time and time transfer - frequency electronics inc - precision

Time and Time Time and Time TransferTransfer

Hugo [email protected]

Jan 2008

2

•• What is TimeWhat is Time•• The Time StandardThe Time Standard

•• Time SourcesTime Sources

•• GPS Time TransferGPS Time Transfer

•• Time DilationTime Dilation

AgendaAgenda

3

What is Time?What is Time?““What then is time? If someone asks What then is time? If someone asks me, I know; If I wish to explain it to me, I know; If I wish to explain it to

someone who asks, I know notsomeone who asks, I know not””..

Bishop of HippoBishop of HippoNorth Africa 5th Century A.D.North Africa 5th Century A.D.

““Time is what a clock readsTime is what a clock reads””Albert EinsteinAlbert Einstein

““The clock, not the steam engine is the The clock, not the steam engine is the key machine of the industrial agekey machine of the industrial age””..

Lewis Mumford*Lewis Mumford*TechnicsTechnics & Civilization& Civilization

* U.S. Social critic and teacher, Stanford; U. of Pennsylvania; MIT; Wesleyan U.; (born 1895)

““Time is Money and Money is TimeTime is Money and Money is Time””(modified sayings of Benjamin Franklin and (modified sayings of Benjamin Franklin and George (Robert) GissingGeorge (Robert) Gissing…… my contribution)my contribution)

4

•• PhilosophicallyPhilosophically

Time is relative to Quality of Life (H.F.)

•• Theoretically/MathematicallyTheoretically/Mathematically

Time is relative to Position (Albert Einstein)

Time and Our UniverseTime and Our Universe

5

•• Universal Time (UT)Universal Time (UT)

Universal Time (UT) is the general designation of time scales based on the rotation of the earth. (GMT may be regarded as the general equivalent of UT)

In applications where imprecision of a few hundredths of a second cannot be tolerated, it is necessary to specify the form of UT:

– UT0 is the mean solar time of the prime meridian obtained fromdirect astronomical observation

– UT1 is UT0 corrected for the effects of small movements of theearth relative to the axis of rotation (polar variation)

– UT2 is UT1 corrected for the effects of a small seasonal fluctuation in the rate of rotation of the earth

(1) Taken in part from letter by, David Allan, NIST, to CCIR Study Group XII U.S. Committee Members. 3-17-89.

Definition of Terms for TimeDefinition of Terms for Time(1)

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•• International Atomic Time (International Atomic Time (TAITAI))

The time scale established by the Bureau International de Poids de Mesure' (BIPM) on the basis of data from atomic clocks operating in several establishments conforming to the definition of the second, the International System of Units (SI)

•• Universal Coordinated Time (Universal Coordinated Time (UTCUTC))

The time scale maintained by the BIPM which forms the basis of acoordinated dissemination of standard frequencies and time signals. It corresponds exactly in rate with TAI, but differs from it by an integral number of seconds

The UTC scale is adjusted by the insertion or deletion of seconds (positive or negative “leap seconds”) to ensure agreement with UT1 to within 0.9 seconds

Definition of Terms for TimeDefinition of Terms for Time(1)(cont)(cont)

7

•• What is a leap second?What is a leap second?A leap second is a second added to Universal Coordinated Time (UTC) to make it agree with astronomical time to within 0.9 second. UTC is an atomic time scale, based on the performance ofatomic clocks. Astronomical time is based on the rate of rotation of the earth. Since atomic clocks are more stable than the rate at which the earth rotates, leap seconds are needed to keep the twotime scales in agreement.

The first leap second was added on June 30, 1972, and they occurat a rate of slightly less than one per year, on average. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (seconds added to UTC because of the slowing of the celestial system.

(2) Taken from the NIST Website http://www.nist.gov/


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•• What is a leap year?What is a leap year?Leap years are years with 366 days, instead of the usual 365. Leap years are necessary because the actual length of a year is 365.242 days, not 365 days, as commonly stated. Basically, leap years occur every 4 years, and years that are evenly divisible by 4 (2004, for example) have 366 days. This extra day is added to the calendar on February 29th

However, there is one exception to the leap year rule involving century years, like the year 1900. Since the year is slightly less than 365.25 days long, adding an extra day every 4 years results in about 3 extra days being added over a period of 400 years. For this reason, only 1 out of every 4 century years is considered as a leap year. Century years are only considered as leap years if they are evenly divisible by 400. Therefore, 1700, 1800, 1900 were not leap years, nor will year 2100. But 1600 and 2000 were leap years, because those year numbers are evenly divisible by 400


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Julian Day NumberJulian Day Number

A number of a specific day from a continuous day count having aninitial origin of 12 hours UT on 1 January 4713 BCE, the Julian Day zero of the Julian Calendar

Julian Date (JD)Julian Date (JD)

The Julian Day Number followed by the fraction of the day elapsed since the preceding noon (12 hours UT).For example: The day extending from 1900 January 0.5 d UT to 1900 January 1.5 d UT has the number 2,415,020.

