What follows below are seleted extracts from the Royal Observatory Bulletins and Greenwich Time Reports which give details of the Greenwich atomic time scales. The Bulletins/Reports were normally issued at three monthly intervals. From the 6 June 1967 onwards, the Greenwich atomic time scales are believed to have been determined from the mean of the available Hewlett-Packard atomic standards at Herstmonceux (of which, in the early days, there was sometimes just one).
Caesium Resonator N.P.L. (Cs1)
By courtesy of the Director, figures relating to the caesium resonator Cs1 at the National PhysicaI Laboratory (1) have been made available month by month to the Observatory. In June 1955 the frequency was measured in terms of the second of U.T. as determined provisionally at the Royal Greenwich Observatory and was found to be 9 192 631 830 cycles per second (2). Due to changes in the rate of rotation of the Earth. which give rise to variations in U.T.2. the measured frequency of the caesium resonance in terms of the second of U.T.2. is not constant. From mid-1955 to mid-1957 the rate of rotation of the Earth progressively decreased, corresponding to an increase in the length of the day of approximately 0.45 msecs per year and an increase of approximately 5 parts in 109 per year in the frequency of the caesium resonance in terms of the U.T.2. second. Since mid-1957 the average rate of rotation of the Earth has remained relatively constant. corresponding to a caesium resonance frequency of approximately 9 192 631 910 c/s.
From comparisons made between the N.P.L. standard Cs1 and Ephemeris Time as derived from observations made with the Markowitz dual-rate Moon position camera at the U.S. Naval Observatory from 1955.50 to 1958.25 the caesium frequency in terms of E.T. was found to be 9 192 631 770 (3) . This provisional value has been widely used, and intercomparisons between observatories are greatly facilitated if the same adopted value is generally accepted. This frequency is commonly referred to as the 'nominal' frequency of caesium. and has been used at the Royal Greenwich Observatory in the derivation of an atomic time scale.
The time signals controlled by an observatory are required to conform closely to U.T.2. Instead of adopting a value for the caesium frequency in terms of the current value of the second of U.T.2, it is more convenient to adopt, from time to time, a value of the 'offset' from the nominal caesium frequency, specified in parts in 1010, which corresponds to the current difference in rate between U.T.2. and the atomic scale. From comparisons between the Herstmonceux P.Z.T. observations and Cs1 during 1959, the offset was found to be approximately 150 parts in 1010. This was in agreement with the comparisons made between the P.Z.T. observations of the U.S. Naval Observatory and the U.S. caesium standards. It was therefore decided to adopt an offset of exactly 150 parts in 1010 throughout the year 1960.
Derivation of Atomic Clock Cs1(EB)
Measures of the frequency of one of the quartz clocks at the N.P.L. in terms of the caesium resonator Cs1 have been communicated to the Royal Greenwich Observatory since mid-1955. Until May 1956 the figures given were monthly means of the actual measures: from June 1956 to June 1959 the figures related to every fifth day, having been interpolated from the actual measures: from July 1959 the measured values have been supplied, and running means of five measures have been taken and interpolated to give values for the Julian 5-day dates. The accuracy of measurement (range) at the N.P.L. is quoted as ±1 part in 1010.
During the first two years that the caesium standard was in operation, the N.P.L. clock whose frequency was measured in terms of Cs1 was compared with the quartz clocks at the Post Office Research Laboratories and at the Royal Greenwich Observatory by means of reception at each of the three establishments of the GBR radio time signals. Some measure of smoothing was introduced to minimise the effects of the errors of radio reception and measurement. Since June 1957 the clocks have been compared by means of the direct line links and the continuously running rotary beat-counters, normally to ±2 parts in 1010. An alternative linkage was provided by reception and measurement at each establishment of the daily standard frequency transmissions of MSF on 60 kc/s. The results obtained via the lines and via MSF do not differ, on the average, by more than ±3 parts in 1010. For a number of selected clocks the frequency differences from Cs1 have been converted into five-day rates, which have then been added to form time comparisons. Due to the various measurement and comparison errors, the summations obtained on four, dIfferent clocks exhibit mutual wanderings which, over the period 1957 to 1960 amounted at tImes to ±2 msecs, but there is no evidence of any real divergence.
