The primary purpose of the FK5 is to provide an absolute optical reference frame against which to measure the positions and motions of other objects. One of the main tasks of meridian circles is to check and improve that reference frame by repeated observation of the fundamental stars. The meridian circle at Bordeaux, France and the Carlsberg Automatic Meridian Circle (CAMC) at La Palma are carrying out programmes of observation of FK5 stars partly with this aim in view.

The positions from which the systematic differences are derived are the means of at least 10, but normally about 30 (Bordeaux) and 50 (CAMC), independent observations of each FK5 star made in the period 1984–87. The systematic differences are displayed in pairs of 3-dimensional plots, Bordeau–FK5 and CAMC–FK5. The similarities of the corresponding pairs of plots is striking, particularly in declination. This implies that systematic errors exist in the FK5 which reach about in places at the epoch 1986.0.

For improved establishment of the dynamical reference frame and tying it to the stellar reference frame, a method to observe close approach (CA) events between satellite and satellite and/or satellite and star is proposed here. The accuracy of measurements of angular distance is 0”.02 – 0”.03 for a single data point. We apply this technique fairly successfully to the CA events of the Galilean satellites and a preliminary orbital longitude correction to the E2X3 constants of J4 is obtained from eight CA observations ranging from 1987 to 1989.

Standard precession theory builds up the precession matrix P, which rotates coordinates from the mean equator and equinox of epoch to the mean equator and equinox of date, by a sequence of three elementary rotations by the accumulated Euler angles ϚA, θA and zA: P = R3(−zA)R2(θA)R3(−ϚA). This scheme works well provided both the epoch and the date are within a few centuries of J2000. For long-term applications, the alternative formulation using the accumulated luni-solar and planetary precession, P = R3(ᵡA)R1(−ѡA)R3(−ψA)R1(ɛ), is more stable.

Yet another formulation for P is possible, using the invariable plane of the Solar System as an intermediate plane: P = R3(−L) R1(−I) R3(−Δ) R1(I0) R3(L0). The angles I0 and L0 are the inclination and ascending node of the invariable plane at epoch; I and L are the same quantities at the date. Only the angle Δ is a function of both times. This scheme works for both short-term and long-term applications.

For the short term, polynomial coefficients for I, L, and Δ are derived from the currently-accepted coefficients of the angles ϚA, θA and zA. For the long term, these angles are expressed as sums of Chebyshev polynomials obtained from analysis of a million-year numerical integration.

If the intersection of the mean equator and the invariable plane were adopted as the origin of right ascensions, the theory would be simplified further: since L0 and L would no longer be required, P would again consist of the minimum three rotations.

For years, interest in precise positions of minor planets has centered on tying the dynamical reference frame with the stellar frame and determining catalog zone errors. Photographic methods are generally used in obtaining observed spherical equatorial coordinates (R.A., Dec.) or crossing-point observations. Estimates of the external precision of the equatorial coordinates are overly pessimistic, while those for crossing-point observations, too optimistic.

It is estimated that equatorial positions for the brighter (m < 11) minor planets can be determined with an external precision not worse than +/−0.2 arcsec (m.e.), and perhaps as low as +/−0.1 arcsec (m.e.), depending on the reference catalog zonal errors.

Intersatellite observations are a type of crossing-point observation in which the images of two different objects appear in the same exposure. This is the most precise type of crossing-point observation and gives an estimate for the lower limit to the external precision of this observation type. Recent studies of satellite observations indicate that this lower limit is in the +/−0.05 to +/−0.08 arcsec (m.e.) range.

Precise positions of optical counterparts of radiosources are needed in connecting optical and radio reference frames. The Cagliari Astronomical Observatory has planned systematic observations with the Danjon astrolabe of objects from the high priority Hipparcos radio-stars list. A preliminary result of the position of radiosource HR 5110 is presented.

A model for the kinematic distribution function of our Galaxy can be used as an independent confirmation that a reference system is free of Earth motions and retains the true kinematics of the stars. Maximum likelihood can simultaneously estimate the parameters required to calibrate distances to the stars, represent the kinematic distribution function, and check on residual Earth rotations in the proper-motion system. The global maximum-likelihood analysis uses all available information: photometry, trigonometric parallax, proper motion, and line-of-sight velocity for a well-defined catalog of stars. Awaiting observations from HIPPARCOS, preliminary testing of the algorithm on available ground-based observations is discussed.

This 5-yr program which began in 1987 is to establish a link between the radio and optical reference frames, using about 400 extragalactic sources which are compact and flat spectrum in the radio and which are observable in the optical. An overview of the program is presented at this meeting by Johnston et al. Various specific aspects of it are also discussed in Ma et al., McCarthy et al. and Reynolds et al.

The observations have been continuing in both hemispheres and in both the radio and the optical. We have observed 348 sources with VLBI. There is at least one astrograph plate available for 369 sources and at least one prime focus plate available for 138 sources. Altogether there are 450 sources with some data.

