Progress in the 2nd realization of ICRF (Invited)

C. Ma

Institution:
Goddard Space Flight Center
USA

The ICRF derived from VLBI observations of extragalactic radio sources up to 1995.6 and effective since 1998.0 was a radical change from the FK5 stellar/equinox celestial system. The number of geodetic/astrometric VLBI observations has since tripled and the number of radio sources with astrometrically useful data has quadrupled. A systematic program for monitoring astrometric sources has been established although the available observing capability limits this to only a fraction of all such sources. Advances in modeling, generation of source position times series, and greater availability of source structure information will permit the identification of better “defining” sources. The VLBA Calibrator Survey will provide a large number of additional sources although most are observed at only one epoch. Working groups have been established by the IAU, IERS and IVS with the goal of presenting the second realization of the ICRF at the IAU General Assembly in 2009.

Selecting ICRF-2 defining sources based on source structure

P. Charlot, A. Fey, A. Collioud, R. Ojha, D. Boboltz, J. Camargo

Institution of the first author:
Observatoire de Bordeaux
France

The intrinsic radio structure of the extragalactic sources is one of the limiting errors in the definition of the International Celestial Reference Frame (ICRF). This paper reports about the ongoing work to monitor the structural evolution of all ICRF sources by using the Very Long Baseline Array and other VLBI telescopes around the world. Based on more than 2000 VLBI images produced from such observations, we have assessed the astrometric suitability (i.e the compactness) of most of the ICRF sources. The number of VLBI images for a given ICRF source varies from 1 for the least-observed sources to more than 20 for the intensively-observed sources. Overall, we identify a subset of about 200 sources that are highly compact at any of the available epochs. We argue that these sources are prime candidates for the realization of the next ICRF with the highest accuracy.

Next ICRF: Single global solution versus combination

Z. Malkin, Ya. Yatskiv

Institution of the first author:
Pulkovo Observatory RAS
Russia

Two approaches to generate the ICRF-2 are compared. The first approach, which has been used for the creation of the current ICRF realization, implies computation of the radio source positions in a single analysis center making use of the most advanced models, software and analysis strategy. However, the latter cannot guarantee that the reference frame is free of systematic errors. The second approach is based on combination of several CRF realizations after investigation and accounting for their stochastic and systematic errors. This procedure is intended to mitigate the systematic errors and improve the precision of the final combined solution. In this presentation, the advantages and shortcomings of both the approaches are discussed.

Time Series Analysis of VLBI Astrometric Source Positions at 24-GHz

D. Boboltz, A. Fey & K-Q VLBI Survey Collaboration

Institution of the first author:
U.S. Naval Obs.
USA

To date there have been 10 VLBI experiments observed over a period spanning 5 years and analyzed for the purpose of establishing a high-frequency (24 GHz) reference frame. The database now contains information on 274 sources and a total of 1052 images. There are currently 87 sources which have been observed in at least 5 experiments. Here we report on an analysis of the time series of astrometric positions determined for these 87 sources and provide a comparison with the X-band time series for the same sources. We discuss the stability of the sources at 24 GHz and the possible implications for ICRF2 source selection as well as the link of the high frequency reference frame to ICRF2.

Reference radio source apparent proper motions

O. Titov

Institution:
Geoscience Australia
Australia

The motions of relativistic jets from the active extragalacit nuclei can reach several hundred microseconds per year and mimic the radio source proper motions observed by geodetic VLBI. Such motions are not correlated and exceed the small systematic effects induced by the rotation of the Solar system around the centre of the Galaxy. In this paper we search the subtle systematic and discuss the results.

Influence of different strategies in VLBI data analysis on realizations of ICRF.

S. Bolotin

Institution:
Main Astronomical Observatory, National Academy of Sciences of Ukraine
Ukraine

Positions of radio sources which are a realization of International Celestial Reference Frame (ICRF) can be estimated from VLBI observations. In this presentation we discuss different approaches in data analysis and the influence of applied strategy on the results. Application of these strategies are illustrated with a set of radio sources catalogs obtained from VLBI data analysis at the Main Astronomical Observatory (Kiev, Ukraine).

Effect of the reference radio source selection on VLBI CRF realization

J. Sokolova

Institution:
Pulkovo Observatory RAS
Russia

Up to now, four stability criteria based on different schemes have been used for selection of reference radio sources. Four lists of reference radio sources based on these schemes have been compiled. Some inconsistency between these lists was found. In our study we analyse the impact of these different selection schemes on the CRF solution as well as on radio source coordinate time-series to construct the optimal approach for reference radio source selection.

Extragalactic Optical-Radio Link Research at USNO

N. Zacharias, M. I. Zacharias, D. Boboltz, A. Fey, R. Gaume, G. Hennessy, K. J. Johnston, R. Ojha

Institution of the first author:
U.S. Naval Observatory
USA

We present the current status and future plans of researchconducted at the U.S. Naval Observatory (USNO) regarding thelink of the optical reference frame to the ICRF using compact,extragalactic sources.Over 500 counterparts of ICRF sources were observed in over20 deep CCD observing runs as part of the USNO CCD AstrographCatalog (UCAC) project, providing a direct link to the Tycho-2stars, thus the Hipparcos Celestial Reference Frame. Abouthalf of that data are reduced now, giving optical-radioposition differences on the 10 to 20 mas level per source.First results will be discussed.A sample of 12 extragalactic ICRF sources are being observedat the 1.55-meter Naval Observatory Flagstaff Station (NOFS)over several years. The optical position stability of thesetargets is monitored on the mas level locally.High resolution imaging of selected sources is planned withthe LuckyCam at the ESO 3.5m NTT and Lick 3-meter AO systemto relate possible source structure at optical wavelengthswith observed optical-radio centroid offsets.As part of the Space Interferometry Mission (SIM) preparatoryscience about 240 bright QSO's are monitored for photometricvariability in B,V,R and I. The goal is to select the mostsuitable sources for future radio-optical link programs.The USNO Robotic Astrometric Telescope (URAT) will be ableto combine deep CCD imaging of all ICRF2 target areas andmillions of anonymous compact galaxies with a stellar,astrometric, all-sky survey of multiple epoch overlaps.

A GAIA oriented analysis of a large sample of quasars

A. H. Andrei, C. Barache, S. Bouquillon, G. Bourda, J.I.B. de Camargo, J.-F. Le Campion, P. Charlot, A.-M. Gontier, S. Lambert, J.J. Pereira Osório, D.N. da Silva Neto, J. Souchay, R. Vieira Martins

Institution of the first author:
(1) Observatorio Nacional/MCT; (2) Observatorio do Valongo/UFRJ
Brasil

GAIA photometric capabilities should distinguish quasars to a high degree of certainty. With this, they should also be able to deliver a clean sample of quasars with a negligible trace of stellar contaminants. However, a purely photometric clean sample could miss a non negligible percentage of ICRF sources counterparts - and this interface is required to align with the ICRS and de-rotate the GCRF (GAIA Celestial Reference Frame), on grounds of continuity. To prepare a minimum clean sample forming the initial quasar catalogue for the GAIA mission, an all sky ensemble was formed containing 128,257 candidates. Among them there is at least one redshift determination for 98.75%, and at least one magnitude determination for 99.20%. The sources were collected from different optical and radio lists. We analyze the redshift, magnitude, and color distributions, their relationships, as well as their degree of completeness.

A VLBI survey of weak extragalactic radio sources for the link with the future GAIA frame

G. Bourda, P. Charlot, S. Garrington, R. Porcas

Institution of the first author:
Observatoire de Bordeaux
France

The space astrometric mission GAIA will construct a dense-optical QSO-based celestial reference frame that will have to be linked to the International Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is found that only about 10% of the ICRF sources are suitable to establish this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional high-quality radio sources for the GAIA link. The sample consists of about 400 sources, typically weaker than the current ICRF sources, which have been selected by cross-correlating optical and radio catalogs. The paper will present the observing strategy and include preliminary results of observation of 224 of these sources with the European VLBI Network in June 2007.

Current Status of the IAU Working Group for Numerical Standards of Fundamental Astronomy (Invited)

B. Luzum, N. Capitaine, A. Fienga, B. Folkner, T. Fukushima, J. Hilton, C. Hohenkerk, G. Krasinsky, G. Petit, E. Pitjeva, M. Soffel, P. Wallace

Institution of the first author:
U.S. Naval Observatory
USA

At the 2006 International Astronomical Union (IAU) General Assembly (GA), a proposal was adopted to form the Working Group (WG) for Numerical Standards of Fundamental Astronomy. The goals of the WG are to update "IAU Current Best Estimates" conforming with IAU Resolutions, the International Earth Rotation and Reference System Service (IERS) Conventions, and the Syst�me International (SI). Initial efforts have concentrated on determining which constants should be considered, the terminology regarding the description of the constants, and the interdependence of the constant estimates on their associated models. The current status of WG activities and the anticipated future directions are presented.

On astronomical constants (Invited)

M. Soffel, S. Klioner

Institution:
Lohrmann-observatory, TU Dresden
Germany

With respect to the ongoing work in the IAU WG 'NSFA' a new classification of astronomical constants into natural constants (defining constants), body constants and coordinate-related constants is suggested. The relation between natural (defining) constants and units of measurements is discussed. The definition of body constants such as masses depends upon the approximation scheme to Einstein's theory of gravity. Finally, coordinate-related constants such as initial values e.g., for center of mass of a planet have a meaning only in a certain coordinate picture of the dynamical problem.Every measurable constant has its meaning only within a certain dynamical framework and a real error related with the involved data set. Finally some remarks concerning the consistency of a set of astronomical constants are given.

