Jan Kryński (red.) - Monographic Series No 10
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New celestial and terrestial reference systems and frames and their mutual relations
Jan Kryński, Jerzy B. Rogowski
Reference Systems and Frames in Geodesy, Geodynamics and Astronomy
A consistent definition of reference systems and their realizations as well as their mutual interrelations that are adequate to positioning systems, are fundamental for geodesy, geodynamics astronomy and navigation. A terrestrial reference system is in a natural way referred to the spin axis of the Earth. Precise determination of that axis requires observations of extraterrestrial objects. Positions of those objects are primarily determined in a celestial reference frame that is a quasi-inertial one, i.e. does not rotate and accelerate with respect to the celestial objects, distant from solar system. Therefore both terrestrial and celestial systems are used in geodesy. It is thus necessary to determine the mutual relationship between those systems. In particular, permanent monitoring of Earth rotation is required. Recently both celestial and terrestrial reference systems are used in global positioning systems and space positioning techniques as well as in monitoring variations of geometry and physical structure of the Earth.
Definitions of reference system, reference frame and coordinate system together with the definition of a kinematic reference system and examples are given in the paper. A review of terrestrial reference systems and terrestrial reference frames, starting from the first concepts is given. Similarly, a review of celestial reference systems, starting from FK3 is presented. Major steps of transformation between celestial and terrestrial reference systems with use of precession and nutation models, sidereal time and polar motion parameters are discussed. A special attention is paid to the problems of determination of the instantaneous pole. Substantial role in maintaining a kinematic terrestrial reference system played by considering crustal deformations of the Earth is underlined. Major sources of crustal deformations are specified together with the estimation of their effect on variations of coordinates of tracking stations. Main changes in definitions and realizations of celestial and terrestrial reference systems are also discussed.
Jerzy B. Rogowski, Mariusz Figurski
Terrestrial Reference Systems and Reference Frames and their Realizations
Terrestrial reference system is the basis for processing observations not only in the framework of geodesy but also in a wide range of Earth’s sciences. Terrestrial reference frame was up to 1988 represented by a conventional reference frame with a primary axis pointing the conventional pole that was determined using time varying due to polar motion – coordinates of the pole, and with the conventional zero-meridian corresponding to the zero-meridian of mean observatory of the International Time Service network of stations. The instantaneous pole as well as time required to determine astronomical longitude were obtained using astrometric observations. Historical review of polar motion determination by ILS, IPMS and BIH is presented in the paper. Since 1988 the coordinates of „instantaneous” pole together with observation station coordinates and their rates are being determined at the high precision level using satellite observations and VLBI technique. Recent realizations of terrestrial reference systems and the role of IERS in their establishment and maintenance are discussed in the paper. Transformations between the realizations of ITRF, from ITRF88 until ITRF2000 with the emphasis on practical aspects are presented. Global models NUVEL1, NUVEL1A and NNRNUVEL-1A of tectonic motion are introduced. ETRF89 and its relation to ITRF and WGS84 are presented. The geodetic reference frames and coordinate systems used in Poland during last 80 years are described. The examples of transformation between ETRF and ITRF2000 are given.
Barbara Kołaczek
Fundamental Celestial Reference Systems and Frames and their Realizations
In the paper a description of the International Celestial Reference System and Frame, ICRS/ICRF, the Hipparcos and Tycho catalogues as well as of the Fundamental Catalogues FK5 and FK6 is given. Connections between the ICRS and the systems of the FK5 and Hipparcos Catalogues are also described.
The definition of the ICRS/ICRF is given and the resolution of the International Astronomical Union recommending the acceptation of the ICRS/ICRF is mentioned. Accuracies of the positions and proper motions of the extragalactic radio sources of the ICRF and of the Hipparcos and Tycho Catalogues are given. Methods of connections between the Hipparcos intermediate system and the ICRS are mentioned too. The history of the development of the catalogues of the German „Fundamental Katalog” series until the FK5 and FK6 Catalogues is outlined. Accuracies of star positions and proper motions of these catalogues are given.
Jan Kryński
Relations between Celestial Reference Systems and the Terrestrial Reference System
Adopted by the XXIV IAU General Assembly in Manchester in 2000, defined in the Resolution B1.3 the Barycentric Celestial Reference System BCRS and the Geocentric Celestial Reference System GCRS that both form the International Celestial Reference System ICRS (Kovalevsky 2002), are formulated on the basis of recent formalism of general relativity. The definitions of the systems contain their metrics expressed by metric tensors as well as time systems. They also contain generalised Lorentz transformation between the BCRS and the GCRS for time coordinate and spatial coordinates independently. The Intermediate Reference System IRS is a system through which transformation between celestial and terrestrial system is performed. The motion of the IRS with respect to the GCRS is determined by the IAU2000 precession/nutation model. Resolution 1.7 of the XXIV IAU General Assembly in Manchester in 2000 defines the Celestial Intermediate Pole CIP that determines the direction of IRS z-axis. Ephemeris origin of the IRS with respect to the GCRS was defined in Resolution B1.8 of that General Assembly as the Celestial Ephemeris Origin CEO, and with respect to the International Terrestrial Reference System ITRS – the Terrestrial Ephemeris Origin TEO, both on the equator of the CIP. The angle measured along the equator of the CIP between the TEO and the CEO, positively in the retrograde direction, is called the Earth Rotation Angle ERA. It replaces GST as a parameter of transformation between the celestial and terrestrial systems. According to new definition of UT1 it is proportional to the ERA. The IRS rotated by the ERA can further be transformed to the ITRS with use of two rotations corresponding to the CIP coordinates in the ITRS provided by the IERS.
