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TAI
B. Guinot
To cite this version:
B. Guinot. STATUS OF THE INTERNATIONAL ATOMIC TIME TAI. Journal de Physique Collo-
ques, 1981, 42 (C8), pp.C8-219-C8-228. �10.1051/jphyscol:1981826�. �jpa-00221721�
JOURNAL DE PHYSIQUE
ColZoque C8, suppliment au n012, Tome 42, dicemhre 1981 page C8-219
STATUS OF THE INTERNATIONAL ATOMIC TIME TAI
B. Guinot
Bureau InternationaZ de ZrHeure, Observatoire de Paris, 62, avenue de 2 rObservatoire, 75014 Paris, France.
Abstract.
-
This paper s u m m a r i z e s the methods for establishing the International Atomic T i m e TAI and s t a t e s i t s stability and accuracy. The various f a c t o r s which l i m i t the quality of TAI a r e considered. Special atten- tion i s given t o s o m e s y s t e m a t i c differences between s o m e of the i n s t r u m e n t s which contribute t o TAI, especially t o the seasonal effects which w e r e r e c e n - t l y found.Introduction.
When the a s t r o n o m e r s w e r e in c h a r g e of determining the t i m e , they enjoyed a privilege : t h e i r clock, the rotation of the E a r t h , w a s unique and available t o everyone. Of c o u r s e , they w e r e faced t o technical and scientific problems in o r d e r to r e a d t h i s clock a t the b e s t , but t h e r e w e r e n o delicate choices. The unique a s t r o n o m i c a l clock had a physical existence, and should be accepted a s i t was, with i t s q u a l i t i e s and defects. In p a r t i c u l a r t h e r e was no r i s k of t i m e drift, o r , in other t e r m s , of frequency e r r o r due t o the processing of the observed data.
On the contrary, a n a t o m i c t i m e s c a l e for worldwide u s e i s based on many i n s t r u m e n t s and for establishing it, one h a s many d e g r e e s of freedom. The cha- r a c t e r i s t i c s of the t i m e s c a l e a r e not imposed by nature ; they a r e , t o a l a r g e extent, linked t o the p m p e r t i e s of experimental data, but they a l s o depend on the appreciation of the u s e r needs.
In t h i s paper, I will d i s c u s s some of the choices m a d e a t the Bureau
International d e 1'Heure (BIH) f o r establishing the International Atomic T i m e TAI.
F o r m of TAI, A c c u r a c y of reading, Availability.
It i s well known that TAI a p p e a r s a s c o r r e c t i o n s t o the readings of some ato- m i c clocks, published by the BIH. The choices m a d e in 1969, and kept since then a r e :
when the clock c o r r e c t i o n s l'TAI-clockll a r e known with the maximum of a c c u r a c y (uncertainties of the o r d e r of a few 0. 1 ps),
.
frequency of t h e s e c o r r e c t i o n s : one e v e r y 10 days,.
frequency of publication : monthly,Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981826
.
delay of sending of t h e publication : f r o m 30 t o 70 d a y s (according t o the date t o which the clock c o r r e c t i o n applies).T h e s e choices a r e explained by the possibility of a n e a s y extrapolation of TAI-clock, allowing t o t i m e a n event in TAI, in r e a l t i m e , with u n c e r t a i n t i e s s m a l - l e r than l p s . It does not s e e m t o e x i s t m o r e stringent n e e d s i n r e a l time. However different r y t h m s could be r e q u i r e d with b e t t e r t i m e comparisons. I s e e no i m p o s s - ibility t o produce TAI daily, f o r instance, using a worldwide computer network ; it i s only a m a t t e r of cost.
The geographical coverage of the a c c e s s t o TAI t o a few 0. 1 p s i s s t i l l l i m i t e d t o c o u n t r i e s b o r d e r i n g the North Atlantic. T h i s limitation i s due t o the coverage of the LORAN-C which r e m a i n s the m a i n s y s t e m f o r long distance clock c o m p a r i s o n s , a f t e r calibration by clock t r a n s p o r t a t i o n . Since 1979, t h e BIH u s e s t h e t r a n s a t l a n - t i c link by the telecommunication s a t e l l i t e SYMPHONIE (1) which i m p r o v e s TAI, but does not extend i t s a c c e s s . T h e l a c k of a c c u r a t e worldwide t i m e c o m p a r i s o n s i s a m a j o r problem in the t i m e metrology.
