formal definition of TAI reads:

“International Atomic Time (TAI) is thetime reference coordinate establishedby the Bureau International de l’Heure(BIH) on the basis of the readings ofatomic clocks operating in acordancewith the definition of the second, thetime of the International System ofUnits.”

(14th CGPM (1971), Resolution 1).

Accurate Time: A SummaryLocal time typically differs from UTC by anintegral number of hours, and UTC differsfrom TAI by an integral number of seconds.Since all standard time broadcasts useUTC by international agreement, almostthe whole world runs on UTC. In many

countries, the legal basis of “standardtime” is UTC. Thus, from a legal point ofview, accurate time must mean UTC,adjusted by the appropriate number ofhours to give local time.

To a navigator on the oceans, however,accurate time really means UT1. UTC maybe close enough (give or take 0.9 seconds)for many navigators but, strictly speaking,navigators need UT1. To a scientist whodoesn’t want to be bothered with leap sec-onds, accurate time means TAI.

So, in a very real sense, accurate time hasdifferent meanings to different people. Theidea of accuracy relates to the use made ofthe time information. Fortunately, most ofus do not need to bother with all these dif-ferent time scales, since the time we getfrom the telephone, radio or television isadequate. If we trace the telephone, radioor television time back to its source, how-ever, we would find that UTC is the masterclock in our lives.

3ExacTime 6000 GPS Time & Frequency Generator

GPS-605

IntroductionIn many applications involving the use of adigital clock or time code generator, theaccuracy of timekeeping is not of para-mount importance. In such cases, all thatmay be required is the ability to preset theclock to a time of day within a second ortwo of UTC time, or the user may want tosimply accumulate elapsed time fromsome initial event. In many cases, veryprecise timekeeping is of extreme impor-tance. Such may be the case whether theuser desires to maintain exact time of day,or precise time from a given starting point.This section discusses two classes of digi-tal clocks and time code generators usingthe general areas described above.

General Purpose Units: Theseunits usually include a medium accuracytime base oscillator, usually a crystaloscillator (sometimes an oven-controlledcrystal to provide improved stability).

Such units have the capability of beingmanually preset to a given time of dayfrom the clock is then started. The startingtime can be set to any value, includingzero. This device usually includes a displayof time that includes hours, minutes andseconds. Day of year is often included.

Precision Units: Precision timekeep-ing units differ from general purpose unitsin that they always operate from an exter-nal reference that is used to synchronizethe unit. A synchronized time code genera-tor will observe a serial time code input(e.g., IRIG B) and synchronize the crystal-driven generator to track the input code.This usually provides a precision of ±2microseconds or better relative to thesource of the time code input.

Another example of a precision unit wouldbe a GPS synchronized time code genera-tor. This unit gets precise time informationfrom the Navstar/ GPS satellite systemand can provide precision of better than±100 nanoseconds relative to UTC, asmaintained by the US Naval Observatory.

Time Base Standards in ClocksThe degree of accuracy with which a digitalclock can be expected to operate (in theabsence of a reference input, such as timecode or GPS data) is dependent on twoprime considerations.

The first factor is that of time baseaccuracy and stability. This considera -tion is of importance whether therequirement is to maintain correct timeof day or to accumulate elapsed timefrom an arbitrary reference point.

The second factor is that of synchroniza -tion to an external time standard. Thisconsideration is of prime importancewhen maintaining time of day. Such synchronization is normally not involvedwhen accumulating elapsed time.

The reference for nearly all working timestandards is based upon a defined fre-quency standard. Since time is the directreciprocal of frequency, the two are directlyinterchangeable and the availability of reli-able frequency makes them ideal as a timebase for digital clocks and time code gen-erators.

Cesium Beam Standards (CS)Although other atomic frequency stan-dards have been developed and are com-mercially available today, the cesium beam(Cs) was selected as the primary standardfor its accuracy and longterm stability.Most commercially available cesium beamstandards are capable of producing a fre-quency to an accuracy of a few parts in1012.

These devices can maintain this accuracyover the life of the cesium tube. The fre-quency stability of the device with meas-urement times from one day to as much asa year does not vary more than a few partsin 1013 of its nominal value. A cesiumbeam standard is an expensive device, butin those instances where autonomous fre-quency accuracy (i.e., with no external ref-erence) for long intervals is required, the

cesium beam standard is the only practicalchoice.

Hydrogen MaserThe hydrogen maser exhibits the bestshort and long-term stability of any stan-dard yet developed. The inherent accuracyof this device is not great compared to thecesium beam. Hydrogen masers are notlikely to become a primary frequencysource unless the accuracy can beimproved and the cost can be substantiallyreduced. The extremely high price of thehydrogen maser limits its use to only themost sophisticated applications requiringultimate stability. The principal areas ofapplication of the hydrogen maser is VeryLong Baseline Interferometry (VLBI), deepspace probes and metrology applications.