Jan Kryński
New Time Scales and the Concept of the Intermediate Reference System
The definitions of some time systems have changed with the adoption of the new celestial reference systems and determination their relation with the terrestrial reference system by the XXIV IAU General Assembly in Manchester in 2000. The changes concern, in particular, definitions of Terrestrial Time TT and Universal Time UT1. The classification of time scales is presented and the relations between them are discussed. The role and use of individual time scales is described. Second part of the paper concerns the concept of the Intermediate Reference System IRS with the Celestial Intermediate Pole CIP. The idea of kinematic definition of Non-Rotating Origin NRO that gives orientation of the IRS is presented. The role of the IRS in process of transformation of the GCRS into the ITRS is discussed. The concept of celestial and terrestrial representation of the IRS was presented. Also definitions of the Celestial Ephemeris Origin CEO and the Terrestrial Ephemeris Origin TEO that determine orientation of both representations of the IRS, were given.
Aleksander Brzeziński
New Precession-Nutation Model
Resolution B1.6 of the XXIV IAU General Assembly (Manchester, 2000) recommends that, beginning on 1st January 2003 the IAU1976 precession model and the IAU1980 theory of nutation be replaced by the new IAU2000 precession-nutation model. This model determines the instantaneous direction of the celestial pole in the Geocentric Celestial Reference System with an accuracy of 0.2 milliseconds of arc (mas). The resolution encourages also the continuation of VLBI observations to monitor the non-modelled nutation residuals including the unpredictable free core nutation signal. The new precession-nutation model relies upon the Souchay et al. (1999) rigid Earth solution and the non-rigid Earth transfer function MHB2000 of Mathews et al. (2002) which improves the IAU1980 theory of nutation by taking into account such effects as mantle anelasticity, ocean tides, electromagnetic coupling produced between the liquid core and the mantle, and by the consideration of nonlinear terms that have been ignored in earlier solutions.
According to the definition of the Celestial Intermediate Pole (CIP) given by Resolution B1.7 of the XXIV IAU General Assembly in Manchester, the IAU2000 model contains all the nutation terms with periods longer than 2 days. Other terms with shorter periods, mainly diurnal and semidiurnal of the total size of about 0.1 mas, frequently designated as ”sub-diurnal nutations”, are interpreted as a change of the direction of the CIP in the terrestrial frame, that is as polar motion. The corresponding model has been worked out and included into the IERS Conventions 2003 as an element of the transformation between the celestial and the terrestrial reference systems.
This paper is an attempt of a concise description of the IAU 2000 precession-nutation model. The theoretical aspects as well as the organizing efforts that led to the consensus on new model are discussed. A list of problems that to the author’s opinion need further investigation is also presented.
Barbara Kołaczek
Monitoring and the Characteristic of Variations of Earth Rotation
A historical review of the development in the determination of Earth Rotation parameters in both instrumental and methodical as well as organisational aspects is given in the first part of the paper. In its second part a characteristic of variations of polar motion in the range of secular, decadal, seasonal and sub-seasonal variations is discussed. Time variability of those oscillations as well as correlations between variations of polar motion, universal time or length of day and angular momentum of atmosphere and ocean are shown.
Marcin Sękowski
Astronomical Almanac of IGiK (Rocznik Astronomiczny IGiK) in the Presence of Newest IAU Resolutions
The IAU General Assembly, Manchester, 2000, adopted a number of resolutions that put in order, make more precise and substantially change a part of concepts and definitions used in astrometry and geodetic astronomy. Simultaneously it has been recommended to implement those new concepts to practice, including astronomic almanacs, on January 1, 2003.
The paper presents the Astronomical Almanac (Rocznik Astronomiczny IGiK) developed and published in the Institute of Geodesy and Cartography, Warsaw, its content and the changes in the newest issue of the Almanac for 2004 due to implementation of the IAU 2000 resolutions.
Wiesław Kosek, Maciej Kalarus, Waldemar Popiński
Comparison of Earth Orientation Parameters Prediction Using Different Methods
Predictions of Earth orientation parameters are needed for determination in real time parameters of transformation between celestial and terrestrial reference frames. Different methods of prediction of x, y coordinates of the pole and UT1 – UTC, such as least squares method (LS), autoregression method (AR), autoregression moving-average method (ARMA), autocovariance method (AC), neural networks method (NN) as well as the methods combining LS with AR, ARMA, AC and NN, are presented in the paper. The method of prediction of pole coordinates data in polar coordinate system was also presented. The method combining LS with AR, i.e. (LS+AR) provides the predicted data with the best accuracy. Prediction errors of the coordinates of Earth’s pole obtained with that method are smaller than those obtained with the method recently used by the IERS Rapid Service/Prediction Centre at USNO. Coordinates of Earth’s pole are once a week predicted using the LS+AR method in the automatic mode and they are transmitted to USNO where they are compared with current IERS predictions. The method will soon be implemented to routine predictions of Earth pole coordinates data at the IERS Rapid Service/Prediction Centre.