Using artificial satellites as transfer objects the project “Coupled Quasar-Satellite-Star Positioning” represents an independent method for linking quasar and stellar reference frames. Optical observations of close approaches between reference stars and satellites yield satellite positions in the stellar reference frame. On the other hand high precision satellite orbits in the International Earth Rotation Service (IERS) terrestrial reference frame are obtained from laser or radiometric observations. Using IERS earth rotation parameters and adopted transformation models the satellite and eventually the star positions can be expressed in the IERS quasar celestial reference frame. In this paper we describe the CQSSP project and assess its capability for providing an accurate tie between tho two metioned celestial reference frames.

The astrometric accuracy of the recent configuration of the (D=2m, f=16.5m) RC telescope of the Rozhen obesrvatory (Bulgaria) was tested. We compared x and y positions from a plate of the RC telescpe with positions obtained from a plate of the (D=0.7m, f=3m) CERGA Schmidt telescope at CERGA (France). The positions coincided on a level of 0.09 arcsec, which is caused mainly by the error of the measurements of the CERGA plate.

The accuracy of Very Long Baseline Interferometry (VLBI), representing one of the most important space techniques of modern geodesy, especially for the determination of the Earth’s rotation parameters and baselines, is steadily increasing. Presently, delay residuals are of the order of 30 - 50 ps, corresponding to an uncertainty in length of about 1 centimeter e.g. in the determination of baselines or the position of the rotation pole. As has already been stressed by many authors, at this level of accuracy a relativistic formulation of the VLBI measuring process is indispensable (e.g. the gravitational time delay for rays getting close to the limb of the Sun amounts to 170 ns!). Starting with the work by Finkelstein et al. (1983) a series of papers has meanwhile been published on a relativistic VLBI theory (Soffelet al., 1986; Hellings, 1986; Zeller et al., 1986; Herring, 1989). However, possibly apart from Brumberg’s treatment in his new monograph (Brumberg, 1990) all of these theories have one fatal drawback: they are not based upon some consistent theory of reference frames, which relates the global, barycentric coordinates, in which the measuring process is primarily formulated and in which positions and velocities of the bodies of the solar system are computed, with the local, geocentric coordinates, comoving with the Earth, in which the geodetically meaningful baselines are defined. Furthermore, none of these theories (including Brumberg’s (1990) treatment) have the accuracy of one picosec which seems desirable with respect to the achieved residual values.

Fundamental catalogues of stars have systematic errors due to the precession error and the equinox correction. The formula for these errors to be applied to the FK4 is presented.

In the framework of CONFOR program the formation of star lists of two intermediate reference star systems is being carried out. The first list, RRS2, contains meridian stars in the fields centered at extragalactic radio/optical sources. The second one is formed on the base of 12-14 magnitude stars. The observations are in progress now. The main purpose of this program is to form a base for investigation of mutual orientation of fundamental reference system and new ones.

The results of research on the Earth’s theory of rotation are presented, comparing observed and theoretical values specially for the luni-solar precession and nutation. Investigations of geodynamics show the strong dependence of the nutation values on the adopted model for the structure of the Earth, while the values referring to the precession are not affected as much. The new observational techniques are able to determine some of these values with greater precision than the classical optical techniques, but it is essential that the observations should be done in a systematic and regular way for several decades.

Recent observation catalogues of extragalactic radio sources obtained by Very Long Baseline Interferometry agree in the mean to a few milliarcseconds (mas). Within this range the position differences show constant, linear and periodic offsets. To cast light on these offsets the differences between some of the representative observation catalogues are plotted. Especially, the periodic variations of declination differences of observation catalogues having significantly different epochs are tentatively explained by an uncertainty of the general precession in declination. In terms of the luni-solar precession this uncertainty is estimated to be of the order of −1to −2 mas per year.

The shift of the celestial pole with respect to its 1980 position in longitude (Δψ) and obliquity (Δε) from the available VLBI measurement of 1984 year within IRIS project is estimated. Formal uncertainties of the angles Δψ and Δε are 5.9 mas and 2.7 mas respectively. The day length variation was obtained from the same data. Then attempt was made to obtain annual and semiannual nutation amplitudes. The values are in reasonable agreement with the determinations of other workers.

Due to the highly accurate determination of the corrections to the nutation terms (the IAU-1980 nutation theory) by VLBI observations, the proposal for adoption of the new nutation theory or standard model of nutation is being discussed. At this point there are two unsolved problems:

1. (a) VLBI observations can’t provide reliable estimates of the principal nutation term. The reason is that the correlation between this term and the precession constant is high due to the insufficient time span of these observations (about 10 years).

2. (b) period of free core nutation (FCN) which depends on the ellipticity of the mantle-core boundary has not been, so far, reliably determined from direct observations.

In the present paper, we would like to draw attention to the results of the analysis of astrometric latitude and time observations which could be useful for resolving these problems.

This paper briefly introduces the current status of the Chinese VLBI Network (CVN) Project and its astrometric and geodetic goals.

Presented is a brief introduction to advances in the first Tokyo PMC program which began in 1985 and is to end in 1991.

Block adjustment (BA) simulations of an entire hemisphere show major improvements in individual as well as systematic accuracy of star positions obtained by photographic astrometry, independent of systematic errors of the reference star catalog used. Results for the accuracy estimates obtained from a patch-like plate pattern in the sky are not valid for closed plate pattern. The importance of BA methods for the realisation of a reference system is stressed.