F r e q u e n c y stability and a c c u r a c y of TAI
The need of ensuring that TAI (and a l s o national o r l a b o r a t o r y t i m e s c a l e s ) be n e v e r i n t e r r u p t e d , r e q u i r e s that it be based on a l a r g e number of devices located in widespread locations. In the p r e s e n t discussion (not f o r g e n e r a l u s e ) , I propose the following terminology.
.
The o r d i n a r y clocks useful f o r TAI, a r e clocks with a good long t e r m s t a b i - l i t y , i. - e . f o r 7: ranging f r o m about 10 to 100 days. The frequency d r i f t s ( o r the deviations with t h e i r modeled values) should be s m a l l : f o r instance, the annual d r i f t should be l e s s than the long t e r m instability. T h e r e i s no r e q u i r e m e n t f o r the a c c u r a c y . In principle, a n y type of clock, even non- a t o m i c , is acceptable ; but, in p r a c t i c e , a l l t h e o r d i n a r y clocks contributing t o TAI a r e i n d u s t r i a l c e s i u m clocks, except one l a b o r a t o r y cesium clock..
F r e q u e n c y s t a n d a r d s will denote p r i m a r y s t a n d a r d s operated in a disconti- nuous mode, which c a n provide a n a c c u r a t e evaluation of the duration of t h es c a l e unit of a t i m e s c a l e , a v e r a g e d o v e r a few days t o a few t e n s of days.
.
P r i m a r v clocks a r e p r i m a r y s t a n d a r d s operated in a continuous o r quasi- continuous mode, so that they provide d i r e c t l y , f o r y e a r s , a t i m e s c a l e with a n a c c u r a t e s c a l e unit, in brief, a n a c c u r a t e t i m e s c a l e .When the t h r e e t y p e s of the above devices co-exist, the derivation of a stable and a c c u r a t e t i m e s c a l e f r o m t h e i r data r e q u i r e s t h r e e s t e p s .
(a) Using the clocks, o r d i n a r y and p r i m a r y , a stable i n t e r m e d i a t e t i m e s c a l e To i s established, with a stability a l g o r i t h m .
(b) Using t h e frequency s t a n d a r d s , including the p r i m a r y clocks considered a s frequency s t a n d a r d s , the duration of the s c a l e unit of To i s evaluated (calibration).
(c) The final t i m e s c a l e T1 i s d e r i v e d f r o m To and the calibrations by applying a s t e e r i n g c o r r e c t i o n which, a t l e a s t , keeps the stability of To, and which gives t o T I the a c c u r a c y of t h e s t a n d a r d s .
In the next p a r a g r a p h s , I will show the application to TAI. But I f i r s t r e c a l l that the establishment of TAI f r o m the data of individual clocks s t a r t e d in 1973.
The s t e e r i n g (c) was introduced in 1977, where we distinguished TAI ( z T I ) f r o m a f r e e t i m e scale denoted EAL ( s To). Between 1973 and 1977, only s t e p (a) was accomplished and TAI 2 EAL.
Stability a l g o r i t h m s - - -
T h e i r main purpose i s
.
to e n s u r e the phase and frequency continuity of the t i m e s c a l e s in spite of changes in the s e t of participating clocks, which have different p h a s e s and r a t e s ,.
to favor the stable clocks by a weighting procedure.However, even when the frequency n o i s e s of the clocks a r e purely random, the a l g o r i t h m s allow indefinitely l a r g e frequency excursions, in the very long t e r m . The steering i s n e c e s s a r y .
Let u s consider the BIH algorithm ALGOS which i s applied t o a l a r g e number of o r d i n a r y clocks (about 60 in 1973, 100 in 1980), and a few p r i m a r y clocks (up to 5 in 1981).
One cannot r e p r e s e n t the p r o p e r t i e s of a random t i m e s e r i e s , even in the c a s e of stationarity, by a single figure. Nevertheless, it i s intuitively c l e a r that a l l the clocks have not the s a m e value for deriving TAI. A s the long t e r m frequen- cy stability of TAI i s e s s e n t i a l , in p a r t i c u l a r for the extrapolation purpose mentio- ned e a r l i e r , the BIH u s e s a weighting based on the sample variance f o r averaging t i m e s of two months with a safeguard against a b n o r m a l behaviour. The weights a r e
re-computed e v e r y two months ; a n upper limitation avoids a feedback effect which would tend t o give a l l the weight to a single clock (Yoshimura (2) has proposed a method of estimating the variance which avoids this feedback).
