Bureau International De l’Heure Research Articles

The first President of the IAU, Benjamin Baillaud

AbstractBenjamin Baillaud was appointed president of the First Executive Committee of the International Astronomical Union which met in Brussels during the Constitutive Assembly of the International Research Council (IRC) on July 28th, 1919. He served in this position until 1922, at the time of the First General Assembly of the IAU which took place in Rome, May 2–10. At that time, Baillaud was director of the Paris Observatory. He had previously been director of the Toulouse Observatory for a period of 30 years and Dean of the School of Sciences of the University of Toulouse. He specialized in celestial mechanics and he was a strong supporter of the “Carte du Ciel” project; he was elected chairman of the permanent international committee of the Carte du Ciel in 1909. He also was the founding president of the Bureau International de l’Heure (BIH) and he was directly involved in the coordination of the ephemerides at an international level. In this paper, we present some of his activities, particularly those concerning international programmes, for which he received international recognition and which eventually led to his election in 1919 to the position of first president of the IAU. We also briefly recount the very first meetings and years of the IAU.

Why the Greenwich meridian moved

In 1884, the International Meridian Conference recommended that the prime meridian “to be employed as a common zero of longitude and standard of time-reckoning throughout the globe” pass through the “centre of the transit instrument at the Observatory of Greenwich”. Today, tourists visiting its meridian line must walk east approximately 102 m before their satellite-navigation receivers indicate zero longitude. This offset can be accounted for by the difference between astronomical and geodetic coordinates—deflection of the vertical—in the east–west direction at Greenwich, and the imposed condition of continuity in astronomical time. The coordinates of satellite-navigation receivers are provided in reference frames that are related to the geocentric reference frame introduced by the Bureau International de l’Heure (BIH) in 1984. This BIH Terrestrial System provided the basis for orientation of subsequent geocentric reference frames, including all realizations of the World Geodetic System 1984 and the International Terrestrial Reference Frame. Despite the lateral offset of the original and current zero-longitude lines at Greenwich, the orientation of the meridian plane used to measure Universal Time has remained essentially unchanged.

Project MERIT and the Formation of the International Earth Rotation Service

AbstractProject MERIT was an international programme to Monitor Earth Rotation and Intercompare the Techniques of observation and analysis. It was conceived by a working group that was set up by the International Astronomical Union in 1978 and was carried through with additional support from the International Union of Geodesy and Geophysics. The first objective was to encourage the development of the use of new techniques, such as laser ranging and radio interferometry, for the regular determination of universal time and polar motion. A successful ‘short campaign’ of observations by six techniques was carried out during the period 1980 August to October. Operational and analysis centres were set up for each technique and a coordinating centre was established at the Bureau International de l’Heure (BIH). The results were reported and discussed at the first MERIT Workshop in 1981.The preparations for the ‘main campaign’, which was held from 1983 September 1 to 1984 October 31, and the plans for the activities that were to follow it were reviewed at the second MERIT Workshop in 1983. Important additional features of the campaign included the use of ‘MERIT Standards’ for the reduction and analysis of the data, the use of electronic techniques for the distribution of data, the comparison of the results with the changes in the angular momentum of the atmosphere and special emphasis on the improvement of the terrestrial reference frame. Proposals for a new International Earth Rotation Service were prepared at the third MERIT Workshop in 1986. The MERIT programme was continued from 1984 until the new service formally started on 1988 January 1. Over the decade the accuracy of the Earth-rotation parameters improved considerably.

