The history, equipment, and current work of the Royal Greenwich Observatory are briefly reviewed

Smith, H. M.

English

NASA Technical Reports Server

F.kCILITIES AND SERVICES IN THE TIME DEPARTMENT OF THE 
ROYAL GREENWICH OBSERVATORY 
H. M. Smith 
Royal Greenwich Observatory 
Haelsham, Sussex, England 
INTRODUCTION 
The Royal Greenwich Observatory plays a unique role in timekeeping, having 
both national and international significance. When the Observatory was founded 
in 1675, two specially-designed clocks by Thomas Tompion were installed to 
enable the first  Astronomer Royal to check on the regularity of the diurnal ro- 
tation of the Earth. In the nineteenthcentury distribution of time signals by 
electric telegraph became increasingly popular and by 1880 Greenwich Mean 
Time (GMT) was adopted a s  the standard time throughout England, Scotland and 
Wales, Followinga meeting of scientific experts in Rome in 1883, the Washington 
Conference of 1884 recommended the use of the Greenwich meridian as the 
reference meridian for the worldwide measurement of time and longitude. 1 
The designation GMT is  still in very widespread use, but in many branches of 
scientific work it has, since 1928, been known as  Universal Time (UT). 
Quartz clocks came into use at Greenwich in about 1938, and by 1944 had com- 
pletely replaced the former pendulum clocks. From the middle of 1955 the 
rates of the Observatory clocks were checked in terms of the caesium beam 
frequency standard at the National Physical Laboratory, and in 1966 a com- 
mercial atomic clock (HP5060A) was installed at the new location of the Royal 
Greenwich Observatory a t  Herstmonceux in Sussex. The time department now 
operates five caesium standards, which are  housed in cellars at sub-basement 
level, Each standard has an independent earthing system, an individual emer- 
gency power supply, and the distribution of time and frequency is  by buffered 
screened balanced lines, On initial installation, each standard is carefully set 
up according to the manufacturers recommended procedure, the C-field being 
adjusted to the optimum level. No subsequent frequency adjustment i s  made by 
off-setting the C-field. In these strictly controlled conditions, the standards a re  
capable of a stability of mean rate over periods of weeks of better than 0.01 
microseconds per day. (1 in 10 l 3  ). The rates of individual standards can differ 
by as  much a s  0.5 microseconds per day, and occasional inexplicable changes of 
rate of a few tenths of a microsecond per day can occur. 
https://ntrs.nasa.gov/search.jsp?R=19750003213 2020-03-23T03:07:05+00:00Z
COMPARISON EQUIPMENT 
In the twenty years  up to around 1955, most of the e uipment used in the time 
service was designed and made in the department 4*(5, but in more recent years  
many electronic instruments have become commerically available. The primary 
system of clock comparisons utilizes a 10 nanosecond time resolution counter, 
programmed by a digital clock, to compare the clocks in groups of three, giving 
both a printed and a punched tape record. These time comparisons a r e  supple- 
mented by continuous records of linear phase comparators with a full scale de- 
flection of one microsecond. 
International comparisons a r e  normally effected by reception and measurement 
of the Loran-C emissions. At Herstmonceux routine measures a r e  made on 
Ejdes (master) and Sylt (slave) of the Norwegian Sea Chain and on Estarti t  (slave), 
Mediterranean Chain, using Austron 2000 C receivers checked differentially and 
by a simulator. Supplementary comparisons a re  made using VLF emissions and 
Tracor 599 and 599T receivers. 
CHECK COMPARISONS 
The Royal Greenwich Observatory i s  indebted to the US Naval Observatory for 
carrying out periodic traveling clock trips, which serve a s  a valuable check on 
the routine linkages via Loran-C. The continuation and extension of this service 
is essential to  the task of the Bureau International de ltHeure (BIH) in the forma- 
tion of an international scale of atomic time. 
Comparisons have also been made by the Office National dlEtudes et de 
Recherclles Aerospatiales: a clock carried in an aircraft was compared with 
clocks on the ground while the aircraft was in flight6; and by the US Naval Re- 
search Laboratory using the clock carried in the Timation I1 satellite. 7 
INTERNATIONAL CO-ORDINATION 
A significant feature of recent developments in timekeeping has been the accept- 
ance of the need for full international co-ordination, and the Royal Greenwich 
Observatory has cooperated fully in these projects8. There a r e  three major 
a reas  of co-ordination: 
a. Astronomical-Corrections for  the effects of polar variation @. v. ) on 
astronomical time determination have been applied to thc RGO obser- 
vations since 1948, and for the seasonal fluctuation in the rate of rota- 
tion of the Earth since 1950. ' The "smoothed" scale of aetronomical 
time thus achieved was designated Provisional Uniform Time (PUT) and 
was the forerunner of UT2 used internationally since 1956. 
