G32A-01 INVITED
Investigation of low-degree gravitational changes from GRACE, Earth rotation, climate models, and satellite laser ranging
We compare several independent time series of variations C21, S21, and C20 in Earth's gravity field for the period April 2002 to May 2007, including estimates from the Gravity Recovery and Climate Experiment (GRACE), Earth rotation variations, climate models (including atmospheric, oceanic, and hydrological models), and satellite laser ranging (SLR). Recently released GRACE release 04 (RL04) solutions show significant improvement relative to earlier results, especially for C21 and S21. At the annual period, all estimates agree remarkably well, and good correlation is found among time series at intraseasonal periods. In general, Earth rotation values for C21 and S21, and SLR values for C20 agree best with GRACE estimates. GRACE RL04 C20 time series are still contaminated by aliased ocean tide model errors. These independent estimates of C21, S21, and C20 variations are important for validating different geodetic techniques and improving the accuracy of low-degree gravity change.
G32A-02 INVITED
GPS reference frame realizations
We examine the effects of the treatment of GPS satellite orbits on the realization of terrestrial reference frames and the effects that these treatments have on regionally dense networks of GPS sites realized in these frames. GPS satellites under go complex orbit perturbations due to the effects of solar radiation pressure acting on the satellite surfaces. The most important surfaces are the solar panels that are controlled to point to the Sun but due to control precision will not always point exactly at the Sun. In some satellites, momentum wheel failures lead to very poor alignment of the panels with the Sun, which leads to large variations in the radiation pressure. In addition to the direct effects of solar radiation, the albedo of the Earth also leads to perturbing forces on the satellites. These variable forces have lead to many different methods of estimating the effects of solar radiation pressure on the GPS satellite orbits. There is an impact of the methods used to estimate the effects of radiation pressure forces on the terrestrial frame realization. In particular, the recovery of the location of the center of mass of the Earth is greatly affected by the treatment of radiation parameters. On the other hand, terrestrial scale is not so affected. We discuss the impact of the radiation parameter estimation on terrestrial frame realization and examine how much of the effect can be accounted for with translation, rotation, and scale of the frame. We also examine the impact of the orbit and reference frame treatments when regional data sets are incorporated into a global reference frame. Here we specifically examine results from the Plate Boundary Observatory (PBO), which includes the 1000-station GPS network spread across the United States.
G32A-03
Defining the Terrestrial Reference Frame from GPS: Results from a JPL Reanalysis
We present results from a reanalysis of over five years of data from a global GPS network using the most recent (2008) version of the GIPSY/OASIS software. In this reanalysis, we have adopted various approaches to determine the terrestrial reference frame from GPS while computing the orbit and clock solutions of the GPS constellation. Alternative models are used to define the phase variations of the GPS transmit and receive antennas, including the calibration standards from the International GNSS Service, transmitter calibrations derived from the GRACE tandem mission, and historical receiver antenna calibrations measured at the Jet Propulsion Laboratory. Initial results show 0.01ppb/year agreement between the scale rate as derived by GPS and ITRF05 (2002-2007), but significant differences in the recovered absolute scale depending on which receiver antenna calibrations are used.
G32A-04
Status of the ITRF Development
It is expected to start preparing for a new version of the International Terrestrial Reference Frame, namely the ITRF2008. Before initiating this new solution, some conditions have to be satisfied, mainly the need of reprocessed and consistent solutions from each technique: a new reprocessed IGS solution involving the absolute PCV models, an improved reanalysis solution from IVS accounting for the mean pole tide correction and better troposphere modeling, an improved ILRS solution taking into account all range bias and other station- dependent corrections, and new DORIS solutions where improvements are expected in the frame Z translation and the scale. It is anticipated to have some of these solutions available so that some initial pre- analysis can be performed and their quality assessed. A particular emphasis will be given to the time behavior of the frame physical parameters (origin and scale). The new reprocessed solutions are expected to change in particular the height component estimate of station positions which will have a direct consequence on the ITRF scale determination. Considering these expected changes, a re-evaluation of the consistency between the available local ties and space geodesy estimates becomes necessary, and in particular their impact on the ITRF datum specification. A specific study will be devoted to quantify the overall frame effects as a consequence of time series discontinuities (especially GPS), including the present adequacy of the available ties.
G32A-05
Influence of the Reference Frame Alignment on Station Positions and Velocities: Global or Regional?
