Geodynamics can be defined as the study of the motions of Earth with respect to its own center of mass and three locally-inertial directions. These motions include rotations of the whole Earth, as well as episodic, cyclic and secular deformations of the crust, and internal fluid circulations. Geodynamics is still largely an observational science, since there is yet no satisfactory theory to account for several of the most interesting of these motions. The observational aspects of geodynamics are basically astronomical, since the most obvious effects of terrestrial rotation and deformation (excepting of course occasional cataclysmic deformations) are seen as variations in the apparent positions of celestial bodies, as observed from the surface of the mobile, non-rigid Earth.

In 1980, Earth rotation parameters have been measured by classical astrometry, Doppler and laser satellite techniques, Lunar Laser Ranging and radio-interferometry. The precision of the series and their systematic differences are investigated; a combination algorithm is applied to the series available throughout the year.

This paper is based on optical observations of 85 instruments (Table 1-a) from January to October 1980. The accuracies of all instruments, 62 for time and 54 for latitude, are shown in Table 1-b, in which:

1. – the roughness ε1i of daily observations is defined by the standard deviation of the observation values of the i-th instrument with respect to the smoothed values of themselves;

2. – the ε2i is the standard deviation of mean observation values during every 0.05 year with respect to the global reference system.

The average accuracy of each type of instrument is given in Table 2, regional averages are given in Table 3.

These tables show that errors in time observations are greater, generally speaking, than those in latitude, and low frequency errors are greater than high frequency ones in time observations.

We have extended our estimates of variation of latitude and UTO from the McDonald Observatory lunar laser ranging (LLR) observations to span the period October 1970 to November 1980. The typical formal uncertainties of our values are about 6 milliarcseconds (mas) and 0.5 milliseconds (ms) of time, respectively. We have compared our values of variation of latitude with those derived from the smoothed Circular D pole positions published by the Bureau International de l’Heure. The root-mean-square (rms) difference about the mean difference is 14 mas. A comparison of our smoothed UTO estimates with those calculated from the smoothed Circular D values of UT1 and pole position gives a corresponding rms difference of 1.5 ms. For the period covered by the MERIT Short Campaign, we have also compared our smoothed UTO values with (unsmoothed) ones derived from daily UT1 and pole-position values obtained by the Goddard Space Flight Center / Massachusetts Institute of Technology / Haystack Observatory group from very-long-baseline interferometric observations spanning two one-week periods. The rms difference about the mean difference is 0.3 ms.

As the techniques of lunar and artificial satellite laser ranging mature, emphasis is being placed upon the use of these observations to monitor the Earth’s rotation. It is important to note, however, that at the present time neither technique alone can furnish all three components of this rotation to an accuracy which surpasses those results obtained from classical techniques. In the case of LAGEOS laser ranging, unmodeled secular orbital effects couple with axial Earth rotation in such a way that these effects are not separable in the analysis of those observations. In the case of lunar laser ranging, observations have been regularly available only from a single station for the past ten years or so with the result that a change in latitude along the McDonald Observatory meridian is not separable into the ordinary (x,y) components of polar motion. The main purpose of this paper is to present the first stages of an investigation to combine LAGEOS and lunar laser ranging observations. It is hoped that the proper implementation of such a process might eliminate the shortcomings inherent in each technique, while accentuating the advantages of each. This has the potential of producing all three components of the Earth’s rotation to an accuracy and precision which is compatible with the present observational uncertainties.

The project EROLD (Earth Rotation from Lunar Distances) was conceived in 1974, in the COSPAR framework, with the goal of demonstrating that the lunar distances technique might be an efficient candidate in a new-generation service for the determination of Earth orientation.

Two years later, it was decided that the computations of the observational residuals and partial derivatives, as well as the analyses themselves, would be done at C.E.R.G.A. on a regular basis, for all the participating stations. Indeed, it was recognized to be important that all the observations be reduced in an homogeneous process. The target chosen for this program was that of Apollo XV which is the easiest to observe. Unfortunately, the operations could not start quickly due to a lack of observations and each station was encouraged to press the completion of the equipment, but many problems slowed the integration of these stations. Still now, only McDonald Observatory transmits observations regularly, and Orroral is operating, but with scattered results; presently this latter station is in course of modifications for improvement of accuracy.

The 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.

The 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.

