The joint program of NAVOBSY/NRL is discussed.

The United States Naval Observatory (NAVOBSY) and the Naval Research Laboratory (NRL) are collaborating in a program to apply radio interferometric techniques to the determination of variations in Earth rotation, polar motion, and improved astronomical position reference systems. Investigations of VLBI and connected interferometer techniques and radio sources for astrometic application have been in progress for several years as part of the NRL radio astronomy program, and currently NRL and NAVOBSY are carrying out experimental programs to investigate VLBI time transfer techniques and UT determination using the connected element interferometer of the NRAO in Green Bank. Some previous results of observations using the Green Bank interferometer and proposed plans for operation as a dedicated system over a period of time to evaluate effectiveness for precise determination of Earth rotation parameters are discussed.

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, approximately or 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.

Considerable improvement in the determination of the motion of the Earth is possible by the potentially high accuracy inherent in very-long-baseline interferometry. Precisions of UT1 are estimated from time delay and fringe frequency measurements of extragalactic radio sources with positional uncertainties at the level. Case studies resulted in standard deviations about one order of magnitude smaller than those obtained by classical astrometric methods. The dependence of estimates on baseline orientations and source declinations is discussed.

Laser tracking of the Lageos spacecraft has been used to derive the position of the Earth's pole of rotation at 5-day intervals during October, November and December 1976. The estimated precision of the results is 0.01 to 0.02 arcseconds in both x and y components, although the formal uncertainty is an order of magnitude better, and there is general agreement with the Bureau International de l'Heure smoothed pole path to about 0.02 arcseconds. Present orbit determination capability of Lageos is limited to about 25 cm rms fit to data over periods of 5 days and about 50 cm over 50 days. The present major sources of error in the perturbations of Lageos are Earth and ocean tides followed by the Earth's gravity field, and solar and Earth reflected radiation pressure. Ultimate accuracy for polar motion and Earth rotation from Lageos after improved modeling of the perturbing forces appears to be of order ± 5 cm for polar motion over a period of about 1 day and about ± 0.2 to ± 0.3 milliseconds in U.T. for periods up to 2 or 3 months.

Using two-day arcs of GEOS-3 laser data, simultaneous solutions for pole position components, xp and yp, and orbit elements have been obtained for the period spanning 3 February to 6 March 1976 using three NASA Goddard Space Flight Center laser stations located near Washington, D.C. (STALAS) and on the islands of Bermuda and Grand Turk. The results are in general agreement with the BIH results. However, because of the locations of the laser sites, the xp solution is weaker than the yp solution. The xp and yp estimates were smoothed with a straight line by weighted least squares using the variance associated with the pole estimates as weights in order to reflect the effect of widely different data distributions. The smoothed yp differs by one meter with respect to the BIH smoothed values and the smoothed xp differs by about two meters. Spectral analysis of the results has identified frequencies associated with the orbital motion indicating the need for further improvements in the model of the physical system.

This paper describes a Lageos laser ranging station design based on single photoelectron detection which could be used to monitor polar motion at all periods, and short-period UT1 variations with high accuracy and at moderately low cost. No comparison has been made with the costs of long baseline radio interferometry (LBI) stations, since the establishment and operation of basic international networks using both approaches seems scientifically well justified. Both LBI and lunar laser ranging results will determine intermediate and long-period UT1 variations and nutation. The LAGEOS laser ranging station design suggested here is similar to the design used in the high-mobility LAGEOS ranging station now being constructed by the University of Texas. It has been shown that 30 cm diameter transmit/receive optics with 10 arc second pointing accuracy and 50 milliwatt average laser power output are usually sufficient to give 70 signal counts in a three minute averaging time, even at 20° elevation angle. Lasers having the desired power at 530 nm wavelength with 10 pps repetition rate and 3 ns pulse length are now reported to be commercially available at moderate cost. The resulting range accuracy for normal points corresponding to the three minute averaging time is three cm, including allowances for absolute calibration of the system, atmospheric refraction correction uncertainties, and Earth tide uncertainties. Much of the pulse-timing electronics needed is commercially available, and many time and latitude stations already have good epoch determination systems. The ten arc second pointing requirement for a 30 cm diameter beam seems considerably easier to meet than the optical requirements for classical observing techniques. A simple beam director with flat mirrors appears quite feasible, with inexpensive encoders on each axis for minicomputer control if desired. A number of special features of the University of Texas high-mobility station would not be needed for a fixed station.