Rubidium Standard (Rb)The rubidium frequency standard findsmany applications in modern time and fre-quency management systems. The rubidi-um standard is built in a much smallerpackage than the cesium beam. The char-acteristics of the rubidium standard, how-ever, are significantly different from that ofthe cesium beam. One reason that thisatomic standard was not selected as a pri-mary standard was an inherent frequencydrift (approximately 1 part in 10-11 permonth for most commercial devices). Therubidium is generally selected for applica-tions in which the quartz crystal oscillatordoes not provide adequate long-term sta-bility, and cost and size considerationsrestrict the use of cesium beam standards.The rubidium has excellent retrace charac-teristics (return to the frequency of interestafter a cold start) and warms up to itsnominal frequency in a matter of minutes.The reliability of modern rubidium is suffi-cient with user reporting in excess of fiveyears of uninterrupted operation. Therubidium’s characteristics have made it anexcellent choice for many precision naviga-tion receiver applications requiring high-stability internal oscillators. It also has

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DIGITAL CLOCK ACCURACY AND SYNCHRONIZATION

found applications in metrology laborato-ries and communication systems.

Quartz OscillatorsThe crystal oscillator has a number ofqualities which make its use as a reliablefrequency standard desirable. When cer-tain crystals are subjected to mechanicalstress, they will produce an electricalpotential across opposite faces and, con-versely, when placed within an electricfield, these crystals will be stressed ordeformed in proportion to the strength ofthe field. This electromechanical relation-ship is know as the piezoelectric effect.

Of all crystalline materials, quartz exhibitsthe best oscillator characteristics. It hasmechanical stability and is relativelyimmune to external environmental condi-tions. When connected into a closed loopelectric circuit in such a manner as to sus-tain oscillations and control the frequencyof oscillation, the quartz oscillator providesa convenient and reliable frequency sourceand time base for clocks and time codegenerators. Almost all atomic standardsincorporate quartz crystal oscillators as anessential element.

Sources of Oscillator ErrorAll crystal oscillators exhibit resonant fre-quency changes. Obviously, since a clockrequires a linear time base for accuratetimekeeping, any frequency change in thetime base will introduce error into the timereading. Frequency changes within aquartz oscillator are attributable to twomajor causes: frequency offset and oscilla-tor aging.

Frequency offset is merely the frequencyerror that exists when the clock is initiallyset. This is most likely due to oscillatoraging since the last time the oscillator wascalibrated, but it is a fixed offset in theerror calculation for any given time period.Oscillator aging is the inherent frequencychange due to small physical changes inthe crystalline structure. This factor iscalled either drift or aging, and is definedas a frequency change over a given periodof time. In all quartz oscillators, this agingrate is very pronounced during the initialoperating period, after which it becomesre l a t i ve ly co n stant. Quartz oscillato rs area va i l a b le with aging ra tes from 1 part in 10- 4

per day to better than 1 part in 10-11 per

day. The aging charistics are, in general, aresult of the quality of the raw quartz, thetype of cut, the manufacturing process,and the aging process allowed by the manufacturer.

The equation for total time error is:

The total time error (E) over any period oftime (t) depends upon four variables.Where …

Eo is the initial time error

fo is the initial frequency setting (offset)

fr is the reference setting (ideal frequency)

a is the oscillator aging rate

t is the elapsed time in seconds

NoteSet t to 86,400 for the daily aging rate of a

crystal oscillator.

The frequency change of the oscillator isassumed to be linear in an ever increasingvalue. The time error increases exponen-tially as the frequency of the oscillatordrifts. If an oscillator is set precisely tocoincide with the reference oscillator, fre-quency offset (fr) is zero and the total timeerror will be a result only of the aging ofthe oscillator. An offset error will vary thetime error on a linear basis.

The nomograph shown on the followingpage can be used as an aid to estimate thetime error accumulated over a period oftime due to aging and frequency offset of acrystal oscillator. In accordance with theabove equation, the two error values areadded to determine the total error for anygiven condition.

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TymMachine 7000

Series 700

Setting the Clock If the purpose of the time code generatoris merely to keep track of a sequence ofevents in a given test, then setting theclock to a precise external reference is notvery important. However, if the time codegenerator must provide time informationthat is to be correlated with data collectedat a different location, then both clocksmust agree. In the second case, setting theclocks to a common time scale (probablyUTC) is not only desirable but necessary.Several methods have been devised toaccomplish this task.

WWV, WWVH, WWVB TimeSynchronization The National Institute of Standards andTechnology (NIST) maintains three radiostations whose function it is to transmitstandard time broadcasts for usersthroughout the world. Radio stations WWVin Colorado and WWVH in Hawaii are highfrequency broadcast stations on carrierfrequencies of 2.5 MHz, 5 MHz, 10 MHz and20 MHz. WWV additionally transmits on 25MHz. In addition to the two high frequencyradio stations, radio station WWVB inColorado broadcasts serial time code at 60kHz carrier for accurate frequency meas-urements within the continental UnitedStates.

