Judah Levine, NIST, CENAM, Sept 2012: 1 Introduction to Time and Timekeeping Judah Levine Time and Frequency Division NIST/Boulder

Judah Levine, NIST, CENAM, Sept 2012: 2 Outline Introductory background Introductory background Requirements of a time service operated by a timing laboratory Requirements of a time service operated by a timing laboratory The error budget for time dissemination The error budget for time dissemination Description of methods with examples Description of methods with examples –advantages and limitations

Judah Levine, NIST, CENAM, Sept 2012: 3 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, …

Judah Levine, NIST, CENAM, Sept 2012: 4 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

Judah Levine, NIST, CENAM, Sept 2012: 5 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

Judah Levine, NIST, CENAM, Sept 2012: 6 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

Judah Levine, NIST, CENAM, Sept 2012: 7 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

Judah Levine, NIST, CENAM, Sept 2012: 8 The Error Budget Internal accuracy of the time source Internal accuracy of the time source –Usually not the limiting factor The transmission delay The transmission delay –This is usually the hard part –Uncertainty often limits traceability Statistics of the user’s clock and the measurement process Statistics of the user’s clock and the measurement process –Is calibration interval consistent with accuracy requirement? –Dynamic, adaptive calibration process

Judah Levine, NIST, CENAM, Sept 2012: 9 Methods of Time Dissemination Simple one-way method Simple one-way method One-way method with model of delay One-way method with model of delay Common-view Common-view Partial two-way method Partial two-way method Full Two-way method Full Two-way method

Judah Levine, NIST, CENAM, Sept 2012: 10 Simple one-way method - 1 Ignore network delay completely Ignore network delay completely –Delay << required accuracy Simple broadcasts Simple broadcasts –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, …) 2.5 MHz, 5 MHz, … delay, coverage variable2.5 MHz, 5 MHz, … delay, coverage variable –Internet service in broadcast mode (NTP) Delay, coverage very variableDelay, coverage very variable

Judah Levine, NIST, CENAM, Sept 2012: 11 Simple one-way method - 2 Simple receiver and transmitter Simple receiver and transmitter Transmission cost does not depend on number of receivers Transmission cost does not depend on number of receivers Receiver is passive Receiver is passive Timing error < 1 s, often < 20 ms Timing error < 1 s, often < 20 ms Traceability possible with adequate log files Traceability possible with adequate log files

Judah Levine, NIST, CENAM, Sept 2012: 12 One-way with delay model GPS Receiver Geometric delay, 65 ms estimated using ephemeris and 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

Judah Levine, NIST, CENAM, Sept 2012: 13 Common-view method Source Rcvr 1Rcvr 2 T1= t(1) – (S +  )T2= t(2) – (S +  )  t= T1-T2= t(1)-t(2) Path delays are nearly equal and cancel in the difference Source clock cancels too The time is S ←δ→←δ→

Judah Levine, NIST, CENAM, Sept 2012: 14 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

Judah Levine, NIST, CENAM, Sept 2012: 15 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

Judah Levine, NIST, CENAM, Sept 2012: 16 All in view melting pot S1S2S3S4S5S6S7S8 Com ref Rcvr 2Rcvr 1 2-C 1-C 3-C4-C5-C6-C7-C 8-C  1 =(S1+S2+S3+S4)/4  2 =(S5+S6+S7+S8)/4  T=  1 -  2

Judah Levine, NIST, CENAM, Sept 2012: 17 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

Judah Levine, NIST, CENAM, Sept 2012: 18 Full Two-way Measure round-trip delay on every calibration Measure round-trip delay on every calibration –Delay is not stable and not white pm over longer periods –Transmission delay is one-half of measured value Delay is symmetric on the averageDelay is symmetric on the average Telephone system using ACTS Telephone system using ACTS Internet using full NTP Internet using full NTP

Judah Levine, NIST, CENAM, Sept 2012: 19 Real-world limitations Inbound and outbound delays are not equal Inbound and outbound delays are not equal –Realized as a two-way physical circuit with some one-way components Physical component dispersionPhysical component dispersion –Realized with a reversible one-way physical circuit Time dispersionTime dispersion –Realized using a packet network Asymmetric queuing and routing delaysAsymmetric queuing and routing delays

Judah Levine, NIST, CENAM, Sept 2012: 20 Effect of Asymmetry - 1 Method assumes one-way delay is one-half of round-trip value. Time error is given by Method assumes one-way delay is one-half of round-trip value. Time error is given by 0≤ k ≤ 1

Judah Levine, NIST, CENAM, Sept 2012: 21 Effect of asymmetry  Round-trip delay→ k=1,  =  /2 k=0,  = -  /2 Smaller delay has smaller asymmetry error

Judah Levine, NIST, CENAM, Sept 2012: 22 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

Judah Levine, NIST, CENAM, Sept 2012: 23 Asymmetry – the bottom line Static asymmetry generally cannot be detected or removed Static asymmetry generally cannot be detected or removed –Limits accuracy of any protocol –Multiply-connected networks sometimes help in detecting asymmetry Apparent time difference over different pathsApparent time difference over different paths

Judah Levine, NIST, CENAM, Sept 2012: 24 Summary - 1 One-way methods are simple and are good enough for many applications One-way methods are simple and are good enough for many applications –Path delay can be ignored –Path delay can be modeled adequately Common-view depends on equality of delays along two one-way paths Common-view depends on equality of delays along two one-way paths –Requires data exchange between stations Neither method can attenuate local effects Neither method can attenuate local effects

Judah Levine, NIST, CENAM, Sept 2012: 25 Summary - 2 Two-way depends on equality of delay in opposite direction along a single path Two-way depends on equality of delay in opposite direction along a single path Limited by the symmetry of the link delay between the transmitter and the receiver Limited by the symmetry of the link delay between the transmitter and the receiver –Magnitude of the delay not important –Message format not important Error in time data proportional to asymmetry and delay Error in time data proportional to asymmetry and delay –Shorter paths will always have smaller errors

Judah Levine, NIST, CENAM, Sept 2012: 26 For more information List of publications of the NIST time and frequency division are in the publications menu of our web page: List of publications of the NIST time and frequency division are in the publications menu of our web page:tf.boulder.nist.gov Many of these publications are on-line Many of these publications are on-line “Time and Frequency Measurement” by C. Hackman and D. B. Sullivan, published by the American Association of Physics Teachers, “Time and Frequency Measurement” by C. Hackman and D. B. Sullivan, published by the American Association of Physics Teachers, 1996.