judah levine, nist, cenam, sept 2012: 1 introduction to time and timekeeping judah levine time and...

Judah Levine, NIST, CENAM, Sept 2012: 1

Introduction to Time and Introduction to Time and TimekeepingTimekeeping

Judah LevineJudah Levine

Time and Frequency DivisionTime and Frequency Division

NIST/BoulderNIST/[email protected]


OutlineOutline

Introductory background Introductory background Requirements of a time service Requirements of a time service

operated by a timing laboratoryoperated by a timing laboratory The error budget for time The error budget for time

disseminationdissemination Description of methods with Description of methods with

examples examples – advantages and limitationsadvantages and limitations


Background

Laboratory is generating a local estimate of UTC: UTC(lab)– Time scale methods in future talk

UTC(lab) is steered to UTC with an uncertainty adequate for the time service 100 s for NTP, telephone services,

radio time signals, …


Requirements - 1

Integrity– Time signals must be protected so

they are not modified or changed during transmission

– Easy:• Telephone service• Radio broadcast service• Authenticated Internet service

– Hard:• Normal Internet services


Requirements - 2

Availability– Service should not have single point

of failure• Multiple sources at different locations

– Minimize Time to Repair– Balanced with Cost– GNSS signals can be jammed or

spoofed• Total reliance on GNSS signals possible

long-term problem


Requirements - 3

Accuracy– Service should transmit UTC(lab) only

when operating correctly– Should transmit nothing or error

message when failed• Some services do not use this principle

– Example: implementations of NTP

• Principle can never be 100% reliable


Requirements - 4 Technical Traceability

– Each link between user and UTC should be calibrated with delay and uncertainty

• Magnitude consistent with user requirements Legal Traceability

– Traceability can be documented and proven in legal proceedings

• Log files and documents show proper operation and also errors

Users are responsible for traceability with assistance from timing laboratory


The Error BudgetThe Error Budget

Internal accuracy of the time sourceInternal accuracy of the time source– Usually not the limiting factorUsually not the limiting factor

The transmission delayThe transmission delay– This is usually the hard partThis is usually the hard part– Uncertainty often limits traceabilityUncertainty often limits traceability

Statistics of the user’s clock and the Statistics of the user’s clock and the measurement processmeasurement process– Is calibration interval consistent with Is calibration interval consistent with

accuracy requirement?accuracy requirement?– Dynamic, adaptive calibration processDynamic, adaptive calibration process


Methods of Time Dissemination Methods of Time Dissemination

Simple one-way methodSimple one-way method One-way method with model of One-way method with model of

delaydelay Common-viewCommon-view Partial two-way methodPartial two-way method Full Two-way methodFull Two-way method


Simple one-way method - 1Simple one-way method - 1

Ignore network delay completelyIgnore network delay completely– Delay << required accuracyDelay << required accuracy

Simple broadcastsSimple broadcasts– Low-frequency services (WWVB, …)Low-frequency services (WWVB, …)

• 60 kHz, 2 × 50 kW covers most of US60 kHz, 2 × 50 kW covers most of US

– Short-wave services (WWV, …)Short-wave services (WWV, …)• 2.5 MHz, 5 MHz, … delay, coverage variable2.5 MHz, 5 MHz, … delay, coverage variable

– Internet service in broadcast mode Internet service in broadcast mode (NTP)(NTP)• Delay, coverage very variableDelay, coverage very variable


Simple one-way method - 2Simple one-way method - 2

Simple receiver and transmitterSimple receiver and transmitter Transmission cost does not Transmission cost does not

depend on number of receiversdepend on number of receivers Receiver is passiveReceiver is passive Timing error < 1 s, often < 20 msTiming error < 1 s, often < 20 ms Traceability possible with Traceability possible with adequateadequate log files log files


One-way with delay model

GPS

Receiver

Geometric delay, 65 ms estimated using ephemerisand known position

65 ns, Ionosphere delay from model or L1-L2 dispersion

5 ns,Troposphere delay from T, RH, or multiple satellites

Geophysical effects,earth models, 1ns Calibration,

Multipath, 10ns


Common-view methodCommon-view method

Source

Rcvr 1 Rcvr 2

T1= t(1) – (S +)T2= t(2) – (S + )

t= T1-T2= t(1)-t(2)

Path delaysare nearly equaland cancel in thedifference

Source clockcancels too

The time is S

←δ→


Common View Sources

GNSS Signals Television Broadcasts

– Synchronization pulse in blank line FM radio signals

– Stereo sub-carrier Phase of mains voltage

– Within building or small area Loran signals (no longer in US) Source is used passively at no cost