Modified Julian Day (MJD)Modified Julian Day (MJD)

Equal to the Julian date shifted so its origin occurs at midnight on November 17, 1858. It differs from the Julian Day by 2,400,000.5 days. For example: The day extending from November 17, 1858 to May 18, 2004 has the MJD number 53,108.


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•• Julian calendarJulian calendarThe solar calendar introduced by Julius Caesar in Rome in 46 B.C., having a year of 12 months and 365 days and a leap year of 366 days every fourth year. It was eventually replaced by the Gregorian calendar.

•• Gregorian calendarGregorian calendarThe solar calendar in use throughout most of the world, sponsored by Pope Gregory XIII in 1582 as a corrected version of the Julian calendar.

Definition of CalendarsDefinition of Calendars

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•• What is TimeWhat is Time

•• The Time StandardThe Time Standard•• Time SourcesTime Sources



AgendaAgenda

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Definition of the Definition of the ‘‘SECONDSECOND’’

• The ‘SECOND’ is the duration of 9,192,631,770

periods of the radiation corresponding to the

transition between the two (unperturbed)

hyperfine levels of the ground state of the 133Cesium atom

• Established in 1967

13

AT&T Phone Monopoly

(Model 625 Qz Osc for timing

Westinghouse)

’50s to ’70s1st Miniature Rb

Vapor Atomic Osc. (Efratom,

Munich)

1969Definition of the

“Second” per 133Cesium atom

(13th Conf. of Wtsand Measures)

1967

Timation Nav Sat (Qz)Transit Nav Sat (Qz)Loran Grd Nav (Cs)

Omega Grd Nav (Cs)WWVB Timing (Cs)

(FEI, HP)

’70s1st (4) GPS Block-I Sats Launched for Nav &

Timing (Rockwell)

Feb 1978

1st GPS Timing RCVR “TANS”

(Trimble)

1981

1st Rb built for Space, for

GPS Block-I Satellites

(Efratom-CA & Rockwell)

1975

1st GPS/Rb/Qz PRS Intro to

Wireless AT&T-Lucent

(Ball-Efratom)

19881st Mil Rb Intro to Mil/Gov (Ball-

Efratom)1983

1985-19891st USA

Com’l Passive Hydrogen

Maser (NIST & Ball-Efratom)

A host of GPS and Qz Companies;

also (4) Rb, (3) Cs, (3) Act Hm, and

(1) Pass Hm Companies

Late ’80s Early ’90s

GPS, Loran, Hm, Cs, Rb, and Qz everywhere in Infrastructure, Telecom, Gov,

Labs, etc.)

1st Com’l Rb Vapor Osc 19”

Rack (HP, GenRad, &

Tracor)

19591964Hi-Perf Qz Osc, Grd & Space (FEI)

World UTC

Defined

(FEI) enters Grd & Space Miniature Rb Market, buys Litton assets

1984

1st Rb Intro to Wired & Wireless

Telecom Nets (Ball-Efratom)

1985

1st GPS-aided Rb-PRS at

AT&T (AT&T & Ball-

Efratom)

1987(FTS & Kernco) enter Grd & Space

Cs Market1979

1st GPS/Rb Discipline

Module (Ball-Efratom)

1982

(EG&G) enters

Miniature Rb Mrkt

----

1st US Active Hydrogen Maser

(John Hopkins & Sigma-Tau)

(FEI &EG&G) enter Mil Rb Mrkt

1986

Active and Passive Hydrogen Masers (KVARZ, Russia)

Late ’60s

(FEI) enters Grd Cs Market

Intro of BITS Clocks to Wireline Telcos (Telecom

Solutions)

Early ’90sNew Mil SAASM GPS Timing Rcvr

(ATK)

1999WAAS GPS-

Augmentation System for the FAA Com’l

Aviation Navigation Infrastructure

2002

Wired and Wireless Telcos,Computer networks,Civil Infrastructure,

Gov/Mil Com, Cmd/Ctrl, and

Space Systems, all using PT&F Components and

Systems

Internet Timing Protocols, NTP, etc.

Mid ’90s

EU Galileo Test Sats Launches (Europe’s GPS

System)

2006

1st Com’l Cs Atomic Beam

Primary Std (HP)

1958

Qz Osc’s for Radars, Radios, & Transmitters

’40s

Precision Time Precision Time & Frequency & Frequency

HistoryHistory

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Atomic Energy LevelsAtomic Energy LevelsElectron Energy Levels - Electrostatic interaction between Proton and Electron (+ and - charges)

Fine Structure - Interaction between electron spin dipole moment and the magnetic field due to the electron’s orbital motion. ~1/50 of the first energy level.

