《航海学》课程参考文献（地文资料）CHAPTER 18 TIME

CHAPTER 18TIMETIMEINNAVIGATION1800.SolarTimemoved to point C in its orbit. Thus, duringthe courseofa daythe sun appears to move eastward withrespect to the stars.The earth'srotation on its axis causes the sun and otherThe apparent positions of the stars are commonly reck-celestialbodies toappeartomoveacross the skyfromeastoned with reference to an imaginary point called the vernalto west each day.If a person located on the earth's equatorequinox,the intersection ofthe celestialequator and theeclip-measured thetime interval between two successivetransitstic.Theperiod oftheearth'srotationmeasured with respecttooverhead ofa very distant star, he wouldbe measuringthethe vernal equinox is called a sidereal day. The period withperiod ofthe earth'srotation.Ifhe then made a similarmea-respect to the sun is called an apparent solar day.surement of the sun,the resultingtimewould be about4When measuringtimebytheearth's rotation,using theminutes longer.This is due to the earth's motion around theactual position ofthe sun results in apparent solartimesun, which continuously changes the apparent place of theUse of the apparent sun as a time reference results insun among the stars.Thus, during the course of a day thetime of non-constantrate for at least three reasons.First,rev-sun appears to move a littleto the east among thestars soolution of the earth in its orbit is not constant. Second, timethat the earth must rotate on its axis throughmore than 3600is measured along the celestial equator and thepath of thein order to bring the sun overhead againreal sun is not along the celestial equator. Rather, its path isSeeFigure1800.If the sun is on the observer's meridianalong the ecliptic, which is tilted at an angle of23°27 withwhen the earth is at point A in its orbit around the sun, it willrespect to the celestial equator.Third, rotation of the earthnotbe onthe observer'smeridian aftertheearthhas rotatedon itsaxis isnotconstant.through360°becausetheearth will havemoved along its or-Toobtaina constantrateoftime,the apparent sun isre-bit to point B. Before the sun is again on the observer'splaced by a fictitious mean sun. This mean sun movesmeridian, the earth must turn still more on its axis. The sunwill be on the observer's meridian again when the earth haseastwardalongthecelestialequatorata uniform speedequalSEGMENTOFELLPTICALPATHOFEARTHSORBITABOUTSUNCBSERVERSMERIDIANFigure 1800.Apparent eastward movement of the sun with respect to the stars287

287 CHAPTER 18 TIME TIME IN NAVIGATION 1800. Solar Time The earth’s rotation on its axis causes the sun and other celestial bodies to appear to move across the sky from east to west each day. If a person located on the earth’s equator measured the time interval between two successive transits overhead of a very distant star, he would be measuring the period of the earth’s rotation. If he then made a similar mea￾surement of the sun, the resulting time would be about 4 minutes longer. This is due to the earth’s motion around the sun, which continuously changes the apparent place of the sun among the stars. Thus, during the course of a day the sun appears to move a little to the east among the stars so that the earth must rotate on its axis through more than 360° in order to bring the sun overhead again. See Figure 1800. If the sun is on the observer’s meridian when the earth is at point A in its orbit around the sun, it will not be on the observer’s meridian after the earth has rotated through 360° because the earth will have moved along its or￾bit to point B. Before the sun is again on the observer’s meridian, the earth must turn still more on its axis. The sun will be on the observer’s meridian again when the earth has moved to point C in its orbit. Thus, during the course of a day the sun appears to move eastward with respect to the stars. The apparent positions of the stars are commonly reck￾oned with reference to an imaginary point called the vernal equinox, the intersection of the celestial equator and the eclip￾tic. The period of the earth’s rotation measured with respect to the vernal equinox is called a sidereal day. The period with respect to the sun is called an apparent solar day. When measuring time by the earth’s rotation, using the actual position of the sun results in apparent solar time. Use of the apparent sun as a time reference results in time of non-constant rate for at least three reasons. First, rev￾olution of the earth in its orbit is not constant. Second, time is measured along the celestial equator and the path of the real sun is not along the celestial equator. Rather, its path is along the ecliptic, which is tilted at an angle of 23° 27' with respect to the celestial equator. Third, rotation of the earth on its axis is not constant. To obtain a constant rate of time, the apparent sun is re￾placed by a fictitious mean sun. This mean sun moves eastward along the celestial equator at a uniform speed equal Figure 1800. Apparent eastward movement of the sun with respect to the stars

288TIMEExample2:See Figure 1801.Determine the time of the up-to the average speed of the apparent sun along the ecliptic.permeridianpassageof the sun on April16,1995Thismeansun,thereforeprovidesauniformmeasureoftime which approximates the average apparent time.TheSolution:FromFigure 1801,uppermeridian passagespeed of themean sunalong the celestial equator is15°perofthe sun on April 16, 1995, is given as 1200. The dividinghourofmeansolartimelinebetweenthevaluesforupperand lowermeridianpas-sage on April 16th indicates that the sign of the equation of1801.EquationOfTimetimechangesbetveen lowermeridianpassageanduppermeridian passage on thisdate; the question,therefore,be-comes: does it become positive or negative? Note that onMeansolartime,ormeantimeasitiscommonlyApril18,1995,uppermeridianpassageisgivenas1159,called, is sometimesahead of and sometimes behindappar-ent solartime.This difference,whichnever exceeds aboutindicating thaton April 18,1995,the equation of time ispositive.All valuesfor the equation oftime on the same side16.4minutes, is called the equation of time.ofthedividingline asAprilI8tharepositive.Therefore,theThe navigatormost often deals with theequation of timeequationof timefor uppermeridianpassageof the sunonwhen determining the time of upper meridian passage of theApril 16,1995 is (+)00m05sUpper meridian passagesun.The sun transits theobserver's uppermeridian at local ap-parent noon.Were it not for thedifference in rate between thetherefore, takes placeat 11h59m55smeanandapparent sun,the sun would beon theobserver'sme-ridian when the mean sun indicated 1200 local time.Theapparent solar time ofupper meridian passage, however, is offsetfromexactly1200meansolartime.Thistimedifference,theSUNMOONDayequation of time atmeridian transit is listed on theright handEqn.ofTimeMerMer.Pass.12h00hPass.UppelLowerAgPhasedailypagesoftheNauticalAlmanacThesignof the equation oftime is positiveif thetime000016605of sun's meridianpassage isearlierthan1200 andnegative170020Oif laterthan1200.Therefore14Apparent Time=Mean Time-(equation of time).Figure1801.The equation oftimefor April 16,17,18,1995Example:Determinethe time of the sun's meridianpassage(Local ApparentNoon)on June16,1994.To calculate latitude and longitude at LAN,the navigatorSolution: See Figure 2007in Chapter 20, the Nauticalseldom requires the time of meridian passage to accuraciesAlmanac's right hand daily pagefor June16,1994.Thegreater than one minute.Therefore,use the time listed underequation of time is listed in thebottom righthand cornerofthe"Mer.Pass."column to estimateLANunless extraordinarythe page.There are two ways to solve theproblem,depend-accuracy is required.ing on the accuracy required for the value of meridianpassage.The time of the sun at meridian passage is givento1802.FundamentalSystemsOfTimethe nearest minute in the"Mer.Pass."column.For June16, 1994, this value is 1201.The first fundamental system of time is EphemerisTodetermine the exacttime of meridian passage,useTime (ET).EphemerisTimeis used byastronomers incal-the value given for the equation of time.This value is listedculating the fundamental ephemerides of the sun, moon,immediatelytotheleff of the"Mer.Pass."column ontheand planets.It is notused bynavigators.dailypages.ForJume16,1994,thevalueisgivenas00m37sThe fundamental system of time of most interesttoUsethe"12h"columnbecausetheproblemasked formerid-navigators is Universal Time (UT). UT is the mean solarianpassageatLAN.Thevalueofmeridianpassagefromthetime on the Greenwich meridian, reckoned in days of 24"Mer.Pass."columnindicatesthat meridian passage oc-mean solar hours beginning with Oh at midnight. Universalcurs afer 1200; therefore, add the 37 second correction toTime, in principle, is determined bythe average rate ofthe1200toobtaintheexacttimeofmeridianpassage.Theexactapparent daily motion ofthe sun relative to the meridian oftimeofmeridianpassageforJune16,1994,is12h00m37s.Greenwich, but in practice the numerical measure of Uni-versal Time at any instant is computed from sidereal timeTheequation of time's maximum value approachesUniversal Time is the standard intheapplication ofastron-16m22sinNovember.omy to navigation.Observations of Universal Times areIftheAlmanacliststhetimeofmeridianpassageasmade by observing the times of transit of stars.1200,proceedasfollows.Examinetheequationsoftimelist-The Universal Time determined directly from astro-nomical observations is denoted UTo.Since the earth'sed intheAlmanactofindthe dividing linemarkingwhere theequation of time changes between positiveand negativeval-rotationisnonuniform,correctionsmustbeappliedtoUToues.Examine thetrendofthe values near this dividingline toto obtain a more uniform time.This more uniform time isdetermine the correct signforthe equationoftime.obtainedbycorrectingfortwoknownperiodicmotions