Radio stations WWV and WWVH have, inaddition to serial time codes, voice timeannouncements and other special voiceannouncements, as well as various tonesignals. Finally, a one-second “tick” is alsobroadcast from both stations for audiodetermination of the 1 PPS “epoch.” Thistick is used to start a time code generatorby first listening to the voice timeannouncement and setting the time of thegenerator to the beginning of the nextminute. Then, when the time approachesthe top of the minute, the local operatorwill arm the generator to use the tick tostart counting. This technique will start atime code generator within one or two mil-liseconds of UTC. By using a dual traceoscilloscope and an advance/retard func-tion (if it is included in the generator), onecan approach UTC to within 100 microsec-onds, provided that the signal propagationtime from the radio station to the user’slocation is well known.

Since the carrier frequency of WWVB is low(60 kHz), the propagation delay remainsrelatively constant and may be more accu-rately calculated compared to WWV andWWVH which have varying transmissionpaths due to atmospheric considerations.A time code with a one-minute time frameis transmitted on WWVB that will providean accuracy of about 5 milliseconds.

Loran CThe Loran C (LOng RAnge Navigation) sys-tem has been used by time and frequencyusers for a variety of reasons. All Loranstations are equipped with triple redundantcesium beam frequency standards. Thetime at every Loran station is periodicallycalibrated by the US Naval Observatory. ALoran C receiver that is specificallydesigned to provide precise time and fre-quency can produce frequency accuracythat is within 1 part in 1012 and a time ref-erence that is always within ±500 nanosec-onds of UTC. The biggest problem associ-ated with using Loran C as a time and fre-quency reference is that it is a coastal sys-tem, intended to provide navigation supportto ships at sea. Loran does not transmitany time scale information, so a local timecode generator must be manually preset toa time, and then kept in synchronization bythe time base frequency and 1 PPS epochderived from the Loran broadcast.

Geostationary OperationalEnvironmental Satellite (GOES)The GOES satellites are used to take pho-tographs of the earth for the study andforecasting of weather, for mapping infor-mation, and collecting data from severalhundred instrumentation platforms spreadthroughout their coverage area. The satel-lites are at an altitude of 36,000 kilome-ters, which give them an orbital period ofexactly 24 hours, thus they appear to befixed in space at one location. A time codemessage is transmitted from a masterclock on the ground at Wallops Island,Virginia up to the satellites. Because of thelength of the path, NIST has elected toadvance transmission of the time codemessage by approximately 260 millisec-onds prior to transmission from WallopsIsland. Thus, GOES timing receiver mayactually receive the time code early. Inorder to provide a precise time code out-

put, a GOES receiver must compensate forthe propagation delay from Wallops Islandto the satellite and back to the user’s loca-tion. The best precision that one canexpect to obtain with a GOES receiver isabout ±10 microseconds. The time scaleused by GOES is UTC.

Transit (Navy NavigationSatellite System, NNSS)The Transit system is a constellation of fivesatellites, all in circular polar orbits,designed to support navigation of ships atsea. The orbital altitude is about 600 miles,providing an orbital period of approximately107 minutes. Commercial equipment hasbeen made for this system, but it has nevergained wide acceptance as a timing sys-tem. Transit was designed for a timingaccuracy of 500 milliseconds, althoughusers have reported a precision of 50microseconds or better. The operator mustmanually enter time within ±15 minutes ofUTC in order to begin the acquisition andtracking process. Timing accuracy at anyuser’s location is dependent largely on thestability of the oscillator in the receiver,since a satellite remains in view only forabout 20 minutes for each pass.

Global Positioning System(GPS)The GPS system provides the ultimate inaccuracy to both the navigator and timinguser. The GPS satellite constellation com-prises four satellites in each of six orbitalplanes for a total of 24 satellites. This sys-tem provides continuous worldwide cover-age for navigational as well as for time/fre-quency users. By observing a minimum offour satellites simultaneously, a GPSreceiver can automatically determine itsown location by triangulation. Each satel-lite transmits a database of informationthat includes (among other data) a precisealmanac that details the orbital details forevery satellite. A GPS receiver can meas-ure the difference in arrival of the satellitesatellite signals from all satellites and thusdetermine their distance, which allowsposition to be determined.

Naturally, this equation requires accuratetime, since the satellites are all in constantmotion. Once the user’s position has beendetermined, a precise time transfer can beaccomplished, with the time at the user’s

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location corrected to within ±100 nanosec-onds relative to the 1 PPS epoch main-tained by the US Naval Observatory.

