G34A-01 INVITED 16:00h
Determination of geocenter motion with Satellite Laser Ranging data: methods, models, results and systematic errors
Mass redistribution within the Earth affects the position of its center of mass whose translation, relative to the International Terrestrial Reference Frame (ITRF), ranges from a few millimetres to centimetres. Satellite space geodetic techniques are able to detect such geocenter motion, Satellite Laser Ranging (SLR) being the most accurate in this respect, since it has produced a long history of valuable observations which are particularly sensitive to the origin of the reference frame. The most recent and updated ASI/CGS analyses of Lageos-1 and Lageos-2 SLR data span two decades and provide time series of fortnightly geocenter offsets with respect to the ITRF. Two different methods have been applied to retrieve the time series: a direct estimation of the degree one geopotential harmonics and a computation of Cartesian coordinate offsets from ITRF. Models and results, together with accuracies and spectral content, will be shown and discussed.
G34A-02 16:15h
Estimating Geocenter motions from GPS measurements
Change in the displacement vector between the Center of Mass (CM) of the Earth system and the Center of the Earth's geometrical Figure (CF) is the most common geocenter motion observed using modern geodetic techniques. Primarily this is due to the location on the Earth's surface of the various satellite tracking networks (DORIS, GPS, SLR, VLBI). Observations of this geocenter motion are fundamental to defining the origin of the International Terrestrial Reference Frame (ITRF) and investigating the surface mass transport between oceans, continent and atmosphere which cause this motion. Estimates of this geocenter motion have been at various levels of agreement both between and within different geodetic techniques. Cross-technique differences can partly be put down to the broad spectrum of technique specific errors but much of the difference lies in the use of different surface tracking networks. What is required is a robust method that is relatively unaffected by the network used to sample the Earth's surface figure and hence CF. With this in mind we explore the different estimation techniques which might be used to estimate the geocenter and investigate the performance for GPS data. We compare and contrast different estimation methods using 7 years of fiducial-free GPS solutions from the IGS Analysis centers each of which use different software and tracking networks. Finally we discuss the implications for the interpretation of geocenter estimates and definition of the ITRF.
G34A-03 16:30h
Estimating Geocenter Variations by Combining GRACE and GPS Data Sets
Understanding the motion of the geocenter is important since these link the underlying motions of a kinematic reference frame defined by site positions and velocities to ongoing dynamics of the Earth system. On seasonal timescales, redistribution of surface water dominates the variation of the geocenter. GRACE gravity field estimates, in contrast to observations from techniques such as SLR, DORIS and GPS, are insensitive to geocenter variations and thus cannot be used alone to determine the seasonal component of these variations. However, the loading associated with this mass redistribution also causes surface deformation, to which the positional space geodetic techniques are sensitive. This sensitivity to deformation defined in two different reference frames, geometric in GPS and geodynamic in GRACE, suggests a new approach for determining and understanding motions of the geocenter. Analysis of results based on combining GPS and GRACE estimates for South America, where the vertical loading signature is $\sim\pm10$~mm, indicates that the annual geocenter variation can be estimated with an accuracy of $\pm2$~mm. A global analysis has the potential to significantly improve this accuracy. We will present such a global analysis and discuss the advantages and limitations of this new methodology.
G34A-04 INVITED 16:45h
Contributions of the fluid layers to the motion of the geocenter
Time-variations of the location of the geocenter relative to the Earth's crust are measured since the early 1990's by precise satellite positioning techniques (SLR, DORIS, GPS). On the other hand, redistributions of mass inside the Earth's system are expected to give rise to such motions. We computed time-series of hydrological contributions to the motion of the Earth's center of mass using global model outputs: for the atmosphere (IB) (ECMWF, NCEP), the oceans (ECCO assimilation) and the continental waters (LaD model). We study their respective contributions at the seasonal, inter-annual and longer time-scales and we compare with geodetic results.
