Training 2012 - Clock data and UTC(k) - BIPM

Bureau International des Poids et Mesures 

Laboratories clock data and UTC(k): 

how do they enter in UTC 


G. Panfilo 

BIPM, Sèvres, 11 September 2012 

1

UTC, UTCr and UTC(k) 

Clock data format 


Outline 

The use of the clock and time and transfer data 

Steps in clock data 

Weight of the clocks 

Rates of the clocks with respect to TAI 

Problems with clock data in UTC and UTCr calculation 

Conclusions and discussion 

2



UTC is calculated by the BIPM in three steps: 

1) EAL is computed as a weighted average of about 

400 atomic clocks spread world-wide 

2) The frequency of EAL is steered to maintain 

agreement with the definition of the SI second, and 

the resulting time scale is TAI 

3) UTC is obtained with the addiction of leap seconds 

to maintain agreement with the time derived from 

the rotation of the Earth 

3



UTCr is calculated by the BIPM in two steps: 

1) A free time scale is computed as a weighted average 

of about 200 atomic clocks spread world-wide (a 

similar UTC algorithm is used) 

2) A steering algorithm is used to maintain UTC close 

to UTCr 

UTCr is a pilot project and the algorithm is under 

study. 

4



UTC(k) – national realization of UTC. 

UTC(k) can be generated in different way: 

1) Single clock 

2) Single clock corrected by the Micro Phase Stepper 

(MPS) 

3) An ensemble of atomic clocks (time scales) 

5

Clock data Format fo UTC 

For UTC generation, monthly files are requested reporting 

clock data at 5 days interval. The time and frequency steps 

are reported in the same table. 

MJD Lab. Code 


TA Code Clock Code 

Time and frequency steps 

UTC(Lab)-Clock 

6

MJD 

Clock data format for UTCr 

For UTCr generation, daily files are requested 

reporting clock data at 1 day interval. The time and 

frequency steps are reported in the same file. 

The file format is the same of the clock participating to 

UTC. 

Lab. Code 


Clock Code UTC(Lab)-Clock 

Time and frequency steps 

7

UTCr (weekly) 

Circular T (monthly) 

BIPM 

Clock weights 

Clock rates 


Data exchange 

Internet (FTP) 

Clock data 

Time transfer data 

Time and frequency 

laboratories and 

Observatories 

8

Clock data in UTC generation 

To generate UTC the clock data and the time link data are 

used. 


k (2 clocks clk1,clk2) 

Ex: 3 labs named p (2 clocks clk3,clk4) - PIVOT 

k 

clk1 clk2 

m (1 clock clk5) 

p 

clk3 clk4 

m 

clk5 

9

EX: data incoming and outcoming 

Clock data: 

UTC(k)-clk 1 

UTC(k)-clk 2 

UTC(p)-clk 3 

UTC(p)-clk 4 

UTC(m)-clk 5 

Time link data: 

UTC(k)-UTC(p) 

UTC(m)-UTC(p) 


Circular T: 

UTC-UTC(k) 

UTC-UTC(p) 

UTC-UTC(m) 

Clock weights 

Clock rates 

Main Product 

Supplementary 

information 

10


Clock differences 

To obtain EAL-clk1,EAL-clk2 etc... we need the 

difference between the clocks. 

p 

k 

clk1 clk2 

Same laboratory: 

clk3 clk4 

clk1-clk2 =(UTC(k)-clk2)- (UTC(k)-clk1) Different laboratories: 

m 

clk5 

clk 1-clk 3 =(UTC(p)-clk 3)- (UTC(k)-clk 1)+(UTC(k)-UTC(p)) 

clk 1-clk 5 =(UTC(m)-clk 5)- (UTC(k)-clk 1)+(UTC(k)-UTC(p))-(UTC(m)-UTC(p)) 

12


Time and Frequency Steps 

An important role in UTC calculation is the time and 

frequency steps affecting UTC(k) or the clocks. 

The time and frequency steps affecting the clocks can 

affect UTC. 

Possible scenarios are possible: 

1) Time or frequency steps in UTC(k) 

2) Time and frequency steps in a single atomic clock 

3) Time and frequency steps in all the clocks 

The BIPM requires that time and frequency steps 

are reported at the end of data clock file 

14

Effect of clock steps in UTC 

UTC is affected by the time and frequency steps of the 

atomic clocks and not declared by the laboratories. 