Every GPS satellite carries a cesium beamfrequency standard. It is essential to theaccuracy of the navigational solution thatthe time and frequency of these devices becontrolled to the maximum extent possible.A one nanosecond error in timing repre-sents about one foot in navigational error.Both the US Naval Observatory and the USAir Force at Falcon AFB, Colorado monitoreach satellite daily. Corrections areuploaded to every satellite as a result ofthese measurements. A user’s GPS receiv-er applies this correction measured data toproduce precise corrected outputs of timeand frequency. The GPS satellite operateson a GPS time scale, which is a continuoustime scale (like UT1). The satellite trans-mits information about the number of leapseconds that must be added to convertGPS time to UTC.

The GPS satellites are in circular orbits atan altitude of about 19,650 kilometers. Atthis altitude, the satellites circle the earthtwice each day (on the sidereal clock).Because the orbital period is based onsidereal time (not solar time), the satellitesappear approximately 4 minutes early eachday.

Symmetricom GPS Time & FrequencyGenerators operate on the L1 frequency(1.57542 GHz) and use the CA (coarseacquisition) code. Many military receiversuse “P” code, which is transmitted on theL2 frequency (1.2276 GHz). P code isencrypted and requires special equipment(and a DoD security clearance) to decryptand use the information. The military hasintroduced an intentional error in the CAcode called Selective Availability (SA). Thepurpose of SA is to deprive an enemy ofthe use of our GPS system to navigate mis-siles and other weapons systems. SA intro-duces a pseudo-random error that effec-tively triples the position fixing capability ofa commercial receiver from about 100 feetto 300 feet (circular error). This causes theinherent GPS (CA) precision of 100nanoseconds to become 300 nanoseconds.Symmetricom GPS receivers average alarge number of position fixes to effectivelyeliminate the error of SA. After positionhas been fixed, the receiver then averagesthe time data from the six satellites that

are the highest in the sky to further reducethe time error.

While processing satellite data normally, atiming GPS receiver will produce precisetime and frequency outputs, regardless ofthe quality of the internal time base oscil-lator, because it is being continuously cor-rected from GPS data. If the receiver isdeprived of satellite data, however, the sta-bility of the internal oscillator is the onlyfactor that determines the amount of timeerror that will develop over time.Symmetricom GPS Time & FrequencyGenerators can be equipped with a basicTCXO crystal, a high-stability oven oscilla-tor or a rubidium oscillator. Each will pro-vide increasing better stability in theabsence of satellite data. All will providethe same time precision of 100 nanosec-onds while processing satellite data.

While the internal operation of a GPSreceiver is extremely complex, the user willfind it to be the simplest system for timeand frequency purposes. The reason is thatthe GPS system is completely self-con-tained and makes all necessary correc-tions automatically. A Symmetricom GPSTime & Frequency Generator need only tobe turned on and allowed to run.

Selection and Use of Time CodeFormats Coded time signals, commonlycalled “time code formats,” some of whichare contained in this document, are prima-rily used to provide time correlation ofmultiple data sources and multiple datapoints during monitoring or recording ofvarious types of data. Precise time, relatedto world-wide time scales, is of criticalimportance in monitoring spaceshiplaunching and tracking activities. Timeintervals, rather than world-wide time, areimportant in monitoring processes: flighttest, weapons test, medical research, etc.Time, when recorded with data, serves as aconvenient means of correlating varioustypes of data recorded on different media.Time also becomes a convenient referencefor data retrieval should analysis of arecording be desired in the future.

Selecting a time code format involves eval-uation of the accuracy and data resolutionbeing recorded, as well as an evaluation ofthe specific recording medium. The band-width of the recording device governs the

rate of the time code format and, therefore,its resolution. The accuracy of the time andtime code format depends primarily on thesource of the time code and its relationshipto world-wide time or on the accuracy ofits individual frequency sources. Thus, vari-ous time codes are defined at differentrates and different resolutions.

Time code formats have been defined tosupport such high speed data acquisitionas camera instrumentation and telemetry.Other formats have been defined to sup-port very low speed data acquisition meth-ods such as oscillographs and strip chartrecorders.

When the time code format is to be trans-mitted or when the specific recordingdevice requires signal conditioning, addi-tional characteristics are usually incorpo-rated. Most common is the use of a sinewave carrier, amplitude modulated by thetime code format. This permits the signalto be conveniently transmitted by means ofa conventional telephone circuit or bybeing recorded on low bandwidth magnetictape recorders.

