The transcripts below are taken from the following three sources. Between them, they form a continuous record of the activities of relating inititlally to the Time Service and then to the Time and Latitude Servicel:
I, The Reports published in the Quarterly Journal of the Royal Astronomical Society (QJRAS) 1964/5–1982
| 1964/5 | 1965/6 | 1966/7 | 1968 | 1969 | |
| 1970 | 1971 | 1972 | 1973 | 1974 | |
| 1975 |
2. The Royal Greenwich Observatory Reports 1974–1980. These reports are not currently available on-line.
1 January 1974 – 30 September 1975*
1 January 1975 – 30 September 1975*
1 October 1976 – 30 September 1976
1 October 1976 – 30 September 1977
1 October 1977 – 30 September 1978
1 October 1978 – 30 September 1979
1 October 1979 – 30 September 1980
* Apart from the prelimiaries and staff lists at the end etc, this Report is the same as that pubished in QJRAS
3. The Royal Greenwich Observatory, Telescopes Instruments Research and Services reports 1980–1987). These reports are not currently available on-line.
1 October 1980 – 30 September 1985
1 October 1985 – 30 September 1987
TIME SERVICE
Observations with the PZT during the period 1958.0 to 1964.2 have been analysed with a view to determining corrections to the star positions adopted in 1958. This analysis has shown that the errors in both coordinates are small and seldom exceed 0"·1 for individual stars. The analysis also confirms the existence of a large closing error in declination, of the order 0"·5, arising from an apparent diurnal variation in latitude. The results suggest that this apparent diurnal change is in part associated with the instrument itself and is probably related to temperature changes; precise differential temperature measurements are now being made within the telescope. Pending the results of these measurements it is not possible to derive new declination corrections to the star places. It has been decided not to apply the adopted right-ascension corrections until the declination corrections have been deduced.
Owing to the effects of precession, a number of stars included in the current programme of observations can no longer be observed with the PZT. These stars, together with a number which have proved too faint for routine observation, are being replaced in the new programme of observations commencing 1965 April 1. The replacement stars have been observed with the PZT in the past two years and the work of reducing their positions to the PZT system of places is almost completed.
The observed rate of rotation of the Earth has continued to decrease, by approximately 0.00026 milliseconds per day per day; the average losing rate relative to Ephemeris Time, made available by atomic standards, was 2.0 milliseconds per day.
By international agreement the offset for the carrier frequencies of the coordinated radio time signals remains at - 150 parts in 1010 for the calendar year 1965; this is equivalent to a losing rate of 1.3 milliseconds per day. The big difference between this rate and the rate of rotation of the Earth has necessitated four adjustments in epoch to the time signals, each of 100 milliseconds retardation. The adjustments were agreed internationally and made on 1964 April 1, September 1, 1965 January 1 and March 1. At the end of August and the end of December the coordinated time signals were emitted in advance of U.T.2 by about 130 milliseconds. British radio time signals were advanced 4.5 milliseconds on 1964 September 1, to bring them into line with other coordinated signals. A systematic error of one part in 1010 gives rise to a cumulative discrepancy of over three milliseconds in one year.
The transatlantic synchronization of radio time signals has become of such importance that three experiments, originating in the U.S.A., have been conducted with flying clocks. In October a quartz clock was compared with WWV at Beltsville (the emitting station), and brought by air to the U.K., where it was compared with MSF at Rugby and with clocks at the Royal Greenwich Observatory. On its return to the U.S.A. the clock was rechecked with WWV. This experiment helped to explain discrepancies of half a millisecond found in the timing of artificial satellites. In February another quartz clock, and later a portable caesium standard, were compared with clocks at the Observatory as part of a world-wide experiment.
If the difference in emission times of the two signals, WWV and MSF, is determined by the travelling clock, then it is possible to calculate the effective surface speed of the radio waves. The experiment made in October gave a value of 287 km/millisecond, and this figure was confirmed by the February experiments; but the reception of WWV was poor in February so that the result was not reliable. This figure is a little higher than that used at present for HF radio waves, i.e. 280 km/millisecond.
TIME AND LATITUDE SERVICE
1. Astronomical observations
A new observing programme with the Photographic Zenith Tube was commenced in 1965 April, details of which are published in R. Obs. Bull. No. 113.
During the twelve months up to 1966 March 31, 101 plates with an average of 16 stars per plate were obtained. Reductions were carried out with the aid of the ICT 1201 computer in H.M.N.A.O. until 1965 September, on the IBM 7094 at the IBM Data Centre in London until 1966 May and thereafter using the new ICT 1909 computer in H.M.N.A.O.
An attempt was made to correlate variations in observed latitude with recorded variations in the horizontal temperature gradient measured by thermocouples within the tube. The measures showed the existence of significant variations of temperature gradient caused by the operation of the extractor fan employed to prevent stratification of air within the tube,
but attempts to correlate these variations with apparent nightly latitude drift failed to yield satisfactory results.
The observed rate of rotation of the Earth has continued to decrease. The average losing rate relative to ephemeris time, made available by atomic standards, was 2.3 milliseconds per day as compared with 2.0 milliseconds per day in the previous 12 month period.
2. Time signals
By international agreement the offset for the carrier frequencies of the coordinated radio time signals was altered to - 300 parts in 1010 for the calendar year 1966; this is equivalent to a losing rate of 2·6 milliseconds per day. The coordinated signals were retarded by 100 milliseconds on 1965 July 1 and again on 1965 September 1. The signals were emitted within 100 milliseconds of UT2 throughout the year. The UK signals were retarded by 1.0 milliseconds on 1966 March 1 to bring them into closer agreement with other coordinated signals.
As a result of experiments carried out with travelling clocks, and in the light of data derived from tracking observations of the 6303 I artificial satellite, a new figure was adopted for the effective surface speed of HF trans-Atlantic radio propagation. From 1966 January 1 the adopted speed was changed from 280 to 285 km/ms.
The GBR 16 kc/s transmitter was taken out of operation for modernization on 1966 January 1. The service of VLF radio time signals is being provided from the reserve transmitter Criggion GBZ on a frequency of 19·6 kc/s.
3. Equipment
A third Sulzer quartz crystal oscillator was installed in November. By the end of the year the drift rates of the first two oscillators were less than 1 part in 1011 per day and that of the new oscillator was 2 parts in 1011 per day.
Two VLF tracking receivers were installed in January for use in the measurement of UK and US radio emissions. A digital counter displays continuously the relative time difference between the phase of the incoming VLF carrier frequency and the phase derived from a local quartz clock to an accuracy of 0.1 microsecond and a recorder gives a continuous record of the phase variations to 0.5 microsecond.
A new frequency comparator has been installed and is capable of measuring to an accuracy of 1 part in 1012 over an interval of one second.
4. Atomic frequency standard
A caesium beam frequency standard was installed in the Time Department on 1966 May 6. The caesium beam tube resonator uses caesium 133 to obtain an atomic resonance frequency which is compared with a multiplied output frequency from a 5 Mc/s crystal oscillator; any difference in frequency is used to correct automatically the crystal oscillator frequency.
A frequency of 100 kc/s 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.