The e n t r y o r the exit of a clock in ALGOS i s conveniently r e p r e s e n t e d , using null weights. Thus the condition of continuity of TAI and d ( ~ ~ ~ ) / d t in t h e s e c a s e s i s p a r t of a g e n e r a l problem : the continuity in c a s e of change of weights. F o r r e a s o n s of simplicity, t h e s e changes a r e made a t fixed d a t e s , e v e r y two months, and t h e s e d a t e s a l s o limit the sampling intervals f o r estimating the v a r i a n c e s - t h e s e r e s t r i c t i o n s imply some l o s s of information. T h e phase continuity i s e a s i l y ensured. But the frequency continuity r a i s e s a new problem, since the frequency evaluation r e q u i r e s a n averaging. In ALGOS we use a frequency prediction exten- ded over two months, which consists in assuming that the frequency r e m a i n s identical t o the mean frequency of the two preceding months.
Thus, in ALGOS, the two-month interval between fixed d a t e s plays s e v e r a l r o l e s , which i s r a t h e r confusing. In i t s generality, such a n algorithm r e q u i r e s the choice of s e v e r a l p a r a m e t e r s , the m o s t important ones being
.rl
, sample time for the weighting,t 2 , i n t e r v a l of r e c u r r e n c e (possibly variable) between the estimations of weights,
t3 , duration of the frequency predictions.
Generally speaking, the p r o b l e m of the a l g o r i t h m s f o r computing t i m e s c a l e s i s f a r f r o m being s a t i s f a c t o r i l y solved
Evaluation of t h e d u r a t i o n of the t i m e - s c a l e unit . . .
T h e p r o b l e m i s t o evaluate the m e a n frequency of a t i m e s c a l e , during a s e - l e c t e d i n t e r v a l , using s o m e frequency c a l i b r a t i o n s during other i n t e r v a l s . T h e t i m e s c a l e i s a f r e q u e n c y m e m o r y , which, of c o u r s e , b e c o m e s bad when the cali- b r a t i o n s a r e too f a r away. The t h e o r e t i c a l study of t h i s problem i s difficult, n e v e r t h e l e s s possible using s o m e a s s u m p t i o n s ; i t s output i s a f i l t e r , a s e r i e s of weights o r , b e t t e r , of coefficients t o be applied t o the calibrations. In the pioneer work of Yoshimura ( 3 ) , the c a l i b r a t i o n s have a p u r e l y random n o i s e ; the BIH h a s developped a m o r e sophisticated m o d e l (4), a s s u m i n g that, in addition t o random n o i s e , t h e s u c c e s s i v e c a l i b r a t i o n s by the s a m e s t a n d a r d have a common s y s t e m a t i c e r r o r . The BIH f i l t e r gave a n e s t i m a t e of t h e frequency of TAI s i n c e 1970, a n d was then c u r r e n t l y u s e d for s o m e y e a r s , when the c a l i b r a t i o n s of EAL/TAI w e r e s p a r s e . But i t s u s e c e a s e d i n 1980 b e c a u s e a l l the p r i m a r y s t a n d a r d s which can be employed f o r evaluating the EAL/TAI frequency, but one, began t o operate in a continuous mode s o t h a t s i m p l e a v e r a g i n g be p o s s i b l e , and a l s o because we d i s c o - v e r e d s y s t e m a t i c frequency v a r i a t i o n s between EAL and the p r i m a r y s t a n d a r d s which a r e not compatible with the t h e o r y of the f i l t e r s .
F r e q u e n c y s t e e r i n g - - -
T h e t i m e s c a l e To given by the stability a l g o r i t h m h a s , hopefully, a frequency s t a b i l i t y which i s optimized f o r s o m e value tS of the s a m p l e t i m e Z:
.
L e t ~2 be the p a i r v a r i a n c e of the frequency f o r Z =8.
Now l e t u s c o n s i d e r a n intervafSof t i m e of duration of t h e o r d e r of tS, f o r which we have a n evaluation of the f r e q u e n - cy of To, based on p a s t data of the p r i m a r y s t a n d a r d s , with a n u n c e r t a i n t yrc
(one s i g m a ) . How can we a c c o m p l i s h the s t e e r i n g during t h i s i n t e r v a l ?
