2016‐09‐28
GPS time transfer and time scales: What does the BIPM do with my data?
Louis Marais 27 September 2016
Technical Workshop for GPS Time‐Transfer and Calibration Techniques
Outline
• Quick overview of GPS time transfer • Let’s look at time scales • BIPM’s role •Our role
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Quick overview of GPS time transfer
• Comparing two local clocks • Comparing with a remote clock • Common view
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Quick overview of GPS time transfer Comparing two local clocks
Our Master Unknown Clock Clock
Counter / Timer
Δt
Unknown Clock = Master Clock + ∆t ± some uncertainty
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Quick overview of GPS time transfer Comparing with a remote clock
Our Master Unknown Clock Clock
Counter / Timer
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Quick overview of GPS time transfer Comparing with a remote clock
Event!Event!
Our Master Unknown Clock Clock
Counter / Timer Counter / Timer
Δt1 Event Event Δt2 recorder recorder Δt1 = Master – Event Δt2 = Unknown – Event
Master – Unknown = Δt1 – Δt2 = Master – Event – (Unknown – Event) ± some uncertainty
6 Animated cannon source: http://www.animationlibrary.com/sc/203/Guns_and_Cannons/?page=2
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Quick overview of GPS time transfer Common view
BIPM UTC UTC(k)
∆UTC = UTC – GPS ∆UTC(k) = UTC(k) – GPS
UTC – UTC(k) = (UTC – GPS) – (UTC(k) – GPS)
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Let’s look at Time Scales
• A bit of history • Universal Time • Atomic Time • Satellite System Time
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Let’s look at Time Scales A bit of history
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
Let’s look at Time Scales A bit of history
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
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Let’s look at Time Scales A bit of history
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
Let’s look at Time Scales A bit of history
TAI
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
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Let’s look at Time Scales A bit of history
TAI
GPS time, GLONASS time
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
Let’s look at Time Scales A bit of history
TAI
GPS time, GLONASS time
EAL
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
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Let’s look at Time Scales A bit of history
TAI
GPS time, GLONASS time
EAL
TT
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
Let’s look at Time Scales A bit of history
TAI
GPS time, GLONASS time
EAL
TT
UTC, UTC(lab)
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
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Let’s look at Time Scales A bit of history
TAI
GPS time, GLONASS time
EAL
TT
UTC, UTC(lab)
TA(lab)
* Source: http://www.bipm.org/en/bipm-services/timescales/time-ftp/scale.html 9
Let’s look at Time Scales A bit of history
• UT – Universal Time (1884) • It is a time standard based on Earth's rotation • It is the mean solar time on the Prime Meridian at Greenwich, London, UK. • It is ambiguous – there are several versions (UT1, UTC, etc.)
Source: http://bestanimations.com/Earth&Space/Earth/Earth.html
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Let’s look at Time Scales Universal Time
• Universal time (UT) scales are based on the rotation of the Earth • It is necessary to specify the form of UT is used, where milliseconds matter: – UT0 is the mean solar time of the prime meridian obtained from direct astronomical observation – UT1 is UT0 corrected for the effects of small movements of the Earth relative to the axis of rotation (polar variation); it corresponds directly with the angular position of the Earth around its axis of diurnal rotation. – UT2 is UT1 corrected for the effects of a small seasonal fluctuation in the rate of rotation of the Earth;
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Let’s look at Time Scales UT1
• UT1 is the form of Universal Time we use the most • It is (conceptually) the mean solar time at 0° longitude • Precise measurements of the sun are difficult • UT1 is computed from observations of celestial bodies – VLBI of quasars, – Laser ranging of the moon and artificial satellites, etc.