The caesium beam standard employs solid-state circuits. It is mounted on a standard 19 in. x 8¾ in. rack panel and is approximately 17 in. deep. The manufacturer's stated frequency stability (standard deviation) is ±2x10-11.
TIME AND LATITUDE SERVICE
Photographic zenith tube. The instrument has been in regular use throughout the period, and 198 plates were obtained. Results are now communicated weekly to the Bureau International de l'Heure and monthly to the International Polar Motion Service.
Astrolabe. An analysis of the Herstmonceux astrolabe observations made at a site adjacent to the PZT in the years 1959–1963 has been prepared for publication. This shows a closing error in latitude of +0".25, which confirms the existence of a local diurnal variation in latitude previously discovered in the PZT observations, and it suggests that the source of the effect is local rather than instrumental.
Rotation of the Earth. During the period covered by this report the observed rate of rotation of the Earth has settled to an approximately constant losing rate of 2.4 msec/day relative to ephemeris time made available by atomic standards.
Time signals. By international agreement the offset for the carrier frequencies of the coordinated radio time signals remained at - 300 parts in 1010 for the whole period; this is equivalent to a losing rate of 2·6 msec/day. There were no step adjustments, and the signals remained within 100 msec of UT2 throughout the period.
The GBR 16 kHz transmitter which had been out of operation for modernization since 1966 January 1 was brought back into use on 1966 December 1; it was again out of operation from 1967 February 1 until 1967 March 6. During these periods the service of VLF radio time signals was provided by the reserve transmitter GBZ on a frequency of 19.6 kHz.
Clocks. A second caesium beam atomic standard was installed in the Time Department in 1967 March. Between 1967 June and 1967 December 31 the divergence between the two atomic standards was two microseconds, corresponding to a frequency difference of 1 part in 1013.
Since 1967 July the time service has been based on these two caesium standards together with the best two of the Sulzer quartz crystal standards. The last and best of the quartz crystal standards designed and built by the Royal Greenwich Observatory, which incorporated an Essen ring crystal, was taken out of service in 1967 May after an oven failure, having been in use from 1954.
Travelling clock comparisons. Comparisons carried out in 1966 November and 1967 March using portable caesium beam atomic standards from the U.S. Naval Observatory showed a change in the difference between the master clocks at the two observatories of 21 microseconds. A visit was also made by a commercial firm in 1967 October in which the U.S.N.O. and R.G.O. standards were compared and fell within the same limits. Use was also made of these visits to check the effective surface speed of HF trans-Atlantic radio propagation. The mean speed was 285 km/ms. The U.S. Coast and Geodetic Survey visited the R.G.O. on seven occasions to check their time standards used in the Western European Triangulation Survey.
Equipment. Considerable progress has been made towards rebuilding obsolete parts of the instrumentation of the Time Department at Herstmonceux.
A complete time-keeping and distribution system, based on commercially available quartz crystal oscillators, was designed and constructed at Herstmonceux, and installed at the Cape during the latter part of 1967.
TIME AND LATITUDE SERVICE
The PZT continued in regular service throughout the year, and 101 plates with an average of 15 stars per plate were obtained. The decrease in the number of plates in comparison with the previous year is indicative of the exceptionally cloudy weather in 1968.
The PZT observations made in the years 1958-67 have been analysed to evaluate the errors in the relative positions adopted for the stars in 1957, using a chain-method. The results were found to be in close agreement with those obtained by comparisons between the Herstmonceux time and latitude results, and the 'Heure définitive' and the x coordinates of the pole, respectively, computed for the epoch of observation from the published results of the Bureau International de l'Heure. In addition, a correction of +0".40, deduced from transit circle observations and from comparisons between the PZT and astrolabe results, has been applied to the adopted declinations of all the stars.
Proper motions of 22 potential PZT stars have been computed by the Meridian Department.
The observations have been communicated weekly to the BIR and monthly to the IPMS.
The published results from 1968 January are based on the revised Constant of aberration (20".496) and on the corrected star positions. Commencing with this year the coordinates of the Instantaneous pole are referred to the Mean Pole of 1900-05, now designated the Conventional International Origin (CIO). On 1968 January I the adopted longitude of the instrument was changed from 1m21s.102 E to 1m21s.0785 E following comparisons made with the BIH. Arising from these changes there was a discontinuity in the time system such that UT2 (Herstmonceux) becomes earlier by 7 milli-seconds
In 1968 a PZT was inaugurated near Calgary in Western Canada by the Dominion Observatory. This instrument is on the same latitude as the Herstmonceux PZT’ the two instruments use the same observing programme, and the observatories work in close co-operation.
The rate of rotation of the Earth has remained at 2.4 ms day-1 losing relative to ephemeris time which is made currently available by atomic standards: there was, however, a rather sudden change towards the end of August which resulted in a loss of time of approximately 20 msecs, followed by a resumption of the former rate.
By international agreement the offset of the carrier frequencies of the coordinated radio time signals remained at - 300 parts in I0I0 (equivalent to a losing rate of 2.6 ms day-1). The signals were advanced by 100 ms on 1968 February 1 and remained within 100 ms of UT2 throughout the period. The HF signals associated with GBR were discontinued from December 31, as their usefulness had decreased and many standard frequency emissions are now available as alternatives.
A third caesium beam atomic standard was installed in August and brought into operational use in November. The Greenwich atomic time scale, which commenced in 1955, is now based entirely on standards at Herstmonceux.
A new series of Circulars was inaugurated containing the daily readings of the relative phase difference between the Herstmonceux atomic standards and the received carriers of selected LF and VLF radio emissions. By using similar results published elsewhere, the atomic time scales of different establishments are compared. Using the additional information provided by travelling clock comparisons, coordination to an accuracy of about 10 μS has been maintained over a period of several months.
In addition to the comparisons made by means of the travelling clocks of the USNO, other organizations have utilized the facilities provided by the RGO to obtain a link with the USNO time system.
Progress in the replacement of obsolete equipment in the Herstmonceux Time Service has been slow but IS continuing.
At the Cape, a 50-ft aerial has been erected, for receiving 10 MHz time signals from Johannesburg. Good signals can now be obtained at any time of day.
TIME AND LATITUDE SERVICE
No change was made in the PZT observing programme and 119 plates with an average of 17 stars per plate were obtained. The star count does not include the observations, also contained on the plates, for the determination of the positions of 22 stars which are to be included in the observing programme for the years 1971–77. The results have been communicated weekly to the BIH and monthly to the IPMS.
The first year's observations of the Calgary PZT mentioned in last year's report are now being studied and the preliminary results are very encouraging,
At the beginning of the year, from about the end of January to the middle of March, there was a gradual change of rate of rotation of the Earth, and it appears to have settled down to a losing rate of 2.7 milliseconds per day relative to ephemeris time which is made currently available by atomic standards.
By international agreement the offset of the carrier frequencies of the coordinated radio time signals remained at -300 parts in 1010 (equivalent to a losing rate of 2.6 milliseconds per day).
The Greenwich atomic time scale, GA2, based on caesium standards at Herstmonceux, has been published quarterly in the Greenwich Time Reports.
