1

Timing with loran

Judah Levine

Time and Frequency Division

NIST/Boulder

jlevine@boulder.nist.gov

(303) 497 3903

ILA31, October 2002

Judah Levine, NIST 2

Outline of the talk

• Transmission requirements for time and frequency

• What is traceability and why is it important?

• Time and frequency user requirements

• Loran performance

• Summary and conclusions

ILA31, October 2002

Judah Levine, NIST 3

Transmission of time

• Time is the primary deliverable

– Applying a time stamp to an event

• Arrival of a seismic signal

– Accuracy of time standard and measurement of absolute channel delay required

ILA31, October 2002

Judah Levine, NIST 4

Transmission of frequency

• Frequency is the primary deliverable

– Synchronous communication channels, power distribution

– Stability of remote standard and channel delay are required

• Accurate measurement of channel delay not important

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Traceability

• A traceable observation can be connected to national or international standards using an unbroken chain of measurements, each of which has a stated uncertainty.

• The adequacy of any such chain can be specified only after the requirements of the end user have been specified

– A measurement technique might be adequately traceable for some applications but not for others

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Need for traceability

• Equity in trade

• Interoperability of systems at both the national and international levels

• Legal requirements

• Research that depends on precision measurements– Pulsars, general relativity, …

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“Strong” traceability

• Realized using a direct measurement of every link in the measurement chain

– The ideal situation

– Cannot always be realized in practice

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“Weaker” traceability – 1• The performance of a link in the

measurement chain is estimated based on measurements of ancillary parameters and a model to relate these other measurements to the datum of interest

– Limited by the accuracy and spatial resolution of the model

• Estimating radio path delay based on measurements of temperature, pressure, …

– Spatial and temporal variation, model approximations, …

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“Weaker” traceability – 2

• The performance of a link in the measurement chain is estimated based on measurements of the datum of interest on another link that is presumed to be equivalent.

– Common-view method

• Simultaneous observations of same signal at multiple locations

• Assumes delay fluctuations along two paths are correlated

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“Weaker” traceability – 3

• Often the only practical solution

• Performance may be degraded compared to more direct methods

– Magnitude of the problem not easily known

• Better than nothing

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Legal traceability

• Traceability with enough additional documentation to support convincing a jury in an adversarial proceeding

– Difficult (perhaps impossible) to realize with a broadcast-only system

• Probably requires a disinterested 3rd party to certify hardware and authenticate documentation

• Essentially no experience at present

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Judah Levine, NIST 12

Time and frequency linksTreaty of the Meter (1875,1921)

International Bureau of Weights and Measures (BIPM)

defines UTC

UTC realized at National Metrology Institutes

and timing laboratories

Distribution system

User equipment

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The problem links

• UTC(lab) to distribution system– Prediction of UTC(USNO) transmitted by GPS

satellites

– Realization of UTC(NIST) at WWVB

– Copy not as good as original

• Distribution transmitter to end-user portal– Model path delay using physical distance and

parameterized index of refraction

– Common view configuration• Estimate path delay using real-time measurements

along another path

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Traceability of loran - 1

• Steer loran transmitter to UTC(USNO) using GPS signals– Depends on GPS system

• Minimal additional equipment

• Steer loran transmitter to UTC(lab) via other method (2-way satellite, fiber, …)– Independent infrastructure with many

realizations

– Significantly more expensive

ILA31, October 2002

Judah Levine, NIST 15

Traceability of loran – 2

• Steer loran transmitter using remote monitor directly linked to UTC(lab)– Independent of GPS (or other transfer link)

– Independent of any one timing laboratory

– Steering incorporates some correction for path delay to end user

• Usefulness depends on isotropy of delay

– Requires secure link back to transmitter

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User’s requirements

• Positioning applications depend on internal synchronization of sub-systems and not on external traceability

– Master/slave relationship in loran

– Satellite clock/system time in GPS

– External traceability is a free parameter that can be driven based on user’s applications