UT1 is required to follow this relationship:
ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians where Tu = (Julian UT1 date - 2451545.0) ERA is Earth Rotation Angle (replaced Greenwich Mean Sidereal Time)
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Let’s look at Time Scales Aside – VLBI
• Geometric technique Measures time difference between the arrival of radio wave fronts from distant quasars • Measurements precise to a few picoseconds Relative positions of antennae to within millimeters, and quasar positions to within fractions of a milli-arcsecond • Track position changes Determines position of Earth in inertial reference frame through antenna positions
13 Source: http://space-geodesy.nasa.gov/techniques/VLBI.html
Let’s look at Time Scales Aside – Laser ranging
• Moon coordinates are measured in mean Earth/polar (ME) axis system • Apollo missions placed Laser Ranging Retro Reflector (LRRR) arrays on the moon • Accurate ranging of these arrays allowed coordinate system to be determined to cm level
Source: https://www.nasa.gov/mission_pages/ LRO/multimedia/lroimages/ lroc-20100413-apollo15-LRRR.html 14
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Let’s look at Time Scales Atomic Time
• Rabi (early 1940s) idea for atomic clock • Harold Lyons (1949) first atomic clock (ammonia) • Not a good clock, but a very important step • Lyons researched caesium as an atomic frequency standard
Harold Lyons (right) and Edward Condon, director of NBS, with ammonia atomic clock (1949) 15 Photograph from IEEE I&M Magazine, Dec 2011
Let’s look at Time Scales Atomic Time
• Louis Essen, NPL England, (1955) first caesium clock
Louis Essen (right) and Jack Parry (left) with the original NPL caesium clock 16 Photograph from IEEE I&M Magazine, Dec 2011
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Let’s look at Time Scales Atomic Time
• NBS-1 in 1958 (Boulder, Colorado) • Finally achieved 1·10-10
Roger Beehler of NBS with NBS-1.
17 Photograph from IEEE I&M Magazine, Dec 2011
Let’s look at Time Scales Atomic Time
• The pioneering work was quickly followed by improved clocks • NBS 2
NBS2
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Let’s look at Time Scales Atomic Time
• The pioneering work was quickly followed by improved clocks • NBS 2 • NBS 3
NBS2
NBS3
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Let’s look at Time Scales Atomic Time
• The pioneering work was quickly followed by improved clocks • NBS 2 • NBS 3 • NBS 4
NBS2
NBS3 NBS4 18
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Let’s look at Time Scales Atomic Time
• NBS 5
NBS5
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Let’s look at Time Scales Atomic Time
• NBS 5 • NBS 6
NBS5 NBS6 19
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Let’s look at Time Scales Atomic Time
• NBS 5 • NBS 6 •NIST 7
NBS5 NBS6 NIST 7 19
Let’s look at Time Scales Atomic Time
• NBS 5 • NBS 6 •NIST 7 •NIST F1
NBS5 NBS6 NIST 7 19
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Let’s look at Time Scales Atomic Time
Other countries
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Let’s look at Time Scales Atomic Time
Other countries
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Let’s look at Time Scales Atomic Time
Other countries
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Let’s look at Time Scales Atomic Time
Other countries
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Let’s look at Time Scales Atomic Time
Other countries
SYRTE FO1
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Let’s look at Time Scales Atomic Time
Other countries
SYRTE FO1 SYRTE FO2
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Let’s look at Time Scales Atomic Time
21 Source: https://www.ncsli.org/c/f/p11/286.314.pdf
Let’s look at Time Scales Atomic Time
21 Source: http://rsta.royalsocietypublishing.org/.../4109
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Let’s look at Time Scales Atomic Time
• Primary caesium clocks are fine for large institutes with substantial research groups, but for smaller laboratories commercial standards are better: – Much cheaper to acquire both in cost and manpower – Much easier to maintain: typical lifetime of more than 20 years – Still a primary standard!
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Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A
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Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A
HP 5061A
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Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A
HP 5061A
HP 5071A 23
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Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A
HP 5061A Microsemi 4310B
HP 5071A 23
Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A
HP 5061A Microsemi 4310B
Microsemi Cs4000 HP 5071A 23
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Let’s look at Time Scales Atomic Time: Commercial clocks
HP 5060A Chip Scale Atomic Clock
HP 5061A Microsemi 4310B
Microsemi Cs4000 HP 5071A 23
Let’s look at Time Scales Redefinition of the second
• 1800s: Second is 1/86 400 of a mean solar day • 1956: Second is 1/31 556 925.9747 of the tropical year 1900 • 1960: CGPM adopts this definition • 1967: Second is redefined in terms of Caesium • 1997: CIPM affirms that the definition relates to Caesium atom in its ground state at 0 K • Future: Optical radiation?