(a) If CC
>>
dS, no p r a c t i c a l solution can b e found and the b e s t i s probably to u s e d i r e c t l y T o : if i t s i n i t i a l frequency h a s been conveniently chosen, a v e r y long t i m e will e l a p s e before i t s frequency h a s d r i f t e d in a n unacceptable way.(b) T h e region w h e r e
O/C
i s of the o r d e r of d i s the domain of the s t e e r i n g . It h a s been shown by work on s i m u l a t e d t i m e s c a l e s (5, 6) t h a t the s c a l e T I obtai- S ned by g r a d u a l c o r r e c t i o n of the frequency of T o , a c c o r d i n g t o the d a t a of the s t a n d a r d s , h a s a p p r o x i m a t i v e l y the s t a b i l i t y of T o f o r+
C tS, a n d a b e t t e r s t a b i - l i t y f o r 't ))Z:S : the a c c u r a c y e n s u r e s the v e r y long t e r m stability.(c) If
rc
d4bS, the situation i s m u c h m o r e complex. T h e p r a c t i c a l solution i s to t r a n s f o r m t h e frequency s t a n d a r d s s o that they become p r i m a r y clocks. But, a s soon a s it i s done, if they r e c e i v e the weight they d e s e r v e in the s t a b i l i t y algo- r i t h m , CS d e c r e a s e s d r a s t i c a l l y , and we come back t o c a s e (b). A p a r t i c u l a r c a s e a r i s e s when no f r e q u e n c y s t a n d a r d s a r e l e f t : T1 c a n be d i r e c t l y d e r i v e d f r o m the p r i m a r y clocks only by a s i m p l e a v e r a g i n g .L e t u s consider the c a s e of TAI.
T h e long t e r m s t a b i l i t y of the i n d u s t r i a l c e s i u m clocks on which ALGOS o p e r a - t e s h a s been only slightly improved s i n c e 1969 ; in c o n t r a s t , s o m e p r i m a r y s t a n - d a r d s became m u c h m o r e a c c u r a t e . Consequently the TAI computation evolved f r o m (a) t o (b), and s o m e s c i e n t i s t s believe t h a t solution (c) would be the best.
The steering p r o c e s s adopted i n 1977 c o n s i s t s in applying a frequency offset t o EAL ( 2 To). T h i s offset i s kept constant a s long a s the frequency of TAI ( T i ) does not differ significantly f r o m the
\--
frequency of the p r i m a r y standards. When chan-g e s a r e n e c e s s a r y , they a r e made by frequency s t e p s of 0 . 2 x 10-13 a t i n t e r v a l s not s h o r t e r than
T
two months. A smoother s t e e r i n g (by intentional frequency drift, for instance) will be applied when
T A l
7'-
b e t t e r t i m e comparisons will be available.- 1 I 1 1 1 1 d 1 1 1 1 1 A c c u r a c y and stability of TAI
. . .
1970 1975 1980
Fig. 1. Normalized frequency according t o the p r i m a r y stan- d a r d s of NBS, NRC and PTB.
Mean annual value s for EA L and TAI. P r i o r t o 1977, TAI was not s t e e r e d and i t was identical with EAL.
F i g u r e 1 shows the normalized frequency of TAI according to the s t a n d a r d s of NBS, NRC and PTB. The s t e e r i n g e n s u r e s that the inaccuracy does not exceed 1 x 10-13 in normalized frequen- cy. The instability, for values of t over two months i s s m a l l e r than 1 x 10-13 ; i t i n c r e a s e s f o r s m a l l e r values of Z on account t o the uncer- t a i n t i e s of the clock comparisons. F o r v e r y l a r g e values of 5 (over s e v e r a l ears), even if one a s s u m e s no instrumental improve- m e n t s , the instability would probably d e c r e a s e . T h i s behaviour, which i s c o n t r a r y t o the usual behaviour of clocks i s due t o the growing influence of the p r i m a r y
s t a n d a r d s when 2 i n c r e a s e s . Ultimately, for the v e r y long t e r m , TAI is based only on t h e s e devices.
