Report of the panel on earth rotation and reference frames, section 7

Objectives and requirements for Earth rotation and reference frame studies in the 1990s are discussed. The objectives are to observe and understand interactions of air and water with the rotational dynamics of the Earth, the effects of the Earth's crust and mantle on the dynamics and excitation of Earth rotation variations over time scales of hours to centuries, and the effects of the Earth's core on the rotational dynamics and the excitation of Earth rotation variations over time scales of a year or longer. Another objective is to establish, refine and maintain terrestrial and celestrial reference frames. Requirements include improvements in observations and analysis, improvements in celestial and terrestrial reference frames and reference frame connections, and improved observations of crustal motion and mass redistribution on the Earth

Analysis strategy issues for the maintenance of the ICRF axes

International audienceIn preparation for the evolution of the definition of the VLBI-based International Celestial Reference Frame (ICRF), various elements of the analysis strategy are investigated, such as reference source selection and the impact of the status of the terrestrial reference frame in the data analysis model. We conclude that including the determination of both the celestial and the terrestrial reference frames in the analysis does not affect the quality of the celestial reference frame. The determination of precession and nutation components is not affected by the status of the terrestrial reference frame but it is affected by the selection of reference radio sources. Extending an earlier study, three lists of reference sources based on progressively larger tolerances are proposed. They include 181, 225 and 247 objects, respectively

Sidereal orientation of the Earth and stability of the VLBI celestial reference frame

International audienceThe consideration of time stability of extragalactic radio sources observed by VLBI is shown to allow the realisation of more consistent celestial reference frames. The impact on the estimation of precession and nutation components is investigated over the time span 1984-2002. The precession correction to the IAU 2000 value that is obtained when excluding the unstable sources reaches 49 ± 5 mu as/year, to be compared to 12 ± 5 mu as/year using the current conventional celestial frame. The determination of the obliquity rate is unaffected and remains at the level of 27 ± 2 mu as/year. The observed correction to the 18.6-year nutation amplitude using the current conventional celestial frame ie sizeably corrupted by the unstable sources. After accounting for this effect, the estimations relative to both sets of reference radio sources confirm a discrepancy with the IAU 2000 nutation model with a total amplitude of 320 ± 100 mu as for the observed nutation in longitude, to be compared to the 80 mu as discrepancy found by Mathews et al. (2002, JGRB, 107, 1029). The discrepancy in obliquity amounts to 50 ± 16 mu as. The effect of source instability is shown to have an impact on the determination of universal time at the one microsecond level. The high and medium frequency nutation terms (up to periods of a few years) are impacted only in the early years of the program. Chapter 7 concerning the observation of the core and inner core free nutations is paralleled by a twin paper (Dehant et al. 2005, A&A, 438, 1149) that proposes a theoretical development for their atmospheric and oceanic excitation

Precession and nutation for a non-rigid Earth: comparison between theory and VLBI observations

International audienceCoefficients of nutation for a non-rigid Earth developed from the Kinoshita & Souchay (1990) theory for a rigid Earth and from Wahr's transformations (Wahr 1980) to describe the action of the Sun, the Moon and the planets on the space orientation of the Celestial Ephemeris Pole is compared to a series of VLBI measurements of Earth rotation initiated in 1979. Estimates of corrections due the imperfections in the modeling for the largest components are given. Linear trends corresponding to a precession in longitude and a rate in obliquity relative to the IERS Celestial Reference System are also estimated (resp. -3.21 mas/year and -0.26 mas/year). The time variability of terms with periods 433d and 365.26d is studied

Precession and nutation for a non-rigid Earth: comparison between theory and VLBI observations

International audienceCoefficients of nutation for a non-rigid Earth developed from the Kinoshita & Souchay (1990) theory for a rigid Earth and from Wahr's transformations (Wahr 1980) to describe the action of the Sun, the Moon and the planets on the space orientation of the Celestial Ephemeris Pole is compared to a series of VLBI measurements of Earth rotation initiated in 1979. Estimates of corrections due the imperfections in the modeling for the largest components are given. Linear trends corresponding to a precession in longitude and a rate in obliquity relative to the IERS Celestial Reference System are also estimated (resp. -3.21 mas/year and -0.26 mas/year). The time variability of terms with periods 433d and 365.26d is studied

Remaining error sources in the nutation at the submilliarcsecond level

International audienceEarths precession and nutations are mainly generated by the luni-solar tidal torque.Diurnal retrograde variations in the atmospheric and oceanic angular momenta in an Earth-fixed reference system induce some additional nutation motions. Observed precession and nutations are derived from very long baseline interferometry (VLBI) data,assuming that the direction of the observed quasars are fixed in space. In this study, we consider the effects of two possible causes for explaining discrepancies between the observed nutations and those modeled in MHB2000 (model adopted by the International Astronomical Union): (1) the time variations in the atmospheric (and potentially oceanic)forcing of the nutations, of the free core nutation (FCN), and of the free inner core nutation (FICN), and (2) the possible contamination of VLBI-derived nutation amplitudes by apparent changes in the directions of the extragalactic radio sources. The robustness ofMHB2000 is tested by perturbing some of the parameters and assessing the validity of the resulting nutation amplitudes against realistic estimations. We show that even allowing for large discrepancies related to atmospheric forcing, the ranges of the possible changes in the FCN and FICN periods and damping factors are small

The International DORIS Service

International audienceThe DORIS system was initially developed for precise orbit determination and precise positioning on the Earth. In continuation of the DORIS Pilot Experiment initiated in 1999, the International DORIS Service (IDS) officially started on July 1, 2003 as an IAG Service after an official acceptance from the IAG Executive Committee at the IUGG General Assembly in Sapporo, Japan. Following this decision, the IERS Directing board accepted the DORIS Service as a new IERS external service. Six satellites carrying DORIS receivers are currently in orbit, permanently observed by 56 well-distributed tracking stations. Among these, three satellites (Jason-1, ENVISAT and SPOT5) are equipped with the new generation of DORIS receivers and were launched between December 2001 and May 2002. The DORIS receivers on these three spacecraft include a navigation function, called DIODE. The permanent tracking network has been constantly improved and specific campaigns of observations have been conducted in Wettzell, Gads and in Antarctica. Recent DORIS performances for precise positioning were improved by this large increase in the satellite constellation, leading to almost 1 cm precision for weekly station coordinates. Significant improvements were also obtained in Polar Motion estimations, leading to 1.0-1.5 mas daily results. In 2003 and 2004, several steps were taken to improve the operations of the IDS, as well as its international cooperation, by organizing several specific analysis campaigns. The International DORIS Service has now started its scientific activity on a routine basis for the International Earth Rotation and Reference Systems Service (IERS) and the Global Geodetic Observing System (GGOS)