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ORBi

An Evaluation of New Estimates from GPS, GRACE and Load Models compared to SLR ; ; et al in Geophysical Research Letters (2010), (37), 5-6 Changes in J2, resulting from past and present changes in Earth’s climate, are traditionally observed by Satellite Laser ranging (SLR). Assuming an elastic Earth, it is possible to infer changes in J2 ... [more ▼] Changes in J2, resulting from past and present changes in Earth’s climate, are traditionally observed by Satellite Laser ranging (SLR). Assuming an elastic Earth, it is possible to infer changes in J2 from changes in Earth’s shape observed by GPS. We compare estimates of non‐secular J2 changes from GPS, SLR, GRACE, and a load model. The GPS and SLR annual signals agree but are different (16%) to the load model. Subtraction of the load model removes the annual variation from GPS, SLR, and GRACE, and the semi‐annual variation in GPS. The GPS and SLR long‐term signals are highly correlated, but GPS is better correlated with the loading model. Subtraction of the load model removes the 1998 anomaly from the GPS J2 series but not completely from the SLR J2 series, suggesting that the SLR anomaly may not be entirely due to mass re‐distribution as has been presumed. [less ▲] Detailed reference viewed: 75 (3 UL)Basis functions for the consistent and accurate representation of surface mass loading ; ; et al in Geophysical Journal International (2007), 171(1), 1-10 Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich ... [more ▼] Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich, ocean-poor distribution of geodetic data, coupled with the predominance of the continental load (water storage and atmospheric pressure) compared with the ocean bottom pressure (including the inverse barometer response). Finer-scale inversion becomes unstable due to the rapidly increasing number of parameters which are poorly constrained by the data geometry. Several approaches have previously been tried to mitigate this, including the adoption of constraints over the oceanic domain derived from ocean circulation models, the use of smoothness constraints for the oceanic load, and the incorporation ofGRACEgravity field data. However, these methods do not provide appropriate treatment of mass conservation and of the ocean’s equilibrium-tide response to the total gravitational field. Instead,we propose a modified set of basis functions as an alternative to standard spherical harmonics. Our basis functions allow variability of the load over continental regions, but impose global mass conservation and equilibrium tidal behaviour of the oceans. We test our basis functions first for the efficiency of fitting to realistic modelled surface loads, and then for accuracy of the estimates of the inferred load compared with the known model load, using synthetic geodetic displacements with real GPS network geometry. Compared to standard spherical harmonics, our basis functions yield a better fit to the model loads over the period 1997–2005, for an equivalent number of parameters, and provide a more accurate and stable fit using the synthetic geodetic displacements. In particular, recovery of the low-degree coefficients is greatly improved. Using a nine-parameter fit we are able to model 58 per cent of the variance in the synthetic degree-1 zonal coefficient time-series, 38–41 per cent of the degree-1 non-zonal coefficients, and 80 per cent of the degree-2 zonal coefficient. An equivalent spherical harmonic estimate truncated at degree 2 is able to model the degree-1 zonal coefficient similarly (56 per cent of variance), but only models 59 per cent of the degree-2 zonal coefficient variance and is unable to model the degree-1 non-zonal coefficients. [less ▲] Detailed reference viewed: 117 (3 UL)Geocenter motions from GPS: A unified observation model ; van Dam, Tonie ; et al in Journal of Geophysical Research (2006), 111(B05), 1-66 We test a unified observation model for estimating surface-loading-induced geocenter motion using GPS. In principle, this model is more complete than current methods, since both the translation and ... [more ▼] We test a unified observation model for estimating surface-loading-induced geocenter motion using GPS. In principle, this model is more complete than current methods, since both the translation and deformation of the network are modeled in a frame at the center of mass of the entire Earth system. Real and synthetic data for six different GPS analyses over the period 1997.25–2004.25 are used to (1) build a comprehensive appraisal of the errors and (2) compare this unified approach with the alternatives. The network shift approach is found to perform particularly poorly with GPS. Furthermore, erroneously estimating additional scale changes with this approach can suggest an apparently significant seasonal variation which is due to real loading. An alternative to the network shift approach involves modeling degree-1 and possibly higher-degree deformations of the solid Earth in a realization of the center of figure frame. This approach is shown to be more robust for unevenly distributed networks. We find that a unified approach gives the lowest formal error of geocenter motion, smaller differences from the true value when using synthetic data, the best agreement between five different GPS analyses, and the closest (submillimeter) agreement with the geocenter motion predicted from loading models and estimated using satellite laser ranging. For five different GPS analyses, best estimates of annual geocenter motion have a weighted root-mean-square agreement of 0.6, 0.6, and 0.8 mm in amplitude and 21°, 22°, and 22° in phase for x, y, and z, respectively. [less ▲] Detailed reference viewed: 128 (0 UL)Effect of gravitational consistency and mass conservation on seasonal surface mass loading models ; ; et al in Geophysical Research Letters (2005), 32(L08306), 1-5 Increasingly, models of surface mass loads are used either to correct geodetic time coordinates by removing seasonal and other ‘‘noise’’, or for comparison with other geodetic parameters. However, models ... [more ▼] Increasingly, models of surface mass loads are used either to correct geodetic time coordinates by removing seasonal and other ‘‘noise’’, or for comparison with other geodetic parameters. However, models of surface loading obtained by simply combining the mass redistribution due to individual phenomena will not in general be self- consistent, in that (i) the implied global water budget will not be mass-conserving, and (ii) the modelled sea level will not be an equipotential surface of Earth’s total gravity field. We force closure of the global water budget by allowing the ‘‘passive’’ ocean to change in mass. This medium-term passive ocean response will not be a uniform change in non- steric ocean surface height, but must necessarily be spatially variable to keep the ‘‘passive’’ ocean surface on an equipotential. Using existing load models, we demonstrate the effects of our consistency theory. Geocenter motion is amplified significantly, by up to 43%. [less ▲] Detailed reference viewed: 111 (3 UL) |
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