The Post Office clock EB was running continuously throughout the period, and the summations based on the five-day rates of EB as determined by Cs1, denoted Cs1(EB), have been adopted as a reference clock keeping ‘atomic time'. The constant adopted in the summation was chosen to bring Cs1(EB) into agreement with U.T.2. in June 1955. By means of this clock comparisons have been made between the atomic time scale and U.T.2. as defined by the current P.Z.T. observatIons. Commencing with this issue of the Bulletin, it is proposed to include in Section 3. Clocks, figures relating to the atomic clock calculated in the manner described.
1. L. Essen and J.V.L. Parry. Phil. Trans. A; 250, 45, 1957.
2. L. Essen and J.V.L. Parry. Nature, London, 176, 280, 1955.
3. W. Markowitz, R.G. Hall, L. Essen and J.V.L. Parry. Phys. Rev. Letters, 1. 105, 1958.
Atomic Clock Cs(H12)
Cs(H12) is one of the four clocks, mentioned on page B 17 of Bulletin No. 22, derived by the summation of the five-day rates as determined by the caesium resonator at the National Physical Laboratory. Cs(H12) commenced in July 1957 and the two clocks Cs(EB) and Cs(H12) were then in agreement to one tenth of a millisecond.
Commencing with this issue the U.T.2 time-system has been determined by passing a smooth curve through the P.Z.T. observations referred to the clock Cs(H12). Corrections for the quart~ clock H12 have been deduced from the time comparisons between H12 and Cs(H12). Corrections for the remaining quartz clocks have been determined by comparison with H12 in the usual way from the readings of the rotary beat-counters.
The Post Office clock EB, used in the derivation of Cs(EB), suffered a disturbance early in 1962. The opportunity has therefore been taken of making increased use of Cs(H12).
Cs(H12) is an atomic time scale based on the summation of the five-day rates of the quartz clock H12, at the Royal Greenwich Observatory, which is calibrated in terms of a caesium resonator at the National Physical Laboratory. The provisionally adopted frequency of the resonance is 9 192 631 770 cycles per second (E.T.) (Markowitz et aI, 1958). For the years for which results are given in this Bulletin the rate of the Cs(H12) time scale thus exceeds that of the U.T.2 time system by approximately 1.2 milliseconds per day. The constant adopted in the summation of the five-day rates was chosen to bring the atomic time scale into agreement with the U.T.2 time system in 1955 June.
Greenwich Atomic Time Scale (G.A.)
The atomic time scale published in the R.O. Bulletins will be known in future as the Greenwich Atomic Time Scale (G.A.). The scale is continuous with the Cs(H12) scale, as described in R.O. Bulletin No. 62, page B157, which is itself continuous with the Cs(EB) scale as described in R.O. Bulletin No. 22, page B17. The constant adopted in the summation was chosen to bring the atomic time scale into agreement with the Provisional Uniform Time System of the Royal Greenwich Observatory (the fore-runner of U.T.2) in 1955 June. The G.A. time scale thus provides a continuous atomic time scale commencing in 1955 June. Since then atomic time has been gaining on U.T.2 and the difference now amounts to about 4.5 seconds.
Since 1955 the atomic time scale has been based on the summations of the rates of quartz clocks calibrated in terms of the caesium atomic frequency standard at the National Physical Laboratory. The provisionally adopted frequency of the resonance is 9 192 631 770 cycles per second (E.T.).
From 1965 January 1, G.A. has been obtained by taking the mean of two determinations based on the summations of the daily rates of the quartz clocks H12 and H21, each calibrated in terms of the National Physical Laboratory caesium resonator. The constant adopted in the summation of H21, which commenced on 1965 January 1, was chosen so that the atomic time scales determined in terms of H12 and H21 were in agreement on that date.
