National Time Service Center. CASNational Time Service Center. CASNational Time Service Center. CASNational Time Service Center. CAS

Time Standard and Ensemble

Pulsar Time Scale

Time Standard and Ensemble

Pulsar Time Scale

Ding Chen, George & Bill, Dick, PPTA teamDing Chen, George & Bill, Dick, PPTA team

20112011 年年 55月月 99日日 , Beijing, Beijing

Outline

Time Standard TAI and UTC Pulsar Time Scale

algorithm

simulation

real data Problem and future improvements Summary

Unit of time (SI)Unit of time (SI) :The second is the duration of 9 192 631 770 The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 the two hyperfine levels of the ground state of the Cesium 133 atom.atom.

Lo

g ( y

())

Log (), seconds

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

1 day 1 month

-9

-10

-11

-12

-13

-14

-15

-16Hydrogen Maser

Rubidium

Quartz

Cesium

The Stabilities of different The Stabilities of different Frequency ScalesFrequency Scales

Ensemble Atomic Time Scale 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 more stable than indiv

idual one clock -- Time comparison to same laboratory (PTB) of different laboratori

es (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 e

stimate the TAI from the frequency correction of EAL in order to be more closer to SI second.

N

The weights of different Lab(k) iThe weights of different Lab(k) i

n TAIn TAI

NTSCUSNO

NICT

NIST

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 a

n 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 pr

evious one. – Post-processed using all available PFS data. – Complete re-processing starting 1993 (change of algorit

hm). – Monthly estimation of the data are smoothed and integra

ted 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.

Ensemble time scale: aiming at 10-

16 and beyond More clocks for time keeper, a 100-fold increase in

clock number would be needed to reach 10-16 . New clock technologies: Cs, Rb fountain, Light clo

ck :100-200 clocks, each with ≈ 5×10-15 stability @ 1 month provide 3-4×10-16 for the ensemble time scale)

Long term stability (more than 3 months or one year) ~ 10-16 or beyond : Combined with a independent more stable time scale (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) :

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

N

kkk PTTAIEPTTAI

1

)(ˆ)( 2

,22 ))(( Tkzkk

Statistics for Clock Stabilities

Allan Deviation: (Allan, D 1987) , SigmaZ: (Matsakis, Taylor et al. 1997)

Fittng the data by X(R)=c0+c1(R-R0)+c2(R-R0)2+

c3(R-R0)3 with minimizing , τ=2-n×T, n=1,2,3,4,5

2]/))([( iii RXR

2/123

2

52)( cz

21

2 )(2

1 nny yy

)( 1 nnn xxy

The observatory clockParkes hydrogen maser -> GPS -> TT(TAI)

• Parkes-GPS • Effelsberg-GPS

What happens if irregularities exist in time-scale? TT(TAI)-TT(BIPM2010)(George’s slide)

New TechniqueDefine clock function to be simple Fourier expansion:

(note: can use other functional forms if needed)

Carry out a standard least-squares fit of pulsar timing model parameters + f(t) as usual, except:

simultaneously fit to multiple pulsarsuse measurement of the covariance in the residuals for a

given pulsar as part of the least-squares-fit fit (to deal with timing noise)

f (t) Ak cos(k 0t)Bk sin(k 0t)

P

est (MTC 1M) 1MTC 1R

Final result (real data)PT(PPTA)-TT(TAI) and TT(BIPM2010)-TT(TAI)

1s

New Clock Reference for Pulsar TimingL: 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

Future workWe will produce the first detailed ensemble pulsar time scale based on international PTA

Combining our data with observations from Europe, USA and China will allow us to make a significant improvement in our time scale

We will publish this new time scale and hope that BIPM will use the pulsar timescale to check/correct long-term timing irregularities

CSIRO. Gravitational wave detection

Some suspicions

Consistency Precision Validity Application

Summary The long term stability of ensemble atomic time scale is limited @ 10-15

Pulsars, being macroscopic astrophysical objects, are completely unrelated to atomic clocks and are governed largely by unrelated physical principles. The high-precision pulsar timing project like PPTA makes it possible to establish the new independent pulsar time scale.

A time standard based upon the rotation of pulsars should be independent of one based on atomic clocks.

The rotation of millisecond pulsars is expected to remain stable for billions of years, so they can provide a very long-term standard. 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!