288 TIME to the average speed of the apparent sun along the ecliptic. This mean sun, therefore, provides a uniform measure of time which approximates the average apparent time. The speed of the mean sun along the celestial equator is 15° per hour of mean solar time. 1801. Equation Of Time Mean solar time, or mean time as it is commonly called, is sometimes ahead of and sometimes behind appar￾ent solar time. This difference, which never exceeds about 16.4 minutes, is called the equation of time. The navigator most often deals with the equation of time when determining the time of upper meridian passage of the sun. The sun transits the observer’s upper meridian at local ap￾parent noon. Were it not for the difference in rate between the mean and apparent sun, the sun would be on the observer’s me￾ridian when the mean sun indicated 1200 local time. The apparent solar time of upper meridian passage, however, is off￾set from exactly 1200 mean solar time. This time difference, the equation of time at meridian transit, is listed on the right hand daily pages of the Nautical Almanac. The sign of the equation of time is positive if the time of sun’s meridian passage is earlier than 1200 and negative if later than 1200. Therefore: Apparent Time = Mean Time – (equation of time). Example 1: Determine the time of the sun’s meridian passage (Local Apparent Noon) on June 16, 1994. Solution: See Figure 2007 in Chapter 20, the Nautical Almanac’s right hand daily page for June 16, 1994. The equation of time is listed in the bottom right hand corner of the page. There are two ways to solve the problem, depend￾ing on the accuracy required for the value of meridian passage. The time of the sun at meridian passage is given to the nearest minute in the “Mer. Pass.”column. For June 16, 1994, this value is 1201. To determine the exact time of meridian passage, use the value given for the equation of time. This value is listed immediately to the left of the “Mer. Pass.” column on the daily pages. For June 16, 1994, the value is given as 00m37s. Use the “12h” column because the problem asked for merid￾ian passage at LAN. The value of meridian passage from the “Mer. Pass.” column indicates that meridian passage oc￾curs after 1200; therefore, add the 37 second correction to 1200 to obtain the exact time of meridian passage. The exact time of meridian passage for June 16, 1994, is 12h00m37s. The equation of time’s maximum value approaches 16m22s in November. If the Almanac lists the time of meridian passage as 1200, proceed as follows. Examine the equations of time list￾ed in the Almanac to find the dividing line marking where the equation of time changes between positive and negative val￾ues. Examine the trend of the values near this dividing line to determine the correct sign for the equation of time. Example 2: See Figure 1801. Determine the time of the up￾per meridian passage of the sun on April 16, 1995. Solution: From Figure 1801, upper meridian passage of the sun on April 16, 1995, is given as 1200. The dividing line between the values for upper and lower meridian pas￾sage on April 16th indicates that the sign of the equation of time changes between lower meridian passage and upper meridian passage on this date; the question, therefore, be￾comes: does it become positive or negative? Note that on April 18, 1995, upper meridian passage is given as 1159, indicating that on April 18, 1995, the equation of time is positive. All values for the equation of time on the same side of the dividing line as April 18th are positive. Therefore, the equation of time for upper meridian passage of the sun on April 16, 1995 is (+) 00m05s. Upper meridian passage, therefore, takes place at 11h59m55s. To calculate latitude and longitude at LAN, the navigator seldom requires the time of meridian passage to accuracies greater than one minute. Therefore, use the time listed under the “Mer. Pass.” column to estimate LAN unless extraordinary accuracy is required. 1802. Fundamental Systems Of Time The first fundamental system of time is Ephemeris Time (ET). Ephemeris Time is used by astronomers in cal￾culating the fundamental ephemerides of the sun, moon, and planets. It is not used by navigators. The fundamental system of time of most interest to navigators is Universal Time (UT). UT is the mean solar time on the Greenwich meridian, reckoned in days of 24 mean solar hours beginning with 0h at midnight. Universal Time, in principle, is determined by the average rate of the apparent daily motion of the sun relative to the meridian of Greenwich; but in practice the numerical measure of Uni￾versal Time at any instant is computed from sidereal time. Universal Time is the standard in the application of astron￾omy to navigation. Observations of Universal Times are made by observing the times of transit of stars. The Universal Time determined directly from astro￾nomical observations is denoted UT0. Since the earth’s rotation is nonuniform, corrections must be applied to UT0 to obtain a more uniform time. This more uniform time is obtained by correcting for two known periodic motions. Day SUN MOON Eqn. of Time Mer. Mer. Pass. 00h 12h Pass. Upper Lower Age Phase ms ms hmhmhm d 16 00 02 00 05 12 00 00 26 12 55 16 17 00 13 00 20 12 00 01 25 13 54 17 18 00 27 00 33 11 59 02 25 14 55 18 Figure 1801. The equation of time for April 16, 17, 18, 1995

TIME289One motion, the motion ofthegeographic poles, is the=104m=60°resultof theaxis of rotation continuouslymoving with re-= 15'60s=1mspectto the earth's crust.The corrections for this motion are=1=60"4squitesmall(±15millisecondsforWashington,D.C.).On= 15"1s=0.25'applying the correction to UTo, the result is UT1, which isthe same as Greenwichmean time(GMT)used in celestialnavigation.Therefore anytime interval can be expressed as anThesecondknownperiodicmotion isthevariation inequivalentamount ofrotation,and vice versa. Interconver-the earth's speed ofrotation due to winds, tides, and othersion of these units can be made bythe relationshipsphenomena.As a consequence,theearth suffers an annualindicated above.variation initsspeedofrotation,ofabout±30milliseconds.WhenUT1 is correctedfor themean seasonal variations inTo convert time to arc:theearth'srate ofrotation,the result is UT2Although UT2 was at one time believed to be a uni-1.Multiply thehours by15 to obtain degrees of arc.form time system, it was later determined that there are2.Divide the minutes of time by four to obtainvariations intheearth'srateofrotation,possibly causedbydegrees.random accumulations of matter in the convection coreof3.Multiply the remainder of step 2 by15 to obtainthe earth.Such accumulations would changetheearth'sminutes of arc.momentofinertiaandthusitsrateofrotation4.Divide the seconds of timeby four to obtain min-Thethird fundamental system oftime,Atomic Timeutes ofarc(AT),isbasedontransitionsintheatom.Thebasicprinci-5.Multiplytheremainderby15toobtainsecondsofarcple of the atomic clock is that electromagneticwaves of a6.Add the resulting degrees, minutes, and seconds.particularfrequencyareemittedwhen an atomictransitionoccurs. The frequency of the cesium beam atomic clock isExample 1:Convert 14h2/m39s toarc.9,192,631,770 cycles per second ofEphemeris Time.The advent of atomic clocks having accuracies betterSolution:than 1 part in 10-13 led in 1961 to the coordination of timeandfrequencyemissionsoftheU.S.Naval Observatoryand=210°00'00"(1)14h×15theRoyal GreenwichObservatory.Themaster oscillators(2)=005°0000"(remainder1)21m+4controllingthe signals werecalibrated in terms ofthe cesium(3)1 × 15=000°1500"standard,and corrections determined at the U.S.Naval Ob-=000°09'00"(remainder3)(4)395+4servatory and the Royal Greenwich Observatorywere made(5)3×15=000°00'45"simultaneouslyat all transmitting stations.The result is Co-ordinated Universal Time (UTC)(6)14h21m39s=215°24'45"1803.TimeAnd ArcTo convert arc to time:One day represents one complete rotation of the earth.Each day is divided into 24 hours of 60 minutes; each1.Dividethe degreesby15to obtain hours.2Multiply the remainder from step 1 by four to ob-minutehas60seconds.Time of dayis an indication of the phase ofrotation oftain minutes of time.3.Divide the minutes of arc by 15 to obtain minutesthe earth.That is, it indicates how much of a day has elapsed,oftime.orwhatpartofarotationhasbeencompleted.Thus,atzerohours the day begins. One hour later, the earth has turned4. Multiply the remainder from step 3 by four to ob-through1/24ofaday,or1/24of360°,or360°+24=15tainseconds oftimeSmallerintervalscanalso bestatedinangularunits5.Divide the seconds of arc by15to obtain secondssince1hour or60minutes is equivalentto15,1minuteofoftime.time is equivalent to 15° + 60 = 0.25°= 15', and 1 second6.Addthe resultinghours,minutes,and secondsoftimeisequivalentto15'+60=0.25'=15"Example2:Convert215°24'45"totimeunitsSummarizingintableform:Solution:TimeArc215°+ 15(1)=14h00m00sremainder 51d=24h=360°(2)5×4=00h20m00s=]h=15°60m24'+15(3)=00ho1m00sremainder9