Svstematic frequency variations
Viewed by the statistician, the p r o g r e s s i v e shift t o a TAI based solely on p r i m a r y clocks
-
the shift f r o m (b) t o (c) above-
should be e a s i l y accomplished if the p r i m a r y clocks d e s e r v e it. The coming back f r o m (b) t o (c) would not r a i s e any difficulty if m o r e stable "ordinary clocks" become operational. T h e r e is no scientific r e a s o n t o f r e e z e the method of computation of TAI. Only the end product m a t t e r s .However, in p r a c t i c e , the BIH i s faced t o a number of problems.
T h e m o s t s e r i o u s one i s the existence of s y s t e m a t i c frequency variations between the various clocks. It is not yet possible t o know with a sufficient c e r t a i n - ty, which clocks a r e right, which a r e wrong.
Another s e r i o u s problem i s the s m a l l number of p r i m a r y clocks : the l i s t of t h o s e with a c c u r a c y a t the level of 1 x 1 0 - l 3 o r b e t t e r follows
1975 May - 1978 Aug.
NRG
G S V1978 Aug. - 1979 Dec. NRC CsV ; P T B CSl
1979 Dec.
-
NRC CsV ; P T B GS1 ; NRC CsVI, A, B, C On the other hand, o t h e r considerations lead t o s o m e "viscosity" in the BIH behaviour. TAI i s not only a scientific product. It i s the output of a worldwide coordination effort of m e n of good will. We cannot play with good will s o e a s i l y a s with figures. The s u c c e s s of the unification of t i m e on TAI and UTC i s , in good p a r t , due t o the feeling of m a n y s c i e n t i s t s and technicians, especially in a v e r a g esized a n d s m a l l c o u n t r i e s , that they effectively contribute t o t h i s worldwide e n t e r - p r i s e . I s h a l l c o m e back on t h i s p r o b l e m in m y conclusions. L e t u s concentrate now on t h e s y s t e m a t i c effects.
T h e s e effects a r e revealed by observing the differences between the data of i n d u s t r i a l c e s i u m clocks on one side, a n d the p r i m a r y clocks and frequency stan- d a r d s on t h e o t h e r s i d e . The m a i n f e a t u r e s , shown by f i g u r e s 1 and 2 , a r e
-
a r e l a t i v e frequency drift,-
a r e l a t i v e annual variation.Fig. 2. T i m e difference between E A L and the p r i m a r y clocks.
F r e q u e n c y d r i f t
- - -
T h e d r i f t of E A L with r e s p e c t t o P T B GS1 w a s found by B e c k e r (7), who a t t r i b u t e d i t to a n a s y m m e t r y of t h e d r i f t s of the i n d u s t r i a l c e s i u m clocks. This explanation w a s confirmed by the a g r e e m e n t with o t h e r frequency s t a n d a r d s , then by t h e p r i m a r y clocks. It i s a l s o supported by the sensitivity of t h e d r i f t of t ~ m e s c a l e s based on i n d u s t r i a l c e s i u m clocks on the type of a l g o r i t h m which i s used f o r t h e i r derivation (8). A s long a s t h i s d r i f t p e r s i s t s regularly, i t can be e a s i l y compensated and it i s not a m a j o r difficulty i n establishing TAI. However the unexpected frequency change between 1979 and 1980 r a i s e s s o m e questions which will be c o n s i d e r e d a f t e r the study of the annual variations.
Annual v a r i a t i o n s
- - -
T h e annual v a r i a t i o n of E A L - p r i m a r y clocks a p p e a r s c l e a r l y on figure 2. I t s explanation by a n annual e r r o r i n t h e t i m e c o m p a r i s o n s can be excluded : it should r e q u i r e a complicated m e c h a n i s m , t o keep the s a m e phase f o r NRC CsV and P T B GS1, and t o show no effect on other t i m e s c a l e s (fig. 3 ) . In addition, a t the end of 1978, t h e t i m e link a c r o s s t h e Atlantic b y LORAN-C w a s r e p l a c e d by a link v i a the telecommunication s a t e l l i t e SYMPHONIE which i s f r e e f r o m annual variation.
T A ( F ) - T A (USNO)
(itncar t e r m r e m o ~ e d )
1 9 7 7 1 9 7 8 1 9 7 9 1 9 8 0 1981
Fig. 3 . T i m e difference between two t i m e s c a l e s based on i n d u s t r i a l cesium clocks (USNO : US Naval Observatory, F : F r a n c e ) .