Collapsing the orbit releases

energy

Expanding orbit requires energy

(Energy out or in is generally in the infrared-visible-UV frequencies)

(Energy out or in is generally in the microwave frequency area)

Hyperfine Structure - Magnetic dipole interaction between the electron spin dipole moment with the nucleus. ~1/1000 of fine structure interaction.

(Energy out or in is generally in the microwave frequencies and below)

f ≈ 1.4 GHz for Hydrogenf ≈ 6.8 GHz for Rubidiumf ≈ 9.2 GHz for Cesium

Zeeman Effect - Magnetic interaction between external magnetic field and electron and proton spin dipoles.

(Energy out or in is generally in the audio frequency area and above)

N

SN

S

(Courtesy: Ed Mattison, Smithsonian from his tutorial 12-92 (a modification of his chart)

15

9.2

0Ener

gy (F

requ

ency

) (G

Hz)

Magnetic Field HO Energy statesat H = HO

(F, mF)(4,4)(4,3)(4,2)(4,1)(4,0)(4,-1)(4,-2)(4,-3)(4,-4)

(3,-3)(3,-2)(3,-1)(3,0)(3,1)(3,2)(3,3)

9.192,631,770 GHz

Cesium Hyperfine Energy LevelsCesium Hyperfine Energy Levels

Courtesy of John Vig, U.S. Army Communications-Electronics Command, Fort Monmouth, NJ

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Atomic state selection Cs atom detection

ATOMIC BEAMSOURCE

ATOMIC BEAM

Cs VAPOR, CONTAINING AN EQUALAMOUNT OF THE TWOKINDS OF Cs ATOMS

VACUUM CHAMBER

MAGNET(STATE SELECTOR)

N

S

KIND 1 - ATOMS(LOWER STATE)

KIND 2 - ATOMS(UPPER STATE)

DETECTOR

DETECTOR

MAXIMUM SIGNAL

NO SIGNAL

S

S

N

N

MAGNET

MAGNET

MICROWAVECAVITY

MICROWAVECAVITY

MICROWAVE SIGNAL(OF ATOMIC RESONANCEFREQUENCY)

STATE SELECTEDATOMIC BEAM

STATE SELECTEDATOMIC BEAM

NO SIGNAL

6-10

Cesium Beam Atomic Frequency StandardCesium Beam Atomic Frequency Standard

Courtesy of John Vig, U.S. Army Communications-Electronics Command, Fort Monmouth, NJ

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Output

State Selection Magnets (7)

(2)

Cesium Source (1)

(5) (6)Beam Tube (3)

Cesium Beam Atomic Frequency StandardCesium Beam Atomic Frequency Standard

Cesium Oven (4)

(1) Active Element of Choice(2) Controlled Environment(3) Element Container(4) Atomic Pump(5) Resonator(6) Irradiation Source(7) State Detector(s)(8) Control Electronics(9) Frequency Source(10) Ion Pump and Getters

Hot Wire Ionizer (7)

5 MHz

~9.2 GHz

DC+Mod Freq

Qz Osc (9)

Servo (8)

Synth(8a)

(10)

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•• The Time StandardThe Time Standard

•• Time SourcesTime Sources•• GPS Time TransferGPS Time Transfer


AgendaAgenda

19

Relative vs. Systems Time SyncRelative vs. Systems Time Sync

Systems time usually refers Systems time usually refers to a Time Sync of the user to a Time Sync of the user

group obtained from an group obtained from an external source.external source.

One User can be selected at random to be the master

for the group

In general, relative time In general, relative time refers to a Time Sync within refers to a Time Sync within

the group of users.the group of users.

Relative Time

UserUser

User

User

User

User

User

(A System)

MC

A Master Station within the system

might be the reference for all

or

Systems TimeSystems Time

User

User

User

User

User

(A System)

User

UTC or

Other

User

UTC or another external world standard is the master for the group

Timing Capabilities From Various SystemsTiming Capabilities From Various Systems

100 (S)

10-3 (MilliS)

10-6 (MicroS)

10-9 (NanoS)

1,500,000 ns3,000,000 ns

1,000,000 ns

30,000 ns

5,000 nsCommunication Network Requirement

1,000 ns500 ns

<10 ns

Sync

h ro n

izat

ion

Acc

u ra c

y

Local Coverage Only

Global Coverage

Fort Collins Radio

GOES Sat

(UTC)

Radio Phone (UTC)

WWVB Radio (UTC)

Transit Nav. Sat.

