G42A-01
GRACE-Based Estimates of GPS Satellite Antenna Phase Variations: Impact on Determining the Scale of the Terrestrial Reference Frame
Treating the GRACE tandem mission as an orbiting fiducial laboratory, we have developed new estimates of the phase and group-delay variations of the GPS transmitter antennas. Application of these antenna phase variation (APV) maps have shown great promise in reducing previously unexplained errors in our realization of GPS measurements from the TOPEX/POSEIDON (T/P; 1992--2005) and Jason-1 (2001--) missions. In particular, a 56 mm vertical offset in the solved-for position of the T/P receiver antenna is reduced to insignificance (less than 1 mm). For Jason-1, a spurious long-term (4-yr) drift in the daily antenna offset estimates is reduced from +3.7 to +0.1 mm/yr. Prior ground-based results, based on precise point positioning, also hint at the potential of the GRACE-based APV maps for scale determination, reducing the spurious scale rate by one half. In this paper, we report on the latest APV estimates from GRACE, and provide a further assessment of the impact of the APV maps on realizing the scale of the terrestrial reference frame (TRF) from GPS alone. To address this, we re-analyze over five years of data from a global (40+ station) ground network in a fiducial-free approach, using the new APV maps. A specialized multi-day GPS satellite orbit determination (OD) strategy is employed to better capitalize on dynamical constraints. The resulting estimates of TRF scale are compared to ITRF2005 in order to assess the quality of the solutions.
G42A-02
The Contribution of Starlette/Stella SLR to Terrestrial Reference Frame Definition
The Lageos-1 (L1) and Lageos-2 (L2) satellites have formed the basis of the ILRS contribution to the ITRF2005. These geodetic cannonball satellites, launched in 1976, and 1992 respectively are well suited for this purpose, by virtue of their high altitude, and lack of sensitivity to either high degree gravity terms or the deleterious effects of atmospheric drag. However, some SLR tracking stations have only limited tracking of the L1 and L2 satellites, with the result that these stations are not well determined in ITRF2005. In addition, we note that the results of analysis of the DORIS contribution to ITRF2005 indicated that the quality of the station positioning was directly proportional to the number of satellites in each weekly solution [Altamimi et al., JoG, 2006]. Therefore, in this paper we test the contribution of additional satellites to SLR-only weekly reference frame solutions. We analyze the SLR data to the Stella (STL) and Starlette (STA) satellites for the period from 1993 to 2006, and first perform a detailed analysis to assess the optimum arc parameterization in order to minimize orbit error, taking advantage of the new high-resolution gravity and ocean tide models that have been developed as a result of the GRACE, TOPEX/Poseidon and Jason missions. We then compare and contrast three series of SINEX solutions: L1+L2 only, STL+STA only, and a four-satellite combination based on L1+L2+STL+STA. We compare the quality of the weekly station position, the geocenter and scale estimates and assess whether there is any improvement or degradation in the estimated EOP.
G42A-03
The GGM03 Mean Earth Gravity Model from GRACE
A new generation Earth gravity field model, called GGM03S, is derived using four years of data spanning January 2003 to December 2006 from the Gravity Recovery and Climate Experiment. Relative to the preceding generation, GGM02, there have been improvements to the data products, the gravity estimation methods and the background models. Based on the calibrated covariance, GGM03S represents a factor of two improvement over the previous GGM02 model. The satellite only model determined from the GRACE data is combined with surface measurements to obtain an improved combined model, referred to as GGM 03C, that is complete to degree and order 360. The optimal combination of the very precise gravity information from GRACE with terrestrial and marine gravity information is a challenging task. Avoiding degradation of the portion of the gravity solution from GRACE is one difficulty, as is the specific solution method to achieve a degree and order 360 model. Traditional techniques require the use of block diagonal or quadrature-like methods. Using a new 'out-of-core' algorithm, a rigorous estimate of a full 360x360 field (with full covariance) has been obtained. By ingesting the terrestrial and marine gravity information directly, assumptions about density required for reducing data to a single surface are not required, nor are there any restrictions on data type, distribution, spacing or weighting. This presentation will briefly describe the two solutions and present preliminary evaluations of the improvement in the model accuracy
G42A-04
Subdaily Earth Rotation Models Estimated From GPS and VLBI Data
Subdaily changes in Earth rotation at diurnal and semi-diurnal periods are mainly caused by ocean tides. Smaller effects are attributed to the interaction of the atmosphere with the solid Earth. As the tidal periods are well known, models for the ocean tidal contribution to high-frequency Earth rotation variations can be estimated from space- geodetic observations. The subdaily ERP model recommended by the latest IERS conventions was derived from an ocean tide model based on satellite altimetry. Another possibility is the determination of subdaily ERP models from GPS- and/or VLBI-derived Earth rotation parameter series with subdaily resolution. Homogeneously reprocessed long-time series of subdaily ERPs computed by GFZ/TU Dresden (12 years of GPS data), DGFI and GSFC (both with 24 years of VLBI data) provide the basis for the estimation of single-technique and combined subdaily ERP models. The impact of different processing options (e.g., weighting) and different temporal resolutions (1 hour vs. 2 hours) will be evaluated by comparisons of the different models amongst each other and with the IERS model. The analysis of the GPS and VLBI residual signals after subtracting the estimated ocean tidal contribution may help to answer the question whether the remaining signals are technique-specific artifacts and systematic errors or true geophysical signals detected by both techniques.