The observations made with the Herstmonceux PZT a r e  of a very high 
standard of accuracy and an interesting development has been the 
establishment in Canada of a PZT at Calgary at the same latitude1'. 
The two instruments employ the same stars ,  the same adopted s ta r  
places and the same basic methods of reduction. Full co-ordination 
permits the separation of e r r o r s  associated with a particular instru- 
ment and site from those common to both. These two instruments to- 
gether make a valuable contribution to  the work of the BIH in computing 
current values of UT and p, v., and to  the work of the International 
Polar Motion Service (IPMS) in the calculation of the definitive polar 
motion. During periods when the BIH data a r e  required with the mini- 
mum delay by the Je t  Propulsion Laboratory for the navigation of space 
vehicles, the Herstmonceux observations a r e  communicated daily to the 
BIH by Telex, 
b. Time Signals-International co-ordination of radio time signal emissions 
was pioneered by the British and United States timc services in 1961 
under the joint direction of the USNO and RGO, using the "offset and 
jump" method introduced in the MSF emissions in 195811. The obvious 
practical advantages of such a scheme led to the recommendation by the 
International Astronomjcal Union (UU 1961) and the International Radio 
Consultative Committee (CCIR, 1962) of the extension of the system to 
worldwide co-ordination under the control of the BIH, This system was 
modified by the elimination of the offset in 1972, and is now universally 
adopted. 
A CCIR Working Party (IWP 7/1), has the continuing task of studying 
the implementation of the system and i ts  possible improvement. 
c. Atomic Time-The Greenwich Atomic Time Scale GA was established in 
1955, and an adjustment was made (GA2) in 1958 to bring it into step 
with other national and international scales. The international scale 
(IAT) formed by the BIH was adopted by the International Committee of 
Weights and Measures in 1972 and is proving to be indispensable, Until 
mid-1973 the IAT scale was formed from seven independent scales (of 
which GA2 was one): since that date it has been formed using the data 
from individual atomic standards. The data a r e  made available to the 
BIH by using Loran-C comparisons. The UTC scale used in rzdio time 
signal emissions i s  based cm LAT, and differs from it by an exact num- 
ber  of seconds. This has become the "de facto" civil timc in most 
countries, and the implications of this a r e  being studied by a working 
party se t  up by t5e Consultative Committee for the Definition of the 
Second (CCDS of the CIPIbl). 
CLXIRENT WORK 
The vital role of the Loran-C system in the formation of IAT and in the co- 
ordination of radio time signal emissions on the UTC system, has led to a re- 
view of the factors affecting the accuracy of Loran-C measurements. Differential 
measures can be made or. a routine basis to  an accuracy of 0.01 microseccmd, 
but there i s  still some confusion regarding absolute measures. It is recom- 
mended by the USNO that calibration of a reception station should be per fo~med 
using a whip aerial: most users  a r e  forced to employ frame aer ials  for  their 
routine operatianal work. Enrapean reception sites suffer a h:& level of mter- 
ference, and cycle identification is difficult. With the cooperation of the USNO, 
and of the US Coastguards (responsible? for the Loran-C system) many tests have 
been made, Some adopted propagation times appear to be in need of revision. 
A puzzling feature is the apparent drift between the b r a n - C  comparisms and 
those made with a travelling clock (see Table 1). 
Another point of interest concerns the IAT scale. This i s  formed almost entirely 
of commercial (Hewlett Packard) standards. The occasional measures made 
with long-beam !aboratory standards (which have a range of the order  of 1 in 
10 :ndicate a systematic departure of IAT from the SI definition of the sec- 
ond12. There is a considerable body of opinion in favour of preserving the uni- 
formity of the IAT scale to the highest degree possible so as to form a continuous 
uniform scale even if this entails a gradual drift from the best contemporary de- 
terminations of the Sl second. It is a matter for discussion whether the IAT 
scale should be re-assessed and, if so, how often. 