Recent progress in concepts, nomenclature and models in fundamental astronomy

N. Capitaine

Institution:
Observatoire de Paris
France

The IAU 2006 resolutions B1, B2 and B3, adopted by the XXVIth IAU General Assembly (August 2006) and endorsed by the XXIVth IUGG General Assembly (July 2007), supplement the IAU Resolutions 2000 on reference systems. The aim of IAU 2006 Resolution B1 is to adopt a precession model as a replacement to the IAU 2000 precession in order to be consistent both with both dynamical theory and the IAU 2000 nutation. The aim of IAU 2006 Resolutions B2 and B3 is to address definition, terminology or orientation issues relative to reference systems and time scales that needed to be specified after the adoption of the IAU 2000 resolutions.The purpose of this presentation is to explain the changes resulting from the joint IAU 2000 and 2006 resolutions with respect to the previous position and to review the consequences on the concepts, nomenclature, models and conventions in fundamental astronomy. We will also report on practical aspects related to the Earth orientation parameters and describe the characteristics of the high accuracy model for the celestial motion of the Earth’s pole.

Recent models of the planet motion and fundamental constants determined from position observations of planets and spacecraft

E. Pitjeva

Institution:
Institute of Applied Astronomy of Russian Academy of Science
Russia

The more recent planet ephemerides constructed at JPL (DE414), IAA RAS (EPM2007) and IMCCE (INPOP06) having about the same accuracy are considered. Common to all ephemerides is a simultaneous numerical integration of the equations of motion of the nine major planets, the Sun, about 300 big asteroids, the Moon and the lunar physical libration performed in the Parameterized Post-Newtonian metric for General Relativity in the TDB time scale, taking into account perturbations due to the solar oblateness and perturbation from a massive ring of small asteroids. The numerical integration includes also the largest trans-Neptunian objects for EPM2007 and the Earth rotation for INPOP06; the software for DE414 permits to solve for many individual asteroid masses. The full set of solution parameters which includes masses of planets and asteroids and the value of AU is presented. All these ephemerides have been oriented to ICRF by using the VLBI measurements ofspacecraft near the planets. Two different versions of constructing ephemerides for the TCB time scale have been done at IAA RAS and JPL. So, some official recommendation should be adopted before recommending the TCB time ephemerides for public.Other methods of determinations of masses of planets and asteroids (from motions of satellites, spacecraft or close encounter asteroids), which can give even more accurate results, are shown. The list of update masses of the 11 planets (including the draft planets Pluto, Eris, Ceres), Pallas, Vesta and the AU value is offered to WG NSFA.

INPOP06. A new numerical planetary ephemeris

J. Laskar, A. Fienga, H. Manche, M. Gastineau

Institution of the first author:
IMCCE, Observatoire de Paris
France

INPOP06 is the new numerical planetary ephemeris developed atthe IMCCE-Observatoire de Paris.INPOP (Intégrateur Numérique Planétaire de l'Observatoire de Paris)is a numerical integration of the motion of the nine planets and the Moonfitted to the most accurate available planetary observations. It alsointegrates the motion of 300 perturbing main belt asteroids, therotation of the Earth and the Moon libration.We will present the latest developments of the INPOP ephemeris.

INPOP: Adjustments to LLR data

H. Manche, S. Bouquillon, A. Fienga, G. Francou, M. Gastineau, J. Laskar

Institution of the first author:
IMCCE, Observatoire de Paris
France

INPOP06 is the latest numerical planetary ephemeris developed at IMCCE and Paris-Observatory. This version has been fitted only to planetary observations (Fienga et al, INPOP06: a new numerical planetary ephemeris, 2007, preprint). The next version currently in development will be directly fitted to Lunar Laser Ranging data. These observations (more than 17000 over 36 years, from 3 different sites) are usefull to determine some physical parameters and inital conditions used in INPOP. We will present our fitting process, the physical effects taken into account to compute residuals and our latest results.

Prospects for Improving the Masses of Asteroids

J. Hilton

Institution:
U.S. Naval Observatory
USA

Among the largest uncertainties in the fundamental constants of astronomy are the masses of the asteroids. They constitute the largest source of uncertainty in the ephemerides of the inner planets. The DE405 planetary ephemerides incorporate the masses of 300 asteroids. However, aside from Ceres, Vesta, those asteroids with satellites, and perhaps Pallas, none of these masses are known with an uncertainty of better than about 50%. Asteroid mass determination requires the discovery of encounters with massive asteroids that are close, and with low relative velocities. The explosion of discoveries of main belt asteroids since the mid-1990s means that it may be possible to discover more such encounters. Thus, the orbits of the perturbed asteroids may be determined with higher accuracy in a shorter period of time. A series of simple filters were developed to determine if any such candidate perturbed bodies exist. Those candidates that made it through the filters can then be subjected to a more rigorous examination to determine the nature of their encounters with a large asteroid. For example, five candidate encounters were discovered for Vesta, all of which have a marginal chance of improving our knowledge of its mass prior to the spacecraft Dawn's arrival at Vesta in 2011. Similarly, a total of 18 encounters were found for Ceres, many of which are almost certainly strong enough to provide a mass estimate with a significantly smaller uncertainty than current estimates prior to Dawn's arrival there in 2015.

Harmonic models of tide-generating potential of terrestrial planets

S. Kudryavtsev

Institution:
Sternberg Astronomical Institute of Moscow State University
Russia

High-accurate harmonic development of the tide-generating potential (TGP) of Mercury, Venus and Mars is made by using a new modification of the spectral analysis method. The TGP harmonic models for Mercury and Venus are developed over 2,000 years (1000-3000) and include 2,345 second-order Poisson series' terms of amplitude exceeding 10^{-8} m^2/s^2 for Mercury and 1,725 analogous terms for Venus. The TGP harmonic model for Mars is done over 200 years (1900-2100) and includes 278 second-order Poisson series' terms of amplitude exceeding 10^{-6} m^2/s^2. The accuracy of the new development of terrestrial planets' TGP is better than that of any previous model. The work is supported in part by grant 05-02-16436 from the Russian Foundation for Basic Research.

Recent advances in modeling precession-nutation (Invited)

V. Dehant, S. Lambert, N. Rambaux, M. Folgueira, L. Koot

Institution of the first author:
Observatoire Royal de Belgique
Belgium

This paper will present recent advances in nutation theory related to what has been done at the Royal Observatory of Belgium by Marta Folgueira, Sebastien Lambert, Laurence Koot, Nicolas Rambaux, and Veronique Dehant, in relation to the Descartes Nutation project. These advances are related to the determination of Earth geophysical parameters from VLBI observations and to better modeling of the phenomena within the Earth. In particular, we will examine the coupling mechanisms at the core-mantle boundary: the electromagnetic coupling, the topographic coupling, and the viscous coupling. Methods related to a time domain approach instead of a frequency domain approach have been examined as well. We also present future developments necessary for a better understanding of the Earth interior and its nutations.

IAU Symposium 78 "Nutation and the Earth's rotation" (held in Kiev,May 1977) as a first step in the consideration concerning the Non-rigid Earth Nutation Theory"

Ya. Yatskiv, A. Korsun'

Institution:
Main Astronomical Observatory
Ukraine

Thirty years ago IAU Symposium №78 "Nutation and the Earth's Rotation" was held (May 23-28,Kiev, Ukraine).The brief history of event and its objectives are described.

Estimation of Earth interior parameters from a Bayesian inversion of nutation time series.

L. Koot, A. Rivoldini, O. de Viron, V. Dehant

Institution of the first author:
Royal Observatory of Belgium
Belgium

The nutation response of the Earth to the external gravitational torque is affected by its internal structure. Because the torque is known very accurately, the high precision nutation observations allow to constrain Earth interior parameters. The non-rigid Earth nutation has been modeled by Mathews et al. (2002). This model is semi-analytical: it depends on parameters, related to the Earth interior, which are adjusted to the observations. The nutation model is frequency-dependent so that, in Mathews et al. (2002), the time-dependent observations are processed before the fit in order to get data in the frequency domain. This procedure implies a loss of information because the parameters are obtained by fitting the model to the twenty dominant terms in the frequency domain and not to the whole time series. In this paper, we develop a new fit procedure which relies on an estimation of the geophysical parameters directly from the nutation data in time series. This allows to use all the information of the time-domain data and to account for the time dependent uncertainties on the data. Rather than the linearized least-squares method used in Mathews et al. (2002), we use the Bayesian inversion method, which is non-linear, and is consequently well-suited for the nutation model. In addition, the Bayesian framework allows to include the possibility of having uncertainties in the nutation model itself. The modeling uncertainties are introduced in terms of a new parameter, estimated jointly with the geophysical parameters. Finally, the results are compared with those obtained by Mathe

VLBI observations of nutation, its geophysical excitations and determination of some Earth model parameters

J. Vondrák, C. Ron

Institution:
Astronomical Institute
Czech Republic

VLBI has been used to observe nutation for more than 25 years, with ever increasing accuracy. The amplitudes and phases of individual nutation terms are sensitive to some parameters of the geophysical Earth model. Dominant is the resonant period P and quality factor Q of retrograde free core nutation that depend on the flattening of the outer fluid core. The period (in celestial frame) is approximately equal to 430 days. In principle, all nutation terms are affected by these resonance effects, but the most affected is the annual retrograde term that is closest to the resonance. Nutation is dominantly driven by external torques, and it contains, as a multiplicative factor, the dynamical ellipticity of the Earth. Relatively small (near-diurnal in terrestrial frame) geophysical excitations are amplified by the resonance, so they also have non-negligible influence on nutation. The aim of the presentstudy is to take these effects into consideration and determine the dynamical ellipticity, and period and quality factor of the Earth at the FCN frequency. An attempt to derive from them also the flattening of the fluid core will be made, taking into account other effects as electro-magnetic coupling between the core and the mantle.