Combinations of Earth orientation measurements: SPACE94, COMB94, and POLE94

A Kalman filter has been used to combine independent measurements of the Earth's orientation taken by the space‐geodetic observing techniques of lunar laser ranging, satellite laser ranging, very long baseline interferometry, and the Global Positioning System. Prior to their combination, the data series were adjusted to have the same bias and rate, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The resulting combination, SPACE94, consists of smoothed, interpolated polar motion and UT1‐UTC values spanning October 6.0, 1976, to January 27.0, 1995, at 1‐day intervals. The Kalman filter was then used to combine the space‐geodetic series comprising SPACE94 with two different, independent series of Earth orientation measurements taken by the technique of optical astrometry. Prior to their combination with SPACE94, the bias, rate and annual term of the optical astrometric series were corrected, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The adjusted optical astrometric series were then combined with SPACE94 in two steps: (1) the Bureau International de l'Heure (BIH) optical astrometric series was combined with SPACE94 to form COMB94, a combined series of smoothed, interpolated polar motion and UT1‐UTC values spanning January 20.0, 1962, to January 27.0, 1995, at 5‐day intervals, and (2) the International Latitude Service (ILS) optical astrometric series was combined with COMB94 to form POLE94, a combined series of smoothed, interpolated polar motion values spanning January 20, 1900, to January 21, 1995, at 30.4375‐day intervals.

Astrometric Latitude and Time Observational Database at Shanghai Observatory

After the closing of Bureau International de L'Heure (BIH) and the establishment of International Earth Rotation Service (IERS) at the end of 1987. Shanghai Observatory has been the institute where the astrometric latitude and time observational data are collected and treated. During the past four years, about 75,293 measurements in latitude or time determination have been obtained by the 64 optical astrometric instruments over the world from which the five-day Earth Rotation Parameters of the 1988–1990 period have still been reduced. Twelve Quarterly Report on the optical ERP have been distributed. Since the beginning of 1991 the regular reduction of the ERP has been stopped but the collecting of the observational data is still going on in Shanghai Observatory in order to meet the requirements of the scientists who are still interested on the studies concerned with these observations. There are still 42 optical astrometric instruments taking part into the regular observations at the moment.

Earth orientation from Doppler tracking of the Navy navigation satellite system

For more than a dozen years, Doppler tracking of the Navy Navigation Satellite System (NNSS) has demonstrated the ability to give solutions for polar motion as a byproduct of precise ephemerides generation. The polar motion estimation process at the Defense Mapping Agency is reviewed and comparisons of the Doppler results with the polar motion series of the newer observational techniques and the Bureau International de l'Heure (BIH) Circular-D are made. This paper extends the analyses and comparisons done during the MERIT Main Campaign for one additional Chandler period.

Seth Carlo Chandler, Jr.: Discoveries in polar motion

On January 1, 1988, the new International Earth Rotation Service (IERS) will become operational. IERS will encompass the services provided by the International Latitude Service (ILS), the International Polar Motion Service (IPMS), and the Bureau International de l'Heure (BIH) for nearly a century and will provide improved services that will be particularly useful to scientific researchers. For example, the IERS will establish and maintain a conventional celestial reference frame defined by a selected set of extragalactic radio sources, establish and maintain a conventional terrestrial reference frame defined by a set of globally distributed stations marked by monuments, determine improved constants of precession and nutation, and collect and distribute ancillary data important to understanding earth rotation, such as atmospheric angular momentum time series.

Determination of the earth rotation parameters from optical astrometry observations, 1962.0–1982.0

The optical astrometric data of the years 1962–1982 have been reduced once again at the Bureau International de l’Heure (BIH) in order to redetermine the Earth Rotation Parameters (ERP). This new reduction is based on serie largely revised by the stations since their use in the operational work of theBIH, and on some series which were not available previously.

Predictability of the Earth’s polar motion

The numerical prediction of the Earth’s polar motion is of both theoretical and practical interest. The present paper is aimed at a comprehensive, experimental study of the predictability of polar motion using a homogeneous BIH (Bureau International de l’Heure) data set for the period 1967–1983. Based on our knowledge of the physics of the annual and the Chandler wobbles, we build the numerical model for the polar motion by allowing the wobble period to vary. Using an optimum base length of six years for prediction, this “floating-period” model, equipped with a nonlinear least-squares estimator, is found to yield polar motion predictions accurate to within 0″.012 to 0″.024 depending on the prediction length up to one year, corresponding to a predictability of 89–82%. This represents a considerable improvement over the conventional fixed-period predictor, which, by its nature, does not respond to variations in the apparent wobble periods (in particular, a dramatic decrease in the periods of both the annual and the Chandler wobbles after the year 1980). The superiority of the floating-period predictor to other predictors based on critically different numerical models is also demonstrated.