G42A-05
Determining the influence of various OAM models on EOP
Earth orientation parameters (EOPs) are effected by a number of geophysical factors. In particular, oceanic angular momentum (OAM) is known to cause changes in polar motion and length of day through changes in the ocean bottom pressure and currents. While these forcings are a significant cause of EOP variation, OAM models have not been used in operational combination and prediction of EOPs because no OAM model is currently being run in an operational mode. This analysis looks at the influence of OAM models on EOPs with particular attention to investigation of the amplitude of the effects and the frequency range over which OAM influences EOPs. Potential benefits to EOP determination and prediction from an operational OAM model are discussed.
G42A-06
High-Resolution Atmosphere Angular Momentum Time Series From the ECMWF
We investigate the application of different classes of data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and different geophysical models for the determination of axial and equatorial effective atmospheric angular momentum functions (EAAMF). Sixty model levels are compared vs. 21 pressure levels (from 1000 down to 1 hPa), the horizontal spatial resolution is decreased up to 0.25 degrees, and the 6-hours temporal resolution is reduced to 3-hours steps from forecasting data. In terms of geophysical models, first we compare the rigorous approach for the matter terms with the simplified surface pressure approach, then we investigate both, IB and non-IB approaches, and finally we check the influence of different height models for the Earth topography upon the AMF. We derive these sets of EAAMF for the CONT05 time span, a 15 days continuous Very Long Baseline Interferometry (VLBI) campaign in September 2005. Geophysical EAAMF are compared to the geodetic ones. The latter are obtained from time series of polar motion and length of day with a temporal resolution of 3 hours. The frequency dependence of both types of functions will be investigated. Finally, we show results from the implementation of the algorithms at the ECMWF for routine processing, and how these can be accessed.
G42A-07
Statistical Analysis of the AAM Contributions to the IERS RS/PC Predictions
The US Naval Observatory Earth Orientation Department, as the IERS Rapid Service/Predictions Center, continues to make improvements to its combination and prediction program. One of the many improvements made in recent years was the inclusion of atmospheric angular momentum (aam) data in the prediction program. The addition of the aam data has increased the accuracy of our UT1-UTC predictions. This presentation will focus on a statistical analysis of the National Oceanic and Atmospheric Administration (NOAA) and the US Navy Operational Global Atmospheric Prediction System (NOGAPS) aam data provided, as well as provide a brief history of the incorporation and use of the aam data, and possible future improvements.
G42A-08
Regional contributions of atmosphere and oceans to transient changes of the Earth's rotation
The time-variable Earth's rotation is precisely monitored by space-geodetic techniques. Beside periodic phenomena, distinct transient signals are apparent, which are predominantly related to near-surface processes of the Earth system. However, since observations of Earth rotation represent an integral signal of the Earth's response to internal and external forces, they are principally not applicable to directly identify the contributions of individual regions or processes. Instead, sophisticated numerical models are required in order to relate observed rotational variations to their causative dynamical processes. Here, a numerical modelling approach of Earth system dynamics is presented allowing for consistent fluxes of mass, momentum and heat between the sub-systems atmosphere, oceans and continental hydrology. Operational atmospheric analyses from ECMWF are used to force a hydrological discharge model as well as a global model for the ocean's baroclinic circulation and ephemeral tides. The unconstrained hydrology and ocean models are coupled via continental discharge in order to close the hydrological cycle. Focussing on the period 2001 - 2006, observed rotational variations are contrasted to simulated individual excitations of atmosphere, oceans, and continental hydrology in order to attribute characteristical signals of Earth rotation to underlying physical processes and to identify regional contributions to the total excitation of Earth rotation. By means of a comparing analysis of observations and simulations it will be discussed to what extent Earth rotation observations can be used to monitor climate relevant dynamics in the atmosphere-hydrosphere system.