Inter Range InstrumentationGroup (IRIG) Time CodeFormatsIn the early 1950’s it became apparent thatefficient interchange of test data betweenthe various test ranges and laboratorieswould require time code standardization.This task of standardization was assignedto the TeleCommunications Working Group(TCWG) of the IRIG in October 1956. Theoriginal IRIG standards were accepted bythe steering committee in 1960. IRIGDocument 104-60 defined the original IRIGformats. This document was later revisedand then reprinted in August 1970 as IRIGDocument 104-70. Later in 1970 the statusof the IRIG Document was upgraded to thatof a Standard, and republished as IRIGStandard 200-70. As of this writing, the lat-est publication is IRIG Standard 200-95. Acopy of the latest version can be obtainedat no cost by writing a request to theSecretariat, Range Commander’s Council,White Sands Missile Range, New Mexico,88002.

8

Description of IRIG FormatsIn the IRIG family, individual time code for-mats are alphabetically designated as A, B,D, E, G and H. These are all defined in IRIGStandard 200-95. IRIG C format was origi-nally defined as a one-minute time code inIRIG Document 104-60, but was subse-quently replaced by IRIG H. Various signalforms are defined in IRIG Standard 20-95.The following identification is taken fromthe IRIG Standard 200-95.

Rate Designation:A) 1,000 PPSB) 100 PPSD) 1 PPME) 10 PPSG) 10,000 PPSH) 1 PPS

Form Designation:0) DCLS (width coded)1) Sine wave carrier (amplitude modu-

lated)

Carrier Resolution:0) No carrier (DCLS)1) 100 Hz (10 mS resolution)2) 1 kHz (1 mS resolution)3) 10 kHz (100 S resolution)4) 100 kHz (10 S resolution)5) 1 MHz (1 S resolution)

Coded Expressions: 0) BCD, CF, SBS1) BCD, CF2) BCD3) BCD, SBS

9

IRIG A IRIG B IRIG D IRIG E IRIG G IRIG H

A000 B000 D001 E001 G001 H001

A003 B003 D002 E002 G002 H002

A130 B120 D111 E111 G141 H111

A132 B122 D112 E112 G142 H112

A133 B123 D121 E121 H121

B150 D122 E122 H122

B152

B153

The following signal combinations are recognized in accordance with IRIG Standard 200-95.

PCI Family of Time & Frequency Processors

bc635/637PCI bc635/637CPCI bc635/637PMC

1. Time Frame: 0.1 second.

2. Code Digit Weighting Options:BCD, SBS or both.a. BCD time of year code

word - 34 bits.

1) Seconds, minutes, hours,days and 0.1 seconds, recycle yearly.

b. Straight binary seconds (SBS) - 17 bits.

1) Seconds time of day, recycles daily(count: 00000 - 86399).

3. Code Word Structure:a. BCD: Word begins at Index

Count 1. BCD bits occurbetween Position IdentifierElements (7 for seconds, 7 forminutes, 6 for hours, 10 fordays, 4 for tenths of seconds)until the code word is com-plete. A Position Identifieroccurs between decimal digitsin each group to provide sepa-ration for visual resolution.

b. SBS: Word begins at IndexCount 80. Seventeen binarybits occur with a PositionIdentifier between the 9th and10th bits. This is a pure binaryinteger representing secondssince midnight.

4. Least Significant Digit:O cc u rs first, except for fra c t i o n a ls e conds digit, which occ u rs fo l lowing the hundreds of days digit.

5. Element Rates Available:a. 1,000 PPS (basic element rate)b. 100 PPS

(Position Identifier rate).c. 10 PPS (Frame rate).

6. Element Identification:a. On-Time is the leading edge

of a pulse.b. Index Marker: 0.2 mS (zero or

uncoded bit).c. Code Digit: 0.5 mS (one).d. Position Identifier: 0.8 mS.e. Reference Marker: Two

consecutive PositionIdentifiers.

7. Resolution:1 mS (DCLS), 0.1 mS (modulated carrier).

8. Carrier Frequency: 10 kHz (when modulated).

10

IRIG Format A1000 PPS Code

IRIG Standard 200-95

1. Time Frame: 1 second.

2. Code Weighting Options: BCD,SBS or both.a. BCD time of year code

word - 30 bits.1) Seconds, minutes, hours

and days, recycle yearly.b. Straight binary seconds

(SBS) - 17 bits.1) Seconds time of day,

recycles daily(count: 00000 - 86399).


Count 1. BCD bits occurbetween Position IdentifierElements (7 for seconds, 7 forminutes, 6 for hours, 10 fordays) until the code word iscomplete. A Position Identifieroccurs between decimal digitsin each group to provide sepa-ration for visual resolution.

b. SBS: Word begins at IndexCount 80. Seventeen binarybits occur with a PositionIdentifier between the 9th and10th bits. This is a pure binaryinteger representing secondssince midnight.