Nanoseconds 

8 

6 

4 

2 

0 

-2 

-4 

0 200 400 600 800 1000 

Time 

If the laboratory declares steps in the clocks the BIPM: 

1) Correct the past frequency data for the weigths 

2) Correct the past clock data to have continous data 

without steps 


Atomic clocks with 

time step 

Resulting time scale 

with time step 

Atomic clocks 

15

Nanoseconds 

500 

400 

300 

200 

100 

0 

1) Example of time step in UTC(k) 

Time Link 

UTC(INPL)-UTC(PTB) 

-100 

55980 55985 55990 55995 56000 

MJD 

56005 56010 56015 56020 


UTC(INPL) is generated by the 

caesium clock 1352480 


The declared time step 

55999.46 1352480 -500.0 0.000 INPL 10061 

Time step 

16

Nanoseconds 

2) Example of a time step in all the clocks 

400 

350 

300 

250 

200 

Time Link 

UTC(IPQ)-UTC(PTB) 

150 

55615 55620 55625 55630 55635 

MJD 

55640 55645 55650 55655 


UTC(IPQ) is generated by the 

caesium clock +MPS 



55645.53 1352169 -200.0 0.000 IPQ 10070 

55645.53 1351797 -200.0 0.000 IPQ 10070 

55645.53 1352012 -200.0 0.000 IPQ 10070 

17

Nanoseconds 

-200 

-210 

-220 

-230 

-240 

-250 

-260 

-270 

3) Example of time steps in a single clock 

Time Link 

UTC(HKO)-UTC(PTB) 

-280 

56040 56045 56050 56055 56060 

MJD 

56065 56070 56075 56080 


UTC(HKO) is generated by the 

caesium clock 1351893 



UTC(HKO) is not affected by time steps 

18


Weight of the clocks 

At each clock is assigned a weight to contribute to UTC. 

The weight attributed to a clock reflects its long-term 

stability. The clocks with deterministic signatures 

like frequency drift or aging are de-weighted and 

considered “bad” clocks. 

In the time scale algorithms clock weights are generally 

chosen as the reciprocals of a statistical quantity 

which characterizes their frequency stability, such as 

a frequency variance (classical variance, Allan 

variance....) 

19

i � N 

� 

i�1 


2 

i 

2 

i 

Check of the data clock 

The weight attributed to clock H i is the reciprocal of the individual 

classical variance σ i 2 

� 

1 

� 

1 

� 

Upper Limit � �MAX 

� 

Two particular situations are checked: 

1. Clock H i shows abnormal behaviour 

2. The weight is bigger then the upper limit fixed to avoid that a 

clock has a predominant role. 

A 

N 

A=2.5 empirical constant 

The weight attributed to clock H i is computed from the frequencies 

of the clock, relative to EAL, estimated over the corrent 30-day 

interval and over the past 11 consecutive 30-day period. The 

weight determination thus uses clock measurement covering one 

year. 

20


Weights of the clocks 

Each month the weights of the 

clocks participating to UTC 

calculation are published on 

the ftp. 

21


Rates of the clocks 

Each month the clock rates are 

calculated with respect to TAI 

for eac clock participating to 

UTC calculation. 

This information ca be used 

by the laboratories to steer 

their clocks to approach 

UTC. 

22

Data clock problems: 


Problems with clock data 

1) Use of commas instead of points 

2) Use of tabulations instead of space 

3) The data format longer than requested 

4) Wrong TA and clock codes 

5) The sign of the time or frequency steps 

6) The time or frequency steps are not reported at the 

end of the clock data file 

7) The format of the time and frequency steps should 

be respected 

8) ......... 

23


The time step sign 

Sometimes the sign reported for the steps is not correct. 

The BIPM requires to evaluate the sign of a step: 

NEW(clockvalue) - OLD(clockvalue) 



NEW(UTC(k)-clock))-OLD(UTC(k)-clock)) = -200.3-855.7 -1050 ns 

The time step has the opposite sign = 1050 ns 

24

Problems with data clocks in UTCr 

The laboratories report data clocks for UTCr and UTC. 

Sometimes the data (of the same clock) reported for 

UTCr are different from the data reported for UTC. 

After UTC calculation UTC-clock data are used in UTCr 

calculation to maintain UTCr close to UTC. 

The clock values used for UTC and UTCr should be 

consistent. 


25

Discussion and conclusions - 1 

The calculation of UTC and UTCr rely on the clock data 

comparison. 

More then 400 atomic clocks are used for UTC and 

more then 200 for UTCr. 

The data clocks are checked and verified before 

entering in the calculation. 

The atomic clocks are weighted with respect to their 

long term stability. 


26

Discussion and conclusions - 2 

Time and frequency steps are particularly important 

due to their impact on UTC quality. 

Each time and frequency steps is checked and verified. 

The BIPM published each month: 

• Circular T 

• The weights of the atomic clock 

• The rates of the atomic clocks with respect to TAI. 


27

Training 2012 - Clock data and UTC(k) - BIPM

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