Atmospheric angular momentum fluctuations, length-of-day changes and polar motion

Variations in the distribution of mass within the atmosphere and changes in the pattern of winds, particularly the strength and location of the major mid-latitude jet-streams, produce fluctuations in all three components of the angular momentum of the atmosphere on timescales upwards of a few days. In a previous study (Hide et al . 1980) it has been shown that variations in the axial component of atmospheric angular momentum during the Special Observing Periods in 1979 of the First GARP Global Experiment (FGGE, where GARP is the Global Atmospheric Research Program) are well correlated with changes in length-of-day. This would be expected if the total angular momentum of the atmosphere and ‘solid’ Earth were conserved on short timescales (allowing for lunar and solar effects) but not if angular momentum transfer between the Earth’s liquid core and solid mantle, which is accepted to be substantial and even dominant on timescales upwards of several years, were significant on timescales of weeks or months. Fluctuations in the equatorial components of atmospheric angular momentum should contribute to the observed wobble of the instantaneous pole of the Earth’s rotation with respect to the Earth’s crust, but this has not been shown conclusively by previous studies. In this paper we re-examine some aspects of the underlying theory of non-rigid body rotational dynamics and angular momentum exchange between the atmosphere and solid Earth. Since only viscous or topographic coupling between the atmosphere and solid Earth can transfer angular momentum, no atmospheric flow that everywhere satisfied inviscid equations (including, but not solely, geostrophic flow) could affect the rotation of a spherical solid Earth. Currently available meteorological data are not adequate for evaluating the usual wobble excitation functions accurately, but we show that partial integration leads to an expression involving simpler functions ─ here called ‘equatorial angular momentum functions’ ─ which can be reliably evaluated from available meteorological data. The length-of-day problem is treated in terms of a similar ‘axial angular momentum function’ ; and ‘effective angular momentum functions’ are defined in order to allow for rotational and surface loading deformation of the Earth. Daily values of these atmospheric angular momentum functions have been calculated from the ‘initialized analysis global database’ of the European Centre for Medium-Range Weather Forecasts (ECMWF). They are presented for the period 1 January 1981─30 April 1982, along with the corresponding astronomically observed changes in length-of-day and polar motion, published by the Bureau International de l’Heure (BIH). Changes in length-of-day during this period can be accounted for almost entirely by angular momentum exchange between the atmosphere and solid Earth, and the existence of a persistent fluctuation in this exchange, with a timescale of about 7 weeks, is confirmed. We also demonstrate that meteorological phenomena provide an important contribution to the excitation of polar motion. Our work offers a theoretical basis for future routine determinations of atmospheric angular momentum fluctuations for the purposes of meteorological and geophysical research, including the assessment of the extent to which movements in the solid Earth associated with very large earthquakes contribute to the excitation of the Chandlerian wobble.

Satellite Laser Ranging Precise Time Transfer Techniques: Some Comments on the Possible Indian Contribution

Following the proposal by Bureau International de L' Heure (BIH), Paris the European Space Agency (ESA) has implemented an experimental space mission on-board SIRIO-2 geosynchronous satellite, a payload package, known as LASSO (Laser Synchronization from Stationary Orbit). The LASSO consists of body-mounted array of corner cube retro-reflectors, laser detectors, event timer, precise time standard, TTL-type memory unit and the communication link between satellite and the ground control. The LASSO/SIRIO-2 experiment is designed to employ laser technique for Intercontinental comparisons of clocks at the sub-nanosecond level and for disseminating reference time (UTC) to the user community on a global basis. The satellite is proposed to be launched by ESA in early 1982. Initially, it is planned to position the satellite at a longitude around 25°W, subsequently the satellite will be moved to 20:E. The latter position of SIRIO-2 (20°Ei is the one which will be used in Indian experiments.As the principle, method,...