4. Least Significant Digit:Occurs first.

5. Element Rates Available:a. 100 PPS (basic element rate)b. 10 PPS (Position Identifier

rate).c. 1 PPS (Frame rate).


of a pulse.b. Index Marker: 2 mS (zero or

uncoded bit).c. Code Digit: 5 mS (one).d. Position Identifier: 8 mS.e. Reference Marker: Two


7. Resolution:10 mS (DCLS), 1 mS (modulatedcarrier); 1 µS (1 MHz modulatedcarrier).

8. Carrier Frequency:1 kHz or 1 MHz (when m o d u l a te d ) .

11

IRIG Format B100 PPS Code


1. Time Frame:1 hour.

2. Code Weighting Options:BCD only.a. BCD time of year code

word -16 bits.1) Hours and days, recycle yearly.


Count 20. BCD bits occurbetween Position IdentifierElements (6 for hours, 10 fordays) until the code word iscomplete. A Position Identifieroccurs between decimal digitsin each group to provide sepa-ration for visual resolution.


5. Element Rates Available:a. 1 PPM (basic element rate)b. 0.1 PPM (Position Identifier

rate).c. 1 PPH (Frame rate).


of a pulse.b. Index Marker: 12 Sec (zero or

uncoded bit).c Code Digit: 30 Sec (one).d. Position Identifier: 48 Sece. Reference Marker: Two


7. Resolution:1 Minute (DCLS), 10 mS (modulated 100 Hz carrier); 1 mS (modulated 1 kHz carrier).

8. Carrier Frequency:100 Hz or 1 kHz (when m o d u l a te d ) .

12

IRIG Format D1 PPM Code


1. Time Frame: 10 Seconds.

2. Code Weighting Options:BCD only.a. BCD time of year code word -

26 bits.

1) Seconds, minutes, hoursand days, recycle yearly.


Count 6. BCD bits occur between Position Identifier Elements (3 for seconds, 7 forminutes, 6 for hours, 10 fordays) until the code word iscomplete. A Position Identifieroccurs between decimal digitsin each group to provide sepa-ration for visual resolution.


5. Element Rates Available:a. 10 PPS (basic element rate)b. 1 PPS (Position Identifier rate)c. 0.1 PPS (Frame rate)



uncoded bit).c. Code Digit: 50 mS (one)d. Position Identifier: 80 mSe. Reference Marker (1 per 10

seconds): Two consecutivePosition Identifiers.

7. Resolution:100 mS (DCLS), 10 mS (modu-lated 100 Hz carrier); 1 mS(modulated 1 kHz carrier)

8. Carrier Frequency:100 Hz or 1 kHz when modulate d )

13

IRIG Format E10 PPS Code


1. Time Frame:0.01 second.


38 bits.1) Seconds, minutes, hours,

days, 0.1 seconds and 0.1 seconds, recycle yearly.


Count 1. BCD bits occurbetween Position IdentifierElements (7 for seconds, 7 forminutes, 6 for hours, 10 fordays, 4 for tenths of seconds,4 for hundredths of seconds)until the code word is com-plete. A Position Identifieroccurs between decimal digitsin each group to provide sepa-ration for visual resolution.

4. Least Significant Digit: Occursfirst, except for fractional sec-onds digits, which occur follow-ing the hundreds of days digit.

5. Element Rates Available:

a. 10,000 PPS (basic element rate).

b. 1,000 PPS (Position Identifier rate).

c. 100 PPS (Frame rate).


of a pulse.b. Index Marker: 0.02 mS (zero

or uncoded bit).c. Code Digit: 0.05 mS (one).d. Position Identifier: 0.08 mS.e. Reference Marker: Two


7. Resolution: 0.1 mS (DCLS), 0.01mS (modulated carrier).

8. Carrier Frequency:100 kHz (when modulated).

14

IRIG Format G1000 PPS Code


1. Time Frame:1 minute.


23 bits. Minutes, hours anddays, recycle yearly.


Count 10. BCD bits occurbetween Position IdentifierElements (7 for minutes, 6 forhours, 10 for days) until thecode word is complete. APosition Identifier occursbetween decimal digits ineach group to provide separa-tion for visual resolution.


a. 1 PPS (basic element rate)b. 0.1 PPS (Position Identifier

rate).c. 1 PPM (Frame rate).


of a pulse.b. Index Marker: 0.2 Sec (zero or

uncoded bit).c. Code Digit: 0.5 Sec (one)d. Position Identifier: 0.8 Sece. Reference Marker: Two


7. Resolution:1 Sec (DCLS), 10 mS ( m o d u l a ted 100 Hz carrier); 1 mS (modulated 1 kHz carrier).

8. Carrier Frequency:100 Hz or 1 kHz (when m o d u l a te d ) .

15

IRIG Format H1 PPS Code


1. Time Frame:1 second.

2. Code Weighting Options:BCDa. BCD time of year code word -

30 bits. 1) Seconds, minutes,hours and days, recycle yearly.