TIME 289 One motion, the motion of the geographic poles, is the result of the axis of rotation continuously moving with re￾spect to the earth’s crust. The corrections for this motion are quite small (± 15 milliseconds for Washington, D.C.). On applying the correction to UT0, the result is UT1, which is the same as Greenwich mean time (GMT) used in celestial navigation. The second known periodic motion is the variation in the earth’s speed of rotation due to winds, tides, and other phenomena. As a consequence, the earth suffers an annual variation in its speed of rotation, of about ± 30 milliseconds. When UT1 is corrected for the mean seasonal variations in the earth’s rate of rotation, the result is UT2. Although UT2 was at one time believed to be a uni￾form time system, it was later determined that there are variations in the earth’s rate of rotation, possibly caused by random accumulations of matter in the convection core of the earth. Such accumulations would change the earth’s moment of inertia and thus its rate of rotation. The third fundamental system of time, Atomic Time (AT), is based on transitions in the atom. The basic princi￾ple of the atomic clock is that electromagnetic waves of a particular frequency are emitted when an atomic transition occurs. The frequency of the cesium beam atomic clock is 9,192,631,770 cycles per second of Ephemeris Time. The advent of atomic clocks having accuracies better than 1 part in 10-13 led in 1961 to the coordination of time and frequency emissions of the U. S. Naval Observatory and the Royal Greenwich Observatory. The master oscillators controlling the signals were calibrated in terms of the cesium standard, and corrections determined at the U. S. Naval Ob￾servatory and the Royal Greenwich Observatory were made simultaneously at all transmitting stations. The result is Co￾ordinated Universal Time (UTC). 1803. Time And Arc One day represents one complete rotation of the earth. Each day is divided into 24 hours of 60 minutes; each minute has 60 seconds. Time of day is an indication of the phase of rotation of the earth. That is, it indicates how much of a day has elapsed, or what part of a rotation has been completed. Thus, at zero hours the day begins. One hour later, the earth has turned through 1/24 of a day, or 1/24 of 360°, or 360° ÷ 24 = 15° Smaller intervals can also be stated in angular units; since 1 hour or 60 minutes is equivalent to 15°, 1 minute of time is equivalent to 15° ÷ 60 = 0.25° = 15', and 1 second of time is equivalent to 15' ÷ 60 = 0.25' = 15". Summarizing in table form: Therefore any time interval can be expressed as an equivalent amount of rotation, and vice versa. Interconver￾sion of these units can be made by the relationships indicated above. To convert time to arc: 1. Multiply the hours by 15 to obtain degrees of arc. 2. Divide the minutes of time by four to obtain degrees. 3. Multiply the remainder of step 2 by 15 to obtain minutes of arc. 4. Divide the seconds of time by four to obtain min￾utes of arc 5. Multiply the remainder by 15 to obtain seconds of arc. 6. Add the resulting degrees, minutes, and seconds. Example 1: Convert 14h21m39s to arc. Solution: To convert arc to time: 1. Divide the degrees by 15 to obtain hours. 2. Multiply the remainder from step 1 by four to ob￾tain minutes of time. 3. Divide the minutes of arc by 15 to obtain minutes of time. 4. Multiply the remainder from step 3 by four to ob￾tain seconds of time. 5. Divide the seconds of arc by 15 to obtain seconds of time. 6. Add the resulting hours, minutes, and seconds. Example 2: Convert 215° 24’ 45" to time units. Solution: Time Arc 1d =24h =360° 60m =1h =15° 4m =1° =60' 60s = 1m = 15' 4s = 1' = 60" 1s = 15" = 0.25' (1) 14h × 15 = 210° 00' 00" (2) 21m ÷ 4 = 005° 00' 00" (remainder 1) (3) 1 × 15 = 000° 15' 00" (4) 39s ÷ 4 = 000° 09' 00" (remainder 3) (5) 3 × 15 = 000° 00' 45" (6) 14h21m39s = 215° 24' 45" (1) 215° ÷ 15 = 14h00m00s remainder 5 (2) 5 × 4 = 00h20m00s (3) 24’ ÷ 15 = 00h01m00s remainder 9

290TIMEofthedate line(east longitude)isIday laterthanthedate im-(4)9×4=00h00m36smediatelytothe east of the line.When solvingproblems,45"+15(5)=00h00m03sconvert local timeto Greenwichtime and then convertthis tolocaltimeontheoppositesideof thedateline215°24'45"(6)=14h2/m39s1806.ZoneTimeSolutions canalsobemadeusing arcto timeconversionAt sea, as well as ashore, watches and clocks are nor-tables inthealmanacs.In theNautical Almanac,thetablemally set to some form of zonetime(ZT).At sea thegiven near the back of the volume is in two parts, permittingnearest meridian exactlydivisibleby15°is usuallyusedasseparate entries with degrees, minutes, and quarter minutesthe time meridian or zone meridian. Thus, within a timeof arc.This table is arranged in this manner because the nav-zone extending 7.5'on each side of thetime meridian theigatorconverts arctotimemoreoftenthanthereversetime is the same, and time in consecutive zones differs byExample 3:Convert 334°1822" to time units, using theexactly onehour.Thetimeischanged as convenient,usual-ly at a whole hour, when crossingthe boundary betweenNautical Almanacarctotime conversiontablezones.Each time zone is identified by the number of timesthe longitude of its zone meridian is divisible by 15°,posi-Solution:tive in west longitude and negative in east longitude.Thisnumber and its sign, called the zone description (ZD), isConvert the22"to the nearest quarter minute of arc forthe number of wholehours that areadded toor subtractedsolution to the nearest second of time.Interpolate if morefrom thezone timetoobtainGreenwichmeantime(GMT)precise results are requiredThe mean sun is the celestial reference point for zone time.See Figure 1806.22h16m00s334°00.00m=Converting ZT to GMT,a positiveZT is added and a000°18.25m00h0|m13s=negative onesubtracted;convertingGMTtoZT,apositiveZD is subtracted, and a negative one added.334°1822"22h17m/3s-Example:TheGMTis15h27m09s1804.TimeAndLongitude(l)ZTatlong.156°24.4W.Required:Suppose a celestial reference point were directly over(2) ZT at long.039°04.8'E.a certain point on the earth.An hour later the earth wouldhaveturnedthrough15°,and thecelestial referencewouldSolutions:be directly over a meridian 15°farther west. Any differenceof longitude betweentwopoints is a measure of the angle(1)GMT15h27m09sthrough which the earth must rotate to separate them.ZD+10h (rev.)Therefore,places east ofanobserverhavelater time,andthose westhave earlier time, and the difference is exactlyZT05h27m09sequal to the difference inlongitude,expressed intime units.Thedifference in time between two places is equal to theGMT(2)15h27m09sdifference of longitude between their meridians,expressedZD-03h (rev.)intimeunits instead ofarc.ZT18h27m09s1805.TheDate Line1807.Chronometer TimeSince time is later towardtheeastandearliertowardtheChronometer time (C) is time indicated by a chronom-west of an observer,time at the lower branch of one's merid-ian is 12 hours earlier or later depending upon the directioneter.Sinceachronometer is setapproximatelytoGMT andofreckoning.Atravelermakinga triparound the worldgainsnot reset until it is overhauled and cleaned about every 3orlosesanentireday.Topreventthedatefrombeing in error,years, there is nearly always a chronometer error (CE), ei-and to provide a starting placefor eachday,a date line isther fast (F) or slow (S).The change in chronometer error infixedby international agreement.This line coincides with the24 hours is called chronometer rate, or daily rate, and des-180th meridian over most of its length.In crossing this line,ignatedgaining orlosing.Witha consistentrateof1sperdaythe date is altered by one day.If a person is traveling east-for three years, the chronometer error would be approxi-mately1gm.Since chronometer error is subjectto change,itward from east longitude to west longitude, time is becominglater, and when the date line is crossed the date becomes 1should bedetermined fromtimetotime,preferablydaily atday earlier.Atanymomentthedate immediatelytothewestsea.Chronometererror isfound byradiotime signal, by