HERSTMONCEUX ATOMIC STANDARD AND GREENWICH ATOMIC TIME SCALE, G.A. On 1966 May 6 a commercial caesium beam atomic standard (Hewlett Packard 5060A) was installed at Herstmonceux to be known as CsA. A frequency of 100 kHz derived from the caesium-controlled crystal oscillator is connected to the beat counters to give comparisons with the other quartz crystal oscillators of the Time Department.
Since 1955 June. the Greenwich Atomic Time Scale has been determined from the summations of the daily rates of quartz crystal clocks calibrated in terms of the long beam caesium atomic frequency standard at the National Physical Laboratory as described in Royal Observatory Bulletin No. l05, page B6.
From 1966 July, G.A. has been determined from the mean of the atomic standard CsA, at Herstmonceux, and a similar standard at the National Physical Laboratory, via the line link. The time derived from CsA was adjusted to within a few microseconds of that derived from the N.P.L. Standard, so that there is no discontinuity in G.A.
Greenwich Atomic Time Scale, G.A. From 1966 October G.A. has been determined from the mean of the atomic standard CsA at Herstmonceux, and a similar standard at the National Physical Laboratory, except as stated below. The line link, which has previously been used to compare clocks at Herstmonceux and the National Physical Laboratory is no longer in use since comparisons made using the MSF 60 kHz transmissions have proved more accurate. At Herstmonceux measurements are made, in microseconds, of the relative phase variations of the carrier wave of MSF and CsA. For this period the National Physical Laboratory has supplied the Royal Greenwich Observatory with the frequency deviations, in parts in 1011, of the MSF transmissions relative to the caesium standard at the N.P.L. averaged over a period of twenty-four hours centred at 1200 U.T. For the period November 11-29 inclusive, G.A. was determined from CsA only, because no record of the MSF phase variations was available at Herstmonceux.
A second caesium beam atomic standard, CsB , was installed at Herstmonceux on March 3 and brought into regular use on April 1. CsA was withdrawn for maintenance from 1967 April 1 to June 6. From April 1-30, G.A. was determined from the mean of CsB and a similar standard at the National Physical Laboratory, Teddington, using MSF 60 kHz emissions as a means of comparison. At Herstmonceux measurement are made of the relative phase variations of the carrier wave of MSF and CsB. At Teddington, measurements were made of the frequency deviation, of MSF relative to the caesium standard from April 1-14, and by a record of the relative phase variations from April 15-30. From 1 May to June 6, G.A. has been determined from CsB only. From June 6-30, G.A. was determined from the mean of CsA and CsB.
Greenwich Atomic Time Scale: Introduction of GA2
The Greenwic atomic times scale, GA, is based on casesium atomic standards. The second is the duration of 9 192 631 770 cycles of the radiation corresponding to the transition between two hyperfine levels of the ground state of the atom of caesium-133, The second was chosen to agree in length with the lenght of the ephemeris second, GA commenced in 1955 June and the initital value was chosen so that GA was in agreement with Greenwich UT2 as currently defined in the mid-June of 1955.
It has been decided to adopt an atomic time scale, GA2, with a constant difference from GA such that
GA-GA2 = 0s.957 700
which is in close accord with AT, (formerly known as A3), the atomic time scale of the Bureau International de l'Heure, which commenced on 1958 January 1.
All figures published in the Bulletins which are referred to GA may be referred to GA2 by applying the above difference.
Greenwich Atomic Time Scale, TA(RGO)
The independent Greenwich atomic time scale, formerly called GA2, was renamed TA(RGO) on 1977 January 1.
The duration of the scale interval was also increased at this time by the equivalent of exactly 45 nanoseconds per day (approximately +5.2 parts in 1013). This change coincided with a change of exactly +10 parts in 1013 in the scale unit of TAI; both were intended to bring the scale units into better agreement with recent determinations of the SI second at sea level.
GA2 had already been referred to as TA(RGO) in publications of the BIH; TA(RGO) should therefore be treated as a continuous scale with a discontinuity of rate at 1977 January 1.
TA(RGO) was determined from the unweighted mean of two or four of the clocks CsF, CsH, CsJ, CsL and CsM wIth rate corrections applied in microseconds per day according to the following table: