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GPS Time Transfer and Time Scales: What Does the BIPM Do with My Data?

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GPS Time Transfer and Time Scales: What Does the BIPM Do with My Data? 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 2 1 2016‐09‐28 Quick overview of GPS time transfer • Comparing two local clocks • Comparing with a remote clock • Common view 3 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 4 2 2016‐09‐28 Quick overview of GPS time transfer Comparing with a remote clock Our Master Unknown Clock Clock Counter / Timer 5 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 3 2016‐09‐28 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) 7 Let’s look at Time Scales • A bit of history • Universal Time • Atomic Time • Satellite System Time 8 4 2016‐09‐28 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 5 2016‐09‐28 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 6 2016‐09‐28 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 7 2016‐09‐28 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 8 2016‐09‐28 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 10 9 2016‐09‐28 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; 11 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) 12 10 2016‐09‐28 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 11 2016‐09‐28 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 12 2016‐09‐28 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 18 13 2016‐09‐28 Let’s look at Time Scales Atomic Time • The pioneering work was quickly followed by improved clocks • NBS 2 • NBS 3 NBS2 NBS3 18 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 14 2016‐09‐28 Let’s look at Time Scales Atomic Time • NBS 5 NBS5 19 Let’s look at Time Scales Atomic Time • NBS 5 • NBS 6 NBS5 NBS6 19 15 2016‐09‐28 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 16 2016‐09‐28 Let’s look at Time Scales Atomic Time Other countries 20 Let’s look at Time Scales Atomic Time Other countries 20 17 2016‐09‐28 Let’s look at Time Scales Atomic Time Other countries 20 Let’s look at Time Scales Atomic Time Other countries 20 18 2016‐09‐28 Let’s look at Time Scales Atomic Time Other countries SYRTE FO1 20 Let’s look at Time Scales Atomic Time Other countries SYRTE FO1 SYRTE FO2 20 19 2016‐09‐28 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 20 2016‐09‐28 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! 22 Let’s look at Time Scales Atomic Time: Commercial clocks HP 5060A 23 21 2016‐09‐28 Let’s look at Time Scales Atomic Time: Commercial clocks HP 5060A HP 5061A 23 Let’s look at Time Scales Atomic Time: Commercial clocks HP 5060A HP 5061A HP 5071A 23 22 2016‐09‐28 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 23 2016‐09‐28 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 24 2016‐09‐28 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 25 Let’s look at Time Scales Leap seconds: UT1 and UTC offsets 26 Sources: USNO and IERS websites 25 2016‐09‐28 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 26 2016‐09‐28 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/..
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