International Earth Rotation Service Research Articles

Survey of Observational Techniques and Hipparcos Reanalysis

AbstractPolar motion and Universal Time have been regularly determined since 1899 and 1956, respectively, at a number of observatories all over the world. Before the International Earth Rotation Service (IERS) was established in 1988, the classical astrometry instruments such as visual zenith-telescopes, PZTs, transit instruments, astrolabes etc. were used. The survey of all these instruments and the methods of observation used is described. The values of instantaneous latitude and UT0-UTC made at a set of selected observatories and based on individual star observations have been collected at the Astronomical Institute in Prague during the past years. They were recalculated using the most recent astronomical standards and the Hipparcos Catalogue, and used to determine the Earth orientation parameters (polar motion, celestial pole offsets and Universal Time). The most recent solution, based on about 4.5 million observations with 47 different instruments at 33 observatories, is described and the results of polar motion presented.

Analysis of Long Time Series of Polar Motion

AbstractTwo long time series of polar motion were analysed with respect to a linear drift, decadal variations, Chandler wobble and annual wobble: the C01 series published by the International Earth Rotation Service (IERS) and the pole series which J. Vondrák, obtained by re-analysis of the classical astronomical observations using the HIPPARCOS reference frame (1899.7–1992.0). By a least-squares fit the linear drift of the pole, usually called ‘secular polar motion,’ was determined to 3.31 milliarcseconds/year (mas/yr) toward 76.1° West longitude. For this fit the a priori correlations within each pair of pole coordinates were taken into account, and the weighting function was calculated by estimation of empirical variance components. The decadal variations of the pole path were determined by Fourier analysis. Using a sliding window analysis, the variability of the periods, the amplitudes and the phases of the Chandler wobble and annual wobble was investigated. The variances of the results and the number of iterations needed to get a convergence in the nonlinear approach show that the new time series by Vondrák is more homogeneous and consistent than the IERS C01 series.

Seth Carlo Chandler Jr.: The Discovery of Variation of Latitude

AbstractIt took Seth Carlo Chandler Jr. just one year to discover the 14 month and annual components of the variation of latitude (polar motion) that had eluded the most talented astronomers for more than a century. He succeeded where those before him had failed because: he used a global set of observations that had been painstakingly collected by astronomers of many nationalities over more than 150 years; he ignored the common wisdom that unexpected annual variations in astronomical observations were always caused by temperature effects on instruments or atmospheric refraction anomalies; and he was not misled by the theory, which predicted that the Eulerian wobble must have a period of approximately 10 months. Chandler was a true amateur only in the sense that he pursued astronomy out of love, while making his living as an actuary. He began his scientific training at the youthful age of 15 under the tutelage of Benjamin Pierce, perhaps the greatest American mathematician of that time. He then became: a private assistant to Benjamin Apthorp Gould, Jr., an aide to Gould while employed by the US Coast Survey, and Assistant Editor to Gould for the Astronomical Journal. It would be impossible to understand and appreciate Chandler’s achievements without recognizing his relationship with Gould, his brilliant mentor and lifelong colleague — the man he once described as his “Magnus Apollo.”In 1901 Chandler announced that the 14 month motion was not a simple oscillation but was in itself a compound motion consisting of the previously discovered 428 day component, and a much smaller 436 day component, whose reality was “beyond reasonable doubt.” The beating of two such components would result in a rapid change in the phase and amplitude of the Chandler motion at intervals of about 80 years. If Chandler was correct, in about 2010, give or take perhaps five years, we should see the next occurrence of this phenomenon. With the International Earth Rotation Service in operation there should be no difficulty in detecting and quantifying such an event, very nearly one century after Chandler’s death.

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.