290 TIME Solutions can also be made using arc to time conversion tables in the almanacs. In the Nautical Almanac, the table given near the back of the volume is in two parts, permitting separate entries with degrees, minutes, and quarter minutes of arc. This table is arranged in this manner because the nav￾igator converts arc to time more often than the reverse. Example 3: Convert 334°18’22" to time units, using the Nautical Almanac arc to time conversion table. Solution: Convert the 22" to the nearest quarter minute of arc for solution to the nearest second of time. Interpolate if more precise results are required. 334° 00.00m = 22h16m00s 000° 18.25m = 00h01m13s 334° 18’ 22" = 22h17m13s 1804. Time And Longitude Suppose a celestial reference point were directly over a certain point on the earth. An hour later the earth would have turned through 15°, and the celestial reference would be directly over a meridian 15° farther west. Any difference of longitude between two points is a measure of the angle through which the earth must rotate to separate them. Therefore, places east of an observer have later time, and those west have earlier time, and the difference is exactly equal to the difference in longitude, expressed in time units. The difference in time between two places is equal to the difference of longitude between their meridians, expressed in time units instead of arc. 1805. The Date Line Since time is later toward the east and earlier toward the west of an observer, time at the lower branch of one’s merid￾ian is 12 hours earlier or later depending upon the direction of reckoning. A traveler making a trip around the world gains or loses an entire day. To prevent the date from being in error, and to provide a starting place for each day, a date line is fixed by international agreement. This line coincides with the 180th meridian over most of its length. In crossing this line, the date is altered by one day. If a person is traveling east￾ward from east longitude to west longitude, time is becoming later, and when the date line is crossed the date becomes 1 day earlier. At any moment the date immediately to the west of the date line (east longitude) is 1 day later than the date im￾mediately to the east of the line. When solving problems, convert local time to Greenwich time and then convert this to local time on the opposite side of the date line. 1806. Zone Time At sea, as well as ashore, watches and clocks are nor￾mally set to some form of zone time (ZT). At sea the nearest meridian exactly divisible by 15° is usually used as the time meridian or zone meridian. Thus, within a time zone extending 7.5' on each side of the time meridian the time is the same, and time in consecutive zones differs by exactly one hour. The time is changed as convenient, usual￾ly at a whole hour, when crossing the boundary between zones. Each time zone is identified by the number of times the longitude of its zone meridian is divisible by 15°, posi￾tive in west longitude and negative in east longitude. This number and its sign, called the zone description (ZD), is the number of whole hours that are added to or subtracted from the zone time to obtain Greenwich mean time (GMT). The mean sun is the celestial reference point for zone time. See Figure 1806. Converting ZT to GMT, a positive ZT is added and a negative one subtracted; converting GMT to ZT, a positive ZD is subtracted, and a negative one added. Example: The GMT is 15h27m09s. Required: (1) ZT at long. 156°24.4’ W. (2) ZT at long. 039°04.8’ E. Solutions: 1807. Chronometer Time Chronometer time (C) is time indicated by a chronom￾eter. Since a chronometer is set approximately to GMT and not reset until it is overhauled and cleaned about every 3 years, there is nearly always a chronometer error (CE), ei￾ther fast (F) or slow (S). The change in chronometer error in 24 hours is called chronometer rate, or daily rate, and des￾ignated gaining or losing. With a consistent rate of 1s per day for three years, the chronometer error would be approxi￾mately 18m. Since chronometer error is subject to change, it should be determined from time to time, preferably daily at sea. Chronometer error is found by radio time signal, by (4) 9 × 4 = 00h00m36s (5) 45" ÷ 15 = 00h00m03s (6) 215° 24’ 45" = 14h21m39s (1) GMT 15h27m09s ZD +10h (rev.) ZT 05h27m09s (2) GMT 15h27m09s ZD –03 h (rev.) ZT 18h27m09s

MHNHHAR1TIMTIMEEVENNUMBEREDZONEOODNUMBEREDZONECOUNTRIESWHERESTANDARD.TIMEDIFFERSHALFANHOURFROMNEIGHBORINGZONES0AAz10o30601503012F0Figure1806.TimeZone Chart

