While calculating low frequency oscillations of the Earth liquid core spherical harmonic representation of the deformation field is usually used [1-3]:

Substitution of (1) into the equations of motion gives an infinite system of differential equations for scalar functions Sɭm and Tɭm . Approximate solutions of such a system are obtained by truncating of the system. But results of [4] show that sometimes such method divergences.

The paper gives a rigorous and purely formal derivation for the relationship between the “nonrotating” origin and the x-axis of the Qt system, i.e., the true equator system. Neglecting nutation, a nonrotatingorigin could also be achieved by putting m=0 in the formula for the time derivative of right ascension.

The U.S. Naval Observatory Navnet program monitors changes in the rotation of the Earth on a regular basis using radio interferometric observations acquired with telescopes in Alaska, Hawaii, Florida, West Virginia and, in the past, Maryland; other radio telescopes have also participated occasionally. These observations have been used to derive a radio interferometric celestial reference system, Navy 1990-5, using two years of dual frequency measurements from 24-hour-duration observing sessions. A total of 84 extragalactic radio sources, mostly quasars, have been observed by the Navnet program to date, of which 70 currently have source position formal errors of one milli second of arc or less. The root mean square of the difference between source position estimates from the Navnet data and an independently derived catalog using completely different data is less than one milli second of arc in both right ascension and declination after the adjustment of an arbitrary rotational offset between the two celestial reference frames.

The influence of optical structure of compact extragalactic radio sources on the definition of their centroids is discussed. Numerical calculations are performed using 4C 31.63 as an example. The results show that the presence of the underlying luminous galaxy does not lead to an unusual decrease of the accuracy in the optical astrometric measurements.

By means of the world function an approximate transformation showing the Riemann tensor between the Fermi coordinates associated to two non-rotating local reference frames is derived in a General Relativistic space-time. One of the observer’s world lines is resticted to be a time-like geodesic of the space-time, and the other is a time-like curve of a general character. The space-time where the transformation is evaluated is supposed to be of small curvature, and the calculations are carried out in a first order of approximation with respect to the Riemann tensor.

We faced the problem of designing and building a Photographic Zenith Tube with mercury basin and lateral plate-holder system providing enough accuracy to allow the determination of astronomical coordinates. This instrumentis intended to work as a prototype of a bigger and more accurate one to be designed and developed in the future (Lopez, A. et al.: 1983).

In this paper, we display an improvement to our process of semi-automatic measuring of astrometric plates, in which the photometric sensor is substituted by a CCD system of image getting and digitalization. The advantages of this method are analyzed taking into account the possibilities of the image analysis in the space and frequency domain.

One of the main problems in positional astronomy is determining the equator and vernal equinox of the reference system.

In this paper we display a method of amendment of minor planets elements taking into account the perturbations in the coefficients of the equations of condition, also including the corrections of the vernal equinox and obliquity in the fitting.

The stability of the Terrestrial Reference Frame (TRF) is studied by analyzing divergences of independently determined Earth rotation series. These series, which have been (nominally) determined within the same TRF, are found to differ by up to 4.7 mas in bias and up to 0.80 mas/yr in rate. These bias-rate differences could reflect errors in the various models used during the data reduction process.

One of the most important problems of modern astrometry is constructing of an ideal inertial coordinate system. A bulk of up–to–date star catalogues is referenced to an equatorial frame, which is far from being really inertial, Some attempts were made to remove this defect by excluding both nutation and precession in right ascention [Guinet 1979, Murray, 1990]. This proposal seams to be a compromise between the existing tradition and the new requirements met by the modern astrometry.

In this paper the corrections of positions and proper motions (J2000) of FK4 were obtained. For different kinds of observational methods the formulae of calculating the influence of systematic differences of FK4 on time and latitude determinations were derived. The influence of systematic differences on the universal time and polar coordinates (1980–1988) of Chinese Joint System (CJS) and the global optical solution of earth orientation parameters (1987–1988) were calculated and their external precisions referred to BIH or IERS were also re–estimated.