GNSS is often used to produce 3D velocity fields aiming at geodynamic interpretations. Due to the small scale deformations in Europe, the accuracy of the estimated relative surface displacements must be at the mm.yr-1 level in the horizontal and vertical components. In this study, we investigate how the reference frame definition affects positions and velocities computed with GPS in a network approach. For this purpose we used ten years (1997-2006) of reprocessed GPS solutions from the ULR contribution to the TIde GAuge (TIGA) Benchmark Monitoring project. The weekly SINEX solutions were stacked in order to estimate accurate 3D velocities for more than 200 stations distributed worldwide and 50 stations located in Europe. This dataset was used to investigate the influence of the reference stations on the estimated 3D velocity field. During our analysis, we used minimum constraint conditions, distinguished between different geographically distributed sets of reference stations (e.g. regional and global) and checked the effect of adding or removing specific reference stations from the datum. First results show differences in the estimated velocities up to the mm.yr-1 level in both horizontal and vertical components and demonstrate that a regional solution is much more sensitive to the choice of the reference stations compared to a global network.
G32A-06
Impact of DORIS Atmospheric Drag Estimation on Terrestrial Reference Frame and Polar Motion
DORIS is one of the many techniques able to define and maintain the terrestrial reference frame. DORIS data are computed on a weekly basis by several analysis centers and then combined in a single solution to participate in the future ITRF2008 realization. As a major improvement was obtained in the past few years in term of precise global gravity field, the atmospheric drag is now becoming more and more a matter of concern by the DORIS analysis groups, using DORIS Low Earth Orbiting satellites (800 km for SPOTs). In particular, atmospheric density model, such as DTM-94 need to be rescaled typically every few hours in standard DORIS data processing. The goal of this presentation is to investigate if the analysis strategies used by all groups in the past few years are still optimum. By selecting a dedicated data set (using few periods of intense geomagnetic activity as well as quieter periods, periods of maximum and minimum of 11- year solar cycle) and by doing single satellite investigation, we are able to show that more frequent estimations (less than 1 hour) of the atmospheric drag provide better results in term of station positions, polar motion and geocenter centering. In addition, the influence of time constrains on these estimated parameters (including a random-walk approach) will be discussed. Finally, a complete year of DORIS data was reprocessed in a multi-satellite mode in order to demonstrate the improvement on the geodetic results toward the current solutions submitted to the International DORIS Service. Finally, the need for a complete homogeneous DORIS reprocessing in view of ITRF2008 will be discussed.
G32A-07
A Kalman Filter for Improved Multi-Technique Estimates of UT1 Variations
We previously described [EGU 2008] a Kalman filter to combine VLBI estimates of UT1-UTC with biased length-of-day (LOD) estimates from GPS. The VLBI results are the analyses of the Goddard Space Flight Center group from the 24-hr multi-station observing sessions several times per week and the nearly daily 1-hr single-baseline sessions. GPS LOD estimates of the International GNSS Service (IGS) are combined with the VLBI UT1-UTC by modeling the natural excitation of LOD as an integration of a white noise process (i.e., as a random walk) and the UT1 variations as the integration of LOD. The variance of the excitation has been determined from the observed UT1 variance and agrees with the value found by Morabito et al. [1988] in their similar filter. To account for GPS technique errors, which express themselves mostly as temporally correlated biases in the LOD measurements, a Gauss-Markov model has been added to assimilate the IGS data, together with a fortnightly sinusoidal term to capture errors in the IGS treatments of tidal effects. Our Kalman filter has been evaluated against independent atmospheric and oceanic axial angular momentum (AAM + OAM) excitations and compared to other UT1/LOD combinations. Ours performs best overall in terms of lowest RMS residual and highest correlation with AAM + OAM over sliding intervals down to 3 days. The IERS C04 and Bulletin A combinations show strong high-frequency smoothing and other problems. The JPL SPACE series suffers in the high frequencies from not including any GPS-based LODs. But care must be taken to handle correlated biases and spurious fortnightly errors to benefit fully from the GPS-based LOD series. We find, somewhat surprisingly, that further improvements are possible in the Kalman filter combination by selective rejection of some VLBI data. The best combined results are obtained by excluding all the 1-hr single-baseline UT1 data as well as those 24-hr UT1 measurements with formal errors greater than 5 microsec (about 18% of the multi-baseline sessions). A rescaling of the VLBI formal errors, rather than rejection, was not an effective strategy. These results suggest that the UT1 errors of the 1-hr and weaker 24-hr VLBI sessions are non-Gaussian and more heterogeneous than expected, possibly due to the diversity of observing geometries used, other neglected systematic effects, or to the much shorter observational averaging interval of the single-baseline sessions.
G32A-08
Loading effects on time-variable gravity and surface displacements
The global circulation of surface geophysical fluids (e.g. atmosphere, ocean hydrology) induce mass redistribution at the Earth's surface, and therefore gravity variations and surface deformation, over a large frequency domain. With the help of Love numbers, or the Green's function formalism, an output from general circulation models (GCM), they can be modeled with a high precision on an operational basis. We present here the computations of time-variable gravity and surface displacements, due to atmospheric, non-tidal oceanic and continental hydrological loading, and show the improvement in terms of reduction of the noise signal in GRACE KBRR residuals, Jason and Topex/Poseidon crossovers, as well as surface displacement observations (GPS, SLR or VLBI).