Two travelling clock comparisons were made with the USNO, one in February and one in July, showing a difference of less than one part in 1013 between the UTC time scales of the RGO and the USNO.
A Loran-C receiver has been installed in the Time Department. Measurements have been made of the pulse from Sylt, one of the slaves in the Norwegian Sea Chain of which Ejdes is the master station. The results have been published in the Circulars, Series B, from April, in the form UTC (RGO) – Ejdes.
TIME AND LATITUDE SERVICE
The results for the year are based on 1998 transit observations obtained on 120 plates. The plates also contain 359 observations of the 22 new stars scheduled for inclusion in the PZT programme from 1971 April. The Herstmonceux observations of these stars have now been reduced and the star place corrections have been computed. The adopted place corrections will take into account the observations of these stars with the Calgary PZT. The observations of the two instruments have also been combined in an investigation of the polar motion; the results are of special interest at present for the study of the improvements that should be achieved in the International Polar Motion Service by the equipment of the ILS stations with PZTs.
The PZT time and latitude observations for the years 1958-1967 were re-reduced this year using the star positions adopted in 1968 and with the revised value of the constant of aberration. The new UT1 results for the same period are now based on the OCI mean pole used in current reductions, since the beginning of 1968. The Cookson observations of the variation latitude at Greenwich between 1916 and 1936 have been combined with the contemporary latitude observations of the Washington PZT in an investigation of the polar motion over the period.
The rate of rotation of the Earth has remained constant from about mid-February 1969 at 2.7 ms per day losing relative to the atomic time scale defined in terms of ephemeris time. By international agreement the offset of the carrier frequencies of the co-ordinated radio time signals remained at - 300 parts in 1010, which is equivalent to a losing rate of 2.6 ms per day.
The Greenwich atomic time scale, GA2, based on caesium standards at Herstmonceux has been published quarterly in the Greenwich Time Reports. The co-ordinated time scale UTC (RGO) is determined arithmetically from GA2. GA2 and thus UTC (RGO) have diverged by less than 3 μS during the year from the corresponding international time scales, AT and UTC, computed by the B.I.H. International comparisons of atomic and UTC time scales have been made to this accuracy by measurements of Loran-C pulses and by travelling atomic clocks. The U.S. Naval Observatory has made three visits with caesium standards on regular flights.
In September the RGO took part in an experiment, operation 'Synchran' (Synchronisation Atlantique Nord) employing a method developed by the French Office National d'Études et de Recherches Aérospatiales for precision intercomparisons between remotely situated clocks. The time difference between a clock carried in an aircraft, whilst the aircraft is in flight, and a clock on the ground, was measured.
The aircraft passed over the Paris Observatory, the Royal Greenwich Observatory, the laboratories of the National Research Council in Canada, and the U.S. Naval Observatory. The scatter of the measures made at Herstmonceux was about 50 ns in bad weather conditions and 30 ns in good weather.
A complete programme of modernization of the time service installation is being continued incorporating recent developments in electronic techniques. An automatic comparator produces a printed output and a punched tape record of time comparisons between the caesium standards and between one standard and the output of the Loran-C receiver, three times daily to an accuracy of one-tenth of a μS. There are now four caesium standards at the RGO one of which has been continuously in use since March 1967 on low beam current.
TIME AND LATITUDE SERVICE
PZT
The PZT continued in service throughout the year and observations of 2400 star transits were obtained on 132 plates. The improved mechanical operation of the plate-carrier drive following the installation of the stepping motor, described in last year's report, has been maintained without loss in accuracy.
The new catalogue contains 120 stars (98 of which appeared in the previous catalogue, used since 1965) and it was introduced simultaneously at Herstmonceux and Calgary in 1971 April. The 22 new programme stars were observed at the two stations in 1969 and 1970 and the corrections to the provisionally adopted positions determined at the two stations were in close agreement. At the same time, corrections to the positions of the 98 stars retained in the new catalogue were also computed and revised positions were adopted. The results have also shown that corrections to the proper motions can be determined with only a few years' observation.
Co-operation with the BIH has continued and the observations were reduced daily in October and November, for the rapid determination of the data supplied by the BIH for the guidance of the Mariner 9 space probe.
Research on the polar motion for the years 1916-33 was completed and the results were presented in IAU Symposium No. 48.
Danjon Astrolabe
The Danjon astrolabe which was returned from South Africa in 1970 was refurbished in the engineering workshop and made ready for service, The method of observation based on photographic recording of the micrometer at preset times has been discontinued and the instrument is now connected to a paper-tape punch for recording the clock times of micrometer contacts,
The Herstmonceux computer program providing for the re-reduction of all the 1965-69 observations on a uniform system independent, to a first approximation, of observer, observing clock, group, the selection of standard stars observed, and time of night, and empirically corrected for variation of prism angle, was completed and proved early in the year. A preliminary analysis of the time and latitude results for the determination of observer and group corrections, and examination of diurnal variations, has now been completed. A preliminary analysis of the prism angle results, and determination of provisional corrections to the adopted places of standard stars is still in progress. Most of this work has been undertaken at the Cape, under the direction of Dr Thomas at Herstmonceux.
Rotation of the Earth
At the end of March there was a change in the rate of rotation of the Earth relative to the atomic time scale from 2.7 ms day-1 to 3.1 ms day-1 losing.
By international agreement the offset of the carrier frequencies of the co-ordinated radio time signals remained at -300 parts in 1010, equivalent to a losing rate of 2.6 ms day-1.
Atomic time scale
The Greenwich atomic time scale GA2 has been published quarterly in the Greenwich Time Reports; it is one of the seven independent atomic time scales from which the International Atomic Time Scale is formed and has been within two and a half microseconds of IAT throughout the year. GA2 is formed from selected caesium clocks at Herstmonceux and, in order to maintain uniformity, rate corrections have been applied to minimize the effects of the introduction or removal of clocks, It follows that GA2 no longer represents the true mean of the clocks on which it is based, and it has therefore been decided to publish five-day values of the individual clocks referred to GA2. No rate corrections have been applied for slow frequency drifts in the clocks. The co-ordinated time scale UTC(RGO) is obtained arithmetically from GA2. To meet the new requirements, additional equipment has been developed and installed and external modifications made to existing units. International comparisons of atomic clocks are made by measurements of Loran-C pulses and by travelling clocks, and additional Loran-C receivers have been brought into service. The USNO has made two visits with caesium clocks on regular flights.
International co-operation
The outstanding feature of current developments is the increasing degree of international co-operation. In order to achieve the standards of accuracy now demanded in the up-to-date knowledge of UT and polar variation, it is necessary to combine the observational results from some 15 to 20 astronomical stations geographically widely spread. The excellent results being attained with the Herstmonceux PZT make a significant contribution to the provisional figures computed by the Bureau International de l'Heure (BIH). In the event of the present BIH programme being discontinued owing to lack of funds, the RGO is able to furnish; by the co-operation of the observatories concerned, provisional values based on the current results at Herstmonceux, Washington, Richmond and Calgary.