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
24 Sources: http://physics.nist.gov http://www.bipm.org
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Let’s look at Time Scales UTC
• Coordinated Universal Time (UTC) is the primary time standard or international time scale by which the world regulates clocks and time • UTC is a stepped atomic time scale – defined by the ITU – determined by the BIPM in cooperation with the IERS – IERS determines the offset between UT1 and UTC – When it reaches a limit, a leap second in introduced
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Let’s look at Time Scales Leap seconds: UT1 and UTC offsets
26 Sources: USNO and IERS websites
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Let’s look at Time Scales Leap seconds: UT1 and UTC offsets
26 Sources: USNO and IERS websites
Let’s look at Time Scales Leap seconds: UT1 and UTC offsets
26 Sources: USNO and IERS websites
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Let’s look at Time Scales Other timescales
• Échelle Atomique Libre (EAL), a free atomic time scale – Produced from data submitted by timing centres • International Atomic Time (TAI) is derived from EAL – By correcting for the scale interval • UTC is obtained from TAI by correcting for leap seconds • Laboratories with primary atomic standards maintain TA(k), there own local atomic time scale • Timing centres maintain their own version of UTC, UTC(k) • Terrestrial Time (TT), defined by the IAU, is an ideal time scale used for astronomical observations from earth
27 Source: http://www.bipm.org/... /7_establishment_TAR15.pdf
Let’s look at Time Scales Satellite System Time
• GPS time – Approximate UTC without leap seconds since 6 January 1980 – Monitored and compared with UTC(USNO) – Offset from UTC is contained in GPS broadcast message • GLONASS time – Approximates UTC(SU) plus 3 hours to within 1 millisecond – Implements leap seconds
28 Sources: www.nist.gov www.navipedia.net
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Let’s look at Time Scales Satellite System Time
• GALILEO time – Synchronised with TAI to within 50 nanoseconds – No leap seconds are implemented – Start epoch is 0 h UTC on 22 August 1999 • Beidou time – Synchronised with UTC to within 100 nanoseconds – No leap seconds are implemented – Start epoch is 0 h UTC on 1 January 2006 •QZSS – Time scale follows GPS time
29 Sources: www.navipedia.net qzss.go.jp directory.eoportal.org
The BIPM’s role
• The Metre Convention • Collecting data • Generation of time scales • Performance of time scales • Rapid UTC
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The BIPM’s role The Metre convention
31 Sources: www.bipm.org en.wikipedia.org
The BIPM’s role The Metre convention
• The Metre Convention created the International Bureau of Weights and Measures (BIPM) • It acts under authority of the General Conference on Weights and Measures (CGPM) • It is supervised by the International Committee of Weights and Measures (CIPM) • It was signed in Paris in 1875 by representatives of 17 nations • The Convention, slightly modified in 1921, is the basis of agreements on units of measurement • The BIPM now has 58 member states
31 Sources: www.bipm.org en.wikipedia.org
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The BIPM’s role The Metre convention
31 Sources: www.bipm.org en.wikipedia.org
The BIPM’s role Collecting data
• BIPM collects data – From more than 500 atomic clocks – Operated by more than 70 timing centres – Maintaining a local UTC, UTC(k) • Data is UTC(k) – Clock – At 5 day intervals – For MJDs ending in 4 and 9 – At 0 h UTC
32 Source: http://www.bipm.org/.../7_establishment_TAR15.pdf
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The BIPM’s role Laboratories and clocks contributing to UTC in July 2016
Clock types PFS/other
H maser
Cs
Clock weights PFS/other
Cs
H maser
33 Slide provided by Dr Michael Wouters
The BIPM’s role Generation of time scales
• An iterative algorithm produces a free atomic time scale, EAL – It is a weighted average of the clock readings – Treats one month of data – Weights and clock frequency predictions are chosen to optimise long term stability – No attempt is made to ensure conformity with the SI second • The scale interval (an EAL second) is evaluated by comparison to – Primary frequency standards – Secondary standards (with corrections)
34 Source: http://www.bipm.org/.../7_establishment_TAR15.pdf
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The BIPM’s role Generation of time scales
• International Atomic Time (TAI) is derived from EAL – By adding a linear function with appropriate slope – Ensures the accuracy of the TAI scale interval (a second) • The frequency offset between TAI and EAL is changed when necessary to maintain accuracy – Changes are same order as frequency fluctuations in instability of EAL – Known as “steering of TAI”