The arguments in favour of changing from a ;.lean of independent scales to a 
statistical mean of Individual standards carried weight: the new system permits 
the use of more standards (including those of establishments having only one o r  
two atomic clocks), eliminates the uncertainties a s  to the procedures used at 
independent establishments in the formation of their own scales, and makes it 
reasonable for the BIH to adopt objective statistical methods of weighting. Never- 
theless, there i s  a feeling that the ideal method has still to b? found. 
A promising development '3 foreseen in the successful tests made, both in the 
U, K. and in Australia, uslng Timation 11. With the improvements planned m 
subsequent satellites, there i s  a prospect of achieving an accuracy no less  than 
that of Loran-C under favcurable canditions, and with the possibility of a full 
worldwide coverage. Inng-term parallel operation of h r a n - C  and Timation i s  
rrecessary. 
Table 1 
Traveling Clock - Loran-C 
unit : microtaecond 
Differences of UTC comparisons with USNO using traveling clock and bran-C. 
It will be evident that in the collaticm of data and in the operation of an 
international service, the BIH is fulfilling an even more important function. 
The existing and predictable demands for accuracy can only be met by combining 
worldwide observatims to determine UT and by syncretizing atomic clocks to 
establish an international standard atomic clock time. If this essential service 
is to he maintained, and is to be developed, adequate financial provision is 
obligatory, but practical assistance in the loan of equipment and staff c0u.d also 
be of assistance. 
Date 
---- . 
1969 Feb. f 4 
Date 
Feb. 25 
OP 
-0.1 
July 11 
Oct, 29 
1970 Feb. 16 
June 22 
A u ~ .  15 
Sep. 10 
Oct. 20 
1971 May 16 
Sep. 23 
1 9 n  Apr. 1: 
D ~ c .  7 
1973 May 7 
Sep. 26 
PTB RGO 
0.0 
-0.4 
-0.1 
0.0 
-0.3 
Date 
+3.4 
0.0 
-0.3 
-0.6 
-0.4 
+0.1 
-1.8 
-1.5 
-2.2 
-1.3 
M.2 
-0.3 
-0.5 
0. a 
-0.6 
-0.6 
-0.4 
-0.8 
-0.3 
-0.6 
-1.0 
-1.3 
-1.1 
July 21 
b ~ t .  13 
Sep. 25 
Apr. 14 
July 10 
Feb. 17 
June 22 
Sep. 14 
29 
Sep. 30 
Apr. 19 
NOV. 30 
May 15 
Sep. 17 
REFERENCES 
hternational Meridian Conference, Washington, for the Purpose of Fixing 
a Prime Meridian and a Universal Day. Washingtcm, D. C. Gibson Bros. 
1884. 
Trans. LA. U. III, Inndon. Cambridge University Press, 1929. 
Quartz Clocks of the Greenwich Time Serice, H. M. Smith, MNRAS - 113, 
p. 67, 1953. 
Precision hfeasurement of Time, H.M. Smith, J. Sc. bst. - 32, p. 199, 
1955. 
L'equipment actuel du Service Horaire de llObservatorie de Greenwich 
H. hi. Smith, J. D. Pope, Proc. Ve Congres Internatiaral de Chm~otnetrie 
Paris, 1954. 
Synchronization Precise de Bases par Simple Survol, J .  Besson and J. 
Cumer, Office National dlEtudes et de Recherches Aerospatiales T. P., 
no. 759, 1969. 
'hternational Time Transfer using the Timation I1 Satellite J. A. Buisson. 
P m .  27th Annual Symposium on Frequency Control U. S. Army Electronics 
Command, p. 277, 1973. 
International Time and Frequency Co-ordination, H. M. Smith, Proc. 
IEEE (USA), 60, p. 479, 1972. 
The Determination of Time and Frequency, H. M. Smith, Proc. IEEE, (UK) 
98, pt. 11, p. 143, 1951. 
-
Comparisons of the Calgary and Herstmonceux Photographic Zenith Tubes, 
N. P. J. O'Hora and R. W. Tanner, The O b s e ~ a b r y ,  90, p. 157, 1970. 
The Disseminatim of Astronomical and Atomic Time, H. M. Smith, Naturc, 
221, No. 5177, p. 221, 1969. 
-
Annual Report BIH, p. A-12, 1971. 
Atomic Time Scales, B. Guinot and M. Granveaud, IEEE Trans. Instr. 
and Meas. IM-21, p. 396, 1972. 
QUESTION AND ANSWER PERIOD 
DR. WINKLER: 
We a r e  open for auestions. 
