Introduction to Time and Timekeeping Judah Levine Time

Introduction to Time and Timekeeping Judah Levine Time and Frequency Division NIST/Boulder jlevine@boulder. nist. gov Judah Levine, NIST, CENAM, Sept 2012: 1

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

Background n Laboratory is generating a local estimate of UTC: UTC(lab) – Time scale methods in future talk n 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: 3

Requirements - 1 n 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: 4

Requirements - 2 n 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 longterm problem Judah Levine, NIST, CENAM, Sept 2012: 5

Requirements - 3 n 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: 6

Requirements - 4 n Technical Traceability – Each link between user and UTC should be calibrated with delay and uncertainty • Magnitude consistent with user requirements n Legal Traceability – Traceability can be documented and proven in legal proceedings • Log files and documents show properation and also errors n Users are responsible for traceability with assistance from timing laboratory Judah Levine, NIST, CENAM, Sept 2012: 7

The Error Budget n Internal accuracy of the time source – Usually not the limiting factor n The transmission delay – This is usually the hard part – Uncertainty often limits traceability n 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: 8

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

Simple one-way method - 1 n Ignore network delay completely – Delay << required accuracy n Simple broadcasts – Low-frequency services (WWVB, …) • 60 k. Hz, 2 × 50 k. W covers most of US – Short-wave services (WWV, …) • 2. 5 MHz, … delay, coverage variable – Internet service in broadcast mode (NTP) • Delay, coverage very variable Judah Levine, NIST, CENAM, Sept 2012: 10

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

One-way with delay model GPS 65 ns, Ionosphere delay from model or L 1 -L 2 dispersion Geometric delay, 65 ms estimated using ephemeris and known position Geophysical effects, earth models, 1 ns 5 ns, Troposphere delay from T, RH, or multiple satellites Receiver Calibration, Multipath, 10 ns Judah Levine, NIST, CENAM, Sept 2012: 12

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

Common View Sources n n GNSS Signals Television Broadcasts – Synchronization pulse in blank line n FM radio signals – Stereo sub-carrier n Phase of mains voltage – Within building or small area n n Loran signals (no longer in US) Source is used passively at no cost Judah Levine, NIST, CENAM, Sept 2012: 14

Common View Limitations n Paths to receivers have very different un-modeled delays – Calibration of local equipment – Atmospheric delay n Receivers too far apart to see the physical transmitter Judah Levine, NIST, CENAM, Sept 2012: 15

Com ref All in view melting pot S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 1 -C 2 -C 3 -C 4 -C 5 -C 6 -C 7 -C 8 -C Rcvr 1 Rcvr 2 1=(S 1+S 2+S 3+S 4)/4 2=(S 5+S 6+S 7+S 8)/4 T= 1 - 2 Judah Levine, NIST, CENAM, Sept 2012: 16

Partial two-way method n 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: 17

Full Two-way n 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 average Telephone system using ACTS n Internet using full NTP n Judah Levine, NIST, CENAM, Sept 2012: 18

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

Effect of Asymmetry - 1 n 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: 20

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

NTP Service model n 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) n Performance limited by delay jitter and asymmetry – Few percent of round-trip measurement • Accuracy < 50 ms, often < 10 ms, maybe ~ 1 ms Judah Levine, NIST, CENAM, Sept 2012: 22

Asymmetry – the bottom line n 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 paths Judah Levine, NIST, CENAM, Sept 2012: 23

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

Summary - 2 n n 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 – Magnitude of the delay not important – Message format not important n Error in time data proportional to asymmetry and delay – Shorter paths will always have smaller errors Judah Levine, NIST, CENAM, Sept 2012: 25

For more information n 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 n “Time and Frequency Measurement” by C. Hackman and D. B. Sullivan, published by the American Association of Physics Teachers, 1996. n Judah Levine, NIST, CENAM, Sept 2012: 26