From the analysis of 24 yr of lunar laser ranges, the correction to the IAU luni-solar precession constant is found to be −3.2 ± 0.3 mas/yr for a total value of 50.3846˝/yr at J2000. The size of the 18.6 yr nutation of the pole needs to be increased by about 2 mas over the 1980 IAU theory.

A new approach to forecasting changes in length-of-day (δl.o.d) with lead times from one to ten days is examined. The approach is based on the high correlation that has been shown to exist between high frequency changes in l.o.d. and those in the atmosphere's angular momentum (M). Because forecasts of tropospheric values of M can be calculated from the zonal wind fields produced by operational numerical weather prediction models, it seems worth investigating whether these forecasts are sufficiently skillful to use to infer the evolution of δl.o.d. Here, we examine the quality of M forecasts made by the Medium Range Forecast (MRF) model of the U.S. National Meteorological Center (NMC). By comparing these forecasts against those based on a simple model of persistence, we find that skillful forecasts of M are being achieved on average by the MRF, although there has been much month-to-month variability in forecast quality. Overall, our results indicate that for prediction lead times of 1–10 days, dynamically-based forecasts of δl.o.d. represent a viable alternative to the empirical approaches currently in use.

Complete ephemerides of the moon and the four inner planets could be created solely from ranging data alone. Such ephemerides would then be independent from any outside astronomical reference system, and, therefore, would define their own unique reference frame. In fact, this is nearly the case with present-day ephemerides; the accuracy of the ranging data tends to dominate most of the least squares adjustment.

This paper outlines the process of creating the lunar and planetary ephemerides along with the orientation of the ephemerides onto the dynamical equinox. The resulting accuracies of these processes are given and a number of uses for the ephemerides are highlighted.

The modern space techniques have achieved such an unprecedented accuracy and precision in position measurements that there is required a rethinking and reformulation of reference frames, both in their concepts and in their realizations. Various reference frames are a natural consequence of the different observational techniques and data types. Not only are the determination and the necessity of these various frames stressed, but also their evolving nature is recognized. The current status of frame ties is highlighted, and the need for improved ties is motivated. We conclude with our concerns and recommendations.

Results from Lunar Laser Ranging data analysis are presented: (a) the values and statistics of UT1 determined by three stations are given; (b) the lunar tidal acceleration n is found to be −24.9 ± 1.0 arc sec/century2; (c) the tidal-effect coefficient k/C for UT1 is shown to be in strong agreement with the theoretical value; and (d) corrections to the IAU values of precession and nutation are estimated.

Analysis of data from new, highly accurate, geodetic techniques reveals rapid polar motions. Comparison of the new geodetic data and meteorological excitation estimates shows that the observed rapid polar motions are correlated with atmospheric pressure changes, and that these changes are related to atmospheric normal modes.

In recent years, a revolution in astronomical position measurements has been taking place with the advent of modern space techniques. These new techniques, which supplement the traditional astrometric measurements, include laser ranging to the moon and artificial satellites, very-long-baseline interferometry (VLBI) of galactic and extra-galactic radio sources and spacecraft, radio tracking of satellites, and radar-ranging and spacecraft tracking during planetary encounters. Impressive accuracies have been achieved and further improvements are forthcoming. Each technique can be expected to establish its own reference frame which is derived from observations of a particular class of objects. The celestial and terrestrial coordinate systems are related through adopted constants and definitions. Contemporary astronomy has led to the development of three principal celestial coordinate systems: the optical frame (FK4/FK5) based on positions of galactic stars; the planetary/lunar ephemeris frame based on the major celestial bodies of the solar system; and the radio frame constructed from observations of extragalactic radio sources (quasars). Each frame is rotated with respect to others; furthermore, the optical frame offset is time variable. It is important that all frames be interconnected and unified. The optical frame is being connected to the radio frame by VLBI observations of radio emitting stars. The radio frame is being tied to the ephemeris frame in several ways – one is via differential VLBI measurements between quasars and planet-orbiting spacecraft.

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 lunar laser range data taken at McDonald Observatory between August 1969 and May 1980 has been analyzed. The simple rms residual for the 2954 ranges is 31 cm. Results of the analysis include GMearth = 398600.45±0.02 km3/sec2 and a secular acceleration of the lunar orbital mean longitude of ṅ = −23.8±1.5“/century2 which yields a Q of 12.3 at semidiurnal frequencies. The lunar harmonic C30 is (−8.7 ± 1.1) × 10−6 and the lunar rotational dissipation k2mT = (4.7 ± 0.5) × 10−3 day. Also resulting from the solution are geocentric coordinates of McDonald accurate to 30 cm, including the first value for the longitude with the new IAU constants and a dynamical equinox.

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.