An Intercomparison of Connected-Element Interferometer and Lunar Laser Earth Rotation Parameters

AbstractThe determination of Earth orientation parameters using techniques which promise an improvement in the precision of their measurements requires a careful evaluation of the systematic errors present in each series of observations. It is now possible to begin to inter-compare two of these sets of observations of the rotation of the Earth (UT1-UTC) obtained routinely and independently. These are the data obtained from laser ranging to the Moon and those derived from the connected-element interferometer information of the U. S. Naval Observatory. Although these data are available for a relatively short time span, it is possible to begin to inter-compare the observations in an effort to derive possible systematic errors existing in the data. This has been done using data from July 1979 to March 1980 in comparison with Bureau International de l’Heure (BIH) data. Using this information, the hypothesis that there is no correlation between the differences of the interferometer and laser ranging data with respect to the BIH must be rejected. This indicates that both sets of observations show that systematic errors do exist in the BIH data. The nature of the correlation of the two series is examined to establish possible models of the systematic errors. A much longer series of data from both techniques is necessary to improve the estimates, and the results will depend critically on adopted models used in the reductions and the degree of smoothing to which the raw data are subjected.

Comparison of Polar Motion Results Using Lunar Laser Ranging

AbstractA comparison of polar motion results from three sources [Bureau International de l’Heure (BIH), Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC-Doppler), the International Polar Motion Service (IPMS)] was performed using lunar laser ranging (LLR) data. The rms errors, both of the LLR data and of the determinations of polar motion by the three services, decreased in recent times. The BIH and Doppler polar motion are comparable in quality (12 ± 6 cm for BIH values taken from August 1976 through May 1980, 14 ± 7 cm for Doppler results taken from June 1977 through May 1980). The IPMS errors were substantially larger (33 ± 7 cm for data taken from August 1976 through May 1980). All three analyses give 13 ± 3 cm as an estimate for the combined LLR modeling, fitting, and instrumental error (noise) for the last four years.

Atmospheric Angular Momentum and the Length of Day

AbstractWe have used lunar laser ranging (LLR) observations from the McDonald Observatory in conjunction with pole positions from the Bureau International de l’Heure (BIH) to determine the length of day (l.o.d.) at five-day intervals over the period 1971 through 1979. We compared the changes in these l.o.d. values to those inferred from determinations of the angular momentum of the global atmosphere, for the years 1976-1979, computed from twice-daily analyses of the zonal wind. These latter analyses were carried out by the National Meteorological Center, Camp Springs, MD. In both the observed (LLR) and the inferred (meteorological) values of changes in l.o.d., we have identified the near 50-day oscillation recently noted by Feissel and Gambis at the BIH. For the period of overlapping dates, the two oscillations agree closely in both amplitude and phase. The typical amplitude is about 0.2 milliseconds or approximately half the amplitude of the annual and semiannual variations in l.o.d.

Intercomparison of Lunar Laser and Traditional Determinations of Earth Rotation

AbstractThe rotational orientation of the earth (UTO at McDonald Observatory) has been determined from lunar laser ranging (LLR) measurements for the interval 1971 to 1980. The results have been differenced from those obtained by conventional means as published by the Bureau International de l’Heure (BIH), on its 1979 system. The difference displays a quasi-seasonal signature, which we ascribe to systematic errors in the conventional measurements. The lunar data are well represented by a smooth curve, which gives UTO at McDonald with a precision of about 3/4 milliseconds or better, and UT1 to within 1 millisecond using BIH polar coordinates.

