Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/research/pulsar/orange10/pdf/DingChenOrange2010.pdf Дата изменения: Wed Sep 29 05:40:11 2010 Дата индексирования: Sun Feb 13 23:18:07 2011 Кодировка: Поисковые слова: ccd

Time Standard and Ensemble Pulsar Time Scale
Ding Chen, George & Dick, Bill
20100928, Orange

National Time Service Center. CAS





24 Cesium clocks 4 Hydrogen clocks


The Stabilities of Frequency Scales
-9

different

-10

z art Qu m d iu i ub R m siu Ce

Log (y())

-11

-12

-13

-14

-15

-16 -3.0

Hydrogen Maser
-2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Log (), seconds

1 day

1 month


Unit of time (SI) :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 Cesium 133 atom.


EAL and TAI
TAI calculation is done each month BIPM firstly computes a free atomic scale: EAL, from around 400 clocks all over the world, to get the optimal high 1-month stability. --AlGOS: weighted average algorithm. --An average of N identical clocks may be N more stable than individual one clock -- Time comparison to same laboratory (PTB) of different laboratories (time transfer GPS CV) -- EAL is stable but may have some values shift to SI second. Every month, primary frequency standard (PFS) are used to estimate the TAI from the frequency correction of EAL in order to be more closer to SI second.


NICT

NIST

USNO

NTSC

The weights of different Lab(k) in TAI


Time Links and Comparation


TAI and UTC: leap second
Coordinated Universal Time (UTC), maintained by the BIPM, is the time scale that forms the basis for the coordinated dissemination of standard frequencies and time signals. The UTC scale is adjusted by the insertion of leap seconds to ensure approximate agreement with the time derived from the rotation of the Earth. Physical realizations of UTC ­ named UTC (k) ­ are maintained in national metrology institutes or observatories contributing with their clock data to the BIPM. The dates of leap seconds of UTC are decided and announced by the International Earth Rotation and Reference Systems Service (IERS), which is responsible for the determination of Earth rotation parameters and the maintenance of the related celestial and terrestrial reference systems.


TT(BIPM)
TAI is computed in real time and will not be updated even an error is discovered, so it is not optimal. Therefore the BIPM computes a post-processed time scale TT(BIPM) Each new version TT(BIPMxx) updates and replaces the previous one. ­ Post-processed using all available PFS data. ­ Complete re-processing starting 1993 (change of algorithm). ­ Monthly estimation of the data are smoothed and integrated to obtain TT(BIPMxx). Significant and time-varying frequency difference between TAI and TT(BIPM) integrates to more than 100 ns/yr, so TAI should not been used as a long-term reference.


10

-13

Allan Deviation: TT-BIPM Scale

10

-14

y() (Hz)
10
-15

10

-16

10

0

(year)


Ensemble time scale: aiming at 10-16 and beyond
More clocks for time keeper, a 10-fold increase in clock number would be needed to reach 10-16 . New clock technologies: Cs, Rb fountain, Light clock :100-200 clocks, each with 5в10-15 stability @ 1 month provide 3-4в10-16 for the ensemble time scale) Long term stability (more than 3 months or one year) ~ 10-16 or beyond : Pulsar Time Scale is a good candidate.


Ensemble Pulsar Time Scale
The arrival time of each Pulsar `k' which is its date in PTK, to be actually measured based on atomic clock, which is date in TAI. So we can obtain Rk=TAI-PTk. which is the timing residuals. Ensemble Pulsar Time Scale(EPT) :

(TAI - EPT ) =
k = (
2 k


k =1

N

^ k (TAI - PTk )

+

2

z ,k

( T ))

-2

PPTA is the best project to establish the new independent ensemble time scale which will take contribution to both GW detection and BIPM.


The Time span of the Observations 20 18 16 14 J0437-4715 J0613-0200 J0711-6830 J1022+1001 J1024-0719 J1045-4509 J1600-3053 J1603-7202 12 10 8 6 4 2 0 J1643-1224 J1713+0747 J1730-2304 J1732+5049 J1744-1134 J1824-2452 J1857+0943 J1909-3744 J1939+2134 J2124-3358 J2129-5721 J2145-0750 5 5.1 5.2 MJD 5.3 5.4 5.5 x 10
4

Individual Observations


1.5

Jump Correction for number variation
2 x 10
-6

The PTA Time Scale(dataset2)

TT-P1 TT-P2 TT-P3 TT-PTA without correction

4

1

The Observation Time Span

3.5

J0711-6830

0.5

TT-PTA

3

2.5

Individual Pulsar

0

J0437-4715

2

-0.5
1.5 J1909-3744

1 -1

0.5

J1713+0747

-1.5 -6000 0
5.1

-5000
tn

-4000
bin

5.3

-3000 MJD

-2000
5.4

-1000
5.5

0

(10 )MJD

4

( TAI - EPT

new

)

t = [ tn ( j ), tn ( j + 1 )]

= ( TAI - EPT )

t = [ tn ( j ), tn ( j + 1 )]

+ ( EPT

tn ( j ) - bin

- EPT

tn ( j )

)


Jump Correction of Different Backends

JUMP ­f e-06 MULTI_fptm -3.54 H-OH_cpsr2n 2.98 H-OH_cpsr2m 0.20 Ref:MULTI_cpsr2m 20cm_fb -24.4 MULTI_cpsr2n 1.28



Statistics for Clock Stabilities
Allan Deviation: (Allan, D 1987) , y n = ( xn - xn-1 ) SigmaZ: (Matsakis, Taylor et al. 1997) Fittng the data by X(R)=c0+c1(R-R0)+c2(RR0)2+c3(R-R0)3 with minimizing [(R - X ( R ))
i i

1 = ( yn - y 2
2 y

n -1

)

2

/ i ]2

z ( ) =



2

25

c

2 1/ 2 , 3

=2-nвT, n=1,2,3,4,5


The Long Term Stability of The Time Scale
-12

10

10

-13

10

-14

10

-15

J0437 J0613 J0711 J1022 J1024 J1045 J1600 J1603 J1643 J1713 J1730 J1732 J1744 J1824 J1857 J1909 J2124 J2129 J2145 TAI-ePT TAI-NTSC TAI-PTB

Stability z()

10

-16

10

0

10 (years)

1


New Clock Reference for Pulsar Timing
L: TAI-TT(BIPM2010); R: TAI-TT(ePT)

JUMP ­f e-07 MJD(20cm_fptm) H-OH_cpsr2m 0.428 H-OH_cpsr2n 1.143 Ref:MULTI_cpsr2n MULTI_fptm -5.819 20cm_fb -40 MULTI_cpsr2m -0.255


Conclusion and Future work
The long term stability of ensemble atomic time scale is limited @ 10-15 The high-precision pulsar timing project like PPTA makes it possible to establish the new independent pulsar time scale. Ensemble Pulsar Time Scale is more stable than the AT of some clock laboratory on long term scale. More algorithms are expected to derive the intrinsic ensemble pulsar time scale. The new interesting and useful applications of EPT like corrected clock for GW detection and new independent time scale for BIPM are on the way.


Thank You!