Polar Motion with Daily and Sub-daily Time Resolution

AbstractSince 1992 the IGS ACs (International GPS Service Analysis Centers) have delivered daily estimates of polar motion (PM) and length of day (LOD) to the IERS (International Earth Rotation Service). The IGS combined PM and LOD series are available since January 1, 1995. Since June 1996 the IGS rapid and the final combined orbits are accompanied with two distinct IGS combined PM series with the same time resolution of one day. Both, the IGS final and rapid combined PM series also include combined PM rate data as well as LOD, starting in March 1997.Since 1995 the CODE Analysis Center of the IGS has produced PM and LOD estimates with a two-hour time resolution on a routine basis. A study of the time series with sub-daily time resolution clearly shows the half-daily and daily tidal signals. The results correspond very well to those obtained from satellite altimetry.These sub-daily time series are based on the same observational data from the IGS tracking network as those time series with a one-day time resolution. It would thus be possible with a minor analysis effort, without additional observational effort, to come up with a long and consistent series of PM and LOD with a time resolution of about 1 to 2 hours.

History of the Bureau International de l’Heure

AbstractAt the beginning of the 20th century, the development of radio time signals offered the possibility of unifying the measurements of Universal Time (UT). A new service, the Bureau International de l’Heure, hosted by the Paris Observatory, was entrusted with this task. The BIH began its operation in 1912, although its statutes were officially settled only in 1919. This paper recalls the activities of the BIH on time determination and various connected fields: Earth rotation, reference frames, atomic time. With the importance of atomic time and the emergence of new techniques for the measurement of Earth rotation, a new organization was required. It was prepared by the MERIT and COTES programs (1978) and by the bodies of the Metre Convention, with the active participation of the BIH. The BIH was dissolved at the end of 1987, its work being shared between the Bureau International des Poids et Mesures and the new International Earth Rotation Service.

SLR Contribution to Investigation of Polar Motion

AbstractThe Satellite Laser Ranging (SLR) technique has been used to determine Earth Rotation Parameters (ERP) for over twenty years. Most of results contributed to the International Earth Rotation Service (IERS) are based on analysis of observations of Lageos 1 & 2 satellites collected by the global tracking network of about 40 stations. Now five analysis centers submit operational (with 2–15 days delay) solutions and about ten analysis centers contribute yearly final (up to 23 years) ERP series. Some statistics related to SLR observations and analysis are presented and analyzed. Possible problems in SLR observations and analysis and ways of its solution are discussed.

The International Latitude Service A Historical Review, From The Beginning To Its Foundation In 1899 And The Period Until 1922

The study of Polar Motion (PM) is associated withthe work of the InternationalLatitude Service (ILS), the International PolarMotion Service (IPMS) and theBureau International de l'Heure (BIH), andthen the International Earth RotationService (IERS). After reviewing briefly thehistory of early PM research, thescientific objective of the ILS including itsrealizing, and the activities of all thecited services and giving information on thePM time series available from mid-19thcentury to the present, important details onthe events and results are presentedover the period until 1922. It is shown, inwhich way the Geodetic Institute Potsdamcontributed to the foundation of the ILS andtook part in its activities through thework of Helmert, Albrecht, Wanach and Mahnkopf.Based on international scientificcooperation, the results were the rectangularcoordinates of the Polar Motion from1890.0 to 1922.7 at 0.1 year intervals, inparticular those derived from the latitudeobservations at independent stations from 1890.0to 1899.8, and those derived fromthe latitude observations at the ILS stationsfrom 1899.9 to 1922.7.

On calculating the Earth's C21 and S21 gravity coefficients in the IERS terrestrial reference frame

Modern models of the Earth's gravity field are developed in the IERS (International Earth Rotation Service) terrestrial reference frame. In this frame the mean values for gravity coefficients of the second degree and first order, C21(IERS) and S21(IERS), by the current IERS Conventions are recommended to be calculated by using the observed polar motion parameters. Here, it is proved that the formulae presently employed by the IERS Conventions to obtain these coefficients are insufficient to ensure their values as given by the same source. The relevant error of the normalized mean values for C21(IERS) and S21(IERS) is 3×10−12, far above the adopted cutoff (10−13) for variations of these coefficients. Such an error in C21 and S21 can produce non-modeled perturbations in motion prediction of certain artificial Earth satellites of a magnitude comparable to the accuracy of current tracking measurements.

