Post on 23-Sep-2020
Bureau International des Poids et Mesures 1
Improvements in ALGOS
G. Panfilo
Bureau International des Poids et Mesures
CCTF, Sèvres, 13-14 September 2012
Bureau International des Poids et Mesures 2
• Clock prediction algorithm
• Quadratic model
• TT as frequency reference for drift evaluation
• Application and effect of the new prediction algorithm :
• EAL frequency drift is almost completely removed
• Effect on TAI and on the weights
• A new model for the weighting procedure
• The prediction is taken into account
• Effect on the UTC stability and on the weight
• Conclusion and discussion
Outline
Bureau International des Poids et Mesures 3 3
EAL prediction algorithm
Until July 2011 the linear prediction had been used to describe
the clock behaviour; the frequecy of the clocks is considered
constant during the calculation period; the frequency drift (or the
aging) is not included in the model.
UTC is calculated with more than 400 clocks of which:
- Caesium clocks 5071 (high performance tube): ~ 270
- H-masers Symmetricom/Sigma tau : more then 50
The ensemble of clocks shows very similar deterministic
signatures (frequency drift or aging).
As a consequence of that EAL shows a frequency drift respect to
TT of about:
-1.3x10-17/day
Bureau International des Poids et Mesures
After test and simulations for a long period (5 years), after
publications and presentations[1,2,3] the new prediction
algorithm for EAL has been validated and implemented.
From August 2011 a quadratic prediction is officially used to
calculate UTC; the frequency drift is considered in the
model for all kind of clocks.
[1] G. Panfilo and E.F. Arias. ”Studies and possible improvements on EAL algorithm”. UFFC, Vol. 57, No.1,
January 2010, pp. 154-160.
[2] G. Panfilo, A. Harmegnies, L. Tisserand. ”A new prediction algorithm for the generation of International
Atomic Time”. Metrologia, 2012, 49, n°1, 49-56.
[3] G. Panfilo, A. Harmegnies, L. Tisserand. Report to CCTF. A new prediction algorithm for EAL.
4
EAL new prediction algorithm
The frequency drift affecting EAL is almost completely
removed.
Bureau International des Poids et Mesures 5
The prediction algorithm for EAL
The main ideas of the new prediction algorithm are:
1. The use of the quadratic model to describe the frequency drift
2. The use of TT (Terrestrial Time, a time scale optimized for
frequency accuracy) as frequency reference to estimate the
frequency drift
2,1,,,
' ˆ2
1ˆ2
1ˆˆ1 kIipkkkIikIipIii ttCttttCttBath
kkkk
Bureau International des Poids et Mesures
Simulation results
6
52000 53000 54000 55000
66
67
68
69
70
71
y(EAL)-y(TT)
MJD
No
rma
lize
d F
req
ue
nc
y 1
0-1
4
Linear Prediction
Quad. Pred. 2008
Quad. Pred. 2006
Two tests were performed on data starting in 2006 and in
2008 until 2010 to validate the new prediction algorithm.
Bureau International des Poids et Mesures
Results: y(EAL)-y(PFS)
7
Starting from August 2011 the new algorithm is officially used in
UTC calculation:
Period of application of the new algorithm
The frequency drift
affecting EAL is almost
completely removed
55000 5600064.5
65
65.5
66
66.5
67
67.5
68
f(EAL)-f(PFS)
MJD
No
rmalized
Fre
qu
en
cy 1
0-14
Bureau International des Poids et Mesures
y(EAL)-y(PFS)
8
54000 54500 55000 55500 56000 5650064.5
65
65.5
66
66.5
67
67.5
68
68.5f(EAL)-f(PFS) only fountains
MJD
No
rmalized
Fre
qu
en
cy 1
0-14
NIST-F1
PTB-CSF1
PTB-CSF2
SYRTE-FO1
SYRTE-FO2
SYRTE-FOM
IT-CSF1
NPL-CsF1NPL-CsF2
NICT-CsF1
NMIJ-F1
Bureau International des Poids et Mesures
55400 55500 55600 55700 55800 55900 56000 56100 562000
1
2
3
4
5
6
7
No
rmalized
Fre
qu
en
cy 1
0-15
MJD
The effect on TAI
9
In the plot the fractional deviation d of the scale interval of TAI from
that of TT based on all available PFS measurements is reported .
Period of application of the new algorithm
The value of d is
decreasing to zero
with some noise
Bureau International des Poids et Mesures
............f(TAI)-f(PFS)
10
Period of
application of the
new algorithm
The value of the difference f(TAI)-f(PFS) should be close to zero. Starting
from the application of the new prediction algorithm this value is
approaching zero with some noise.
54000 55000 56000-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0f(TAI)-f(PFS)
MJD
No
rma
liz
ed
Fre
qu
en
cy
10-1
4
Bureau International des Poids et Mesures
54000 54500 55000 55500 56000 565000
10
20
30
40
50
60
70
80
90
100
To
tal
we
igh
t in
pe
rce
nta
ge
te
rms
MJD
Cs-clock
H-maser
54000 54500 55000 55500 56000 565000
10
20
30
40
50
60
MJD
Nu
mb
er
of
H-m
asers
wit
h m
ax. w
eig
ht
Cs clock
H-maser
The effect on the weights
11
The weighting algorithm has not yet been changed. The new
prediction algorithm has no effect on the weights.