Astrometric accuracies of a few tens of milliarcseconds are expected to be attainable within five years by calibrating astrograph plates with optical observations of Global Positioning System (GPS) satellites against a stellar background. The line of sight from an observer on the Earth’s surface to a GPS satellite may be calculated with high accuracy. Motion on each day of the line of sight to the satellite and changes from day to day in the apparent path of the satellite are sufficiently slow to make it possible to reduce atmospheric errors by averaging. Advanced ground-based optical sensors, probably using charge coupled device technology, will be required for GPS optical astrometry.

HIPPARCOS is now well settled on its 10h40m eccentric orbit and the data is received by three telemetry stations that are also transmitting commands: Odenwald (F.R.G.), Perth (Australia) and Goldstone (California). Although during about 80 to 90% of the time, data are recovered, in practice only the observations made from higher than the outer Van Allen belt are usable. Sometimes even at that height, on-board attitude determination does not converge due to the noise that covers the star mapper signal. Finally, about between 55 and 65% of the mission time is usable for reduction. The actual routine data recovery started on November 27, 1989.

Relations among the three concepts (TAI, coordinate time, and proper time)are discussed and comments of the definition of the TAI are given.

On the basis of the data observed with the Photoelectric Astrolabes in China observed during the period from 1982 to 1990, the general catalogue is compiled. With magnitudes ranging from 0m.l to 7m.3, the general catalogue consists of 2577 stars spreading from δ=−3° to δ= 69°, in which 955 are FK5 stars and 1622 are GC stars. The mean precisions of 2577 Δα’s and 1892 Δδ’s are ±4.0 ms and ±0”.065, respectively. The mean epoch of the catalogue is 1987.4. Finally systematic corrections of (CAT—FK5) are given.

This article presents a new method for observing minor planets. The observation is operated with a photo–electronic imaging devise CCD and a lower latitude meridian circle.

The CCD is mainly used except it is during the favourable opposition of the minor planet when the meridian circle is mainly used in this method. The method can improve precision of observation of planetary position and enlarge scope of observation of planetary orbit. Therefore, the measured precision of zero point of stellar coordinate could be increased. The key of succeeding is that more precise result is got using the CCD.

The experiment in this article indicates that this method is a good way:While a minor planet and calibration stars locate in the same image of the CCD, the measured precisions of the minor plant are

σα = ±0″.03, σ´ ±0″.035;

While the minor planet and calibration stars are located in different images of the CCD, the precisions are

σα = ±0″.06, σδ = ±0″.08.

An analysis of the differences between observed nutation angles and the 1980 IAU Nutation Theory shows the existence of currently unmodeled effects. An empirical set of corrections to the 1980 IAU Nutation Theory is presented and compared with current geophysical models. A prograde periodic variation (period ≈ 420 days), which may be related to the theoretical free core nutation, is apparently seen.

A cooperative program has been established to produce a reference frame of at least 400 suitable extragalactic sources. This is an extension of collaborative efforts (Johnston et al., Russell et al., Ma et al., and Reynolds et al.) reported at this meeting. In Phase One, a catalog of radio source positions will be constructed using original VLBI observations already obtained by NASA, NGS, NRL, JPL and USNO. Observations will be re-reduced in a consistent system to avoid some of the more serious problems associated with the formation of compilation catalogs. In Phase Two, the USNO will maintain the system, including the monitoring of source structure and/or variation. During both phases cooperation among the various agencies will continue. A list of sources will be made available to optical observers so that the frame will have benchmarks in both the optical and radio regimes.

Automated photoelectric meridian circles are able nowadays to have the full set of graduation errors of the decimation circle determined within a few days. Thus, the modern meridian circles can be monitored, and the annual and secular changes of the graduations can be easily detected to provide the graduation corrections at any date. In order to remove systematic declination errors in the form Δδα from absolute catalogs, these continuous changes should be taken into account.