TIME 291 Figure 1806. Time Zone Chart

292TIMEcomparisonwithanothertimepieceofknownerror,orbyerror,watch error (WE),labeledfast (F)or slow(S)to indicatewhether thewatch is ahead of or behind the correct time.orbyapplyingchronometerrateto previousreadingsof thesame instrument.It is recordedto the nearest wholeor halfsecIfawatch is tobe setexactlytoZT orGMT, set it toond.Chronometerrate isrecorded tothenearest0.1secondsome wholeminute slightlyahead of the correct time andstopped.When thesettime arrives,startthewatch andExample: AtGMT1200 on May 12 the chronometer readscheck itforaccuracy.12h04m215.AtGMT 1600 on May 18 it reads 4h04m255.The GMT may be in error by 12h, but if the watch is grad-uated to 12 hours, this will not be reflected.If a watch with aRequired: 1. Chronometer error at 1200 GMT May 12.24-hour dial is used, the actual GMT should be determined2.Chronometererrorat1600GMTMay18.To determine watch error compare the reading of the3.Chronometerrate.watch with that of the chronometer at a selected moment.4.ChronometererroratGMT0530,May27ThismayalsobeatsomeselectedGMT.Unlessawatchisgraduatedto24 hours, itstimeisdesignatedambefore noonSolutions:andpmafternoon.1.12h00m00sGMTMay 12Even thougha watch is set to zone time approximately,c12h04m2/sits error on GMT can be determined and used for timing ob-CE(F)4m2]sservations.In this case the 12-hour ambiguity in GMTshould beresolved,anda timediagram usedto avoid error.2.GMT16h00m00sMay 18This method requires additional work, and presents a great-C04 04 25er probability oferror, without compensating advantages.CE(F)4m25sIfastopwatchisusedfortimingobservations,itshouldbe started at some convenient GMT, such as a whole 5m or3.18d16hGMT10m.The time ofeach observation is then the GMT plus the12d12hGMTdif.06d04h = 6.2dwatchtime.Digital stopwatches andwristwatches areidealCE(F)4m2]s1200May12forthispurpose,astheycanbe setfroma convenient GMTCE(F)4m25s1600May18and read immediatelyafterthealtitude is takendif.4s (gained)0.6s (gain)dailyrate1809.Local MeanTime4.GMT27d05h30mLocal mean time (LMT), like zone time, uses the18d16h00mGMTmean sun as the celestial reference point.It differs fromdif.08d13h30m (8.5d)zone time in that the local meridian is used as theterrestrial(F)4m25sCE1600 May 18reference,rather than a zone meridian.Thus, the local mean(+)0m05scorr.diff.xrateCE(F)4m30stimeat eachmeridian differsfromevery other meridian,the0530May27difference being equal to the difference of longitude ex-pressed in time units.At each zone meridian, including 0°BecauseGMT is on a24-hour basis and chronometerLMTandZTareidenticaltime on a 12-hour basis,a 12-hour ambiguity exists.This is igIn navigation the principal use ofLMT is in rising, set-noredinfindingchronometererror.However,ifchronometerting, and twilight tables. The problem is usually one oferror is applied to chronometer timetofind GMT,a12-hourconverting theLMT taken from the table to ZT.At sea, theerror can result.This can be resolved by mentally applying thedifference between the times is normally not more thanzonedescriptiontolocal timetoobtain approximateGMT.A30m,and the conversion ismade directly,without findingtime diagram can beused for resolving doubt as to approxi-GMT as an intermediate step.This is done byapplying amateGMT andGreenwichdate.Ifthe sunforthekind oftimecorrection equal to the difference of longitude.If the ob-used (meanorapparent)isbetweenthelowerbranchesoftwoserver is west of thetime meridian,the correction isaddedtimemeridians(as the standard meridian forlocal time,and theand if east of it, the correction is subtracted.If GreenwichGreenwich meridianfor GMT),the date at the place farthertime is desired, it is found from ZTeast is one day later than at the placefarther westWhere thereis an irregular zoneboundary,thelongitude1808.WatchTimemaydifferbymorethan7.5°(30m)fromthetimemeridian.If LMT is to be corrected to daylight saving time, theWatchtime(WT)isusuallyanapproximationofzonedifferencein longitudebetweenthelocal and timemeridiantime,except that fortiming celestial observations it is easi-canbeused,ortheZTcanfirstbefoundandthenincreasedby one hour.est to set a comparing watch to GMT.If the watch has asecond-setting hand, the watch can be set exactly to ZT orConversion of ZT (including GMT)to LMT is theGMT, and the time is so designated. If the watch is not setsameas conversion in the oppositedirection,exceptthattheexactlyto oneof thesetimes,thedifferenceisknownassign of difference of longitude is reversed.This problem is

292 TIME comparison with another timepiece of known error, or byerror, or by applying chronometer rate to previous readings of the same instrument. It is recorded to the nearest whole or half sec￾ond. Chronometer rate is recorded to the nearest 0.1 second. Example: At GMT 1200 on May 12 the chronometer reads 12h04m21s. At GMT 1600 on May 18 it reads 4h04m25s. Required: 1. Chronometer error at 1200 GMT May 12. 2. Chronometer error at 1600 GMT May 18. 3. Chronometer rate. 4. Chronometer error at GMT 0530, May 27. Solutions: Because GMT is on a 24-hour basis and chronometer time on a 12-hour basis, a 12-hour ambiguity exists. This is ig￾nored in finding chronometer error. However, if chronometer error is applied to chronometer time to find GMT, a 12-hour error can result. This can be resolved by mentally applying the zone description to local time to obtain approximate GMT. A time diagram can be used for resolving doubt as to approxi￾mate GMT and Greenwich date. If the sun for the kind of time used (mean or apparent) is between the lower branches of two time meridians (as the standard meridian for local time, and the Greenwich meridian for GMT), the date at the place farther east is one day later than at the place farther west. 1808. Watch Time Watch time (WT) is usually an approximation of zone time, except that for timing celestial observations it is easi￾est to set a comparing watch to GMT. If the watch has a second-setting hand, the watch can be set exactly to ZT or GMT, and the time is so designated. If the watch is not set exactly to one of these times, the difference is known as watch error (WE), labeled fast (F) or slow (S) to indicate whether the watch is ahead of or behind the correct time. If a watch is to be set exactly to ZT or GMT, set it to some whole minute slightly ahead of the correct time and stopped. When the set time arrives, start the watch and check it for accuracy. The GMT may be in error by 12h, but if the watch is grad￾uated to 12 hours, this will not be reflected. If a watch with a 24-hour dial is used, the actual GMT should be determined. To determine watch error compare the reading of the watch with that of the chronometer at a selected moment. This may also be at some selected GMT. Unless a watch is graduated to 24 hours, its time is designated am before noon and pm after noon. Even though a watch is set to zone time approximately, its error on GMT can be determined and used for timing ob￾servations. In this case the 12-hour ambiguity in GMT should be resolved, and a time diagram used to avoid error. This method requires additional work, and presents a great￾er probability of error, without compensating advantages. If a stopwatch is used for timing observations, it should be started at some convenient GMT, such as a whole 5m or 10m. The time of each observation is then the GMT plus the watch time. Digital stopwatches and wristwatches are ideal for this purpose, as they can be set from a convenient GMT and read immediately after the altitude is taken. 1809. Local Mean Time Local mean time (LMT), like zone time, uses the mean sun as the celestial reference point. It differs from zone time in that the local meridian is used as the terrestrial reference, rather than a zone meridian. Thus, the local mean time at each meridian differs from every other meridian, the difference being equal to the difference of longitude ex￾pressed in time units. At each zone meridian, including 0°, LMT and ZT are identical. In navigation the principal use of LMT is in rising, set￾ting, and twilight tables. The problem is usually one of converting the LMT taken from the table to ZT. At sea, the difference between the times is normally not more than 30m, and the conversion is made directly, without finding GMT as an intermediate step. This is done by applying a correction equal to the difference of longitude. If the ob￾server is west of the time meridian, the correction is added, and if east of it, the correction is subtracted. If Greenwich time is desired, it is found from ZT. Where there is an irregular zone boundary, the longitude may differ by more than 7.5° (30m) from the time meridian. If LMT is to be corrected to daylight saving time, the difference in longitude between the local and time meridian can be used, or the ZT can first be found and then increased by one hour. Conversion of ZT (including GMT) to LMT is the same as conversion in the opposite direction, except that the sign of difference of longitude is reversed. This problem is 1. GMT 12h00m00s May 12 C 12h04m21s CE (F)4m21s 2. GMT 16h00m00s May 18 C 04 04 25 CE (F)4m25s 3. GMT 18d16h GMT 12d12h diff. 06d04h = 6.2d CE (F)4m21s 1200 May 12 CE (F)4m25s 1600 May 18 diff. 4s (gained) daily rate 0.6s (gain) 4. GMT 27d05h30m GMT 18d16h00m diff. 08d13h30m (8.5d) CE (F)4m25s 1600 May 18 corr. (+)0m05s diff. × rate CE (F)4m30s 0530 May 27