E A L - N R C Cs V I
1 9 8 0 1981
L e t u s denote by TA(PC) the time s c a l e based on the a v e r a g e of NRC CsV and P T B CS1. The annual t e r m of EAL-TA(PC) shows a minimum in J a n u a r y and a flat
m a x i m u m in June-August ; the peak to peak a m - plitude i s about 0 . 5 p s , t o which c o r r e s p o n d s
1 3 peak t o peak frequency variations of about 1x10- ,
the maximum and minimum frequency differen- c e s taking place in spring and autumn.
Which type of instrument i s responsible for t h e s e seasonal effects ? A study m a d e a t BIH (9)
s e e m s t o exclude a t e m p e r a t u r e influence on in- d u s t r i a l cesium clocks. But the explanation by a n humidity influence is proposed by Becker (com munication t o URSI, 1981), the quasi-totality of t h e industrial cesium clocks being not located in humidity controlled rooms. However, i n the a b s e n c e of humidity m e a s u r e m e n t s no s t a t i s t i c s can bring a support to this explanation. On the o t h e r hand, the NRC CsVI, A, B, C p r i m a r y clocks which w e r e s t i l l under development bring little light on this problem : only clock A s e e m s t o a g r e e with NRC CsV and P T B CS1 (fig. 4). It i s a l s o s u r p r i s i n g that no effect can be seen between two l a b o r a t o r y t i m e s c a l e s based on in- d u s t r i a l clocks such a s TA(F) and TA(USNO),
(fig. 3 ) , although the clocks a r e submitted to r a t h e r different climates and that the effect should not be b l u r r e d by the geographical d i s p e r - Fig. 4. T i m e difference between
sion of the clocks.
EAL and the p r i m a r y clocks NRC Cs VI.
Although I have some sympathy for t h e idea t h a t m e t r o l o g y should be b a s e d on r e a l m e t r o l o g i c a l i n s t r u m e n t s , such a s t h e p r i m a r y clocks which a r e fully evalua- t e d , I a m not yet convinced that they a r e f r e e f r o m the s e a s o n a l effects.
I r r e g u l a r i t i e s - - -
Whichever i s t h e c a u s e of the s e a s o n a l effect, i t i s i n t e r e s t i n g t o r e m o v e i t e m p i r i c a l l y in o r d e r to study other i r r e g u l a r i t i e s . In f i g u r e 5, a purely periodic
E A L
-
T A ( P C ) + a n n u a l c o r r e c t i o n1 9 7 8 1979 1980 1981
Fig. 5. TA(PC) i s the m e a n of NRC CsV and P T B CSl.
T h e s e a s o n a l v a r i a t i o n i s removed by adding a p u r e l y p e r i o d i c c o r r e c t i o n .
P T B CS 1
-
N R C C s V1 9 7 8 1 9 7 9 1980 1981
Fig. 6. T i m e difference between the p r i m a r y clocks.
c o r r e c t i o n h a s been added. An i n t e r e s t i n g f e a t u r e i s the change of frequency a t the end of 1979, which is of the o r d e r of 1. 5 t o 2.0 x 10-13. No change of such a n amount o c c u r r e d a t t h i s date i n the d i f f e r e n c e s EAL-TA(ICC), w h e r e TA(1CC) r e p r e s e n t t i m e s c a l e s b a s e d on i n d u s t r i a l c e s i u m clocks (9). A l m o s t s i m u l t a - neously , a s h a r p frequency change between NRC CsV and P T B CS1 a p p e a r e d , a l s o of the o r d e r of 1. 5 to 2.0 x 10-13 (fig. 6)
On the o t h e r hand, the probable change of frequency of EAL in two months c a n be e s t i m a t e d f r o m the o b s e r v e d frequency change of e a c h clock with r e s p e c t to E A L during the i n t e r v a l ; i t h a s been found equal to 1 x 10-14 (one sigma). T h u s a change by 1 x 1 0 - l 3 h a s a v e r y s m a l l probability, u n l e s s t h e r e i s s o m e common c a u s e of frequency variation.
Thus, a t l e a s t one of the p r i m a r y clocks which o p e r a t e s s i n c e a few y e a r s m a y have f r e q u e n c i e s i r r e g u l a r i t i e s of 1 to 2 x 10-13.