35 Source: http://www.bipm.org/.../7_establishment_TAR15.pdf
The BIPM’s role Generation of time scales
36 Source: www.bipm.org/.../Time_annual_report_2015.pdf
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The BIPM’s role Current TWSTFT links
SP NPL VSLPTBAOS OP CH NIST IT NIM USNO KRIS ROA NICT
KPGO (Hawaii) TL
AMC-1 T-11N Eutelsat-172A KPGO
37 Slide provided by Dr Michael Wouters
The BIPM’s role Performance of time scales
38 Source: www.bipm.org/.../Time_annual_report_2015.pdf
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The BIPM’s role Performance of time scales
38 Source: www.bipm.org/.../Time_annual_report_2015.pdf
The BIPM’s role Performance of time scales
39 Source: www.bipm.org/.../Time_annual_report_2015.pdf
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The BIPM’s role Performance of time scales
40 Source: www.bipm.org/.../Time_annual_report_2015.pdf
The BIPM’s role Performance of time scales
41 Source: www.bipm.org/.../Time_annual_report_2015.pdf
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The BIPM’s role Performance of time scales
42 Source: www.bipm.org/.../Time_annual_report_2015.pdf
The BIPM’s role Aside: Rapid UTC (UTCr)
• UTCr allows laboratories to monitor steering of clocks – Shorter time intervals than Circular T • A subset of laboratories contribute data – Voluntary contributions • Comparison with UTC – Its offset to UTC is less than 2 ns
43 Sources: www.bipm.org ftp2.bipm.org
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The BIPM’s role Aside: Rapid UTC (UTCr)
44 Sources: www.bipm.org ftp2.bipm.org
The BIPM’s role Aside: Rapid UTC (UTCr)
45 Sources: www.bipm.org ftp2.bipm.org
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Our role
• Submission of data • Ensuring delays are accurate • Calibration of receivers • An example
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Our role Submission of data
• BIPM needs data of UTC(k) against GPS • We send daily GPS common view data to the BIPM • We measure all of our other clocks against UTC(AUS) – 3 local clocks measured against master clock – 1 remote clock compared using GPS common view – Reduced to 5 day averages – Sent to BIPM once a month
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Our role Submission of data
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Our role Submission of data
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Our role Submission of data
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Our role Submission of data
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Our role Submission of data
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Our role Ensure delays are accurate
Antenna Delay Cs Clock Antenna 1PPS 10MHz Cable Delay Cs Clock 1PPS Delay GPS Rx 1 Ext Ref 1PPS 2 Timer / Counter GPS Receiver GPS Receiver 1PPS Delay Internal Delay
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Our role Calibration of receivers
• Calibration of receivers • BIPM calibration receivers are sent to group 1 laboratories – In APMP •TL •NICT •NIM • Regional calibrations link laboratories in RMOs • This exercise is part of such a regional calibration
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Our role Calibration of receivers
54 Source: http://www.bipm.org/jsp/en/TimeCalibrations.jsp
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Our role Calibration of receivers
55 Source: 1001-2014_GPSP3_Initial-Group1-trip.pdf
Our role An example: From raw data to Bulletin T
GPS observations Timing data
Processing software
cggtts file
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Our role An example: From raw data to Bulletin T
GPS observations Timing data
Processing software
cggtts file
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Our role An example: From raw data to Bulletin T
Processing software
cggtts file
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Our role An example: From raw data to Bulletin T
cggtts file (daily) Clocks file (monthly)
BIPM
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Our role An example: From raw data to Bulletin T
• BIPM uses daily cggtts files to generate ppp files • These are used with clock files from all contributing time centres to generate time scales • Then UTC – UTC(k) are determined and published in Circular T
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Our role An example: From raw data to Bulletin T
ppp file
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Our role An example: From raw data to Bulletin T
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Questions
Department of Industry, Innovation and Science | National Measurement Institute 36 Bradfield Rd PO Box 264 West Lindfield Lindfield NSW 2070 NSW 2070 Telephone +61 2 8467 3543 [email protected]
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