The scale of atomic time is formed by summation from an adopted initial epoch. There is no independent check which can be applied to determine the amount of any accumulated time error. While the AT scale may conform to ET, there are possible theoretical differences, and the accuracy with which ET can be determined is comparatively poor. It is therefore essential to form a mean of a large number of independent atomic standards, preferably at widely-separated locations, to establish a reference scale of AT. Again this is a matter of co-operative action, co-ordinated by the BIH. In this continuing commitment, the RGO again plays its full part.
Radio time signals
The desirability of international synchronization of radio time signals has been evident from their inception over 60 years ago, and has now been realized by the general adoption of a new form of Co-ordinated Universal Time (UTC) from 1972 January 1. Mr H.M. Smith has been largely involved in the preparations for the new system, and continues to be concerned with its implementation, through active participation in the work of the IAU and the Consultative Committee for the Definition of the Second set up by the International Committee of Weights and Measures (CIPM), as Chairman of the Directing Board of the BIH and as Chairman of Working Party 7/1 of the International Radio Consultative Committee (CCIR).
Following discussions and negotiations over a number of years, the following international agreements were adopted:
CCIR Recommendation 460 (New Delhi, 1970); lAU Resolution 1 of Commission 31 (Brighton, 1970); CCIR Report 517 (Geneva, 1971); and CGPM Resolution 1 (1967), Resolutions 1 and 2 (1971).
As a result of these arrangements the new system of co-ordinated Universal Time (UTC) was introduced at midnight on December 31. Under this system standard frequency emissions and radio time signals will be broadcast without offset so that the rate is in accordance with the IAT Scale and the time intervals between pips correspond exactly to the second as defined in 1967. In order that the broadcast time scale shall not depart from UT1 by more than 0.7 s, the time signals will be adjusted by the introduction of a leap second, when required, preferentially on June 30 or December 31. The approximate value of the difference UT1 -UTC will be given in code in the time signals to 0.1 s.
A special adjustment was made to the time signals at midnight on December 3 I so that they differ from the International Atomic Scale by an integral number of seconds.
TIME AND LATITUDE SERVICE
Photographic Zenith Tube
The PZT was maintained in service without major interruption, and observations of 2298 star transits were obtained on 137 plates. Cooperation with the BIH and the IPMS has continued and the results are regularly reported to these agencies. Research in methods of improving the instrument performance has made some progress: new equipment for effecting rotary reversals has been developed, and a Moiré fringe linear displacement transducer for monitoring the motion of the photographic plate is now being assembled.
Analysis of the time and latitude observations made between 1958.0 and 1970.0 has revealed effects of systematic deflexions of the vertical that depend on the hour angle and declination of the Moon. Preliminary results of this study are in press, and further work is in progress. Analyses have also shown the existence of fortnightly terms, with amplitudes of the order of 0”.01, in both the time and latitude observations. From a review of the results over the years 1958.0 to 1971.2 corrections have been derived, in both co-ordinates, to the adopted proper motions of the catalogue stars. These corrections are now being utilized to compute corrections for the adopted mean positions of the stars. This work is being carried out in collaboration with the Calgary PZT Observatory, which continues to work in close cooperation with the RGO.
Danjon astrolabe
Tests on the Danjon astrolabe showed that its optical quality had greatly deteriorated since it was previously used at Herstmonceux (1964). The transmission optics were returned to the manufacturers (SOPELEM, Paris) for investigation, and defects discovered in these components were rectified. Re-assembly has just been completed and the instrument is now ready for further tests.
Gravimetric recording
A recording gravimeter (North American Type) employed in a programme of determination of gravity profiles across Europe was installed in the sub-basement of the West Building at Herstmonceux in October. Collaboration of the RGO in this work was organized by P. Melchior, and the results obtained to date are very encouraging.
Rotation of the Earth
Until the end of September, the average rate of rotation of the Earth was 3.2 ms per day losing, relative to the atomic time scale; since then there appears to have been a change of rate in the sense that there was a small reduction in the losing rate.
Atomic time scale
The Greenwich atomic time scale, GA2, has been determined from the mean of selected caesium standards at Herstmonceux, and is one of seven atomic time scales from which the International Atomic Time Scale is determined. Atomic clocks are compared internationally by measurements of Loran-C pulses and by travelling clocks. Measurements commenced of the Loran-C pulses from Estartit, in the Mediterranean chain, in June, and from Ejdes, the master in the Norwegian Sea chain, in August. These results, together with those from Sylt, which have been measured for several years, have been published each month in the Time Service Circulars, Series B. Staff from the USNO have made two visits with caesium clocks on regular flights.
At the end of July, the Time Department took part in an experiment to compare atomic clocks at the US Naval Research Laboratory, Washington, and the RGO, using a clock carried by a satellite. The satellite, known as Timation II, is in a circular orbit 500 nautical miles above the surface of the Earth and at an inclination of 70° to the equator. Members of the staff of the US Naval Research Laboratory brought the satellite receiving equipment to the RGO, and signals received from the satellite were measured at RGO and NRL, Washington.
Radio time signals
In accordance with international agreements, the new UTC system was brought into operation on 1972 January 1 at 0000 UT. Under this system the emitted pips differ from the International Atomic Time Scale, determined by the Bureau International de I'Heure, by an integral number of seconds, and the intervals between seconds pips corresponds to SI second. In order that the time signals shall not depart from UT1 by more than about 0.7 s, leap seconds are applied when necessary. A positive leap second was applied as the last second of June 30 and again as the last second of December 3I.
TIME AND LATITUDE SERVICE
Photographic Zenith Tube
The PZT continued in service throughout the year, and 2960 star transits were observed on 161 nights, i.e. 54 per cent of the nights scheduled. Up-to-date results of the time and latitude observations have been communicated weekly to the Bureau International de l'Heure, and the collaboration with the International Polar Motion Service has also been maintained. The quality of the observations is recognized in the exceptionally high weights assigned to the Herstmonceux PZT results by the BIH. Maintenance of this high standard is becoming increasingly difficult because of delays in the installation of the Moiré fringe timing system and new control equipment.
Mr N.P.J.O'Hora has completed his analysis of the effects of deflexions of the vertical, referred to in last year's Report. Further analysis of the PZT observations by O'Hora has confirmed the existence of the quasi-diurnal term predicted by Atkinson. This term arises from the use of nutation corrections computed for the Earth's pole of rotation. The widely publicized notion that the great solar storm at the beginning of 1972 August significantly affected the rate of rotation of the Earth has been shown by O'Hora and Miss C.J.A.Penny to be inconsistent with the observations made by six national observatories, including the RGO. This work has led to a more extensive examination of geophysical phenomena that might affect the rotation.
All star transits observed at Herstmonceux since 1958 and at Calgary since 1968 have been analysed for the determination of improved star positions; this work is now nearly complete and improved positions and proper motions for the catalogue stars should be available for use in 1974. The availability of improved star co-ordinates will warrant re-reduction of all previous observations, and the subsequent processing of such a large number of observations has emphasized the need for computer methods of smoothing results. The implications of Vondrak's method of smoothing non-uniform time series have been investigated by Dr D.V.Thomas, and O'Hora has devised a computer-application of this method to the PZT observations.