A new ΔLOD series in monthly intervals (1892.0–1997.0) and its comparison with other geophysical results

Using the ΔT (integrated variation of the Earth's rotation measured in terestrial time) series (1891.5–1955.5) derived from lunar occultation observations and the UT1–UTC (universal time–coordinated universal time) series (1955.5–1997.5) of the Bureau International de L'Heure/International Earth Rotation Service, a new ΔLOD (variation of the length of day) series in monthly intervals from 1892.0 to 1997.0 is calculated. Using digital filtering, the interannual and decadal components of the ΔLOD series are separated and then compared with those inferred from other geophysical quantities. It is shown that, on the interannual time scale, atmospheric processes can play an important role in exciting astronomical ΔLOD. However, the main oscillation with a mean period of about 5.8 years and peak-to-peak amplitude of about 0.3 ms in the residuals of ΔLOD(Astr) −ΔLOD(Wind) for 1968.0–1997.0 suggests that about half of the amplitude in astronomical ΔLOD must be excited by other geophysical processes, while on the decadal time scale the atmospheric excitation is too small. Geomagnetic core–mantle coupling may be a plausible source of the excitation of ΔLOD on the decadal time scale, but the geomagnetic data are still insufficient and an improved model of core–mantle coupling is required.

IGS Earth Rotation Parameters

Since its official start in January 1994, the International GPS Service (IGS) has been distributing, as part of its product combination, two distinct Earth rotation parameter (ERP) series: the IGS Rapid series and the IGS Final series. Initially, the IGS Rapid ERP values were interpolations of the International Earth Rotation Service (IERS) Bulletin A, whereas the IGS Final ERP series was based on the IERS Bulletin B. Since June 1996, the IGS has been generating its own Final ERP series consistent with the IGS combined orbit products and based on weighted means of individual IGS analysis center (AC) solutions. At first, only the polar motion (PM) coordinates and their rates were combined. Length of Day (LOD) and Universal Time (UT) solutions, also based on separate weighted mean combinations, followed in March 1997. Currently, the IGS Rapid and Final combinations are produced and made available within 17 hours and 11 days, respectively, after the last observation. Both IGS and the best AC series are consistent and precise at the 0.1-milliarcsecond (mas) level for PM and at about 30 μs for LOD. Biases in some AC solutions may exceed these consistency levels. Comparisons of both IGS ERP series with external standards, such as the IERS multitechnique Bulletins and atmospheric angular momentum series, confirm the estimated precisions. © 1999 John Wiley & Sons, Inc.

Estimation of nutation using the Global Positioning System

In the past the estimation of corrections to nutation models was uniquely reserved to very long baseline interferometry (VLBI) and lunar laser ranging (LLR) data processing. Although satellite space‐geodetic measurements have been used to determine UT1‐UTC rates (or length of day) for many years now, the estimation of nutation rates was not performed. There is no fundamental difference, however, between the estimation of rates in UT1‐UTC and nutation rates in obliquity and longitude from satellite data. A simple variance‐covariance analysis shows that significant contributions to nutation by the Global Positioning System (GPS) are possible for periods below about 16 days. Since March 1994, daily nutation rates have been computed at the Center for Orbit Determination Europe using the data collected by the global GPS network. The series of nutation rates now covers 3.5 years. It is used to compute corrections for a set of 34 nutation periods between 4 and 16 days. The formal uncertainties of the estimated nutation coefficients in obliquity Δε and longitude Δψ sin ε0 grow linearly with period from several microarcseconds (μas) at periods of a few days to about 30 μas at periods of 16 days. The comparison of the GPS‐derived coefficients with the International Earth Rotation Service 1996 nutation model shows an overall agreement of 10 μas (median). The GPS results are also in very good agreement with the most recent model by Souchay and Kinoshita (1997.2), better than most of the VLBI and LLR results reported in the literature. GPS thus allows, although limited to high frequencies because of the satellite orbits involved, an independent check of the validity of theoretical nutation models and of results obtained from VLBI and LLR.

Introduction of JD 3 ON ‘PRECESSION, NUTATION AND ASTRONOMICAL CONSTANTS IN THE DAWN OF THE 21ST CENTURY’.