Determining coordinates of the rotational pole using satellite data from four sites

This study investigates how precisely the terrestrial coordinates of the rotational pole can be determined from satellite data from four sites in the United States. The five satellites used are those of the Navy Navigation Satellite System (Transit). Tracking data for these satellites are Doppler observations from sites in Maine, Minnesota, California, and Hawaii. The data used in the study are intermediate data products of the near‐real‐time orbit determination procedure, which is an integral part of the navigation system operations. Six 5‐day intervals were selected for study, one every other month during 1976. Coordinates were determined daily from five satellites. Five‐day averages were used as final estimates. Comparisons were made with extrapolated and final coordinates from the Bureau International de l'Heure (BIH) and final coordinates from the Doppler Polar Motion Service (DPMS). The final coordinates determined in the study agree as well with the final values of BIH as do the results of DPMS. The study implies that low‐cost and near‐real‐time estimates of the terrestrial coordinates of the pole can be made available daily as a by‐product of the routine support needed for the Navy Navigation Satellite System.

Geodesy and astrometry by transatlantic long base line interferometry

We report geodetic and astrometric results from the analysis of fringe frequency observations from a series of three long base line interferometry (LBI) experiments carried out in 1973 between the 46‐m antenna of the Algonquin Radio Observatory, Lake Traverse, Canada, and the 2‐m antenna at Chilbolton Field Station, Chilbolton, England. The rms deviation from the mean of the estimates of the length and orientation of the 5251‐km equatorial component of the base line from all three experiments is 1.05‐m and 0.015″, respectively. The experiments also yielded positions of five extragalactic radio sources. The reported positions, each of which is from only a single experiment, have uncertainties of about 0.2″ in declination (except for low declination sources) and about 0.01 s in right ascension. The LBI determination of the length and orientation of the equatorial component of the base line is compared to the corresponding values derived from Naval Weapons Laboratory 9D (NWL‐9D) coordinates for the antennae. The two length measurements agree in scale within quoted experimental errors, however, the NWL‐9D coordinate frame is found to be rotated 0.867″±0.1″ to the east relative to the average terrestrial frame of the Bureau International de l'Heure (BIH), (LBI coordinate frame). This is in good agreement with the expected misalignment of 0.65″±0.2″. The differences in the rates of the clocks used at each end of the base line were also determined and compared to Loran‐C observations.

Recent Results of Radio Interferometric Determinations of a Transcontinental Baseline, Polar Motion, and Earth Rotation

Radio interferometric observations of extragalactic radio sources have been made with antennas at the Haystack Observatory in Massachusetts and the Owens Valley Radio Observatory in California during fourteen separate experiments distributed between September 1976 and May 1978. The components of the baseline vector and the coordinates of the sources were estimated from the data from each experiment separately. The root-weighted-mean-square scatter about the weighted mean (“repeatability”) of the estimates of the length of the 3900 km baseline was approximately 7 cm, and of the source coordinates, approximatelyor less, except for the declinations of low-declination sources. With the source coordinates all held fixed at the best available,a posteriori, values, and the analyses repeated for each experiment, the repeatability obtained for the estimate of baseline length was 4 cm. From analyses of the data from several experiments simultaneously, estimates were obtained of changes in the x component of pole position and in the Earth's rotation (UT1). Comparison with the corresponding results obtained by the Bureau International de l'Heure (BIH) discloses systematic differences. In particular, the trends in the radio interferometric determinations of the changes in pole position agree more closely with those from the International Polar Motion Service (IPMS) and from the Doppler observations of satellites than with those from the BIH.

Velocity of the Motion of the Terrestrial Pole

The results of the spectral analysis of the velocity of the polar motion based on the data of the International Latitude Service (ILS), International Polar Motion Service (IPMS),. Bureau International de l'Heure (BIH), and the Doppler Polar Motion Service (DPMS) are presented.

A New System of the U.S.S.R. Standard Time for 1955–1974 and its Application in the Study of the Earth's Rotation

For the study of the Earth's rotation and geodetic measurements the longest possible series of homogeneous Universal Time (UT) data is required. For various reasons this remains a difficult problem. Periodically it is necessary to revise past observations and redetermine the UT scale. This was done by the Bureau International de l'Heure (BIH) in 1968 when the FK 4 catalog, revised values of astronomical longitudes, and new techniques were standardized for the reduction of astronomical observations. This system known as the 1968 BIH System has been in use since 1968 and has also been used for the re-reduction of observations back to 1962.0.