G34A-05 INVITED 17:00h
Hydrological Data for Geodetic Research
Explaining and predicting Earth rotation variations and geocenter motion requires a detailed accounting of the sources of mass variability at Earth's surface, including the water stored on and within the continental crust. This terrestrial water storage is composed of groundwater, soil moisture, surface waters, snow and ice, and biomass. The presentation will describe modern, global hydrological datasets which are currently available and the degree of accuracy that can be expected in each. These datasets include ground and satellite observation based products and output from sophisticated land surface modeling and assimilation systems. The latter may be the best hope for reliably constraining terrestrial water storage contributions to geodetic phenomena because they are able to integrate data from disparate sources and fill spatial and temporal gaps in the observational record.
G34A-06 INVITED 17:15h
Atmospheric Angular Momentum Time Series: Characterization of Their Internal Noise and Construction of a Combined Series
Variations in the rotational speed as well as in the direction of the Earth rotation axis are due to the gravitational torque exerted by the Sun, the Moon and the other planets and to the interactions between the solid Earth and the geophysical fluids, namely, the atmosphere, oceans, hydrology and the fluid core. The effects of the atmosphere on Earth rotation are classically computed using the so-called "angular momentum approach". In this method, the variations in the rotation of the Earth are estimated from the (opposite) variations in the atmospheric angular momentum (AAM). Several AAM time series are available, from different meteorological centers. However, the estimation of atmospheric effects on Earth rotation differs strongly when using one atmospheric model or the other. The purpose of our work is to build an objective criterion which justifies the use of one series in particular or one combined series. Because the atmosphere is not the only cause of Earth rotation variations, this criterion cannot rely only on a comparison of AAM series with Earth rotation data. We determine the quality of each series by making an estimation of their noise level, using a generalized formulation of the "three-cornered hat method". We show the existence of a link between the noise of the series and their correlation with Earth rotation. This link exists both between the series, a noisy series is usually less correlated with Earth rotation variations, and inside the series, when looking at the time variable noise and correlation. As the quality of the series varies in time, we construct a combined AAM series, using time dependent weights chosen so that the noise level of the combined series is minimal. To test our combined time series, we computed its correlation with Earth rotation data. We noted that the combined series is always amongst the best correlated series: the quality criterion, while totally independent on Earth rotation observations, appears to be convincing when atmospheric and Earth rotation data are intercompared.
G34A-07 17:30h
Decadal Variations in Earth Rotation and Mechanisms of Core-Mantle Coupling
Lengthy observational records have enabled the detection of decadal variations in both the orientation and magnitude of the rotation of the solid Earth. Rotational variations that do not arise from a forcing external to the planet as a whole represent interaction between the mantle and some other subsystem of the Earth (the atmosphere, oceans, outer or inner core). Changes in mantle rotation correspond to a change in angular momentum and require the application of a torque. The observed decadal rotational variations likely involve interaction with the core. Study of rotational variations can thus provide information on the dynamics of the core, that is the motions associated with the changing angular momentum, and the mechanisms of core-mantle coupling, that is the processes which produce the torque. Decadal changes in the magnitude of solid Earth rotation, observed as changes in length-of-day, are generally considered to arise due to torsional oscillations of the fluid core. This view is based on estimates of core angular momentum, constrained by geomagnetic secular variation data, that correlate well with the length-of-day time series. The orientation of solid Earth rotation also varies on decadal timescales and this data series has been found to correlate with the length-of-day time series suggesting that torsional oscillations and core-mantle interactions may also be responsible for decadal variations in polar motion. Core-mantle coupling can occur through electromagnetic, topographic and gravitational mechanisms. This study uses a torsional oscillation model to investigate these coupling mechanisms in order to estimate both their magnitude and spectral signature and compares the results with observed properties of rotational variations.
G34A-08 17:45h
Celestial Motion of Earth's Spin Axis Derived From the Combination of VLBI and GPS Observations
The motion of Earth's spin axis in space is monitored by VLBI, and since 1994 also its rate is measured by GPS. The method of "combined smoothing", developed recently at the Astronomical Institute in Prague, enables to combine both series. The solution, based on observations in 1979.6-2004.5, is compared with the recently adopted IAU2000 model of precession-nutation. The analysis reveals statistically significant deviations from the model. These differences can be identified with free core nutation (FCN) and several forced nutation terms.