TIME293notnormallyencountered innavigation.Aries,and occasionallythemoon,arecommonlyusedHourangles are usually expressed in arc units,and are1810.SiderealTimemeasured from the upper branch of the celestial meridianTime is customarily expressed in time units. Sidereal time isSidereal time uses thefirst point of Aries (vernal equi-measuredfromtheupperbranchofthecelestialmeridianlikenox) as the celestial reference point. Since the earthhour angle,but solartimeismeasuredfromthelowerbranchrevolves aroundthe sun,and since thedirection of theThus, LMT=LHAmean sun plus or minus 180,LAT=LHAearth'srotationandrevolutionarethesame,itcompletesaapparent sunplusorminus180,andLSTLHAAries.rotationwith respect tothestars in less time (about3m56.6sAs withtime,local hourangle(LHA)attwoplaces differsofmean solar units)than withrespect to the sun,and duringby their difference in longitude, and LHA at longitude oo isonerevolution aboutthe sun(Iyear)itmakesonecompletecalled Greenwich hour angle (GHA). In addition, it is often con-rotationmorewithrespecttothe starsthanwiththe sunvenienttoexpresshourangleintermsoftheshorterarcbetweenThis accounts for thedailyshift ofthestars nearly1°west-the local meridian and the body.This is similar to measurementwardeachnight.Hence.siderealdaysareshorterthan solaroflongitudefromtheGreenwichmeridianLocalhourangledays,and its hours,minutes,and secondsare correspond-measured in this way is called meridian angle (t), which is la-inglyshorter.Becauseof nutation,sidereal time is notquitebeled east or west, like longitude,to indicate thedirection ofconstant in rate.Time based upon the average rate is calledmeasurementAwesterlymeridianangleisnumericallyegualtomean sidereal time,when it is to be distinguished from theLHA,while an easterly meridian angle is equal to 360°LHAslightly irregular sidereal time.The ratio ofmean solar timeLHA=t(W), and LHA=360°-t (E).Meridian angle is used inunitstomeansiderealtimeunitsis1:100273791the solution of the navigational triangleA navigator very seldom uses sidereal time.Astrono-Example:FindLHAandtof thesunatGMT3h24m16sonmers use it to regulate mean time because its celestialJume1.1975.forlong.118°48.2W.referencepointremainsalmostfixed inrelationtothestars.Solution:1811.TimeAnd HourAngleGMT3h24m16sJune13h225°35.7Both time and hour angleareameasureof thephase of6°04.0'24m16srotation of the earth, since both indicate the angular dis-GHA231°39.7"tance of a celestial reference point west of a terrestrial入118°48.2'Wreference meridian.Hour angle, however, applies to anyLHA112°51.5point on the celestial sphere.Time might be used in this re112°51.5'Wspect, but only the apparent sun, mean sun, the first point oftRADIODISSEMINATIONOETIMESIGNALS1812.DisseminationSystemsthe time it takes the signal to pass through the receiverInmostcases standardtimeandfrequencyemissionsOfthemanysystemsfortimeandfrequencydissemina-as received are more thanadequate for ordinary needstion, themajority employ some type ofradio transmission,However, many systems exist for the more exacting scien-either in dedicated timeandfrequencyemissions orestab-tificrequirements.lished systems such as radionavigation systems. The mostaccuratemeans of timeandfrequencydisseminationtoday1813.CharacteristicElements OfDisseminationis by the mutual exchange oftime signals through commu-Systemsnication(commonlycalled Two-Way)and bythemutualobservationofnavigation satellites (commonly called Com-Anumberofcommonelementscharacterizemosttimemon View).andfrequencydisseminationsystems.Amongthemoreim-Radio time signals can be used either to perform aportant elements are accuracy,ambiguity,repeatabilityclock'sfunction or to setclocks.When using aradiowavecoverage,availabilityoftimesignal,reliability,ease ofuse,instead of a clock, however,new considerations evolve.cost to the user, and the number of users served. No singleOne isthedelaytimeof approximately3microsecondspersystem incorporates all desired characteristics.The relativekilometer it takes the radio wave to propagate and arrive atimportanceof thesecharacteristicswill varyfromoneuserthe reception point.Thus, a user1,o00kilometersfrom ato the next, and the solution for one user may not be satis-transmitter receivesthetime signal about3millisecondslaterthantheon-timetransmittersignal.Iftimeisneededtofactorytoanother.Thesecommonelementsarediscussedbetterthan3milliseconds,acorrectionmustbemadeforinthefollowingexaminationofa hypothetical radiosignal

TIME 293 not normally encountered in navigation. 1810. Sidereal Time Sidereal time uses the first point of Aries (vernal equi￾nox) as the celestial reference point. Since the earth revolves around the sun, and since the direction of the earth’s rotation and revolution are the same, it completes a rotation with respect to the stars in less time (about 3m56.6s of mean solar units) than with respect to the sun, and during one revolution about the sun (1 year) it makes one complete rotation more with respect to the stars than with the sun. This accounts for the daily shift of the stars nearly 1° west￾ward each night. Hence, sidereal days are shorter than solar days, and its hours, minutes, and seconds are correspond￾ingly shorter. Because of nutation, sidereal time is not quite constant in rate. Time based upon the average rate is called mean sidereal time, when it is to be distinguished from the slightly irregular sidereal time. The ratio of mean solar time units to mean sidereal time units is 1:1.00273791. A navigator very seldom uses sidereal time. Astrono￾mers use it to regulate mean time because its celestial reference point remains almost fixed in relation to the stars. 1811. Time And Hour Angle Both time and hour angle are a measure of the phase of rotation of the earth, since both indicate the angular dis￾tance of a celestial reference point west of a terrestrial reference meridian. Hour angle, however, applies to any point on the celestial sphere. Time might be used in this re￾spect, but only the apparent sun, mean sun, the first point of Aries, and occasionally the moon, are commonly used. Hour angles are usually expressed in arc units, and are measured from the upper branch of the celestial meridian. Time is customarily expressed in time units. Sidereal time is measured from the upper branch of the celestial meridian, like hour angle, but solar time is measured from the lower branch. Thus, LMT = LHA mean sun plus or minus 180°, LAT = LHA apparent sun plus or minus 180°, and LST = LHA Aries. As with time, local hour angle (LHA) at two places differs by their difference in longitude, and LHA at longitude 0° is called Greenwich hour angle (GHA). In addition, it is often con￾venient to express hour angle in terms of the shorter arc between the local meridian and the body. This is similar to measurement of longitude from the Greenwich meridian. Local hour angle measured in this way is called meridian angle (t), which is la￾beled east or west, like longitude, to indicate the direction of measurement. A westerly meridian angle is numerically equal to LHA, while an easterly meridian angle is equal to 360° – LHA. LHA = t (W), and LHA = 360° – t (E). Meridian angle is used in the solution of the navigational triangle. Example: Find LHA and t of the sun at GMT 3h24m16s on June 1, 1975, for long. 118°48.2’ W. Solution: RADIO DISSEMINATION OF TIME SIGNALS 1812. Dissemination Systems Of the many systems for time and frequency dissemina￾tion, the majority employ some type of radio transmission, either in dedicated time and frequency emissions or estab￾lished systems such as radionavigation systems. The most accurate means of time and frequency dissemination today is by the mutual exchange of time signals through commu￾nication (commonly called Two-Way) and by the mutual observation of navigation satellites (commonly called Com￾mon View). Radio time signals can be used either to perform a clock’s function or to set clocks. When using a radio wave instead of a clock, however, new considerations evolve. One is the delay time of approximately 3 microseconds per kilometer it takes the radio wave to propagate and arrive at the reception point. Thus, a user 1,000 kilometers from a transmitter receives the time signal about 3 milliseconds later than the on-time transmitter signal. If time is needed to better than 3 milliseconds, a correction must be made for the time it takes the signal to pass through the receiver. In most cases standard time and frequency emissions as received are more than adequate for ordinary needs. However, many systems exist for the more exacting scien￾tific requirements. 1813. Characteristic Elements Of Dissemination Systems A number of common elements characterize most time and frequency dissemination systems. Among the more im￾portant elements are accuracy, ambiguity, repeatability, coverage, availability of time signal, reliability, ease of use, cost to the user, and the number of users served. No single system incorporates all desired characteristics. The relative importance of these characteristics will vary from one user to the next, and the solution for one user may not be satis￾factory to another. These common elements are discussed in the following examination of a hypothetical radio signal. GMT 3h24m16s June 1 3h 225°35.7' 24m16s 6°04.0’ GHA 231°39.7’ λ 118°48.2’ W LHA 112°51.5’ t 112°51.5’ W