Danjon astrolabe
This instrument is at present being modified in the Engineering Department to enable observations to be carried out with a reflecting objective prism of Cer-Vit in place of the transmission prism. The advantages of the use of such a prism were set out several years ago by Thomas. A satisfactory method of mounting the prism has now been devised through collaboration of the Time and Engineering Departments.
Rotation of the Earth
Mean Solar time was losing on the atomic time scale by an average of 3.2 ms per day until mid-September when the rate changed to 2.8 ms per day losing.
Atomic time scale
The Greenwich atomic time scale GA2 is an independent atomic time scale based on the mean of selected caesium standards at Herstmonceux. On 1958 January 1 GA2 was in agreement with the International Atomic Time Scale (TAI) of the Bureau International de l'Heure; at the end of 1973 the difference TAI-GA2 was approximately +27 μs, and the frequency of GA2 was 0·5 parts in 1012 less than that of TAI. In 1971 evidence from three independent laboratories using primary standards indicated that the frequency of TAI was about one part in 1012 too high. For the past four years GA2 has been one of the seven independent atomic time scales from which TAI has been determined. A new method of determining TAI has been adopted by the BIH since 1973 July I; it uses the results of individual atomic clocks which are compared by means of measurements of the reception times of pulses of the Loran-C radio-navigation system. The results of all the RGO atomic standards are sent to the BIH for inclusion in TAI, which is then published a month or two in arrear. GA2 is available as a national scale on a day-to-day basis.
Staff of the US Naval Observatory have made two visits to RGO with portable caesium clocks in order to check the differences between atomic clocks and to investigate the cause of small discrepancies, of the order of 3 μs, between clock comparisons made by travelling clocks and by reception of Loran-C pulses in Europe and America.
Radio time signals
In accordance with international agreements, and in order to keep broadcast time scales within agreed limits of UTI, a positive leap second was inserted in the UTC time signals emitted by Rugby and the BBC as the last second of 1973 December 31.
Time and Latitude Service
Photographic Zenith Tube. Under the supervision of Mr. N. P. J. O'Hora, 2688 star transits were observed with the PZT on 146 nights, and the time and latitude results communicated weekly to the Bureau International de l'Heure (BIH) and the office of the International Polar Motion Service. An improved catalogue of positions and proper motions has been derived by O'Hora, Mr. J. V. Carey and Wheeler (Calgary PZT Observatory) from an analysis of the PZT observations made at Herstmonceux in 1958-1972 and at Calgary in the years 1968-1972. All observations to date are being re-reduced using the new stellar data and improved reduction techniques.
The design and manufacture of components required for modification of the PZT to enable it to operate completely under automatic control have continued. Preliminary work on the use of a mini-computer for the on-line monitoring and recording of plate-measuring has also been carried out.
Danjon Astrolabe. The modifications to the instrument, associated with the replacement of the original objective prism by a reflecting prism made of Cer-Vit, have been completed. Tests of the new configuration have been delayed by the shortage of scientific staff.
Rotation of the Earth. The comparison between universal time (UT1) and atomic time shows that the average rate of rotation of the Earth remained constant during the year, so that the departure between the two scales remained at the value of 2.7 ms d-1, to which it changed over quite a short interval in 1973 October. This change is the first long-term increase in the rate of rotation since monitoring of the Earth's rotation against atomic time began in 1955.
Atomic Time. Under the supervision of Miss C. J. A. Penny, the Greenwich atomic time-scale (GA2) was formed throughout the year by combining the scales of individual caesium-beam time standards (atomic clocks) at RGO. The results of the comparisons of the individual clocks with the time pulses of the radio signals of the Loran-C navigation system were communicated to the BIH for use in the formation of the international atomic time scale TAI.
The atomic clocks purchased since the beginning of 1972 have not proved to be as reliable as earlier clocks. Thus GA2 was based on five clocks for less than one month; for 2½ months the scale was based on only two clocks, one of which has been in operation since 1967. In spite of these difficulties, the aim of providing a uniform, independent time-scale has been realized, as is shown by the comparisons with TAI, which is currently based on the mean of some 70 clocks, but which is published about two months in arrears. The frequency of GA2 was about 3 parts in 1013 lower than that of TAI in 1974, whereas it was about 5 parts in 1013 lower in 1973.
The frequency of TAI has been compared with the frequencies of primary laboratory standards in other countries. It appears that the difference was almost constant throughout 1974, but the frequency of TAI was about 1 part in 1012 high.
Staff of the US Naval Observatory made one visit with a portable atomic clock in order to establish more precisely the difference between the clocks at Herstmonceux and elsewhere and to investigate the causes of the small discrepancies between the time-differences derived from intercomparisons via the Loran-C signals and those via a portable clock.
A new battery system and a new frequency distribution system, designed by Mr. H. G. Gill, were both installed during the year.
Radio Time Signals. The fiftieth anniversary of the inception of the "six-pips" time signals occurred on 1974 February 5. During the year the time signals, on the scale of coordinated universal time (UTC), were provided to the BBC by land-line without interruption. Throughout 1974 the difference TAI-UTC was 13 seconds, but by international agreement a positive leap second was inserted in UTC as the last second of December 31.
The national radio time signals emitted by the Post Office from Rugby (GBR) have been monitored at Herstmonceux, and the reserve transmissions (GBZ) were calibrated for use in early April. Other international time signals have also been monitored regularly.
Time and Latitude Service
Photographic Zenith Tube (PZT). The programme for the determination of universal time (UT) and the variation of latitude by astronomical observations with the PZT was continued under the supervision of Mr. N. P. J. O'Hora. 2022 star transits were observed on 101 nights and the results were sent weekly to the Bureau International de l'Heure (BIH) and the International Polar Motion Service. The use, from January 1, of an improved catalogue of stellar positions and proper motions and better reduction techniques has led to reductions of about 10 per cent in the external standard error of the results from a single plate and about 15 per cent in the internal standard errors for each star.
A new rotary drive, powered by a stepping motor, has been constructed in the Observatory workshop, and tests on the PZT indicate that it will prove much more efficient than the present drive. Design studies of the necessary alterations for fully-automatic operation are nearly complete and some of the new equipment has already been ordered. The time distribution equipment at the Spencer Jones Group terminal is also being redesigned.
Danjon Astrolabe. It has not been possible, because of a shortage of staff, to make the observations required to evaluate the performance of the new reflecting prism in the Danjon astrolabe. The roof of the observing hut was replaced in August by a new roof assembled in the Observatory workshops.
Atomic Time. Under the supervision of Miss C. J. A. Penny, the Greenwich Atomic time-scale (GA2) was formed throughout the period by combining the scales of the individual caesium-beam time standards (atomic clocks) at RGO. The GA2scale was based on four clocks for most of the period.
The results of the comparisons of the clocks with the time pulses of the radio signals of the Loran-C navigational system were sent to the BIH for use in the formation of the international atomic time-scale (TAI).
A sixth caesium standard was brought into operational use; it has a high-performance beam-tube, but it is too early yet to assess its long-term stability. Prototype equipment for making clock comparisons with an accuracy of better than one nanosecond (10-9 s) has been designed and tested by Mr. H. G. Gill; he was assisted in the production and initial evaluation of the equipment by Mr. R. Zerrander of the Swedish Research Institute for National Defence.