Lunar ranging data have been routinely available since September of 1970, but many problems of a varying nature have delayed the establishment of a world-wide lunar ranging network. As a result, we must re-examine the role which this program can play in the determination of the Earth's rotation and polar motion. Although there are many technical difficulties now inhibiting the widespread use of this technique there seems little doubt but that we can overcome these problems and achieve routine, accurate orientation determinations. The more difficult questions concern how an Earth rotation campaign should now be configured to use the equipment and resources in the best way.

Despite considerable progress by other techniques, the failure to develop a lunar capability for Earth orientation determinations would result in a serious loss of information. Lunar monitoring of long-term effects in the Earth's rotation rate and the relationship of the lunar orbital parameters to a stellar reference frame are two tasks for which there is little redundancy. However, the most cost-effective usage of station resources may not require daily measures, but only periodic, accurate snapshots of the Earth's rotational position relative to the Moon. If some satellite laser ranging facilities could treat the Moon as an object of opportunity, but were able to elevate the program to priority status when global conditions were favorable, the incremental cost of gathering the required lunar data might be drastically reduced. Even though these cost savings could not be achieved without daily communication between cooperating stations, such a detailed interaction is not unreasonable to consider.

Several new “space” techniques have been used for episodic determination of Earth rotation parameters, usually the variation in apparent longitude (UT0) and apparent latitude of an observing station. Earth rotation services require more than episodic determinations; they need near-daily determinations. Since 1975, planning has been underway for a demonstration of the viability of lunar laser ranging for such a usage. The observing campaign named Earth Rotation from Lunar Distances (EROLD) was organized with the proposed activity to cover the years 1977–78. Progress has not been so rapid as hoped, but it remains true that lunar ranging has produced more Earth rotation information than other new techniques.

The status of the EROLD campaign has already been described in another paper (Mulholland, 1979). Since the beginning of 1977 all of the computer software has been ready to work. With the expectation that at least one other station would be able to operate before the end of 1977, it appeared not very interesting to use the single-station data for the study of UT0 during the last year since the other parameters of the Earth's rotation can be determined only with multiple stations. However, as this situation of a single operating station continues, it seemed to be preferable to attempt some determinations of UT0, even if that represents only a part of the goals envisioned from this campaign. We give here a brief resume of our preliminary results.

Doppler observations of Navy Navigation Satellites have been used to compute pole positions on a daily basis since 1969. Limited results exist for the period 1964 to 1969. Based on Doppler observations from four or five satellites, the standard error for a five-day mean pole position is less than 20 cm. Comparisons are made between BIH, IPMS and ILS results and those obtained from Doppler. It is shown that the six years of reliable Doppler data since 1972 contribute little in finding the Chandler period. Using observations from the three astronomical sources over 12 years yields a Chandler period of 432.0 ± 0.2 days.

The independent series of coordinates of the pole obtained by the Defense Mapping Agency by observations of Transit satellites (DOP) and by the BIH using classical astrometry only (AST) are considered. They mainly differ by a periodic annual term. After removal of the mean annual difference from AST, the two series are compared. They show common and remarkable irregularities. A derivation of the excitation functions from DOP and AST shows also a good correlation. In the course of this study the constant improvement of DOP, which is now much better than AST, was noticed. The author expresses the opinion that the annual difference between DOP and AST is due to AST, which should therefore be calibrated against DOP.

Observations of Earth rotation and polar motion are now performed on a routine basis by classical astronomical methods and by the satellite Doppler technique.