Count 1. BCD elements occurbetween Position IdentifierElements in four-bit groupsuntil the code word is com-plete. A Code Digit occursbetween decimal digits ineach group to provide separa-tion for visual resolution.


5. Element Rates Available:a. 100 PPS (basic element rate)b. 1 PPS (Frame rate)



uncoded bit).c. Code Digit: 6 mS (one)d. Position Identifier: 6 mS

(same as one).e. Reference Marker: Five

consecutive PositionIdentifiers followed by anIndex Marker.

7. Resolution:10 mS(DCLS), 1 mS (modulatedcarrier).

8. Carrier Frequency:1 kHz (when modulated).

16

NASA 36-BitOne-Second Time Code

1. Time Frame: 1 minute.

2. Code Weighting Options: BCD

a. BCD time of year code word -23 bits. Minutes, hours anddays, recycle yearly.

3. Code Word Structure: BCD:Word begins at Index Count 1.BCD elements occur betweenPosition Identifier Elements infour-bit code groups until thecode word is complete. A CodeDigit occurs between decimaldigits in each group to provideseparation for visual resolution.


5. Element Rates Available:

a. 2 PPS (basic element rate)

b. 1 PPM (Frame rate)

6. Element Identification:

a. On-Time is the leading edge of a pulse.

b. Index Marker: 100 mS (zero oruncoded bit).

c. Code Digit: 300 mS (one)

d. Position Identifier: 300 mS (same as one)

e. Reference Marker: Fiveconsecutive PositionIdentifiers followed by anIndex Marker.

7. Resolution: 0.5 Sec (DCLS), 10mS (modulated 100 Hz carrier);1 mS (modulated 1 kHz carrier)

8. Carrier Frequency: 100 Hz or 1kHz when modulated).

17

NASA 28-BitOne-Minute Time Code

1. Time Frame:1 hour.

2. Code Weighting Options:BCDa. BCD time of year code word -

16 bits. Hours and days, recycle yearly.

3. Code Word Structure:BCD: Word begins at IndexCount 1. BCD elements occurbetween Position IdentifierElements in four-bit code groupsuntil the code word is complete.A Code Digit occurs betweendecimal digits in each group toprovide separation for visualresolution.


5. Element Rates Available:a. 1 PPM (basic element rate).b. 1 PPH (Frame rate).


of a pulse.b. Index Marker: 12 Sec (zero or

uncoded bit).c. Code Digit: 36 Sec (one).d. Position Identifier: 3 Sec

(same as one).e. Reference Marker: Five

consecutive PositionIdentifiers followed by anIndex Marker.

7. Resolution:1 Minute (DCLS), 10 mS (modulated 100 Hz carrier); 1 mS (modulated 1 kHz carrier)

8. Carrier Frequency:100 Hz or 1 kHz (when modulate d ) .

18

NASA 20-BitOne-Hour Time Code

1. Time Frame:1 second.

2. Code Weighting Options:BCDa. BCD time of day code word -

20 bits. Hours, minutes andseconds, recycle daily.

3. Code Word Structure:BCD: Word begins at IndexCount 1. BCD elements occurbetween Position IdentifierElements consecutively (6 forhours, 7 for minutes, 7 for sec-onds) until the code word iscomplete.

4. Most Significant Digit:Occurs first.

5. Element Rates Available:a. 25 PPS (basic element rate)b. 1 PPS (Frame rate)

6. Element Identification:a. On-Time is the trailing edge of

a pulse.b. Index Marker: 12 mS (zero).c. Code Digit: 24 Sec (one).d. Reference Marker: 36 mS

(once per frame).

7. Resolution:1 Second (frame rate).a. 4 mS - modulated XR3 code.b. 1 mS - modulated 2137 code.

8. Carrier Frequency:a. 250 Hz for XR3 code.b. 1 kHz for 2137 code.

19

25 PPS-One SecondTime Code

(250 Hz - XR3)(1 kHz - 2137)

This special bi-level slow code isdesigned to provide a convenientand efficient time reference forstrip chart and oscillographrecords. Five different and uniquecodes are provided to enable selec-tion of the desired resolution thatis consistent with the chart paperspeed.

The code is a width-modulated,BCD format. Amplitude modulationis also included to aid in the visualseparation of the coded informa-tion.

Each code group is separated by aspace as a further aid to fast visualrecognition of the coded time infor-

mation without loss of resolutionor inclusion of extraneous pulses.Coded information represents timeof year in seconds, minutes, hoursand days.

20

Bi-Level — 5-RateSlow Code



30 bits. Minutes, hours, days,Daylight Savings Time and UTCorrection.

3. Code Word Structure:BCD: Word begins at IndexCount 10 + 30 mS. BCD elements occur betweenPosition Identifier Elements (7for minutes, 6 for hours, 10 fordays, 1 for DST and 7 for UT correction) until the code wordis complete. Position Identifieroccurs between decimal digits in each group to provide separa-tion for visual resolution.