About observability of the free core nutation

L. Petrov

Institution:
NVI, Inc./NASA GSFC
USA

Neither astronomical technique, including VLBI, can measure nutationdirectly. Estimates of parameters of the nutation model are produced by solving the LSQ problem of adjusting millions parameters using estimatesof group delay. The choice of the mathematical model for nutation used in the estimation process of analysis of group delays affects our ability to interpret the results. Ignoring these subtleties and using parameters of the nutation model either in the form of time series, or in the form of empirical expansion as "VLBI measurement of nutation", opens a room for misinterpretation and mistakes. Detailed analysis of the problem reveals that the separation of forced nutations, atmospheric nutations, ocean nutations,and the free core nutation requires invoking some hypotheses, and beyond a specific level becomes uncertain. This sets a limit of our ability tomake an inference about the free core nutation.

Some issues in the Earth's interior exploration with VLBI

S. Lambert, V. Dehant, A.-M. Gontier

Institution of the first author:
Royal Observatory of Belgium
France

We used several homemade nutation series derived from VLBI delays using different constraint on the radio source coordinates. Residuals with respect to the MHB 2000 nutation model corrected from various effects (e.g., non linear) have been analyzed in order to retrieve the Earth's interior parameters. In particular we have examined the value of the FCN and FICN resonant periods and quality factors in terms of physics of the Earth's core and the accuracy with which the parameters can be retrieved considering the current VLBI developments. We have also looked at the inner core parameters and have determined the impact of VLBI uncertainties of the physics of the inner core, and in particular on the inner core magnetic field.

Concise algorithms for precession-nutation

P. Wallace, N. Capitaine

Institution of the first author:
Rutherford Appleton Laboratory
United Kingdom

The precession-nutation models based on the IAU 2000A nutation and IAU 2006 precession involve several thousand amplitude coefficients, many under 1 microarcsecond in size, and the sines and cosines of 1500 angles. For the many applications that do not require the utmost accuracy this represents an unnecessary or even excessive computational overhead. The IAU 2000B model offers one alternative, an order of magnitude smaller than IAU 2000A and delivering classical nutation components of 1 mas accuracy in the current era. We look at other options, based on series for the CIP coordinates X,Y and the CIO locator s and with the GCRS to CIRS rotation matrix as the end product. Truncation of the series provides most of the savings, but certain other measures can be taken also. Three example formulations are presented that achieve 1 mas, 16 mas and 0.4 arcsec accuracy throughout 1995-2050 with computational costs 1, 2 and 3 orders of magnitude less than the full models.

Relativity in Fundamental Astronomy: solved and unsolved problems (Invited)

S. Klioner

Institution:
Lohrmann Observatory
Germany

Nowadays it is no longer necessary to justify the importance of consistent relativistic modelling in the field of fundamental astronomy. Although in the last 20 years the theoretical foundations of relativistic modelling have been elaborated with a lot of care, there are a number of issues, mostly of practical character, that still require both theoretical discussions and practical implementations. This 'gray' areas of the modelling include modelling of rotational motion of celestial bodies, correct inclusion of multipole structure of the bodies in the translational equations of motion, interplay between numerical accuracy and analytical "order of magnitude" of various relativistic terms, relativistic scaling of astronomical quantities and units of measurements. An overview of the relativistic issues in the field of fundamental astronomy will be given from this critical point of view.

Local tests of GR and dynamical reference frame linking with Gaia astrometry of asteroids

D. Hestroffer, S. Mouret, F. Mignard, J. Berthier, P. Tanga

Institution of the first author:
IMCCE/Paris observatory
France

The Gaia satellite, an ESA cornerstone mission to be launched at the end of 2011, will observe a large number of celestial bodies including also small bodies of the solar system (mainly near-Earth objects and main-belt asteroids). The scanning telescope will observe all objects brighter than magnitude V≤20 providing, over the 5 years mission, high precision photometry and astrometry with an unprecedented accuracy – ranging roughly from ≈0.3 to 3 milli-arcescond on the CCD level, and depending on the target's magnitude. In addition, several hundreds of QSOs directly observed by Gaia will provide the kinematically non-rotating reference frame in the visible light, resulting in the construction of a 'Gaia-ICRF'. The positions of the asteroids hence enable to relate the dynamical reference frame to the kinematic one, and to further check the non-rotating consistency between both frames’ definition. Here we show the results of a variance analysis obtained from a realistic simulation of observations for such link. The simulation takes into account the time sequences and geometry of the observations that are particular to Gaia observations of solar system objects, as well as the instrument sensitivity and photon noise. Additionally, we show the achievable precision for the determination of a possible time variation of the gravitational constant. Taking into account the non-completeness of the actually known population of NEOs, we also give updated values for the nominal precision of the joint determination of the solar quadrupole J2 and PPN parameter beta.

Geodetic (Relativistic) Rotation of the Solar System Bodies

V. Pashkevich, G. Eroshkin

Institution:
Central (Pulkovo) Astronomical Observatory of Russian Academy of Sciences
Russia

The problem of the geodetic (relativistic) rotation of the major planets, the Moon, and the Sun is studied by using DE404/LE404 ephemeris. For each body the files of the ecliptical components of the vectors of the angular velocity of the geodetic rotation are determined over the time span from AD1000 to AD3000 with one day spacing. The most essential terms of the geodetic rotation are found by means of the least squares method and spectral analysis methods.

Towards the relativistic theory of precession and nutation

S. Klioner, M. Soffel, C. Le Poncin-Lafitte

Institution:
Lohrmann Observatory
Germany

A numerical theory of Earth rotation has been constructed using the model of post-Newtonian rigidly rotating multipole moments. The theory is construted by a numerical integration and treats all sprectrum of relativistic issues in a consistent way: (1) relativistic times scales, (2) relativistic scaling of astronomical constants, (3) relativistic torques and (4) rigorous treatment of geodetic precession as an additonal torque in the equations of rotational motion with respect to the kinematically non-rotating GCRS. In the quasi-Newtonian limit our theory reproduces SMART97 within the accuracy of the latter. The effects of various relativistic factors will be demonstrated and discussed.

Activities of the IERS Working Group on Prediction (Invited)

W. Wooden

Institution:
U. S. Naval Observatory
USA

The International Earth Rotation and Reference Systems Service (IERS) established a Working Group (WG) on Prediction to investigate what IERS prediction products are useful to the user community in addition to making a detailed examination of the fundamental properties of the different input data sets and algorithms. The major goals and objectives of the WG are to determine the desired Earth orientation prediction products, the importance of observational accuracy, which types of input data provide an optimal prediction, the strengths and weaknesses of various prediction algorithms, and the interactions between series and algorithms that are beneficial or harmful. To focus the research efforts of the WG, the user community was polled to ascertain what prediction products are needed and at what level of accuracy. The current status of WG activities and the anticipated future directions are presented.

Rigorous combination to ensure ITRF and EOP consistency (Invited)

Z. Altamimi, D. Gambis, Ch. Bizouard

Institution of the first author:
IGN-France
France

The ITRF2005 is the first rigorous combination ensuring ITRF and EOP consistency, based on time series of station positions and Earth Orientation Parameters (EOPs). The objective of the presentation is the development of combination strategy allowing to ensure the ITRF2005 and IERS 05 C04 consistency with time. Combining additional input time series after the release of ITRF2005 from the four techniques (VLBI, SLR, GPS and DORIS) allows the assessment of two main procedures: (1) using CATREF combination model and (2) using EOP-only combination method. The main features of the two combination strategies are presented together with numerical application. Comparisons of results obtained by the two procedures will be discussed in an attempt to evaluate the current accuracy of EOP determination by space geodesy techniques and to guide our conclusion for future development.

Forecasting of the Earth orientation parameters – comparison of different techniques

W. Kosek, M. Kalarus, T. Niedzielski

Institution:
Space Research Centre, Polish Academy of Sciences
Poland

The Earth orientation parameters (EOP) are determined by space geodetic techniques with very high accuracy. However, the accuracy of their prediction, even for a few days in the future, is several times lower and still unsatisfactory for practical use. The main problem of each prediction technique is to predict simultaneously long and short period oscillations of the EOP. It has been shown that the combination of the prediction methods which are different for deterministic and stochastic part of the EOP can provide the best accuracy of prediction. Several prediction techniques, e.g. combination of the lease-squares with autoregressive, neural networks methods, Kalman filter, multivariate autoregressive, as well as combination of wavelet transform decomposition and autocovariance prediction, were used to predict x, y pole coordinates and UT1-UTC or LOD data. The advantages and disadvantages of these prediction techniques are discussed by considering the difference between the EOP data and their predictions at different starting prediction epochs as well as by comparing the mean EOP prediction errors.

Current results of the Earth Orientation Parameters Prediction Comparison Campaign

M. Kalarus, W. Kosek, H. Schuh

Institution of the first author:
Space Research Centre, PAS
Poland

The precise transformation between the celestial (ICRF) and terrestrial (ITRF) reference frames is needed for many advanced geodetic and astronomical tasks. To perform this transformation for the time moment of observation the precise EOP data and their predictions have to be known. This paper presents the current status of the Earth Orientation Parameters Prediction Comparison Campaign (EOP PCC), which started in October 2005 under the umbrella of the IERS (International Earth rotation and Reference systems Service). The ultra-short term, short term and medium term EOP predictions submitted since then by different groups/algorithms were evaluated by means of the same statistical analysis. The mean prediction errors of the EOP for each group/algorithm were computed with respect to IERS C04 data to show the performance in each prediction category. In October 2006 the EOP PCC rules were slightly changed however all prediction results before this moment were transformed according to the new conventions.

Recent Improvements at the IERS Rapid Service/Prediction Center

N. Stamatakos, B. Luzum, W. Wooden

Institution:
US Naval Observatory
USA

The International Earth Rotation and Reference System Service (IERS) Rapid Service/Prediction Center (RS/PC) has made several improvements to its combination and prediction products. These improvements are due to the inclusion of new input data sources as well as modifications to the combination and prediction algorithms. These changes and their impact on the users of the RS/PC data are presented.

