Dynamical constants and other fundamental reference parameters for Mars have been derived from analyses of Viking lander ranging and Doppler tracking data covering a time span of nearly four years. Precise values have been obtained for the coordinates of the spin axis and for the rotation rate, suggesting that these Viking-derived values are definitive and are suitable for adoption by the IAU. Preliminary results have been obtained for a small seasonal variation in the rotation rate, and progress has been made toward a direct determination of the precession constant.

The orientation of the FK4 coordinate axes was made by two methods: using the classical interpretation of the orientation problem and by the new method suggested and developed at the Main Astronomical Observatory.

The formulation of the fundamental reference system to be represented by the FK5 includes the determination of the equinox and equator on the basis of planetary dynamics and the application of the new expressions for the general precession in longitude adopted in the IAU (1976) System of Astronomical Constants. The role of hypotheses which entered the determination of the lunisolar precession is explained. Results are presented for the equinox and equator of the FK5 which are based on observations of the Sun, planets, and lunar occultations.

The HIPPARCOS Satellite, to be launched by the European Space Agency, will provide a stellar reference frame over the whole celestial sphere with an average accuracy of ± 0002 in each coordinate and component of annual proper motion, for some 100,000 stars.

The origin of coordinates will be arbitrary. Absolute rotation of the system of proper motions can be obtained by measuring quasars relative to stars in the HIPPARCOS catalogue, either with the NASA Space Telescope or by conventional ground based astrometric observations.

A concept of dynamical equinox and its relation to the analytical form of the adopted theory of the Sun is discussed. Connection between the FK4 equinox and the dynamical equinox is determined by comparing two analytical theories of the Sun (the adopted Newcomb’s theory and a new one (AT-1) constructed at the Instituted of Theoretical Astronomy) with solar meridian observations made at US Naval Observatory (1911-1971 yr.). Corrections Δα to the FK4 right ascensions, Δδ to declinations and Δε to the angle between the equator and the ecliptic are:

Secular variations in Δα and Δε may be explained by improvement of observational conditions after reconstruction of the telescope pavilion in the forties.

The Jet Propulsion Laboratory of the California Institute of Technology has been developing a radio-astrometric catalogue for use in the application of radio interferometry to interplanetary navigation and geodesy. The catalogue consists of approximately 100 compact extragalactic radio sources whose relative positions have formal uncertainties of the order of 0”.01. The sources cover nearly all of the celestial sphere above -40° declination. By using the optical counterparts of many of these radio sources, we have tied this radio reference frame to the FK4 optical system with a global accuracy of approximately 0”.01. This paper describes the status of this work.

A radio astronomical reference frame has been established from four interferometric position surveys of extragalactic objects. The resulting systematic differences between surveys were studied with different weighting schemes and averaged systematic corrections were derived.

Twenty-two radio sources, being bright objects of the Galaxy are observable with astro labe (plus six for those having photoelectric devices). It is about half of the total number (forty-six, figure 2) of objects of the list of galactic objects given by the working group of Commission six of the IAU. Seventeen astro labe stations (figure 2) are interested in a cooperation which final objective is to link the radio and optical reference systems.

A method of comparison of the FK4 system to a system based on extragalactic sources is tested on actual data for 140 sources observed by radio-interferometric and whose optical counterparts have positions referred to FK4. The difference of coordinates is analyzed in spherical harmonics series. The computation show that the method gives satisfactory results. However, the precision of the data prevents from any interpretation in terms of systematic errors of the FK4.

After commenting on certain definitions related to both the terrestrial and celestial (quasi-inertial reference systems (CTS and CIS) to clarify terminology, the geodynamic requirements for such systems are reviewed. This is followed by a discussion of certain problematic aspects of the two systems. The article concludes with a list of required actions aimed to assure that the reference system issue is resolved early. The list is proposed for the joint IAG/IAU working group to be established in accordance with the Colloquium resolution printed elsewhere in this volume.