Period of application of the new algorithm
Bureau International des Poids et Mesures
Weight
12 12
EAL: Weighting Algorithm
In the current weighting algorithm 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.
The H-masers have a small weight due to frequency drift
affecting the behaviour.
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....)
Bureau International des Poids et Mesures
A new proposed weight algorithm
After the implementation of the new prediction algorithm the
deterministic signatures are eliminated or minimized in the
time scale.
The main idea of the new weighting algorithm is that a good clock
is not a stable clock but a predictable clock[1].
In this new version of the weighting algorithm 1 year of the
difference between the predicted and real frequencies of the
atomic clocks is evaluated. A filter is used to give a bigger role
to the new measurement with respect to old ones.
[1] Levine J. “Introduction to time and frequency metrology”. Review of Scientific Instruments, Vol. 70, No.
6, pp.2567-2596, 1999.
13
Bureau International des Poids et Mesures
Effect of the weight distribution – H-maser
14
A test was performed using 6 years of data: 2006-2011
By applying the new weighting algorithm for the H-masers:
Total weight: ~ 15% ~ 30%
Number of clocks at maximum weight: ~ 10 ~30
53500 54000 54500 55000 55500 560005
10
15
20
25
30
35
40
To
tal w
eig
ht
in p
erc
en
tag
e t
erm
s
MJD
H-ms orig weight
H-ms new weight
53500 54000 54500 55000 55500 560005
10
15
20
25
30
35
40
45
Nu
mb
er
of
H-m
asers
wit
h m
ax. w
eig
ht
MJD
H-ms orig weight
H-ms new weight
Bureau International des Poids et Mesures
Effect on the weight distribution – Cs clocks
15
By applying the new weighting algorithm for the caesium clock:
Total weight: ~ 85% ~ 70%
Number of clocks at maximum weight: ~ 55 ~20
53500 54000 54500 55000 55500 5600055
60
65
70
75
80
85
90
95
To
tal w
eig
ht
in p
erc
en
tag
e t
erm
s
MJD
Cs orig weight
Cs new weight
53500 54000 54500 55000 55500 560000
10
20
30
40
50
60
Nu
mb
er
of
ce
siu
m c
lock
s w
ith
ma
x. w
eig
ht
MJD
Cs orig weight
Cs new weight
Bureau International des Poids et Mesures
Results and effect on EAL
16
A test was performed using 6 years of data: 2006-2011 and by
using the quadratic prediction algorithm.
52000 53000 54000 55000
67
67.5
68
68.5
69
69.5
70
70.5
y(EAL)-y(TT)
MJD
No
rma
lize
d F
req
ue
nc
y 1
0-1
4
original weight
new weight
Bureau International des Poids et Mesures
Stabilty of f(EAL)-f(TT)
17
By using the new weighting algorithm the long term stability of EAL is
not affected by the frequency drift of the H-Masers.
Due to short term correlation between EAL and TT we have only long
term information from this analysis.
105
106
107
108
10-17
10-16
10-15
10-14
Frequency Stability
Ov
erl
ap
pin
g A
lla
n D
ev
iati
on
Averanging Time, Seconds
Linear Prediction
original weight
new weight
Bureau International des Poids et Mesures
Stability of EAL and TT vs USNO Rb fountain
18
The data coming from USNO Rb fountain MJD 55634-55924 are
used to evaluate EAL long term stability.
By using the new weighting
algorithm the short and
the long term stability of
EAL is improved.
simulated EAL (new weight)
simulated EAL (old weight)
105
106
107
10-16
10-15
10-14
Frequency Stability
Ov
erl
ap
pin
g A
lla
n D
evia
tio
n
Averanging Time, Seconds
Rb-EAL(new)
Rb-EAL(old)
Sigma/Tau
[d]
5 10 20 40 80
EAL 1.3E-15 8.7E-16 6.6E-16 3.9E-16 2.6E-16
EAL new 1.0E-15 6.9E-16 4.5E-16 2.5E-16 2.0E-16
Bureau International des Poids et Mesures 19
Discussion and Conclusions - 1
Starting from August 2011 the new prediction algorithm has
been officially used for UTC.
The used mathematical model takes in to account the
treatment of the frequency drift.
No change in the weighting strategy has been implemented.
After 1 year of the application of the new algorithm the
positive effects are evident.
The frequency drift of EAL is almost completely removed.
Bureau International des Poids et Mesures
Discussion and Conclusions - 2
The new prediction algorithm has no impact on the weights.
A revision of the weight algorithm has been presented to
increase the role of the H-masers in the time scale
ensemble.
The new weighting algorithm gives a good weight to the H-
masers.
Complementary studies are necessary to validate new
weighting algorithm.
20
Bureau International des Poids et Mesures
In the framework of the Rapid UTC pilot project data clock
and time transfer data are available at 1 day interval.
At 1 day interval the dominant noise is given by the time
transfer system if the the H-masers are correctly used.
In this situation it can be interesting to test the effect of
the new weighting algorithm with 1 day interval data to
observe the short term contribution of the H-masers to the
ensemble.
21
Future developments – Rapid UTC
Bureau International des Poids et Mesures 22