294TIMEcharacteristic of timeand frequency dissemination is reli-ability, i.e., the likelihood that a time signal will beavailablewhenscheduled.Propagationfadeoutcan some-timespreventreceptionofHF signals.1814.RadioPropagationFactorsRadio has been used to transmit standard time and fre-quency signals sincetheearly 1900's.As opposed to thephysical transferof timeviaportableclocks,thetransferofinformationbyradioentailspropagationofelectromagneticenergy through some propagation medium from a transmit-tertoadistantreceiverIn a typical standard frequency and time broadcast, thesignals are directly related to some master clock and aretransmittedwithlittleornodegradation in accuracy.Ina vac-uum and with a noisefreebackground,the signals should bereceived atadistantpoint essentiallyas transmitted, exceptfor a constant path delay withtheradio wave propagatingnearthespeedoflight (299,773kilometersper second).TheFigure1813.Singletone time dissemination.propagation media,including the earth,atmosphere,and ion-osphere,as well as physical and electrical characteristicsofConsideraverysimplesystemconsistingofanunmod-transmitters and receivers,influence the stability and accura-ulated 10-kHz signal as shown in Figure1813.This signal,cyofreceivedradiosignals,dependentuponthefrequencyofleavingthetransmitter at 0000UTC,will reachthereceiverthetransmission and lengthof signal path.Propagation de-at a later timeequivalent to the propagation delay.The userlays areaffected invaryingdegrees byextraneous radiationsmustknowthis delaybecausethe accuracy of his knowl-in thepropagationmedia,solardisturbances,diurnal effects,edge oftime canbenobetter thanthe degreeto which theand weather conditions, among others.delayisknownSinceall cvclesofthesignal areidentical.Radio dissemination systems can be classified in athesignalisambiguousandtheusermustsomehowdecidenumber of differentways.One way is todivide those carrierwhich cycle is the"on time"cycle.This means, in the casefrequencies low enoughto be reflected bythe ionosphereof the hypothetical 10-kHz signal,that theuser must know(below30MHz)fromthose sufficientlyhightopenetratethetimeto±50microseconds(half theperiodofthesigthe ionosphere(above30MHz).Theformercanbeob-nal).Further, the user may desire to use this system, sayserved at great distances from the transmitter but sufferonce a day,foran extended period of time to check hisfrom ionosphericpropagationanomalies that limit accura-clock or frequency standard.However,if the delay variescy:the latter are restricted to line-of-sight applications butfrom onedayto the next without the userknowing,accura-showlittleornosignal deteriorationcausedbypropagationcywill be limited by the lack of repeatability.anomalies.Themost accuratesystemstendtobethoseManyusers are interested inmaking timecoordinatedwhich use the higher,line-of-sight frequencies,whilemeasurements over large geographic areas.Theywouldbroadcasts of the lowercarrierfrequencies showthegreat-likeallmeasurementstobereferencedtoonetimesystemestnumberofusers.toeliminatecorrectionsfordifferenttimesystemsusedatscattered or remote locations.This is a very important1815.StandardTimeBroadcastspractical consideration when measurements are undertak-en in the field. In addition, a one-reference system, suchTheWorld AdministrativeRadioCouncil (WARC)as a singletimebroadcast,increases confidencethatallmeasurementscanberelatedtoeachotherinsomeknownhas allocated certain frequencies infivebands for standardway.Thus, the coverage ofa system is an important con-frequency and time signal emission.For such dedicatedcept.Anotherimportantcharacteristicofatimingsvstemstandardfreguencytransmissions,theInternationalRadicis the percentoftime available.Theman on the street whoConsultativeCommittee(CCIR)recommendsthatcarriehas tokeep an appointment needstoknowthetimeper-frequencies be maintained so that the average daily frac-haps to a minute or so. Although requiring only coarsetional frequency deviations from the internationallytimeinformation, hewants it ondemand, sohecarriesadesignated standard for measurement of time intervalshould not exceed 1X 10-10.TheU.S.Naval Observatorywristwatch thatgives the time 24 hours a day.On the otherhand, a user who needs time to a few microseconds em-Time Service Announcement Series 1,No.2,gives charac-ploys a very good clock which only needs an occasionalteristics of standard time signals assigned to allocatedbands,as reportedbytheCCIR.update, perhaps only once or twice a day.An additional

294 TIME Consider a very simple system consisting of an unmod￾ulated 10-kHz signal as shown in Figure 1813. This signal, leaving the transmitter at 0000 UTC, will reach the receiver at a later time equivalent to the propagation delay. The user must know this delay because the accuracy of his knowl￾edge of time can be no better than the degree to which the delay is known. Since all cycles of the signal are identical, the signal is ambiguous and the user must somehow decide which cycle is the “on time” cycle. This means, in the case of the hypothetical 10-kHz signal, that the user must know the time to ± 50 microseconds (half the period of the sig￾nal). Further, the user may desire to use this system, say once a day, for an extended period of time to check his clock or frequency standard. However, if the delay varies from one day to the next without the user knowing, accura￾cy will be limited by the lack of repeatability. Many users are interested in making time coordinated measurements over large geographic areas. They would like all measurements to be referenced to one time system to eliminate corrections for different time systems used at scattered or remote locations. This is a very important practical consideration when measurements are undertak￾en in the field. In addition, a one-reference system, such as a single time broadcast, increases confidence that all measurements can be related to each other in some known way. Thus, the coverage of a system is an important con￾cept. Another important characteristic of a timing system is the percent of time available. The man on the street who has to keep an appointment needs to know the time per￾haps to a minute or so. Although requiring only coarse time information, he wants it on demand, so he carries a wristwatch that gives the time 24 hours a day. On the other hand, a user who needs time to a few microseconds em￾ploys a very good clock which only needs an occasional update, perhaps only once or twice a day. An additional characteristic of time and frequency dissemination is reli￾ability, i.e., the likelihood that a time signal will be available when scheduled. Propagation fadeout can some￾times prevent reception of HF signals. 1814. Radio Propagation Factors Radio has been used to transmit standard time and fre￾quency signals since the early 1900’s. As opposed to the physical transfer of time via portable clocks, the transfer of information by radio entails propagation of electromagnetic energy through some propagation medium from a transmit￾ter to a distant receiver. In a typical standard frequency and time broadcast, the signals are directly related to some master clock and are transmitted with little or no degradation in accuracy. In a vac￾uum and with a noise free background, the signals should be received at a distant point essentially as transmitted, except for a constant path delay with the radio wave propagating near the speed of light (299,773 kilometers per second). The propagation media, including the earth, atmosphere, and ion￾osphere, as well as physical and electrical characteristics of transmitters and receivers, influence the stability and accura￾cy of received radio signals, dependent upon the frequency of the transmission and length of signal path. Propagation de￾lays are affected in varying degrees by extraneous radiations in the propagation media, solar disturbances, diurnal effects, and weather conditions, among others. Radio dissemination systems can be classified in a number of different ways. One way is to divide those carrier frequencies low enough to be reflected by the ionosphere (below 30 MHz) from those sufficiently high to penetrate the ionosphere (above 30 MHz). The former can be ob￾served at great distances from the transmitter but suffer from ionospheric propagation anomalies that limit accura￾cy; the latter are restricted to line-of-sight applications but show little or no signal deterioration caused by propagation anomalies. The most accurate systems tend to be those which use the higher, line-of-sight frequencies, while broadcasts of the lower carrier frequencies show the great￾est number of users. 1815. Standard Time Broadcasts The World Administrative Radio Council (WARC) has allocated certain frequencies in five bands for standard frequency and time signal emission. For such dedicated standard frequency transmissions, the International Radio Consultative Committee (CCIR) recommends that carrier frequencies be maintained so that the average daily frac￾tional frequency deviations from the internationally designated standard for measurement of time interval should not exceed 1 X 10-10. The U. S. Naval Observatory Time Service Announcement Series 1, No. 2, gives charac￾teristics of standard time signals assigned to allocated bands, as reported by the CCIR. Figure 1813. Single tone time dissemination