Radio Time Signals. The national time signals (GBR) emitted by the Post Office from Rugby, and some international time signals, have been monitored regularly at Herstmonceux. A special series of clock comparisons with Loran-C emissions has been made at the request of the US Naval Observatory in order to measure directly the difference between the Norwegian Sea Chain and the North Atlantic Chain, and so to increase the accuracy of the transfer of time across the Atlantic.
Travelling-Clock Comparisons. Travelling clock comparisons were made with an RGO caesium standard as follows: in January, at the request of Government Communications Headquarters, Cheltenham, to synchronize their timing system; in March, at the request of the Nuffield Radio Astronomy Laboratories, to synchronize their clock with the US Naval Observatory time scale in connection with a joint Jodrell Bank-USA programme of very long baseline interferometric observations.
Time Comparisons by Satellite. A joint US-Australian-British experiment to evaluate the use of the Navigational Technology Satellite (NTS-I, previously known as Timation-3 for global clock synchronization at the sub-microsecond level is now in progress. The satellite carries two rubidium oscillators and one specially-developed quartz crystal oscillator, and emits time signals which are compared with the clocks at the ground stations. The observations are analysed to improve the knowledge of the satellite's orbit and clock performance and hence to improve the precision of the comparisons between the time-scales of the ground stations. (If the observations are made simultaneously from two stations the comparison between their time-scales is independent of the performance of the satellite clock, but depends on the accuracy with which the travel times can be calculated.) Aerials and the associated timing and recording equipment, lent by the US Naval Research Laboratory, were installed at RGO under the direction of Mr. H. G. Gill. Satellite passes occur at intervals of about eight hours, and measurements are made at Herstmonceux for about two hours during each pass; 50 passes were observed between July 2 and August 28, but observations were then suspended while the receiving equipment was being modified.
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Time Service Circulars A (weekly)
Time Service Circulars B (monthly)
Time Service Notices (irregularly)
Greenwich Time Reports 1974 January-September.
Time and Latitude Service
Photographic Zenith Tube (PZT). The programme for the determination of universal time (UT) and the variation of latitude by astronomical observations with the PZT has continued under the supervision of Mr. N. P. J. O'Hora: 2664 star transits were observed on 138 nights and the results were sent weekly to the Bureau International de I'Heure (BIH) and the International Polar Motion Service (IPMS). Preliminary work for the publication of the catalogue of PZT stars has now been completed. The PZT was taken out of operational use in August to allow the fitting of the new rotary drive and moiré-fringe equipment to monitor the displacements of the photographic plate. Work on the automation of the plate-measuring has continued slowly; a sandwich student has recently been appointed to assist in this work.
Danjon Astrolabe. Commissioning of the modified Danjon astrolabe. with its reflecting prism, has been indefinitely postponed because of the shortage of staff.
Atomic Time. Under the supervision of Miss C. J. A. Penny, the Greenwich Atomic Time-scale (GA2) was formed throughout the period by combining the scales of a selection of the six caesium-beam time-standards (atomic clocks) at RGO. The number of clocks used in forming GA2 varied from five down to only two, for a period of about three months. Comparison of individual RGO clocks with the time pulses of the Loran-C radio navigational system has continued: the results are used by the BIH in the formation of the international atomic time-scale (TAI).
Studies of ways of comparing the clocks at RGO with those at the National Physical Laboratory have been started. Both establishments have begun to measure the times of arrival of selected field-synchronization pulses on BBC TV. The accuracy so far achieved, about 0.5 μS, is not yet adequate, but improvement may be expected.
The results of the high-performance caesium-beam tubes show a significant improvement over those of the standard tubes: the precision on a day-to-day basis is about 7 ns, compared with 50 ns. So far. however, there is no evidence that the long-term stability and reliability are better. The installation of the equipment for making more precise clock comparisons is not yet complete.
Radio Time-signals. The RGO has continued to provide the BBC "6-pips" time-signal, and to monitor the radio time-signals on GBR. and a small number of other signals. In addition the USNO has provided a fourth Loran-C receiver for monitoring the synchronization of the emissions, and hence the accuracy of the navigation system.
Travelling-Clock Comparisons. An RGO caesium clock was taken to the Paris Observatory at the time of a visit of a clock from US Naval Observatory in order to compare the time-scales of RGO, USNO and BIH to better than 1 μS, and to calibrate the regular use of the Loran-C radio-navigation signals for time comparisons. A clock was also taken to Jodrell Bank to synchronize the station clock with UTC(USNO), for use in VLBI observations. Assistance in the commissioning of a Loran-C receiver was given, and a clock was brought from the USNO to the RGO at the beginning of July to complete the calibration.
Time Comparisons by Satellite. Within the limited regions of their coverage, the Loran-C radio-navigation signals continue to provide the principal means for the international comparison of atomic clocks, and hence for the establishment of TAI. The RGO has continued to collaborate in the testing of the use of the Navigational Technology Satellite (NTS-1) for global clock comparisons at the submicrosecond level. The analysis of the early data (1975 July) has shown this technique is potentially more precise than that of using Loran-C, but at present the system is only giving comparable precision. RGO staff have collaborated with the engineers of the US Naval Research Laboratory (which has lent the equipment) in the improvement of the receiver and aerial. The precision of the comparison in the course of a pass between the RGO and satellite clocks has been reduced from 45 to 15 ns. The uncertainty in the comparison between the clocks at the ground stations is affected by the uncertainties in the knowledge of the orbit, as well as by the uncertainties in the ionospheric delays.
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Time Service Circulars A (weekly)
Time Service Circulars B (monthly)
Time Service Notices (irregularly)
Greenwich Time Reports (quarterly) 1974 October-1975 December.
Time and Latitude Service
Photographic Zenith Tube (PZT). The programme for the determination of universal time and the variation of latitude by astronomical observations with the PZT has continued under the supervision of Mr. N. P. J. O'Hora. The results obtained from 1648 star transits recorded on 107 nights were sent weekly to the Bureau International de I'Heure (BIH) and the International Polar Motion Service (IPMS). The catalogue of PZT stars, based on observations made with the Herstmonceux and Calgary PZTs, has been published (Greenwich Time Report 1976 January-March) and a new determination has been made of the amplitudes of some short- period terms by an analysis of all observations made with the PZT since 1958. The measurements made on the photographic plates are now recorded and monitored automatically, using the Zeiss Ascorecord (see page 9). Modifications to the rotary drive of the PZT, which were begun during the previous report year, took much longer than expected and have not yet led to satisfactory reliability. Installation of the moiré fringe system for more accurate monitoring of the motion of the plate carriage during exposures has therefore been postponed.
Atomic Time. Under the direction of Dr. J. D. H. Pilkington, a local atomic time scale was formed throughout the period by combining the scales of two to four of the six caesium beam atomic clocks at RGO. On 1977 January 1, by international agreement, the duration of the scale unit of TAI was increased by exactly 10 parts in 1013 (about 32 μS per year) to bring it into closer conformity with recent determinations of the SI second at sea level; the opportunity was taken to make a similar but smaller adjustment in the rate of the local atomic scale,. and to change its name from GA2 to TA(RGO). Four Loran-C receivers (one on loan from USNO) have been used to monitor the Synchronization of the emissions, and hence the accuracy of the navigation system; these receivers also link TA(RGO) to TAI.