Common View Limitations

Paths to receivers have very different un-modeled delays– Calibration of local equipment– Atmospheric delay

Receivers too far apart to see the physical transmitter


All in view melting pot

S1 S2 S3 S4 S5 S6 S7 S8

Com ref

Rcvr 2Rcvr 1

2-C1-C 3-C 4-C 5-C 6-C 7-C 8-C

1=(S1+S2+S3+S4)/4 2=(S5+S6+S7+S8)/4

T=1-2


Partial two-way method

Delay is stable and is white pm– Measure only occasionally– Unique to PTP/1588– Useful only in special cases

• Problems in wide-area networks• False-tickers and the trust problem


Full Two-wayFull Two-way

Measure round-trip delay on every Measure round-trip delay on every calibrationcalibration– Delay is not stable and not white pm Delay is not stable and not white pm

over longer periods over longer periods – Transmission delay is one-half of Transmission delay is one-half of

measured valuemeasured value• Delay is symmetric on the averageDelay is symmetric on the average

Telephone system using ACTSTelephone system using ACTS Internet using full NTP Internet using full NTP


Real-world limitationsReal-world limitations

Inbound and outbound delays are Inbound and outbound delays are not equalnot equal– Realized as a two-way physical circuit Realized as a two-way physical circuit

with some one-way componentswith some one-way components• Physical component dispersionPhysical component dispersion

– Realized with a reversible one-way Realized with a reversible one-way physical circuitphysical circuit• Time dispersionTime dispersion

– Realized using a packet networkRealized using a packet network• Asymmetric queuing and routing delaysAsymmetric queuing and routing delays


Effect of Asymmetry - 1Effect of Asymmetry - 1

Method assumes one-way delay is one-Method assumes one-way delay is one-half of round-trip value. Time error is half of round-trip value. Time error is given bygiven by

fractionoutboundk

delaytripround

k

)5.0(

0≤ k ≤ 1


Effect of asymmetry - 2Effect of asymmetry - 2

0 Round-trip delay→

k=1, =/2

k=0, = -/2

Smaller delay has smaller asymmetry error


NTP Service model

Operate servers at many locations– Minimizes delay error for all users– No single point of failure– How are remote servers synchronized?

• Time link to source of UTC(lab) Performance limited by delay jitter

and asymmetry– Few percent of round-trip measurement

• Accuracy < 50 ms, often < 10 ms, maybe ~ 1ms


Asymmetry – the bottom lineAsymmetry – the bottom line

Static asymmetry generally cannot Static asymmetry generally cannot be detected or removedbe detected or removed– Limits accuracy of any protocolLimits accuracy of any protocol– Multiply-connected networks Multiply-connected networks

sometimes help in detecting sometimes help in detecting asymmetryasymmetry• Apparent time difference over different Apparent time difference over different

pathspaths


Summary - 1Summary - 1

One-way methods are simple and are One-way methods are simple and are good enough for many applicationsgood enough for many applications– Path delay can be ignoredPath delay can be ignored– Path delay can be modeled adequatelyPath delay can be modeled adequately

Common-view depends on equality of Common-view depends on equality of delays along two one-way pathsdelays along two one-way paths– Requires data exchange between stationsRequires data exchange between stations

Neither method can attenuate local Neither method can attenuate local effectseffects


Summary - 2Summary - 2 Two-way depends on equality of delay in Two-way depends on equality of delay in

opposite direction along a single pathopposite direction along a single path Limited by the symmetry of the link delay Limited by the symmetry of the link delay

between the transmitter and the receiverbetween the transmitter and the receiver– Magnitude of the delay not importantMagnitude of the delay not important– Message format not importantMessage format not important

Error in time data proportional to Error in time data proportional to asymmetry and delayasymmetry and delay– Shorter paths will always have smaller Shorter paths will always have smaller

errorserrors


For more informationFor more information

List of publications of the NIST time List of publications of the NIST time and frequency division are in the and frequency division are in the publications menu of our web page:publications menu of our web page:

tf.boulder.nist.govtf.boulder.nist.gov Many of these publications are on-line Many of these publications are on-line ““Time and Frequency Measurement” Time and Frequency Measurement”

by C. Hackman and D. B. Sullivan, by C. Hackman and D. B. Sullivan, published by the American Association published by the American Association of Physics Teachers, 1996.of Physics Teachers, 1996.

judah levine, nist, cenam, sept 2012: 1 introduction to time and timekeeping judah levine time and...

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