Due to the adoption of the new International Celestial Reference Frame (ICRF), the Earth’s Ori-entation Parameters (EOP) will be revised and their definitions will need to be re-examined and clarified. This implies that precession/nutation formulation will be also revised in the future.The precession/nutation theories for a non-rigid Earth suffer from a lack of dissipation in the core and from a mismodeling of the ocean and of the atmospheric effects. The scientific community is examining these questions. The IAU community is consequently not yet ready to adopt a new precession/nutation geophysical model but the users may use the International Earth Rotation Service (IERS) empirical series.In order to review those questions and prepare the future research, the Scientific Organizing Committee (SOC: P. Bretagnon, V.A. Brumberg, N. Capitaine, V. Dehant (Chair), T. Fukushima, E. Groten, H. Kinoshitä, B. Kolaczek, D.D. McCarthy, P.K. Seidelmann and P.T. Wallace) has proposed invited talks on the current situation concerning: (1)the formulation of precession/nutation (N. Capitaine, see paper 1),(2)the planetary theories and their relation to precession/nutation (P. Bretagnon, see paper 2),(3)the precession/nutation for a rigid Earth (J. Souchay and H. Kinoshita, see paper 3),(4)the DExxx JPL ephemerides precision and accuracy (E.M. Standish, see paper 4),(5)the observations of the Celestial Ephemeris Pole (CEP) and in particular the pole offset from which precession/nutation corrections can be derived (M. Feissel and A.M. Gontier, see paper 5),

New transfer functions for nutations of a nonrigid Earth

There are differences between the observed values of nutation and the computed ones based on the International Astronomical Union (IAU) 1980 adopted nutation series. These differences can be expressed in the frequency domain where they may reach several milliarc seconds, a level that is too large for practical use. This paper aims to resolve part of these differences by computing a new theoretical model accounting for additional geophysical effects. A new transfer function is computed, based on an Earth initially in a nonhydrostatic equilibrium corresponding to the steady state associated with the present mantle convection. The mantle mass anomalies are deduced from seismic tomography data, and the flow‐induced boundary deformations are computed from internal loading for an Earth made up of a viscous inner core, a liquid outer core, a viscous mantle, and a solid lithosphere. In this way, a new core‐mantle boundary (CMB) flattening is obtained, which gives the observed free core nutation (FCN) period. Furthermore, the global Earth dynamical flattening induced by the mass anomalies in the mantle associated with tomography and by the mass anomalies due to the computed boundary deformations, is in agreement with the J2 form factor (or the observed precession constant). In addition to this nonhydrostatic initial state, the rheology of the mantle is considered as inelastic. The transfer function for nutation is then obtained by numerical integration of motion equations from the Earth's center up to the surface to provide a model which is completely self‐consistent. In order to validate our model, the transfer function is convolved with new rigid Earth nutations, ocean corrections are applied and the final results are then compared with the observed nutations or with the International Earth Rotation Service (IERS) nutation series. The residuals between our model and the observation are about 3 times smaller than those between the IAU 1980 adopted model and the observation. However, our model still presents residuals above the observational error; this is particularly true for the out‐of‐phase part of the residuals, while the in‐phase part gives very small residuals (improvement of about 1 order of magnitude). A further step in this study is a refinement of the modeling of geophysical fluids (core, ocean, and atmosphere).

Study of Earth's rotation and geophysical fluids progresses

Variations in the Earth's speed of rotation and the position and orientation of the rotation axis—known collectively as Earth orientation parameters—are observed by a variety of space geodetic techniques coordinated by the International Earth Rotation Service (IERS). Observations of Earth orientation parameters provide fundamental information about the physical properties of the Earth's interior and the variability of fluids in the atmosphere, oceans, and liquid core, and on land.Participants at a workshop held October 14–18, 1996, at the Paris Observatory, where IERS is headquartered, called for extending IERS activities [Reigber and Feissel, 1997] to include the monitoring of geodetically important properties of the oceans, the liquid core, and terrestrial water to supplement current atmospheric angular momentum variation (AAM) measurements. Given the success of the IERS, expansion of its activities into other areas is not only natural for understanding the observed Earth orientation parameters, but essential for fulfilling its mission of improving the underlying space geodetic observations.