GGOS-D: A German Project on the Integration of Space Geodetic Techniques

A. Nothnagel, M. Rothacher, D. Angermann, T. Artz, S. Boeckmann, H. Drewes, M. Kluegel, D. Koenig, R. Koenig, B. Meisel, B. Richter, W. Schwegmann, V. Tesmer, D. Thaller

Institution of the first author:
Inst. of Geodesy and Geoinformation, Bonn University
Germany

Since September 2005 the German Ministry for Research and Education is funding a group of scientists at GeoForschungsZentrum (GFZ) Potsdam, Deutsches Geodätisches Forschungsinstitut (DGFI) München, Bundesamt für Kartographie und Geodäsie (BKG) Frankfurt a.M. and Institut für Geodäsie und Geoinformation, Universität Bonn (IGGB) in a project related to the integration of space geodetic techniques. These groups comprise experience in GPS, SLR and VLBI observing techniques as well as in large scale combinations. They cooperate with the aim of producing reference frames and time series which are consistent across techniques by adapting software packages to common standards and by refining combination procedures. In this presentation we introduce the project and show first results.

Three years solution of EOP by combination of results of different space techniques

V. Stefka, I. Pesek, J. Vondrák

Institution:
Astronomical Institute of the Czech Republic
Czech Republic

The method of non-regular combination of results of different techniques, namely GPS, VLBI, SLR and Doris, to obtain unique values for both the Earth orientation parameters (EOP) and the station coordinates needs the EOP at the adjacent epochs to be suitably constrained to each other. Modified smoothing algorithm was used as this constraint. Weighting controls smoothness of the combined EOP, so that it can also be used to filter out undesirable frequencies from the solution. To do it, a transfer function was empirically estimated from three years combination of EOP and will be presented here.

Advances in inertial Earth rotation measurement - new data from the Wettzell 'G' ring laser

T. Kluegel, U. Schreiber, A. Velikoseltsev, W. Schlüter

Institution of the first author:
BKG, Wettzell
Germany

The world's most precise inertial rotation sensor, the "G" ring laser at the Fundamentalstation Wettzell in Germany, has undergone a technical upgrade in 2006. The former resolution of 10^-8 with respect to the Earth rotation rate could be further improved giving more insight to signals affecting inertial Earth rotation measurements. This implies a better determination of diurnal polar motion terms, the detection of a curious signal at a period of 8 hours, and local and regional atmospheric deformation effects. The recent noise level of the "G" ring laser in the diurnal to semi-diurnal frequency band is in the order of 0.2 ms in LOD or 0.5 mas in polar motion.

ILRS contributions to near-real time EOP series

E.C. Pavlis

Institution:
JCET/UMBC and NASA Goddard
USA

The ILRS is embarking in a pilot project to deliver on a daily basis, estimates of polar motion and variations in Earth rotation rate. With the majority of the high-yield and high accuracy stations submitting their data to the ILRS archives immediately after they were collected, we are now able to analyze the data in almost real time. As a first step, the products will be based on weekly arcs that span the seven-day period that ends with the day we will report the EOP for. Each day the orbital arc will be shifted by a day and a new analysis that will consider the data from the additional day will be performed. All other aspects of the analysis will remain the same as the standard weekly product that we deliver to IERS nominally on Thursday of every week. The fact that we will have such products updated on a daily basis raises the possibility that the official product may be made available to IERS even earlier than Thursday, possibly as early as Tuesday of each week. As the pilot project matures, we will investigate the possibility of even more frequent updates, if these are deemed useful to the user community.

Atmospheric excitation of Earth rotation/polar motion at high temporal resolution

D. Salstein, J. Nastula, K. Quinn, D. Macmillan, and P. Mendes Cerveira

Institution of the first author:
Atmospheric and Environmental Research
USA

We have calculated excitations of Earth rotation/polar motion with hourly resolution. These series were based on a data assimilation system run at the NASA Global Modeling and Data Assimilation Office using the GEOS4 atmospheric model. Excitations from five of the six hours are based on model forecasts, and at each six-hour epoch they are based on a mixture of model and observations. This heterogeneous procedure can cause some discontinuities at the six-hour mark if the model is not skillful. We have developed approaches to mitigate the effect of the discontinuities. The polar motion excitation terms based on winds, which are known to have a strong diurnal signal due in part to tidal fluctuations, is nevertheless reasonably continuous. However, the phase of the diurnal signal appears to vacillate during this period. The resulting excitation functions agree reasonably well with independent data sets based on other 6-hour data assimilations. We chose October 2002 as it encompasses the CONT 2002 observing period for Earth rotation. Time-spectra of these wind-based, pressure-based terms, and geodetic terms reveal diurnal and semidiurnal signals with other sub-daily spectral peaks. NASA is currently in the process of updating its analysis to a new system called the “Modern Era Retrospective-Analysis Research and Applications” (MERRA). The MERRA approach should eliminate the discontinuities at the 6-hour intervals. The MERRA system will be multi-year and should give us the opportunity to investigate high temporal resolution excitations for other periods, including the more rec

On the influence of diurnal atmospheric tides on Earth rotation

A. Brzezinski

Institution:
Space Research Centre, Polish Academy of Sciences
Poland

The diurnal cycle in solar heating give rise to variation in the atmospheric angular momentum (AAM) with main components S1 (period 24 hours) and S2 (12 hours). The S1 and S2 signals are subject to seasonal modulations producing the side lobes with frequencies shifted by +/-1 or +/-2 cycles per year from the main spectral lines. Moreover, similar components as in the AAM can be seen in the nontidal ocean angular momentum (OAM) due to the ocean response to the atmospheric forcing. These diurnal and subdiurnal variations in the AAM and OAM excite small, below 1 milliarcsecond, but already well detectable variations in all components of Earth rotation including precession-nutation, polar motion and UT1. Here we give a general description of the perturbations of Earth rotation caused by diurnal thermal tides in the atmosphere and the oceas and an overview of the observation and modeling efforts. We also report on own estimation using the VLBI observations and the available high resolution atmospheric and oceanic excitation data.

Empirical validation of the conventional model for length of day variations due to zonal Earth tides

S. Englich, R. Weber, H. Schuh

Institution:
Institute of Geodesy and Geophysics, Vienna University of Technology
Austria

The deformations of the Earth caused by the zonal part of the tidal potential induce fluctuations in the length of day (LOD) with periods from ~5 days to 18.6 years. LOD variations were derived from 22 years of VLBI observations, using the VLBI software package OCCAM61E. We also calculated a second LOD series of two years with the Bernese GPS Software 5.0 for the years 2005 and 2006, analysing data of a global GPS network composed of 113 stations of the IGS05 reference frame. The atmospheric influence on LOD was computed from atmospheric angular momentum functions provided by the NCEP and subtracted from the original series. Different sets of tidal terms were estimated in a least squares adjustment introducing the remaining LOD variations as pseudo-observations. The resulting amplitudes were compared with the values of the model recommended in the IERS Conventions 2003 by Defraigne and Smits (1999).

Geophysical excitation of LOD/UT1 estimated from the output of the global circulation models of the atmosphere - ERA-40 reanalysis and of the ocean - OMCT

A. Korbacz, A. Brzezinski, M. Thomas

Institution of the first author:
Space Research Centre, PAS
Poland

The axial component of Earth rotation which is expressed by the length of day (LOD) and universal time (UT1), is subject to changes with periods ranging from a fraction of a day to decades. The low frequency variations are excited mainly by interactions between the core and the mantle. Changes with shorter periods are mostly driven by the dynamically coupled system atmosphere-oceans. Here we use new estimates of the global atmospheric and oceanic angular momenta (AAM, OAM) to study the influence on LOD/UT1. The AAM series was calculated from the output fields of the atmospheric general circulation model ERA-40 reanalysis. The OAM series is an outcome of global ocean model OMCT (Ocean Model for Circulation and Tides) simulation excited by global fields of the atmospheric parameters from the ERA-40 reanalysis. The excitation data cover the period between 1963 and 2001. Our calculations concern atmospheric and oceanic effects in LOD/UT1 over a broad band of frequencies, starting from variations with periods of several days, through seasonal oscillations, up to decadal changes.

Geophysical excitation of diurnal prograde polar motion derived from different OAM and AAM data

M. Kudryashova

Institution:
Astronomical Institute of St. Petersburg University
Russia

Short period variations in polar motion are mostly caused by the system atmosphere-ocean impact. In this work we compare the geodetic and geophysical excitation. Geodetic excitation have been estimated on the basis of polar motion series with sub-diurnal temporal resolution obtained from VLBI observations. For our comparison we used two sets of geophysical data: first one is that of atmospheric and oceanic angular momenta (AAM, OAM) calculated during ERA-40 reanalysis project; second one contains AAM series derived from NCEP/NCAR reanalysis project and corresponding OAM series from the barotropic OGCM (Oceanic Global Circulation Model) model. Our analysis covers prograde diurnal frequency band.

Coherence of satellite gravimetric observations with the polar motion

L. Seoane, C. Bizouard, D. Gambis

Institution:
SYRTE
France

Polar motion is interpreted as the effect of the Earth's inertia moment changes and relative angular momentum taking place in the terrestrial frame. Thanks to the GRACE and in a less extent LAGEOS missions, those inertia moment variations of the Earth are now determined undependently from any geophysical model. The purpose of our work is to compare that gravimetric excitation to the equivalent one found in observed polar motion. This is done for the last releases of the gravity field changes : those of the GFZ, CSR, JPL and GRGS. Global agreement is satisfactory, especially for the y-component of the equatorial excitation. When considering the residuals after removing the modeled oceanic and atmospheric excitation, which reflects the non-modeled excitation, mostly of hydrological nature, coherence is maintained in seasonal band, especially the retrograde one, but lost for other frequencies. On the other side hydrological excitation derived from CPC contains more power in seasonal band than residual polar motion excitation, but is well correlated with this later one.