Conclusions
TAI, a s it i s established, has a long t e r m stability and a n a c c u r a c y c h a r a c t e r i - zed by frequency fluctuations, with r e s p e c t to the ideal frequency s m a l l e r than 1 x 10-13. But the a t t e m p t s to improve i t m e e t s e v e r a l difficulties.
F i r s t , the improvement of the t i m e comparisons, in a c c u r a c y and geographi- c a l coverage, would be n e c e s s a r y . A reasonable goal would be t o reduce tile un- c e r t a i n t i e s t o 10 n s on the whole E a r t h .
Another n e c e s s i t y i s t o develop a s e t of frequency standards with homogeneous a c c u r a c i e s , which should be operated e i t h e r quasi-continuously ( p r i m a r y clocks) o r a t frequent intervals (once a month, for instance). The situation, which might be the a c t u a l one, where one single standard i s ten t i m e s m o r e a c c u r a t e than any other one, r a i s e s some unsolvable p r o b l e m s : the best standard, the p e r f o r m a n c e s of which cannot be a s s e s s e d by i n t e r c o m p a r i s o n s , cannot be p r o p e r l y used. At p r e s e n t , the a c c u r a c y of 1 x 10- 13 does not suffice t o elucidate the problems which a r o s e . New frequency s t a n d a r d s and p r i m a r y clocks should r e a c h the a c c u r a c y of about 1 x 10-14.
T h e respective role of p r i m a r y clocks and other clocks i s a subject which can be debated indefinitely. Fortunately, the p r a c t i c a l solution i s guided by the p r o p e r - t i e s of a c t u a l instruments. F r o m a purely scientific point of view, it s e e m s that TAI should be based on a s e t of about ten p r i m a r y clocks, having s i m i l a r p e r f o r - m a n c e s , and located in s e v e r a l l a b o r a t o r i e s , because they a r e fully metrological instruments. An additional advantage of t h i s solution would be t o simplify the work a t BIH. However, a t BIH, our opinion i s that our duty i s t o include in the establish- ment of TAI the data of clocks located in a s many a s possible l a b o r a t o r i e s , even if it does not bring a n improvement of the stability of TAI. We believe that TAI i s not only a t i m e s c a l e f o r scientific u s e , but a s y s t e m which o p e r a t e s a l l the better when i t r e s u l t s f r o m a common effort. In addition, a l a r g e b a s i s e n s u r e s the fia- bility, which i s e s s e n t i a l .
In the p r e s e n t situation, we have some doubts concerning the influence of cli- m a t i c environment on the numerous industrial cesium clocks which contribute t o TAI. But even if the seasonal frequency variation which i s observed h a s t o be entirely attributed t o t h e s e clocks, it r e s u l t s in a seasonal time i r r e g u l a r i t y of
f
0. 3 p s (extreme values), which do not bring much nuisance to the u s e r s , and which could be e a s i l y c o r r e c t e d when i t s origin i s known. On the other hand, we have s e e n than the p r i m a r y clocks m a y have frequency i r r e g u l a r i t i e s of a n amount such a s the industrial cesium clocks contribute to reduce the impact of t h e s e i r r e g u l a r i t i e s on TAI.F o r t h e s e r e a s o n s , m a j o r changes of the methods for establishing TAI a r e not contemplated a s long a s the experimental data will keep the s a m e level of quality.
R e f e r e n c e s
(1) Costain, C. C. e t a l . , P r o c 33rd Annual Frequency control Symp., Atlantic-City, (1979) 473.
( 2 ) Yoshimura, K . , Metrologia pj(1980) 133.
(3) Yoshimura, K . , NBS Technical Note 626 (1972).
(4) Azoubib, J., Granveaud, M. , Guinot, B. , Metrologia
13
(1977) 87.(5) Guinot, B., P r o c . Sd Cagliari int. Meeting on time determination, disse- mination and synchronization, Enslin and Proverbio ed. (1975) 15.
( 6 ) Granveaud, M. , Azoubib, J . , Comitk Consultatif pour l a Dkfinition de l a
Seconde 8e session, BIPM ed. (1977) S 40.
(7) Becker, G . , PTB-Mitt. 2 (1973) 319.
(8) Guinot, B . , Granveaud, M . , Azoubib, J . , J. Institution of Electronics and Telecommunications Engineers, New Delhi (in p r e s s , 1981).
(9) Guinot, B., Azoubib, J . , IEEE-IM (1980) 226.