In September and October 1980, the National Geodetic Survey, jointly with the National Aeronautics and Space Administration and several other agencies and institutions, conducted a series of astronomical radio interferometry (VLBI) observing sessions to support the IAU/IUGG MERIT short campaign. A total of 14 days of observations, organized into two 7-day sessions, was collected by three observatories in the United States (Harvard Radio Astronomy Station (HRAS), Haystack Observatory, and Owens Valley Radio Observatory) and the Onsala Space Observatory in Sweden. Chilbolton Observatory, England, and Effelsberg Observatory, West Germany, also participated on some days. Immediately following the MERIT campaign, NGS initiated a series of 24-hour observing sessions, spaced at approximately 2-week intervals, as a pilot program to project POLARIS. All of these sessions included two observatories, HRAS and Haystack, and Onsala participated in about half of the sessions. The MERIT and POLARIS observations were made with the third generation MARK III VLBI system using procedures and schedules designed to yield high quality geodetic information, including Earth rotation values. This paper briefly traces the planning, observing, and data processing activities, and presents the Earth rotation information thus far derived from the data.

Laser ranging collected during the period from May 7 1976 through May 1981, has been used to determine polar motion and length of day (LOD) variations. Independent short-arc solutions of five days in length are used to obtain the polar motion and LOD results. The solutions obtained with these approaches are compared with BIH and Doppler solutions. With the Vondrak smoothing parameter of 10−7, the weighted RMS of the raw minus smoothed results is 0010 in x, 0008 in y and 0.28 msec in LOD. Finally, the results obtained during the MERIT campaign, an international polar motion intercomparison experiment, using both “quick-look” and the final processed laser range data are discussed.

A 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.

On the basis of 223 passes of Lageos Laser Tracking data taken in October 1980 during the MERIT short arc campaign from 13 tracking sites, the pole position was determined in the orbit correction program MGM along with the initial state vector. This analysis was done for a varying time resolution (5-1 days).

Basis of our computation is the GRIM3P gravity model and a station position set derived by UTEX and partly by the DGF1. The formal sigma of the 5 and 2.5 mean values for the pole coordinates is generally about 0.005 arc-seconds.

Polar motion coordinates obtained during the Merit short campaign by various techniques, as well as their variations, have been compared directly and through a smoothing procedure. In the direct comparison, values at selected dates were differenced in all combinations and the rms values computed. Through the smoothing procedure the amplitudes of the annual and Chandler motions as well as the centers of the polhodes were estimated and compared with predicted values based on the previous six years of polar motion data. The results are somewhat inconclusive, mainly due to the shortness of the campaign and the (practically) real time data reduction techniques. To resolve the issue of possible systematic differences, the proposed MERIT main campaign seems an absolute necessity.

The origin, objectives and programme of Project MERIT, which is a special programme of international collaboration to Monitor Earth-Rotation and Intercompare the Techniques of observation and analysis, were described briefly at IAU Colloquium No. 56 (Wilkins, 1981). Further details of the project and reviews of the techniques to be used were published in a special report (Wilkins, 1980). The MERIT Short Campaign of observations was held during the period 1980 August 1 to 1980 October 31 and the preliminary results obtained will be published by the Bureau International de l’Heure in its Annual Report for 1980. The main objective of the campaign was to provide a realistic test of the operational arrangements that will be required during the MERIT Main Campaign in 1983/4. The first MERIT Workshop was held at Grasse on 1980 May 19-21 to review the operational aspects of the short campaign and to continue the planning for the main campaign. Some of the results obtained during the short campaign were presented on the following day at IAU Colloquium No. 63, and are reported in this volume. The proceedings of the Workshop will be published by the Working Group in a report that will also contain the principal results of the short campaign and information about the availability of the observational data.

If no perturbation exists, the motion of the Earth around its center of mass would be a rigid rotation around a fixed axis in space with constant angular velocity.

In fact, many perturbations disturb this ideal motion and produce variations in both the celestial orientation of the rotation axis and the Earth’s angular velocity.

The mechanisms responsible for these perturbations are the changes in the total angular momentum due to external torques and also the changes in the inertia tensor of the Earth (due to deformations or motions of matter) or in the relative angular momentum in the terrestrial frame (due for instance to winds or to turbulent flow inside the core).

We 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.

Using current procedures, polar motion and Earth’s rotation can be computed from 7 days of observations from four stations to four Global Positioning System Satellites to an accuracy of 1.5 m and .3 msec/day, respectively. Improved computational techniques or instrument accuracy and/or measurements from additional satellites or stations would give significant improvements in accuracy.

In recent years, efforts of Astro-Geodynamics research in China have mainly been concerned with the exploration of the regularity and mechanism of the rotational and crustal motion of the Earth as well as with the possible relationships between astronomical factors and earthquakes. In the meantime, our observational devices are under modification and some new techniques have been established.