Omega Portable Clock Trips

GPS (UTC)

(UTC) Traceable to Universal Coordinated Time

Navigation Requirement Long and Short Range

Shut

DownShut

Down

No

Longer

Practical

Common View

(DGPS) WAAS

EGNOS MSAS

GAGAN

<10 ns

~500 ns

eLoran (UTC)

10-7

10-8

<100 nsGlobal

Close to

being

shut

down

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•• GPS Time TransferGPS Time Transfer•• Time DilationTime Dilation

AgendaAgenda

22

UTC UTC ““CommonCommon--ViewView”” Time Transfer via GPSTime Transfer via GPS

- BIPM (International Time Bureau)- TAI (International Atomic Time)

*UTC is maintained within 0.9 sec. of UT-1. If exceeded, UTC is

corrected + one second (“Leap-Second”)

Russia

Germany

EnglandBIPM BIPM ParisParis

TAI TAI Time Time (PaperScale)

Atomic Clocks and Time Scales UTC* UTC*

Time Scale

Etc.

China

UTUT--11Time Time (Earth

Rotation)

Common ViewTime Transfer

GPS Satellites

Com

mon

Vie

wTi

me

Tran

sfer

Others

USNOUSNO

GPS Master GPS Master Control Station Control Station

(MCS)(MCS)

Data Exchange

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GPS Time Keeping InfrastructureGPS Time Keeping Infrastructure

GPS Satellites

USNO Wash. DC

NRL

GPS TimeGPS Time≤100 ns of UTC (USNO-Master Clock)

Monitor Monitor StationStation

Cs Clocks (Diego Garcia)

Satellite Control Uplink


Cs Clocks (Hawaii)

Monitor Monitor Station Station

Cs Clocks (Ascension)


Cs Clocks (Kwajalein)

Satellite L-Band Downlink

GPS Satellites

GPS Master GPS Master Control StationControl Station

(MCS-Schriever AFB)


Cs Clocks (Schriever)

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Separating Sat. Clock and Sat. Position ErrorSeparating Sat. Clock and Sat. Position Error

GPS Satellites


Cs Clocks (Diego Garcia)


Cs Clocks (Hawaii)


Cs Clocks (Ascension)


Cs Clocks (Kwajalein)

GPS Satellites


Cs Clocks (Schriever)

Ephemeris

25

UTC is related to PositionUTC is related to Position

Assume User #1 and User #2 are receiving the same satellite signal.

Both receive time but it will not be to <100 Nanoseconds of UTC.

GPS Satellite If signal has to travel 1000

miles further to reach User #2, then propagation delay of about 5.3 ms must be calibrated out.

User must first resolve his position before <100 Nanosec off UTC (USNO-MC) is possible.

∆ Miles

User #2

User #1

1

4

3

2

26






AgendaAgenda

27

Developed by Albert Einstein in 1905. He used a “Thought Experiment”in which he rode through space on a beam of light.

Stationary observer not on the train ∆tR = (1.6911886) (10 sec) = 16.911 sec

EinsteinEinstein’’s Special Theory Of Relativitys Special Theory Of Relativity

21

2R

CV1

tt

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−

∆=∆

Train travels

150,000Mi/Sec

Clock

Flashlight

Path oflight pulse

Mirror

10 SECONDS PASS; 5 pulses down and back up10 SECONDS PASS; 5 pulses down and back up

Observerin the train

Path ofLight Pulse

Train travels

150,000Mi/Sec

16.911 SECONDS PASS; 5 pulses down and back up16.911 SECONDS PASS; 5 pulses down and back up

One of (2) main effects (Special and General Theory of Relativity)

28

Clock Relativity ConsiderationsClock Relativity Considerations

2000 4000 6000 8000 10,000 12,000Altitude (nmi)

-6

GPS = 10,898 nmi

Circular Orbits

14,000

6

2

Rel

ativ

istic

Effe

ct, ∆

f/fx1

0-10

-2

-4

4

0

Special Theory of RelativitySpecial Theory of Relativity

112

12

−⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−=

∆CV

ff s

21

ss V

V ⎥⎦

⎤⎢⎣

⎡ µ=

1010x83.0ff −−=

∆ For GPS:

1010x3.5ff −=

∆For GPS:

General Theory of Relativity

⎥⎦

⎤⎢⎣

⎡−+=

∆

srer1

2Cer

egff

ZeroEffect

ZeroEffect

Total Relativistic Effect on GPS Clocks ∆f/f 4.47E-10

Total Relativistic Effect on GPS Clocks ∆f/f 4.47E-10

10

.sec

1046.4400,86 −∗=∆

∗=∆ xfftt

dayoner nsec621,38=∆ rt Original Clocks set to:

10,229,999.995433 Hz

time and time transfer - frequency electronics inc - precision

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