Progress in the 2nd realization of ICRF (Invited)

C. Ma

Institution:
Goddard Space Flight Center
USA

The ICRF derived from VLBI observations of extragalactic radio sources up to 1995.6 and effective since 1998.0 was a radical change from the FK5 stellar/equinox celestial system. The number of geodetic/astrometric VLBI observations has since tripled and the number of radio sources with astrometrically useful data has quadrupled. A systematic program for monitoring astrometric sources has been established although the available observing capability limits this to only a fraction of all such sources. Advances in modeling, generation of source position times series, and greater availability of source structure information will permit the identification of better “defining” sources. The VLBA Calibrator Survey will provide a large number of additional sources although most are observed at only one epoch. Working groups have been established by the IAU, IERS and IVS with the goal of presenting the second realization of the ICRF at the IAU General Assembly in 2009.

Selecting ICRF-2 defining sources based on source structure

P. Charlot, A. Fey, A. Collioud, R. Ojha, D. Boboltz, J. Camargo

Institution of the first author:
Observatoire de Bordeaux
France

The intrinsic radio structure of the extragalactic sources is one of the limiting errors in the definition of the International Celestial Reference Frame (ICRF). This paper reports about the ongoing work to monitor the structural evolution of all ICRF sources by using the Very Long Baseline Array and other VLBI telescopes around the world. Based on more than 2000 VLBI images produced from such observations, we have assessed the astrometric suitability (i.e the compactness) of most of the ICRF sources. The number of VLBI images for a given ICRF source varies from 1 for the least-observed sources to more than 20 for the intensively-observed sources. Overall, we identify a subset of about 200 sources that are highly compact at any of the available epochs. We argue that these sources are prime candidates for the realization of the next ICRF with the highest accuracy.

Next ICRF: Single global solution versus combination

Z. Malkin, Ya. Yatskiv

Institution of the first author:
Pulkovo Observatory RAS
Russia

Two approaches to generate the ICRF-2 are compared. The first approach, which has been used for the creation of the current ICRF realization, implies computation of the radio source positions in a single analysis center making use of the most advanced models, software and analysis strategy. However, the latter cannot guarantee that the reference frame is free of systematic errors. The second approach is based on combination of several CRF realizations after investigation and accounting for their stochastic and systematic errors. This procedure is intended to mitigate the systematic errors and improve the precision of the final combined solution. In this presentation, the advantages and shortcomings of both the approaches are discussed.

Time Series Analysis of VLBI Astrometric Source Positions at 24-GHz

D. Boboltz, A. Fey & K-Q VLBI Survey Collaboration

Institution of the first author:
U.S. Naval Obs.
USA

To date there have been 10 VLBI experiments observed over a period spanning 5 years and analyzed for the purpose of establishing a high-frequency (24 GHz) reference frame. The database now contains information on 274 sources and a total of 1052 images. There are currently 87 sources which have been observed in at least 5 experiments. Here we report on an analysis of the time series of astrometric positions determined for these 87 sources and provide a comparison with the X-band time series for the same sources. We discuss the stability of the sources at 24 GHz and the possible implications for ICRF2 source selection as well as the link of the high frequency reference frame to ICRF2.

Reference radio source apparent proper motions

O. Titov

Institution:
Geoscience Australia
Australia

The motions of relativistic jets from the active extragalacit nuclei can reach several hundred microseconds per year and mimic the radio source proper motions observed by geodetic VLBI. Such motions are not correlated and exceed the small systematic effects induced by the rotation of the Solar system around the centre of the Galaxy. In this paper we search the subtle systematic and discuss the results.

Influence of different strategies in VLBI data analysis on realizations of ICRF.

S. Bolotin

Institution:
Main Astronomical Observatory, National Academy of Sciences of Ukraine
Ukraine

Positions of radio sources which are a realization of International Celestial Reference Frame (ICRF) can be estimated from VLBI observations. In this presentation we discuss different approaches in data analysis and the influence of applied strategy on the results. Application of these strategies are illustrated with a set of radio sources catalogs obtained from VLBI data analysis at the Main Astronomical Observatory (Kiev, Ukraine).

Effect of the reference radio source selection on VLBI CRF realization

J. Sokolova

Institution:
Pulkovo Observatory RAS
Russia

Up to now, four stability criteria based on different schemes have been used for selection of reference radio sources. Four lists of reference radio sources based on these schemes have been compiled. Some inconsistency between these lists was found. In our study we analyse the impact of these different selection schemes on the CRF solution as well as on radio source coordinate time-series to construct the optimal approach for reference radio source selection.

Extragalactic Optical-Radio Link Research at USNO

N. Zacharias, M. I. Zacharias, D. Boboltz, A. Fey, R. Gaume, G. Hennessy, K. J. Johnston, R. Ojha

Institution of the first author:
U.S. Naval Observatory
USA

We present the current status and future plans of researchconducted at the U.S. Naval Observatory (USNO) regarding thelink of the optical reference frame to the ICRF using compact,extragalactic sources.Over 500 counterparts of ICRF sources were observed in over20 deep CCD observing runs as part of the USNO CCD AstrographCatalog (UCAC) project, providing a direct link to the Tycho-2stars, thus the Hipparcos Celestial Reference Frame. Abouthalf of that data are reduced now, giving optical-radioposition differences on the 10 to 20 mas level per source.First results will be discussed.A sample of 12 extragalactic ICRF sources are being observedat the 1.55-meter Naval Observatory Flagstaff Station (NOFS)over several years. The optical position stability of thesetargets is monitored on the mas level locally.High resolution imaging of selected sources is planned withthe LuckyCam at the ESO 3.5m NTT and Lick 3-meter AO systemto relate possible source structure at optical wavelengthswith observed optical-radio centroid offsets.As part of the Space Interferometry Mission (SIM) preparatoryscience about 240 bright QSO's are monitored for photometricvariability in B,V,R and I. The goal is to select the mostsuitable sources for future radio-optical link programs.The USNO Robotic Astrometric Telescope (URAT) will be ableto combine deep CCD imaging of all ICRF2 target areas andmillions of anonymous compact galaxies with a stellar,astrometric, all-sky survey of multiple epoch overlaps.

A GAIA oriented analysis of a large sample of quasars

A. H. Andrei, C. Barache, S. Bouquillon, G. Bourda, J.I.B. de Camargo, J.-F. Le Campion, P. Charlot, A.-M. Gontier, S. Lambert, J.J. Pereira Osório, D.N. da Silva Neto, J. Souchay, R. Vieira Martins

Institution of the first author:
(1) Observatorio Nacional/MCT; (2) Observatorio do Valongo/UFRJ
Brasil

GAIA photometric capabilities should distinguish quasars to a high degree of certainty. With this, they should also be able to deliver a clean sample of quasars with a negligible trace of stellar contaminants. However, a purely photometric clean sample could miss a non negligible percentage of ICRF sources counterparts - and this interface is required to align with the ICRS and de-rotate the GCRF (GAIA Celestial Reference Frame), on grounds of continuity. To prepare a minimum clean sample forming the initial quasar catalogue for the GAIA mission, an all sky ensemble was formed containing 128,257 candidates. Among them there is at least one redshift determination for 98.75%, and at least one magnitude determination for 99.20%. The sources were collected from different optical and radio lists. We analyze the redshift, magnitude, and color distributions, their relationships, as well as their degree of completeness.

A VLBI survey of weak extragalactic radio sources for the link with the future GAIA frame

G. Bourda, P. Charlot, S. Garrington, R. Porcas

Institution of the first author:
Observatoire de Bordeaux
France

The space astrometric mission GAIA will construct a dense-optical QSO-based celestial reference frame that will have to be linked to the International Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is found that only about 10% of the ICRF sources are suitable to establish this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional high-quality radio sources for the GAIA link. The sample consists of about 400 sources, typically weaker than the current ICRF sources, which have been selected by cross-correlating optical and radio catalogs. The paper will present the observing strategy and include preliminary results of observation of 224 of these sources with the European VLBI Network in June 2007.

Current Status of the IAU Working Group for Numerical Standards of Fundamental Astronomy (Invited)

B. Luzum, N. Capitaine, A. Fienga, B. Folkner, T. Fukushima, J. Hilton, C. Hohenkerk, G. Krasinsky, G. Petit, E. Pitjeva, M. Soffel, P. Wallace

Institution of the first author:
U.S. Naval Observatory
USA

At the 2006 International Astronomical Union (IAU) General Assembly (GA), a proposal was adopted to form the Working Group (WG) for Numerical Standards of Fundamental Astronomy. The goals of the WG are to update "IAU Current Best Estimates" conforming with IAU Resolutions, the International Earth Rotation and Reference System Service (IERS) Conventions, and the Syst�me International (SI). Initial efforts have concentrated on determining which constants should be considered, the terminology regarding the description of the constants, and the interdependence of the constant estimates on their associated models. The current status of WG activities and the anticipated future directions are presented.

On astronomical constants (Invited)

M. Soffel, S. Klioner

Institution:
Lohrmann-observatory, TU Dresden
Germany

With respect to the ongoing work in the IAU WG 'NSFA' a new classification of astronomical constants into natural constants (defining constants), body constants and coordinate-related constants is suggested. The relation between natural (defining) constants and units of measurements is discussed. The definition of body constants such as masses depends upon the approximation scheme to Einstein's theory of gravity. Finally, coordinate-related constants such as initial values e.g., for center of mass of a planet have a meaning only in a certain coordinate picture of the dynamical problem.Every measurable constant has its meaning only within a certain dynamical framework and a real error related with the involved data set. Finally some remarks concerning the consistency of a set of astronomical constants are given.