DR. REDER: 
Mr. Smith, wouldn't i t  be good, If you do make this co~nparison which you 
showed in the last two slides, to have at least one of those equipments which a r e  
used in France brought to your place o r  the U. S. Naval 9bservator.y for corn- 
parison to be sure it i s  not due to an equipment difference ? 
MR. SMITH: 
I am sorry, hut in the limited time a t  my disposal, I couldn't explain the experi- 
ment in detail. 1 s b u l d  have said that the receivers and full measuring e q u i p  
ment from the United States Naval Observatory were taken both to Herstmonceux 
and to Paris. 
The agreement between the onsite equipment a t  Herstmonceux and Paris  with 
that which was brought from the USNO was absolutely first  class, and thereforc 
there was the common element which you so rightly p i n t  out is necessary to an 
experiment of this type. 
DR. WINKLER: 
Mr. Lavanceau ? 
MR. LAVANCEAU: 
In regard to the Loran C measurements of 1973 on two different occasions in 
Europe (Paris, Royal Greenwich Observatory, Germany, and alsc in the Faeroe 
Island, near the master station of the Norwegian Loran C chain), extreme care 
was taken in taking the measurements, i. e. , during the same week, using the 
very same equipment, including antenna cables. All measurements made in 
Paris, BIH, and the Royal Greenwich Observatory did agree very, very well. 
I think one should realize and recognize that Loran C is a marvelous transfer 
tool. When one wants to make rektioc time transfer measurements, extreme 
precision can be achieved, such a s  perhaps tenths of microseconds o r  better. 
B&t, when one i s  trying to make absolute measuremeilts, there a r e  so many 
variations, so many variables which sometime cannot be controlled that the 
scatter of the data, which we have seen here, i s  actually not really very large. 
In most cases, it i s  0.5 microseconds, and if one realized that some of this 
linkage i s  built by using 5 o r  G different sources of data, like for instance, 
when one i s  trying to relate measurements made a t  the U. S. Naval Obsenratory 
to measurements made at the same time across the Atlantic. 
You know, one may not appreciate the fact that any small e r ro r s  which one will 
encounter a t  one of thes.e locations will of course add in some statistical way, 
and perhaps create that scatter. The Loran C system is  capable of achieving 
greater accuracy, but a considerable price may have to be paid for that. This 
may not be easy to cope with. 
DR. WNKLER: 
I would like to make a comment myself. There a r e  really two issues here. 
Mr. Lavanceau mentioned the small (0.5 microseconds) scatter in the measure- 
ments across the No141 Atlantic. 
The question which we a r e  also debating is the difference between making 
absolute measurements and relative measurements which you caiibrate by 
visiting portable clocks, and this is what we have done from the beginning of the 
use of Loran C in the international system of atomic time keeping. 
The observatory, for that very reason, has sent portable clocks to various 
establishments about twice every year. 
We have kept a check on the propagation delays. But when you talk about 
absolute time transfer, i. e., when you set up equipment a t  the new location, 
and compare the computed delays with your own calibrated delays including the 
equipment, what i s  then the error ,  in an absolute sense of your t:irl~ transfer? 
We a r e  talking about a ~ r o b l e m  which is  common to every synchronization 
system in existence. J t  i s  only the magnitude which is different for the various 
systems. 
If you take a VLF timine; system, the magnitude i s  ten times larger. The 
differences between co~r~puted delays and actually calibrated delays a r e  a s  large 
a s  10 microseconds. 
In a satellite system you also have exactly the same situation that one must 
calibrate a path o r  location. I think we will hear more about that this afternoon. 
Again, s.ve have to understand that difference ktwee.1 an absolute and a relative 
measurement where you calibrate overall by bringing a portable clock to the 
p i n t  of use, from time to time, to check everything, to certify the operation, 
d that it is one operation which eesentialiy will have to be done in every 
system of time transfer. 
Now, tbe magnitude of the specific effects we a re  here concerned with is for me 
startiingly great. I m i d  think that what we have to accomplish urgently is a 
standardizztion of methods. 
We have found between the various stations which w9 have visited that if we do 
the same things, with the same equipment, everywhere, we fiad a very high 
degree of conformity and repeatability of results, 
But the general problem arises, I think, in using equipment which has been 
calibrated in entirely different ways, and then, of course, you could expect such 
discrepancies as  discussed before. The discrepancy, in fact, has been two and 
a half microseconds roughly. 
I am afraid we will lave to stop here, because time is  pressing. I will have to 
defer any further discussion to the outside o r  after the break. 


Facilities and services in the time department of the Royal Greenwich Observatory

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