ILA31, October 2002

Judah Levine, NIST 17

Realizing traceability

• Time applications need rapid steering to minimize RMS time errors– Resulting frequency excursions are the price

of admission

• Frequency applications benefit from slow steering to keep frequency smooth– Resulting time dispersion is larger and has

longer persistence • Ok, timing requirements are less stringent

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Frequency

• Power-line frequency, stratum-1 telecom

– Fractional frequency accuracy 110-11

• Calibrate best commercial cesium

– Fractional frequency stability 210-14

• Calibrate best commercial H maser

– Fractional frequency stability 110-15

• Frequency transfers have implied averaging times (more later)

ILA31, October 2002

Judah Levine, NIST 19

Time

• Stratum-1 Network Time, time services, …

– Time accuracy at server: 1 ms

• Fault detection, LAN timing, …

– Time accuracy: 500 ns – 1 s

• International time coordination

– Time accuracy: 1 ns best, 5-10 ns typical

• Time transfers often cannot exploit averaging

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How well does current system perform?

• Data from 9610 master at Boise City, OK

– Monitored by NIST at Boulder, CO

– Monitored by USNO at Flagstaff, AZ

• Data from 9960 master at Seneca, NY

– Monitored at LSU (Loran Support Unit), Wildwood, NJ

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ILA31, October 2002

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UTC(GPS)-9610 Master (Data provided by USNO)

-600

-400

-200

0

200

400

600

52250 52300 52350 52400 52450 52500 52550

MJD (1 Jan 2002 - 29 Sep 2002)

ns

April, 2002

June, 2002

-1X10-13

Feb, 2002

+1X10-13

23

ILA31, October 2002

Judah Levine, NIST

Loran - NIST & USNO

-600

-500

-400

-300

-200

-100

0

100

200

300

400

52100 52150 52200 52250 52300 52350 52400 52450 52500 52550 52600

MJD

ns

NIST

NIST

USNO

USNO

24

ILA31, October 2002

Judah Levine, NIST

y(1day), Loran - UTC(NIST)

-1.00E-12

-5.00E-13

0.00E+00

5.00E-13

1.00E-12

1.50E-12

52100 52150 52200 52250 52300 52350 52400 52450 52500 52550

MJD

Frac

tiona

l Fre

quen

cy

ILA31, October 2002

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LOCUS Timing Data10 second average data from LSU at 9960 rate

-800

-600

-400

-200

0

200

400

600

0 2 4 6 8 10 12 14 16

hours

ns (m

ean

rem

oved

)

•60 ns RMS

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Summary and conclusions

• Principal assumption:

– Boise City 9610 data for 2002 are “typical”

• Ignore 9610 data from 2000 and 2001– 15X worse in time, 50X worse in frequency

• Ignore older data from Seneca– 50X worse in time and frequency– Significant number of synchronization failures

• Comparable to best 9960 data

– Assume the best is “typical”

ILA31, October 2002

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Time from loran• Better than 1 s 100% of the time

• Sometimes much better than this – can reach 60 ns RMS– Significant variability with time and location

– Your mileage may vary• Caveat emptor …

• Can support almost all routine civilian timing applications

• Scientific, research, national labs, will need something better

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Frequency from loran• One-day average

– Fractional frequency accuracy of

• 1X10-12 100% of the time

• 5X10-13 90% of the time

– Supports telecom stratum-1 (1X10-11)

• Assumes reference clock has adequate holdover stability consistent with 1-day averaging time

– Inadequate for research, technical, high end users

• Cannot support high-end cesium device– 2X10-14 with 1 day of averaging

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Other measurement strategies

• Combine data from several transmitters

– Signal averaging

• Uncorrelated effects improve only as n– Cost and complexity may increase as n

• Correlated effects unaffected

– Outlier detection

• Useful as a glitch detector

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Thank you for data …

• Tom Celano, Timing Solutions Corp.

• Harold Chadsey, US Naval Observatory

• Mike Lombardi, NIST