Recent progress in concepts, nomenclature and models in fundamental astronomy

N. Capitaine

Institution:
Observatoire de Paris
France

The IAU 2006 resolutions B1, B2 and B3, adopted by the XXVIth IAU General Assembly (August 2006) and endorsed by the XXIVth IUGG General Assembly (July 2007), supplement the IAU Resolutions 2000 on reference systems. The aim of IAU 2006 Resolution B1 is to adopt a precession model as a replacement to the IAU 2000 precession in order to be consistent both with both dynamical theory and the IAU 2000 nutation. The aim of IAU 2006 Resolutions B2 and B3 is to address definition, terminology or orientation issues relative to reference systems and time scales that needed to be specified after the adoption of the IAU 2000 resolutions.The purpose of this presentation is to explain the changes resulting from the joint IAU 2000 and 2006 resolutions with respect to the previous position and to review the consequences on the concepts, nomenclature, models and conventions in fundamental astronomy. We will also report on practical aspects related to the Earth orientation parameters and describe the characteristics of the high accuracy model for the celestial motion of the Earth’s pole.

Recent models of the planet motion and fundamental constants determined from position observations of planets and spacecraft

E. Pitjeva

Institution:
Institute of Applied Astronomy of Russian Academy of Science
Russia

The more recent planet ephemerides constructed at JPL (DE414), IAA RAS (EPM2007) and IMCCE (INPOP06) having about the same accuracy are considered. Common to all ephemerides is a simultaneous numerical integration of the equations of motion of the nine major planets, the Sun, about 300 big asteroids, the Moon and the lunar physical libration performed in the Parameterized Post-Newtonian metric for General Relativity in the TDB time scale, taking into account perturbations due to the solar oblateness and perturbation from a massive ring of small asteroids. The numerical integration includes also the largest trans-Neptunian objects for EPM2007 and the Earth rotation for INPOP06; the software for DE414 permits to solve for many individual asteroid masses. The full set of solution parameters which includes masses of planets and asteroids and the value of AU is presented. All these ephemerides have been oriented to ICRF by using the VLBI measurements ofspacecraft near the planets. Two different versions of constructing ephemerides for the TCB time scale have been done at IAA RAS and JPL. So, some official recommendation should be adopted before recommending the TCB time ephemerides for public.Other methods of determinations of masses of planets and asteroids (from motions of satellites, spacecraft or close encounter asteroids), which can give even more accurate results, are shown. The list of update masses of the 11 planets (including the draft planets Pluto, Eris, Ceres), Pallas, Vesta and the AU value is offered to WG NSFA.

INPOP06. A new numerical planetary ephemeris

J. Laskar, A. Fienga, H. Manche, M. Gastineau

Institution of the first author:
IMCCE, Observatoire de Paris
France

INPOP06 is the new numerical planetary ephemeris developed atthe IMCCE-Observatoire de Paris.INPOP (Intégrateur Numérique Planétaire de l'Observatoire de Paris)is a numerical integration of the motion of the nine planets and the Moonfitted to the most accurate available planetary observations. It alsointegrates the motion of 300 perturbing main belt asteroids, therotation of the Earth and the Moon libration.We will present the latest developments of the INPOP ephemeris.

INPOP: Adjustments to LLR data

H. Manche, S. Bouquillon, A. Fienga, G. Francou, M. Gastineau, J. Laskar

Institution of the first author:
IMCCE, Observatoire de Paris
France

INPOP06 is the latest numerical planetary ephemeris developed at IMCCE and Paris-Observatory. This version has been fitted only to planetary observations (Fienga et al, INPOP06: a new numerical planetary ephemeris, 2007, preprint). The next version currently in development will be directly fitted to Lunar Laser Ranging data. These observations (more than 17000 over 36 years, from 3 different sites) are usefull to determine some physical parameters and inital conditions used in INPOP. We will present our fitting process, the physical effects taken into account to compute residuals and our latest results.

Prospects for Improving the Masses of Asteroids

J. Hilton

Institution:
U.S. Naval Observatory
USA

Among the largest uncertainties in the fundamental constants of astronomy are the masses of the asteroids. They constitute the largest source of uncertainty in the ephemerides of the inner planets. The DE405 planetary ephemerides incorporate the masses of 300 asteroids. However, aside from Ceres, Vesta, those asteroids with satellites, and perhaps Pallas, none of these masses are known with an uncertainty of better than about 50%. Asteroid mass determination requires the discovery of encounters with massive asteroids that are close, and with low relative velocities. The explosion of discoveries of main belt asteroids since the mid-1990s means that it may be possible to discover more such encounters. Thus, the orbits of the perturbed asteroids may be determined with higher accuracy in a shorter period of time. A series of simple filters were developed to determine if any such candidate perturbed bodies exist. Those candidates that made it through the filters can then be subjected to a more rigorous examination to determine the nature of their encounters with a large asteroid. For example, five candidate encounters were discovered for Vesta, all of which have a marginal chance of improving our knowledge of its mass prior to the spacecraft Dawn's arrival at Vesta in 2011. Similarly, a total of 18 encounters were found for Ceres, many of which are almost certainly strong enough to provide a mass estimate with a significantly smaller uncertainty than current estimates prior to Dawn's arrival there in 2015.

Harmonic models of tide-generating potential of terrestrial planets

S. Kudryavtsev

Institution:
Sternberg Astronomical Institute of Moscow State University
Russia

High-accurate harmonic development of the tide-generating potential (TGP) of Mercury, Venus and Mars is made by using a new modification of the spectral analysis method. The TGP harmonic models for Mercury and Venus are developed over 2,000 years (1000-3000) and include 2,345 second-order Poisson series' terms of amplitude exceeding 10^{-8} m^2/s^2 for Mercury and 1,725 analogous terms for Venus. The TGP harmonic model for Mars is done over 200 years (1900-2100) and includes 278 second-order Poisson series' terms of amplitude exceeding 10^{-6} m^2/s^2. The accuracy of the new development of terrestrial planets' TGP is better than that of any previous model. The work is supported in part by grant 05-02-16436 from the Russian Foundation for Basic Research.

Recent advances in modeling precession-nutation (Invited)

V. Dehant, S. Lambert, N. Rambaux, M. Folgueira, L. Koot

Institution of the first author:
Observatoire Royal de Belgique
Belgium

This paper will present recent advances in nutation theory related to what has been done at the Royal Observatory of Belgium by Marta Folgueira, Sebastien Lambert, Laurence Koot, Nicolas Rambaux, and Veronique Dehant, in relation to the Descartes Nutation project. These advances are related to the determination of Earth geophysical parameters from VLBI observations and to better modeling of the phenomena within the Earth. In particular, we will examine the coupling mechanisms at the core-mantle boundary: the electromagnetic coupling, the topographic coupling, and the viscous coupling. Methods related to a time domain approach instead of a frequency domain approach have been examined as well. We also present future developments necessary for a better understanding of the Earth interior and its nutations.

IAU Symposium 78 "Nutation and the Earth's rotation" (held in Kiev,May 1977) as a first step in the consideration concerning the Non-rigid Earth Nutation Theory"

Ya. Yatskiv, A. Korsun'

Institution:
Main Astronomical Observatory
Ukraine

Thirty years ago IAU Symposium №78 "Nutation and the Earth's Rotation" was held (May 23-28,Kiev, Ukraine).The brief history of event and its objectives are described.

Estimation of Earth interior parameters from a Bayesian inversion of nutation time series.

L. Koot, A. Rivoldini, O. de Viron, V. Dehant

Institution of the first author:
Royal Observatory of Belgium
Belgium

The nutation response of the Earth to the external gravitational torque is affected by its internal structure. Because the torque is known very accurately, the high precision nutation observations allow to constrain Earth interior parameters. The non-rigid Earth nutation has been modeled by Mathews et al. (2002). This model is semi-analytical: it depends on parameters, related to the Earth interior, which are adjusted to the observations. The nutation model is frequency-dependent so that, in Mathews et al. (2002), the time-dependent observations are processed before the fit in order to get data in the frequency domain. This procedure implies a loss of information because the parameters are obtained by fitting the model to the twenty dominant terms in the frequency domain and not to the whole time series. In this paper, we develop a new fit procedure which relies on an estimation of the geophysical parameters directly from the nutation data in time series. This allows to use all the information of the time-domain data and to account for the time dependent uncertainties on the data. Rather than the linearized least-squares method used in Mathews et al. (2002), we use the Bayesian inversion method, which is non-linear, and is consequently well-suited for the nutation model. In addition, the Bayesian framework allows to include the possibility of having uncertainties in the nutation model itself. The modeling uncertainties are introduced in terms of a new parameter, estimated jointly with the geophysical parameters. Finally, the results are compared with those obtained by Mathe

VLBI observations of nutation, its geophysical excitations and determination of some Earth model parameters

J. Vondrák, C. Ron

Institution:
Astronomical Institute
Czech Republic

VLBI has been used to observe nutation for more than 25 years, with ever increasing accuracy. The amplitudes and phases of individual nutation terms are sensitive to some parameters of the geophysical Earth model. Dominant is the resonant period P and quality factor Q of retrograde free core nutation that depend on the flattening of the outer fluid core. The period (in celestial frame) is approximately equal to 430 days. In principle, all nutation terms are affected by these resonance effects, but the most affected is the annual retrograde term that is closest to the resonance. Nutation is dominantly driven by external torques, and it contains, as a multiplicative factor, the dynamical ellipticity of the Earth. Relatively small (near-diurnal in terrestrial frame) geophysical excitations are amplified by the resonance, so they also have non-negligible influence on nutation. The aim of the presentstudy is to take these effects into consideration and determine the dynamical ellipticity, and period and quality factor of the Earth at the FCN frequency. An attempt to derive from them also the flattening of the fluid core will be made, taking into account other effects as electro-magnetic coupling between the core and the mantle.