5. Element Rates Available:a. 1 PPS (basic element rate)b. 1 per 20 seconds (Position

Identifier).c. 1 PPM (Frame rate).

6. Element Identification:a. On-Time reference for each

element is 30 mS before the element.

b. Index Marker: 170 mS (zero oruncoded element).

c. Code Digit: 470 mS (one)d. Position Identifier: 770 mS.d. Reference Marker: 770 mS

(once per frame)

7. Resolution:1 Second unmodulated; 0.01second modulated 100 Hz (theoretical).

8. Carrier Frequency:100 Hz - 100% modulation.

9. Time Scale:UTC with coded correction toUT1.

21

National Institute of Standards and TechnologyWWV / WWVH Time Code

Standard Time Code Envelope



35 bits. Minutes, hours, days,UT Correction, Daylight SavingTime and leap year.

3. Code Word Structure:BCD: Word begins at IndexCount 10 + 30 mS. BCD elements occur betweenPosition Identifier Elements (7 for minutes, 6 for hours, 10for days, 7 for UT correction, 1for DST and 1 for leap year) until the code word is complete.Position Identifier occursbetween decimal digits in eachgroup to provide separation forvisual resolution.


5. Element Rates Available:a. 1 PPS (basic element rate)b. 1 per 10 seconds (Position

Identifier).c. 1 PPM (Frame rate).

6. Element Identification:a. On-Time reference for each

element is the leading edge.b. Index Marker: 200 mS (zero or

uncoded element).c. Code Digit: 500 mS (one).d. Position Identifier: 800 mS.d. Reference Marker: 800 mS

(once per frame)

7. Resolution:1 Second unmodulated; 0.01second modulated 100 Hz (theoretical).

8. Carrier:60 Hz - 1:10 modulation ratio.

9. Time Scale:UTC with coded correction toUT1.

22

National Institute of Standards and TechnologyWWVB Time Code

1 PPS Code Envelope

The Power System RelayingCommittee of the IEEE PowerEngineering Society has publisheda document (IEEE Std 1344-1995)that discusses unique synchroniz-ing issues that are encountered inthe Power Utilities. As a part ofthat document (Annex F), a specific

utilization of the ‘Control Bit’ seg-ment of IRIG B time code isdefined. Symmetricom has deter-mined that this definition has meritfor many traditional users of timecode. Accordingly, new timingproducts that are designed andmanufactured will be ‘IEEE 1344

Compliant,’ which means that the27 control bits in a standard IRIG Bserial time code format are used toprovide additional information asset forth by the IEEE standard. Thefollowing table defines each of thebits in the Control Function field ofthe IRIG B code format.

23

IEEE:1344 COMPLIANCE

P50 1 Year, BCD 1

P51 2 Year, BCD 2

P52 3 Year, BCD 4

P53 4 Year, BCD 8

P54 5 Not Used Unassigned

P55 6 Year, BCD 10

P56 7 Year, BCD 20

P57 8 Year, BCD 40

P58 9 Year, BCD 80

P59 — P6 Position Identifier #6

P60 10 Leap Second Pending (LSP) Becomes 1 up to 59 sec BEFORE leap second insertion

P61 11 Leap Second (LS) 0=Add leap second; 1=Subtract leap second

P62 12 Daylight Savings Pending (DSP) Becomes 1 up to 59 BEFORE DST change

P63 13 Daylight Savings Time (DST) Becomes 1 during DST

P64 14 Time Zone Offset - Sign Bit

P65 15 Time Zone Offset - Binary 1 hr

P66 16 Time Zone Offset - 2 Hr

P67 17 Time Zone Offset - Binary 4 Hr

P68 18 Time Zone Offset - Binary 8 Hr


P70 19 Time Zone Offset - 0.5 Hr 0=None; 1=Additional 0.5 Hr time offset

P71 20 Time Quality

P72 21 Time Quality

P73 22 Time Quality

P74 23 Time Quality

P75 24 Parity Odd parity for all data bits from the frame start to CF Bit 23





IEE 1344 Conrol Bit Assignments

Last 2 digits of year in BCD

Last 2 digits of year in BCD

Offset between UTC and the time in the IRIG B time codeframe (the offset will change during Daylight Savings Time)

4-bit code representing approximate clock time error

0100=Clock locked, accuracy <1 microsecond1111=Clock failed, data unusable

©2003 Symmetricom. Symmetricom and the Symmetricom logo are registered trademarks of Symmetricom, Inc. All specifications subject tochange without notice. DS/CS4000/D/0703/500

SYMMETRICOM, INC.2300 Orchard ParkwaySan Jose, California 95131-1017tel: 408.433.0910fax: [email protected]

timing & time code reference

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