The joint reception of TV synchronizing pulses for comparison of atomic clocks at the RGO and at the National Physical Laboratory has continued. No effort has been available for technical improvements at RGO, and the accuracy of about ±0.3 μS is still inferior to that achieved by the use of Loran-C. However, the TV results provide an independent check on those obtained from Loran. Precise time is now available relatively simply to potential users over a wide area of south-east England by reception of TV signal and reference to the regular RGO calibrations.
Radio Time-signals. The RGO has continued to provide the BBC "6-pips" time-signal, and to monitor the radio time-signals on GBR. and a small number of other signals.
Time Comparisons by Satellite. Regular observations have been made of radio signals from the Navigational Technology Satellite NTS-1 and from NTS-2, which was launched on June 23, using equipment lent by the US Naval Research Laboratory. Experience gained with NTS-1, which used quartz frequency standards during this period, indicates that the use of the NTS-2 satellite, which is equipped with caesium beam clocks, should give substantially better accuracy in intercontinental time transfer than can be achieved by the use of Loran-C. Current work at RGO, directed by Dr. J. D. H. Pilkington and Mr. H. G. Gill, is aimed at improving the performance of the receiver so that ionospheric delays can be estimated directly and more accurate predictions of the orbit obtained.
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Time Service Circulars A (weekly).
Time Service Circulars B (monthly).
Time Service Notices (irregularly).
Greenwich Time Reports (quarterly) 1976 January-December.
Time and Latitude Service
Photographic Zenith Tube. The programme for the determination of universal time and the variation of latitude by astronomical observations with the PZT has continued under the supervision of O'Hora. The results obtained from 2374 star transits recorded on 112 nights were sent weekly to the Bureau International de l'Heure and monthly to the International Polar Motion Service.
Atomic Time. Under the supervision of Pilkington, the local atomic time scale, TA(RGO), was formed throughout the period by combining the scales of from two to four of the six caesium-beam atomic clocks at RGO. Corrections derived from inter-comparison of all the clocks were applied, as usual, to maintain the continuity and uniformity of TA(RGO).
Radio Time Signals. The RGO has continued to provide the BBC "6-pips" time signal, and to monitor the radio time-signals on GBR and a small number of other signals. Four receivers (one on loan from the US Naval Observatory) have been used to monitor the timing of the Loran-C emissions, and hence the accuracy of this navigation system; these receivers also provide the link through which RGO clocks contribute to international atomic time, TAI. Monitoring of TV synchronizing pulses has continued, in order to provide data for precise timing within the UK by the use of simple equipment.
Time Comparisons by Satellite. Regular observations have been made of the radio signals from the Navigation Technology Satellites NTS-1 and NTS-2. Most of the equipment used has been lent by the US Naval Research Laboratory, which has also funded the observations since April. These observations are making a valuable contribution to the development of the NAVSTAR-GPS system that is expected to become operational in the mid-1980s and will provide world-wide facilities for position determination and time comparison with precisions of around 10 m and 100 nS respectively; it should also provide information on Earth rotation.
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Greenwich Time Reports (quarterly) 1977 January-December.
Time Service Circulars A (weekly) and B (monthly).
Time Service Notices (irregularly) -14 issues.
Time and Latitude Service
Photographic Zenith Tube. The programme for the determination of universal time and the variation of latitude by astronomical observations with the PZT was continued under the direction of O'Hora; 2653 star transits were recorded on 123 nights and the results were sent weekly to the Bureau International de l'Heure (BIH) and monthly to the International Polar Motion Service (IPMS). Work on the development of an automatic control system for the PZT has continued while keeping the telescope in regular operation
Project MERIT. Wilkins, as Chairman of the IAU Working group on the Determination of the Rotation of the Earth took part in the preparation of a proposal for a special period of international collaboration in monitoring of Earth rotation and the intercomparison of techniques (known as Project MERIT). This proposal, which was endorsed at the IAU General Assembly in August, is primarily concerned with the introduction of modern techniques, such as laser ranging and VLBI, for the regular determination of the variations in the rotation of the Earth; there will be a special campaign during 1983/4 when the classical and modern techniques will be used simultaneously.
Atomic Time. Under the direction of Pilkington, the local atomic time scale, TA(RGO), was formed throughout the period by combining the scales of from two to four of the six caesium-beam atomic clocks at RGO. Corrections derived from inter-comparison of all the clocks were, as usual, applied to maintain the continuity and uniformity of TA(RGO). Some preparatory work for the rebuilding of the electronics was carried out.
Radio Time Signals. The RGO continued to provide the BBC 6-pips time signal (UTC), and to monitor the radio time-signals on GBR and a small number of other signals. Four receivers (one on loan from USNO) were used to monitor the timing of the Loran-C emissions, and so contribute to the accuracy of this navigation system; the results are used to link the RGO clocks and time-scales to international atomic time, TAI.
Time Comparisons by Satellite. Regular observations were made of the radio signals from the Navigation Technology Satellites NTS-I and NTS-2; most of the equipment used was lent by the US Naval Research Laboratory, which also funded the observations. Observations of NTS-2 were a contribution to the development of the NAVSTAR-GPS system that is expected to become operational in the mid-1980s to provide world-wide facilities for position determination and. time comparison, with precisions of about 10 m and 100 ns respectively, and will also yield information on Earth rotation. Observations of NTS-1 were resumed after NTS-2 failed in order to support users requiring precise time in remote locations. Emissions from NTS-1 were terminated in mid-September. A control system based on a microcomputer was developed at RGO and used to permit unmanned operation of the receiver; a tracking antenna-mount to be controlled by the same system was installed on the roof of the West Building
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Greenwich Time Reports (quarterly) 1978 January-December.
Time Service Circulars A (weekly) and B (monthly)
Time Service Notices (irregularly) – 11 issues.
Time and Latitude Service
Photographic Zenith Tube. The programme for the determination of Universal Time and the variation of latitude by astronomical observations with the PZT was continued under the direction of O'Hora; 2063 star transits were recorded on 98 nights and the results were sent weekly to the Bureau International de I'Heure (BIH) and monthly to the International Polar Motion Service (IPMS). The PZT was taken out of service in mid-December for renewal of much of the wiring and the electronic control system. Some mechanical and optical renovation was done at the same time. Regular observations were resumed in early April.
Observations have been scheduled for every night during August and September in support of the preliminary campaign of Project Merit.
Project MERIT. Wilkins, as chairman of the Joint IAU/lUGG Working Group on the Determination of the Rotation of the Earth, presented the proposal for Project MERIT at the IUGG General Assembly (in December) and took part in the planning of the short campaign of observation and analysis, which started In August. Preliminary reports suggest that the basic objectives of the campaign will be met; results are being obtained regularly from laser ranging, and VLBI observations are being made.