About observability of the free core nutation

L. Petrov

Institution:
NVI, Inc./NASA GSFC
USA

Neither astronomical technique, including VLBI, can measure nutationdirectly. Estimates of parameters of the nutation model are produced by solving the LSQ problem of adjusting millions parameters using estimatesof group delay. The choice of the mathematical model for nutation used in the estimation process of analysis of group delays affects our ability to interpret the results. Ignoring these subtleties and using parameters of the nutation model either in the form of time series, or in the form of empirical expansion as "VLBI measurement of nutation", opens a room for misinterpretation and mistakes. Detailed analysis of the problem reveals that the separation of forced nutations, atmospheric nutations, ocean nutations,and the free core nutation requires invoking some hypotheses, and beyond a specific level becomes uncertain. This sets a limit of our ability tomake an inference about the free core nutation.

Some issues in the Earth's interior exploration with VLBI

S. Lambert, V. Dehant, A.-M. Gontier

Institution of the first author:
Royal Observatory of Belgium
France

We used several homemade nutation series derived from VLBI delays using different constraint on the radio source coordinates. Residuals with respect to the MHB 2000 nutation model corrected from various effects (e.g., non linear) have been analyzed in order to retrieve the Earth's interior parameters. In particular we have examined the value of the FCN and FICN resonant periods and quality factors in terms of physics of the Earth's core and the accuracy with which the parameters can be retrieved considering the current VLBI developments. We have also looked at the inner core parameters and have determined the impact of VLBI uncertainties of the physics of the inner core, and in particular on the inner core magnetic field.

Concise algorithms for precession-nutation

P. Wallace, N. Capitaine

Institution of the first author:
Rutherford Appleton Laboratory
United Kingdom

The precession-nutation models based on the IAU 2000A nutation and IAU 2006 precession involve several thousand amplitude coefficients, many under 1 microarcsecond in size, and the sines and cosines of 1500 angles. For the many applications that do not require the utmost accuracy this represents an unnecessary or even excessive computational overhead. The IAU 2000B model offers one alternative, an order of magnitude smaller than IAU 2000A and delivering classical nutation components of 1 mas accuracy in the current era. We look at other options, based on series for the CIP coordinates X,Y and the CIO locator s and with the GCRS to CIRS rotation matrix as the end product. Truncation of the series provides most of the savings, but certain other measures can be taken also. Three example formulations are presented that achieve 1 mas, 16 mas and 0.4 arcsec accuracy throughout 1995-2050 with computational costs 1, 2 and 3 orders of magnitude less than the full models.

Relativity in Fundamental Astronomy: solved and unsolved problems (Invited)

S. Klioner

Institution:
Lohrmann Observatory
Germany

Nowadays it is no longer necessary to justify the importance of consistent relativistic modelling in the field of fundamental astronomy. Although in the last 20 years the theoretical foundations of relativistic modelling have been elaborated with a lot of care, there are a number of issues, mostly of practical character, that still require both theoretical discussions and practical implementations. This 'gray' areas of the modelling include modelling of rotational motion of celestial bodies, correct inclusion of multipole structure of the bodies in the translational equations of motion, interplay between numerical accuracy and analytical "order of magnitude" of various relativistic terms, relativistic scaling of astronomical quantities and units of measurements. An overview of the relativistic issues in the field of fundamental astronomy will be given from this critical point of view.

Local tests of GR and dynamical reference frame linking with Gaia astrometry of asteroids

D. Hestroffer, S. Mouret, F. Mignard, J. Berthier, P. Tanga

Institution of the first author:
IMCCE/Paris observatory
France

The Gaia satellite, an ESA cornerstone mission to be launched at the end of 2011, will observe a large number of celestial bodies including also small bodies of the solar system (mainly near-Earth objects and main-belt asteroids). The scanning telescope will observe all objects brighter than magnitude V≤20 providing, over the 5 years mission, high precision photometry and astrometry with an unprecedented accuracy – ranging roughly from ≈0.3 to 3 milli-arcescond on the CCD level, and depending on the target's magnitude. In addition, several hundreds of QSOs directly observed by Gaia will provide the kinematically non-rotating reference frame in the visible light, resulting in the construction of a 'Gaia-ICRF'. The positions of the asteroids hence enable to relate the dynamical reference frame to the kinematic one, and to further check the non-rotating consistency between both frames’ definition. Here we show the results of a variance analysis obtained from a realistic simulation of observations for such link. The simulation takes into account the time sequences and geometry of the observations that are particular to Gaia observations of solar system objects, as well as the instrument sensitivity and photon noise. Additionally, we show the achievable precision for the determination of a possible time variation of the gravitational constant. Taking into account the non-completeness of the actually known population of NEOs, we also give updated values for the nominal precision of the joint determination of the solar quadrupole J2 and PPN parameter beta.

Geodetic (Relativistic) Rotation of the Solar System Bodies

V. Pashkevich, G. Eroshkin

Institution:
Central (Pulkovo) Astronomical Observatory of Russian Academy of Sciences
Russia

The problem of the geodetic (relativistic) rotation of the major planets, the Moon, and the Sun is studied by using DE404/LE404 ephemeris. For each body the files of the ecliptical components of the vectors of the angular velocity of the geodetic rotation are determined over the time span from AD1000 to AD3000 with one day spacing. The most essential terms of the geodetic rotation are found by means of the least squares method and spectral analysis methods.

Towards the relativistic theory of precession and nutation

S. Klioner, M. Soffel, C. Le Poncin-Lafitte

Institution:
Lohrmann Observatory
Germany

A numerical theory of Earth rotation has been constructed using the model of post-Newtonian rigidly rotating multipole moments. The theory is construted by a numerical integration and treats all sprectrum of relativistic issues in a consistent way: (1) relativistic times scales, (2) relativistic scaling of astronomical constants, (3) relativistic torques and (4) rigorous treatment of geodetic precession as an additonal torque in the equations of rotational motion with respect to the kinematically non-rotating GCRS. In the quasi-Newtonian limit our theory reproduces SMART97 within the accuracy of the latter. The effects of various relativistic factors will be demonstrated and discussed.

Activities of the IERS Working Group on Prediction (Invited)

W. Wooden

Institution:
U. S. Naval Observatory
USA

The International Earth Rotation and Reference Systems Service (IERS) established a Working Group (WG) on Prediction to investigate what IERS prediction products are useful to the user community in addition to making a detailed examination of the fundamental properties of the different input data sets and algorithms. The major goals and objectives of the WG are to determine the desired Earth orientation prediction products, the importance of observational accuracy, which types of input data provide an optimal prediction, the strengths and weaknesses of various prediction algorithms, and the interactions between series and algorithms that are beneficial or harmful. To focus the research efforts of the WG, the user community was polled to ascertain what prediction products are needed and at what level of accuracy. The current status of WG activities and the anticipated future directions are presented.

Rigorous combination to ensure ITRF and EOP consistency (Invited)

Z. Altamimi, D. Gambis, Ch. Bizouard

Institution of the first author:
IGN-France
France

The ITRF2005 is the first rigorous combination ensuring ITRF and EOP consistency, based on time series of station positions and Earth Orientation Parameters (EOPs). The objective of the presentation is the development of combination strategy allowing to ensure the ITRF2005 and IERS 05 C04 consistency with time. Combining additional input time series after the release of ITRF2005 from the four techniques (VLBI, SLR, GPS and DORIS) allows the assessment of two main procedures: (1) using CATREF combination model and (2) using EOP-only combination method. The main features of the two combination strategies are presented together with numerical application. Comparisons of results obtained by the two procedures will be discussed in an attempt to evaluate the current accuracy of EOP determination by space geodesy techniques and to guide our conclusion for future development.

Forecasting of the Earth orientation parameters – comparison of different techniques

W. Kosek, M. Kalarus, T. Niedzielski

Institution:
Space Research Centre, Polish Academy of Sciences
Poland

The Earth orientation parameters (EOP) are determined by space geodetic techniques with very high accuracy. However, the accuracy of their prediction, even for a few days in the future, is several times lower and still unsatisfactory for practical use. The main problem of each prediction technique is to predict simultaneously long and short period oscillations of the EOP. It has been shown that the combination of the prediction methods which are different for deterministic and stochastic part of the EOP can provide the best accuracy of prediction. Several prediction techniques, e.g. combination of the lease-squares with autoregressive, neural networks methods, Kalman filter, multivariate autoregressive, as well as combination of wavelet transform decomposition and autocovariance prediction, were used to predict x, y pole coordinates and UT1-UTC or LOD data. The advantages and disadvantages of these prediction techniques are discussed by considering the difference between the EOP data and their predictions at different starting prediction epochs as well as by comparing the mean EOP prediction errors.

Current results of the Earth Orientation Parameters Prediction Comparison Campaign

M. Kalarus, W. Kosek, H. Schuh

Institution of the first author:
Space Research Centre, PAS
Poland

The precise transformation between the celestial (ICRF) and terrestrial (ITRF) reference frames is needed for many advanced geodetic and astronomical tasks. To perform this transformation for the time moment of observation the precise EOP data and their predictions have to be known. This paper presents the current status of the Earth Orientation Parameters Prediction Comparison Campaign (EOP PCC), which started in October 2005 under the umbrella of the IERS (International Earth rotation and Reference systems Service). The ultra-short term, short term and medium term EOP predictions submitted since then by different groups/algorithms were evaluated by means of the same statistical analysis. The mean prediction errors of the EOP for each group/algorithm were computed with respect to IERS C04 data to show the performance in each prediction category. In October 2006 the EOP PCC rules were slightly changed however all prediction results before this moment were transformed according to the new conventions.

Recent Improvements at the IERS Rapid Service/Prediction Center

N. Stamatakos, B. Luzum, W. Wooden

Institution:
US Naval Observatory
USA

The International Earth Rotation and Reference System Service (IERS) Rapid Service/Prediction Center (RS/PC) has made several improvements to its combination and prediction products. These improvements are due to the inclusion of new input data sources as well as modifications to the combination and prediction algorithms. These changes and their impact on the users of the RS/PC data are presented.