TIME2951816.Time Signalsbe maintained, or begun at least three days prior to depar-ture, if conditions permit.Error and rate are entered in theThe usual method of determining chronometer errorchronometer record book (orrecord sheet)eachtimetheyand daily rate is byradio time signals,popularly called timeare determined.ticks.Mostmaritime nations broadcasttime signals severalThe various time signal systems used throughout thetimes dailyfrom one or more stations,and avesselworld are discussed in Pub.No.117,RadioNavigationalequippedwithradioreceivingequipmentnormallyhasnoAids,andvolume5 of AdmiraltyListof Radio Signals.difficulty in obtaining a time tick anywhere in the world.Only the United States signals are discussed here.Normally,thetimetransmitted is maintained virtually uni-The National Institute of Standards and Technologyform with respect to atomic clocks.The Coordinated(NIST)broadcasts continuous timeand frequency refer-Universal Time (UTC)as received bya vessel may differencesignalsfromWWV,WWVH,WWVB,andtheGOEsfrom(GMT)byasmuchas 0.9second.satellite system.Because of their wide coverage and rela-Themaiorityofradiotimesignals aretransmittedautive simplicity,theHF servicesfromWWVand WWVHtomatically,beingcontrolledbythestandardclockofanare used extensivelyfor navigation.astronomical observatory or a national measurement stan-StationWWVbroadcastsfromFortCollins,Coloradodards laboratory.Absolute reliance maybe had in theseattheinternationallyallocatedfrequenciesof2.5,5.0,10.0,signals because they are required to be accurate to at least15.0,and20.0MHz, stationWWVHtransmitsfromKauai,0.001s astransmittedHawaii on the same frequencies with the exception of 20.0Otherradiostations,however,havenoautomatictrans.MHz.Thebroadcast signals include standard time and fre-mission system installed,and thesignalsaregiven by handquencies, and various voice announcements.Details ofIn this instance the operator is guided by the standard clockthesebroadcastsaregiven inNISTSpecialPublication432at the station.The clock is checked byastronomical obser-NISTFrequency and Time Dissemination Services.Bothvationsor radiotimesignals and is normallycorrectto 0.25second.HFemissionsaredirectlycontrolledbycesiumbeamfre-quency standards with periodic reference to the NISTAt sea,a spring-driven chronometer should be checkeddaily by radio time signal, and in port daily checks shouldatomicfrequencyand timestandardsBrpadcast FommatPidiaToePoe NirbeSTATIONID440HE1-HOURMARYNESTRESERVEDNO952.50727.07VANDARDBRDADCASTFREQUENCIESREPORTS0AOOWE2.8之OMHD10W2MHE-108W415OCGAREPORTUT1CORRECTIONSINFORMATIONOONTACTNISTRAOIDSTATIONWW2000EASTCOUNTYRD.SSODIBEGINNING CFEACHROURIS IDENTIFIED BYSECONDLONGL1SOO-HETONDBEGRNINGCFEACHMNUTEISSDENTIFEDBYDBSECONDLONS1SDDHETONEOTHE20CONDPULSESCFEACHINUTEAREOMITTEDSTATONIDD448HETONEISCMTTEDDURINGFIRSTINUTESHOUROFENCHDAYFigure1816a.BroadcastformatofstationWWV

TIME 295 1816. Time Signals The usual method of determining chronometer error and daily rate is by radio time signals, popularly called time ticks. Most maritime nations broadcast time signals several times daily from one or more stations, and a vessel equipped with radio receiving equipment normally has no difficulty in obtaining a time tick anywhere in the world. Normally, the time transmitted is maintained virtually uni￾form with respect to atomic clocks. The Coordinated Universal Time (UTC) as received by a vessel may differ from (GMT) by as much as 0.9 second. The majority of radio time signals are transmitted au￾tomatically, being controlled by the standard clock of an astronomical observatory or a national measurement stan￾dards laboratory. Absolute reliance may be had in these signals because they are required to be accurate to at least 0.001s as transmitted. Other radio stations, however, have no automatic trans￾mission system installed, and the signals are given by hand. In this instance the operator is guided by the standard clock at the station. The clock is checked by astronomical obser￾vations or radio time signals and is normally correct to 0.25 second. At sea, a spring-driven chronometer should be checked daily by radio time signal, and in port daily checks should be maintained, or begun at least three days prior to depar￾ture, if conditions permit. Error and rate are entered in the chronometer record book (or record sheet) each time they are determined. The various time signal systems used throughout the world are discussed in Pub. No. 117, Radio Navigational Aids, and volume 5 of Admiralty List of Radio Signals. Only the United States signals are discussed here. The National Institute of Standards and Technology (NIST) broadcasts continuous time and frequency refer￾ence signals from WWV, WWVH, WWVB, and the GOES satellite system. Because of their wide coverage and rela￾tive simplicity, the HF services from WWV and WWVH are used extensively for navigation. Station WWV broadcasts from Fort Collins, Colorado at the internationally allocated frequencies of 2.5, 5.0, 10.0, 15.0, and 20.0 MHz; station WWVH transmits from Kauai, Hawaii on the same frequencies with the exception of 20.0 MHz. The broadcast signals include standard time and fre￾quencies, and various voice announcements. Details of these broadcasts are given in NIST Special Publication 432, NIST Frequency and Time Dissemination Services. Both HF emissions are directly controlled by cesium beam fre￾quency standards with periodic reference to the NIST atomic frequency and time standards. Figure 1816a. Broadcast format of station WWV

296TIMEOMHBrpadcastFormatO0OH21-HOURMAHSTATIONNSTRESERVEDr95ESLRTHOKaroaEQUENCESOEAS2e0NDSMK-10AW15.N01/10604014152OUHDSCSWareandUT1CORECTIONSFORUATONCCNTACTaNSTRAGOSTATIONWHNEORC0782o(208335-438NGOFCACHHOURISIDCNTFICOYS8SCONDLCNG1SOOHTONBPPAIDENTWIEDSYBBSECONDLONO.100OHTOMEDTHE2SECONDPULSESOFEACHMINUTE ARIC ONTTEE30TATIONIDTTEDSURINGFIRSTMIPTESD44HOUROFEACHDAYFigure1816b.Broadcastformatof stationWWVHThetimeticks intheWWVand WWVHemissions are1817.Leap-SecondAdjustmentsshown inFigure 1816a and Figure 1816b.The 1-secondUTCmarkersaretransmittedcontinuouslybyWWVandByinternationalagreement,UTCis maintained withinWWVH,exceptforomissionofthe29thand59thmarkerabout 0.9 seconds of the celestial navigator's time scaleeach minute. With the exception of the beginning tone atUT1.The introduction of leap seconds allows a good clockeachminute(800milliseconds)all 1-secondmarkersareofto keep approximate step with the sun.Because ofthe vari-5 milliseconds duration.Each pulse is preceded by 10 mil-ations in the rate of rotation of the earth, however,thelisecondsofsilenceandfollowedby25millisecondsofoccurrencesoftheleapsecondsarenotpredictable indetail.silence. Time voice announcements are given also at 1-TheCentral Bureau of the International EarthRotationminute intervals.All time announcements areUTCService (IERS)decides uponand announcesthe introductionPub.No.I17.Radio Navigational Aids,shouldbe re-ofa leap second.The IERS announces the newleap secondferred to forfurther information on time signalsat least several weeks in advance.A positive or negative leapEventLeapsecondDesignation of the date of the event1-21111-1--57101565859603430June.23h59m60.6sUTC30 June, 23h 59m-1July.OhOm1Figure1817a.Datingofevent inthevicinityof apositive leap second

296 TIME The time ticks in the WWV and WWVH emissions are shown in Figure 1816a and Figure 1816b. The 1-second UTC markers are transmitted continuously by WWV and WWVH, except for omission of the 29th and 59th marker each minute. With the exception of the beginning tone at each minute (800 milliseconds) all 1-second markers are of 5 milliseconds duration. Each pulse is preceded by 10 mil￾liseconds of silence and followed by 25 milliseconds of silence. Time voice announcements are given also at 1- minute intervals. All time announcements are UTC. Pub. No. 117, Radio Navigational Aids, should be re￾ferred to for further information on time signals. 1817. Leap-Second Adjustments By international agreement, UTC is maintained within about 0.9 seconds of the celestial navigator’s time scale, UT1. The introduction of leap seconds allows a good clock to keep approximate step with the sun. Because of the vari￾ations in the rate of rotation of the earth, however, the occurrences of the leap seconds are not predictable in detail. The Central Bureau of the International Earth Rotation Service (IERS) decides upon and announces the introduction of a leap second. The IERS announces the new leap second at least several weeks in advance. A positive or negative leap Figure 1816b. Broadcast format of station WWVH. Figure 1817a. Dating of event in the vicinity of a positive leap second