GPS High Accuracy Geodetic Networks in Mexico

Recently, Mexico's Instituto Nacional de Estadistica, Geografia e Informatica (INEGI) adopted a geocentric three-dimensional coordinate system as the basis for all its national geodetic and mapping needs. The selected frame is the International Earth Rotation Service (IERS) Terrestrial Reference Frame of year 1992 (ITRF92), epoch 1988.0. The geometric surface of choice is the ellipsoid of the Geodetic Reference System of 1980 (GRS80). The implementation of this geocentric datum permits the integration of all geodesy, engineering surveys, mapping, and land information systems of Mexico into a modern framework consistent with present accuracies obtainable through the global positioning system (GPS). The practical realization of the spatial reference system was done in cooperation with the National Oceanic and Atmospheric Administration's (NOAA) National Geodetic Survey (NGS) through a series of coordinated GPS projects. As a result, Mexico established 14 permanent GPS trackers (A-order stations) from which ...

Path of the Mean Rotational Pole From 1899 to 1994

SUMMARY Historical sources of polar motion are analysed together with modern data in order to compile a set of coordinates of the mean pole in a reference system consistent with that of the International Earth Rotation Service. the trend and quasi-periodic motion of the pole are investigated, and we find that the rotational pole appears to be moving at the rate of 0.333 arcsec century−1 in the direction of 75.0° west longitude. Modern data also appear to be consistent with the possible existence of very low-frequency periodic motion (Markowitz wobble).

A Radio Reference Frame

view Abstract Citations (155) References (65) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Radio Reference Frame Johnston, K. J. ; Fey, A. L. ; Zacharias, N. ; Russell, J. L. ; Ma, C. ; de Vegt, C. ; Reynolds, J. E. ; Jauncey, D. L. ; Archinal, B. A. ; Carter, M. S. ; Corbin, T. E. ; Eubanks, T. M. ; Florkowski, D. R. ; Hall, D. M. ; McCarthy, D. D. ; McCulloch, P. M. ; King, E. A. ; Nicolson, G. ; Shaffer, D. B. Abstract A catalogue is presented based on the radio positions of 436 extragalactic sources distributed over the entire sky. The positional accuracy of the sources is better than 3 milliarcsec (mas) in both coordinates, with the majority of the sources having errors better than 1 mas. This catalogue is based upon a general solution of all applicable dual frequency 2.3 and 8.4 GHz Mark-III VLBI data available through the end of 1993 consisting of 1,015,292 pairs of group delay and phase delay rate observations. Details and positions are also given for an additional 124 objects that either need further observation or are currently unsuitable for the definition of a reference frame. The final orientation of the catalogue has been obtained by a rotation of the positions into the system of the International Earth Rotation Service and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117571 Bibcode: 1995AJ....110..880J Keywords: REFERENCE SYSTEMS full text sources ADS | data products SIMBAD (633) NED (610) CDS (1)

Earth Orientation - The Current and Future Situation

Sub-milliarcsecond astrometry often requires an accurate characterization of the orientation of the Earth in a quasi-inertial reference frame. The International Earth Rotation Service (IERS) standards provide the current state of the art in the transformation between celestial and terrestrial reference systems. Improvements in the determination of Earth orientation parameters which describe this transformation continue to be made. Current and future capabilities are given.

Improvement and Maintenance of the Extragalactic Reference Frame

Very Long Baseline Interferometry (VLBI) is at present the most powerful technique to construct the best approximation to an inertial reference frame. After more than a decade of VLBI observations several hundreds of extragalactic objects have positions known within ±0.0003″. Since 1988 the International Earth Rotation Service (IERS) elaborates a global extragalactic celestial reference frame (IERS Celestial Reference Frame, ICRF) that is tied to the international terrestrial reference frame through the high precision monitoring of the Earth's rotation. The direction of the ICRF axes relative to the IAU definitions are known within better than ±0.001″ for the polar axis and ±0.003″ for the origin of right ascensions. The FK5 axes are consistent with the ICRF ones within their uncertainties (0.050″-0.100″). The maintenance of this high accuracy extragalactic frame will be necessary for the long term programs, such as the future monitoring of the tie of the Hipparcos galactic frame as well as of the dynamical planetary frame (millisecond pulsars, lunar laser ranging).