Doppler observations of satellites. The. Time Department has participated in two international campaigns of simultaneous radio-observations of the US Navy (Doppler) Navigation Satellites, one organized by the Institut für Angewandte Geodäsie (FDR) and one by the Austrian Academy of Sciences. Both aimed to establish precise relative positions of European sites involved in timekeeping and/or primary geodetic observations. The receiver used at RGO was lent by the Oxford University Department of Surveying and Geodesy, and help was also given by Decca Survey Ltd.
Atomic Time. Under the direction of Pilkington, the local atomic time scale, TA(RGO) was formed throughout the period by combining the scales of from two to four of the six caesium-beam atomic clocks at RGO. Corrections derived from inter-comparison of all the clocks were, as usual, applied to maintain the continuity and uniformity of TA(RGO).
Radio Time Signals. The RGO continued to provide the BBC 6-pips Greenwich time signal (UTC), and to monitor the radio time-signals on GBR and a small number of other signals. Four receivers (one on loan from USNO) were used to monitor the timing of the Loran-C emissions, and so to contribute to the accuracy of this navigation system; the results also link the RGO clocks and time-scales to international atomic time, TAI.
Use of Satellites. During the year the Time Department has also monitored the performance at RGO of a clock controlled by a time-code relayed from the GOES-3 meteorological satellite as a service of the US National Bureau of Standards. This satellite is very close to the horizon at RGO but its intended area of coverage includes the Canary Islands and It is hoped to Use a similar clock to provide a time-reference at the La Palma Observatory. Observations have confirmed that the clock usually remains within 100μs of UTC, but it is sometimes rendered unserviceable by transfer of operations to a spare satellite that is below the horizons of both RGO and La Palma.
Publications. The results of the time and latitude service have been published as follows during the period of this report:
Greenwich Time Reports (quarterly) 1979 January-December.
Time Service Circulars A (weekly) and B (monthly)
Time Service Notices (irregularly) – 12 issues.
Greenwich Time Service
Throughout the period of this report the primary objectives of the Time Department have remained unchanged: to establish a stable atomic timescale, primarily for use as a reference in astronomy; to monitor the rotation of the Earth, on which timekeeping was originally based; and to make the results of both activities accessible to users of all kinds, both inside and outside the Observatory.
Today work in these 'service' activities is the subject of almost complete international coordination; the most generally useful results are produced in central bureaux by combining results submitted by many independent establishments. This cooperation has certainly improved the accuracy of the overall reference system, but it has also had the effect of reducing the unfavourable local impact of withdrawal of support from one contributor among many. During a period of increasing competition for resources this has been unfortunate for the RGO's atomic time service; in future the RGO must expect to be a customer for precise time rather than a major timing centre.
The activities of the RGO that relate to atomic time have been:
(a) Operation of a group of commercial caesium-beam clocks in a stable environment under conditions intended to justify as far as is feasible the assumption that each generates an independent good approximation to an ideal atomic timescale. The clocks are kept in six locked, temperature-controlled rooms in an air-conditioned sub-basement and have individually stabilized external DC power supplies.
(b) Evaluation of the performance of individual clocks and identification of any showing significant deviations from 'normal' behaviour. This requires the frequent precise intercomparison of at least three independent clocks of comparable quality. In principle they need not all be at the same site, but in practice the uncertainties introduced by external timing links still make it impossible to perform comparable evaluations of remote clocks. Methods now exist to reduce these uncertainties, but only after substantial investment of funds and effort.
(c) Formation of a composite 'paper' timescale by suitable combination of the readings of the clocks currently identified as being most predictable. This timescale, TA(RGO), is intended to be uniform, is not influenced by seasonal or other variations in the propagation of signals over long distances, and is immediately accessible. Because it is a free scale, not subject to control by an evaluable frequency standard, it must be subject to a slow random walk in frequency and will gradually diverge from other similar scales and from the internationally adopted reference timescale TAI. It is also potentially subject to ageing effects, or residual seasonal effects, which would invalidate the assumption that the clocks operate independently. But in the medium- and long-term its stability and uniformity are much better than those available from individual caesium-beam clocks. It can, at any time, be related to similar local timescales in Europe and North America with an expected uncertainty of less than 1 μS. It is a national facility and it is still in everyone's interest that it should be properly exploited.
(d) Use of this timescale as a reference in measurement of the reception of selected radio signal, and regular publication and distribution of the results. If the same signals can be received at other locations these results can be used there to obtain retrospective access to TA(RGO) and to other related timescales, time intervals and frequencies,
(e) Contribution of clock data and signal-reception data to the Bureau Internation[al] de L’Heure for use in the computations for defining TAI, which are currently performed two months in arrears. TAI in definitive form, and UTC which differs from ATI by an integral number of seconds, are thus accessible only after a two-month delay as time-offsets from the readings of real clocks participating in the scheme coordinated by the BIH. Times recorded in terms of a local clock that can be referred to TA(RGO) can be subsequently referred to TAI.
The signals that have been particularly relevant in (d) are those emitted from Rugby on 16 kHz (GBR) and 60 kHz (MSF), which were timed and phase-tracked at RGO, and the 100 kHz groundwave-propagated pulsed and phase-encoded emissions of the Loran-C navigation aid, which is still the mainstay of the international system of time comparisons and can yield total uncertainties of less than 1 μS. Four distinct Loran-C transmissions are monitored at RGO and our results are used in operational control of the Loran-C system for both time-transfer and navigation. We can also make measurements of TV synchronizing pulses for use in the calibration of precise timing equipment in southeast England, but at present we know of no demand for this service. Publication of GBR and MSF phase results ceased in June 1985.
This description of the work of the atomic-time service at RGO could have been given at any time during the past few years. Although a high-level meeting in 1982 between representatives of SERC and NPL concluded that RGO's atomic-time service complemented that provided by NPL, with parallel operation providing valuable insurance against technical or other difficulties at either establishment, funds and staff at RGO have both been reduced to a level that has enforced a fundamental revision of objectives and a concentration upon the RGO's own immediate requirements.
The Earth-rotation related activities of the Time Department have also undergone drastic revision since 1980, buy here the changes have been more positive. The 'classical' zenith telescope was removed from service in June 1984 because effort was no longer available to operate it and reduce the results. Instead the Satellite Laser Ranging system has been producing much more precise but not directly comparable observations of the Earth's rotation since October 1983. These observations are best analyzed in combination with similar observations made at other sites, and offer the prospect of a greatly improved understanding of factors influencing the Earth's rotation in the short term, as well as other topics. But they do not provide direct access to a non-rotating celestial reference frame; in future this will be primarily the responsibility of geodetic users of the radio astronomical technique with Very Long Baseline Radio Interferometry. The excellent quality and quantity of RGO’s SLR data, and the fact that can provide a basis for research in many fields, make us hopeful that this instrument will remain in operation, funded as it is now by MoD, DTI and NERC as well as SERC, for the foreseeable future. It is now the main on-site ‘customer’ of the atomic-time service. J.D.H. Pilkington
[There is also a separate section in the Report on Satellite Laser Ranging (p74-75), which has not been transcribed here.]
[There was no contributuion from the Time Service to this Report.]
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