GGOS-D: A German Project on the Integration of Space Geodetic Techniques

A. Nothnagel, M. Rothacher, D. Angermann, T. Artz, S. Boeckmann, H. Drewes, M. Kluegel, D. Koenig, R. Koenig, B. Meisel, B. Richter, W. Schwegmann, V. Tesmer, D. Thaller

Institution of the first author:
Inst. of Geodesy and Geoinformation, Bonn University
Germany

Since September 2005 the German Ministry for Research and Education is funding a group of scientists at GeoForschungsZentrum (GFZ) Potsdam, Deutsches Geodätisches Forschungsinstitut (DGFI) München, Bundesamt für Kartographie und Geodäsie (BKG) Frankfurt a.M. and Institut für Geodäsie und Geoinformation, Universität Bonn (IGGB) in a project related to the integration of space geodetic techniques. These groups comprise experience in GPS, SLR and VLBI observing techniques as well as in large scale combinations. They cooperate with the aim of producing reference frames and time series which are consistent across techniques by adapting software packages to common standards and by refining combination procedures. In this presentation we introduce the project and show first results.

Three years solution of EOP by combination of results of different space techniques

V. Stefka, I. Pesek, J. Vondrák

Institution:
Astronomical Institute of the Czech Republic
Czech Republic

The method of non-regular combination of results of different techniques, namely GPS, VLBI, SLR and Doris, to obtain unique values for both the Earth orientation parameters (EOP) and the station coordinates needs the EOP at the adjacent epochs to be suitably constrained to each other. Modified smoothing algorithm was used as this constraint. Weighting controls smoothness of the combined EOP, so that it can also be used to filter out undesirable frequencies from the solution. To do it, a transfer function was empirically estimated from three years combination of EOP and will be presented here.

Advances in inertial Earth rotation measurement - new data from the Wettzell 'G' ring laser

T. Kluegel, U. Schreiber, A. Velikoseltsev, W. Schlüter

Institution of the first author:
BKG, Wettzell
Germany

The world's most precise inertial rotation sensor, the "G" ring laser at the Fundamentalstation Wettzell in Germany, has undergone a technical upgrade in 2006. The former resolution of 10^-8 with respect to the Earth rotation rate could be further improved giving more insight to signals affecting inertial Earth rotation measurements. This implies a better determination of diurnal polar motion terms, the detection of a curious signal at a period of 8 hours, and local and regional atmospheric deformation effects. The recent noise level of the "G" ring laser in the diurnal to semi-diurnal frequency band is in the order of 0.2 ms in LOD or 0.5 mas in polar motion.

ILRS contributions to near-real time EOP series

E.C. Pavlis

Institution:
JCET/UMBC and NASA Goddard
USA

The ILRS is embarking in a pilot project to deliver on a daily basis, estimates of polar motion and variations in Earth rotation rate. With the majority of the high-yield and high accuracy stations submitting their data to the ILRS archives immediately after they were collected, we are now able to analyze the data in almost real time. As a first step, the products will be based on weekly arcs that span the seven-day period that ends with the day we will report the EOP for. Each day the orbital arc will be shifted by a day and a new analysis that will consider the data from the additional day will be performed. All other aspects of the analysis will remain the same as the standard weekly product that we deliver to IERS nominally on Thursday of every week. The fact that we will have such products updated on a daily basis raises the possibility that the official product may be made available to IERS even earlier than Thursday, possibly as early as Tuesday of each week. As the pilot project matures, we will investigate the possibility of even more frequent updates, if these are deemed useful to the user community.

Atmospheric excitation of Earth rotation/polar motion at high temporal resolution

D. Salstein, J. Nastula, K. Quinn, D. Macmillan, and P. Mendes Cerveira

Institution of the first author:
Atmospheric and Environmental Research
USA

We have calculated excitations of Earth rotation/polar motion with hourly resolution. These series were based on a data assimilation system run at the NASA Global Modeling and Data Assimilation Office using the GEOS4 atmospheric model. Excitations from five of the six hours are based on model forecasts, and at each six-hour epoch they are based on a mixture of model and observations. This heterogeneous procedure can cause some discontinuities at the six-hour mark if the model is not skillful. We have developed approaches to mitigate the effect of the discontinuities. The polar motion excitation terms based on winds, which are known to have a strong diurnal signal due in part to tidal fluctuations, is nevertheless reasonably continuous. However, the phase of the diurnal signal appears to vacillate during this period. The resulting excitation functions agree reasonably well with independent data sets based on other 6-hour data assimilations. We chose October 2002 as it encompasses the CONT 2002 observing period for Earth rotation. Time-spectra of these wind-based, pressure-based terms, and geodetic terms reveal diurnal and semidiurnal signals with other sub-daily spectral peaks. NASA is currently in the process of updating its analysis to a new system called the “Modern Era Retrospective-Analysis Research and Applications” (MERRA). The MERRA approach should eliminate the discontinuities at the 6-hour intervals. The MERRA system will be multi-year and should give us the opportunity to investigate high temporal resolution excitations for other periods, including the more rec

On the influence of diurnal atmospheric tides on Earth rotation

A. Brzezinski

Institution:
Space Research Centre, Polish Academy of Sciences
Poland

The diurnal cycle in solar heating give rise to variation in the atmospheric angular momentum (AAM) with main components S1 (period 24 hours) and S2 (12 hours). The S1 and S2 signals are subject to seasonal modulations producing the side lobes with frequencies shifted by +/-1 or +/-2 cycles per year from the main spectral lines. Moreover, similar components as in the AAM can be seen in the nontidal ocean angular momentum (OAM) due to the ocean response to the atmospheric forcing. These diurnal and subdiurnal variations in the AAM and OAM excite small, below 1 milliarcsecond, but already well detectable variations in all components of Earth rotation including precession-nutation, polar motion and UT1. Here we give a general description of the perturbations of Earth rotation caused by diurnal thermal tides in the atmosphere and the oceas and an overview of the observation and modeling efforts. We also report on own estimation using the VLBI observations and the available high resolution atmospheric and oceanic excitation data.

Empirical validation of the conventional model for length of day variations due to zonal Earth tides

S. Englich, R. Weber, H. Schuh

Institution:
Institute of Geodesy and Geophysics, Vienna University of Technology
Austria

The deformations of the Earth caused by the zonal part of the tidal potential induce fluctuations in the length of day (LOD) with periods from ~5 days to 18.6 years. LOD variations were derived from 22 years of VLBI observations, using the VLBI software package OCCAM61E. We also calculated a second LOD series of two years with the Bernese GPS Software 5.0 for the years 2005 and 2006, analysing data of a global GPS network composed of 113 stations of the IGS05 reference frame. The atmospheric influence on LOD was computed from atmospheric angular momentum functions provided by the NCEP and subtracted from the original series. Different sets of tidal terms were estimated in a least squares adjustment introducing the remaining LOD variations as pseudo-observations. The resulting amplitudes were compared with the values of the model recommended in the IERS Conventions 2003 by Defraigne and Smits (1999).

Geophysical excitation of LOD/UT1 estimated from the output of the global circulation models of the atmosphere - ERA-40 reanalysis and of the ocean - OMCT

A. Korbacz, A. Brzezinski, M. Thomas

Institution of the first author:
Space Research Centre, PAS
Poland

The axial component of Earth rotation which is expressed by the length of day (LOD) and universal time (UT1), is subject to changes with periods ranging from a fraction of a day to decades. The low frequency variations are excited mainly by interactions between the core and the mantle. Changes with shorter periods are mostly driven by the dynamically coupled system atmosphere-oceans. Here we use new estimates of the global atmospheric and oceanic angular momenta (AAM, OAM) to study the influence on LOD/UT1. The AAM series was calculated from the output fields of the atmospheric general circulation model ERA-40 reanalysis. The OAM series is an outcome of global ocean model OMCT (Ocean Model for Circulation and Tides) simulation excited by global fields of the atmospheric parameters from the ERA-40 reanalysis. The excitation data cover the period between 1963 and 2001. Our calculations concern atmospheric and oceanic effects in LOD/UT1 over a broad band of frequencies, starting from variations with periods of several days, through seasonal oscillations, up to decadal changes.

Geophysical excitation of diurnal prograde polar motion derived from different OAM and AAM data

M. Kudryashova

Institution:
Astronomical Institute of St. Petersburg University
Russia

Short period variations in polar motion are mostly caused by the system atmosphere-ocean impact. In this work we compare the geodetic and geophysical excitation. Geodetic excitation have been estimated on the basis of polar motion series with sub-diurnal temporal resolution obtained from VLBI observations. For our comparison we used two sets of geophysical data: first one is that of atmospheric and oceanic angular momenta (AAM, OAM) calculated during ERA-40 reanalysis project; second one contains AAM series derived from NCEP/NCAR reanalysis project and corresponding OAM series from the barotropic OGCM (Oceanic Global Circulation Model) model. Our analysis covers prograde diurnal frequency band.

Coherence of satellite gravimetric observations with the polar motion

L. Seoane, C. Bizouard, D. Gambis

Institution:
SYRTE
France

Polar motion is interpreted as the effect of the Earth's inertia moment changes and relative angular momentum taking place in the terrestrial frame. Thanks to the GRACE and in a less extent LAGEOS missions, those inertia moment variations of the Earth are now determined undependently from any geophysical model. The purpose of our work is to compare that gravimetric excitation to the equivalent one found in observed polar motion. This is done for the last releases of the gravity field changes : those of the GFZ, CSR, JPL and GRGS. Global agreement is satisfactory, especially for the y-component of the equatorial excitation. When considering the residuals after removing the modeled oceanic and atmospheric excitation, which reflects the non-modeled excitation, mostly of hydrological nature, coherence is maintained in seasonal band, especially the retrograde one, but lost for other frequencies. On the other side hydrological excitation derived from CPC contains more power in seasonal band than residual polar motion excitation, but is well correlated with this later one.