In the near future the laser is likely to realize its old objective of determining ER and PM with an accuracy of a few milliseconds of arc through ranging to Lageos. VLBI is still in development.

To permit investigation of possible systematic differences, all these techniques should be used simultaneously for a period whose length should not be set in advance.

The Doppler method can still be improved and could be easily introduced at many observatories in support of the Medoc campaign. We present here two possible methods of data condensation which, if implemented, would permit much of the data analysis to be done at the tracking stations and would reduce to a very few the number of parameters required by the central computing agency.

It is already well known that the coordinates of the pole can be derived not only by the classical astronomical methods but also from analysis of the orbits of artificial Earth satellites. The MEDOC experiment (Motion of the Earth by Doppler Observation Campaign) has been initiated by the Groupe de Recherches de Géodésie Spatiale (GRGS) to provide observations and undertake independent analysis for this purpose. Several organisations are participating.

The data on the irregular fluctuations in the length of the day and the motion of the pole is of great significance in the geophysicist's task of constructing a model of the earth's interior.

Short term instabilities in the dynamo generating the earth's magnetic field produce what is observed at the surface as the secular variation. These changes induce currents in the lower mantle and the resulting torques appear to be the cause of the irregular fluctuations in the length of the day, although some quantitative problems remain.

The excitation of the Chandler wobble could result from impulsive torques applied to the mantle by very short period (a year or less) local magnetic field disturbances coming to the surface of the core. The alternative mechanism by earthquakes has been much investigated and the possibility of this is still obscure. A test however is available in the polar motion data: a disturbance in its path displaces the subsequent centre of its Chandler motion in the latter theory, but only its amplitude on the former theory. The behaviour of the pole around 1968 supports the core theory, but much more analysis of the polar motion is required.

The parameters of the Earth's polar motion have been estimated by a method which accounts for observational error and which treats two possible statistical models for the polar motion excitation process. Both a 100 year and a 78 year long polar motion series have been used to estimate the Chandler wobble frequency, Fc, and quality factor, Qc; the noise level in the polar motion data; and the magnitude of the excitation process. Discussion of the annual component of polar motion has been omitted because its frequency (1 cycle per year (cpy)) and cause (annual motion of air and water) are presumed known.

There have been many papers discussing the rotation of the Earth (Jeffreys and Vicente, 1957; Molodenskij, 1961; Rochester, 1973; Smith, 1974; Shen and Mansinha, 1976). This report summarizes the application of the perturbation method of celestial mechanics to calculate the rotation of the Earth (Takagi, 1978). In this solution the Earth is assumed to consist of three components: a mantle, liquid outer core, and a solid inner core, each having a separate rotational velocity vector. Hamiltonian equations of motion were constructed to solve the rotational motion of the Earth.

The comparison of astronomical time observations with the theory of solid-Earth tides makes it possible to determine the Love number, k, which characterizes the elastic properties of the Earth. In addition, the comparison of values of k determined from different tidal waves allows us to judge the accuracy of the nutational theory in astronomical observations since both tides and the Earth's nutation are produced by the same causes.

The main problems of the hydrodynamical integrations are analyzed. New results are presented for the effect of an extreme ice age.

In connection with the problem of continental drift some authors voice a supposition of the existence of progressive and rotational motion of continental blocks, particularly in seismically active regions. Azimuths of some terrestrial objects determined over long time intervals were used to investigate these motions. Naturally, the reliability of the result depends on the length of these intervals.

The axis of the meridian instrument of Uloug-Beg Observatory in Samarkand, installed 550 years ago (1420-30) is the most ancient meridian direction on the Earth. Using measurements made by the author in 1941, 1956, and present-day observations, it was found that the direction of the axis had moved through an angle of . Allowing for observational errors in the pre-optical period one can accept a change of 5′ or per year. To ascertain if the observed change is caused by rotation of the continental block or local deformation repeated measurements of azimuths of triangulation stations first measured forty years ago were made. The results show the absence of continental block rotation in the region of Middle Asia within a possible accidental error of .