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See detailA comparison of interannual hydrological polar motion excitation from GRACE and geodetic observations
Meyrath, Thierry UL; van Dam, Tonie UL

in Journal of Geodynamics (2016), 99

Continental hydrology has a large influence on the excitation of polar motion (PM). However, these effects are far from being completely understood. Current global water storage models differ ... [more ▼]

Continental hydrology has a large influence on the excitation of polar motion (PM). However, these effects are far from being completely understood. Current global water storage models differ significantly from one another and are unable to completely represent the complex hydrological cycle, particularly at interannual scales. A promising alternative to study hydrological effects on PM is given by the GRACE satellite mission. In this study, we assess the ability of GRACE to investigate interannual hydrological PM excitations. For this purpose, we use the latest GRACE Release-05 data from three different processing centers (CSR, GFZ, JPL) that we convert into estimates of hydrological PM excitation, $\chi_1^H$ and $\chi_2^H$. In addition to these gravimetric excitations, we also consider geodetic hydrological excitations, which we calculate by removing modelled atmospheric and oceanic effects from precise observations of full PM excitations. We remove signals with frequencies $\geq 1$ cpy from the series and compare the resulting estimates of interannual hydrological excitations for the period 2004.5 - 2014.5. The comparison between geodetic and gravimetric excitations reveals some discrepancies for $\chi_1^H$, likely to be related to inadequately modelled atmospheric and oceanic effects. On the other hand, good agreement is observed for $\chi_2^H$. For both components, the best agreement between geodetic and gravimetric excitations is obtained for the estimate from CSR. Very good agreement is obtained between GRACE-derived excitations from different processing centers, in particular for CSR and JPL. Both the comparisons between geodetic and gravimetric, and the comparisons between the different gravimetric excitations give substantially better results for $\chi_2^H$ than for $\chi_1^H$, leading to the conclusion that geodetic and gravimetric $\chi_2^H$ can be more reliably determined than $\chi_1^H$. Although there are still some discrepancies between geodetic and gravimetric interannual hydrological excitations, we conclude that GRACE and potential follow-on missions are valuable tools to study the interannual effects of continental hydrology on the excitation of PM. [less ▲]

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See detailSingular spectrum analysis for modeling seasonal signals from GPS time series
Chen, Qiang; van Dam, Tonie UL; Sneeuw, Nico et al

in Journal of Geodynamics (2013), 72

Seasonal signals in GPS time series are of great importance for understanding the evolution of regional mass fluctuations, i.e., ice, hydrology, and ocean mass. Conventionally these signals quasi-annual ... [more ▼]

Seasonal signals in GPS time series are of great importance for understanding the evolution of regional mass fluctuations, i.e., ice, hydrology, and ocean mass. Conventionally these signals quasi-annual and semi-annual signals are modeled by least-squares fitting harmonic terms with a constant amplitude and phase. In reality, however, such seasonal signals are modulated, i.e., they will have a time-variable amplitude and phase. Recently, Davis et al.(2012) proposed a Kalman filter based approach to capture the stochastic seasonal behavior of geodetic time series. Singular Spectrum Analysis (SSA) is a non-parametric method, which uses time domain data to extract information from short and noisy time series without a priori knowledge of the dynamics affecting the time series. A prominent benefit is that trends obtained in this way are not necessarily linear. Further, true oscillations can be amplitude and phase modulated. In this work, we will assess the value of SSA for extracting time-variable seasonal signals from GPS time series. We compare our SSA-based results to those obtained using 1) least-squares analysis and 2) Kalman filtering. Our results demonstrate that SSA is a viable and complementary tool for extracting modulated oscillations from GPS time series. [less ▲]

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See detailObserving and understanding the Earth system variations from space geodesy
Jin, Shuanggen; van Dam, Tonie UL; Wdowinski, Shimon

in Journal of Geodynamics (2013), 72

The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere, cryosphere, lithosphere, and other fluids in Earth's interior, influence the Earth's shape, gravity ... [more ▼]

The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere, cryosphere, lithosphere, and other fluids in Earth's interior, influence the Earth's shape, gravity field and its rotation (the three pillars of geodesy). The effects of global climate change, such as sea level rise, glacier melting, and geoharzards, also affect these observables. However, observations and models of Earth's system change have large uncertainties due to the lack of direct high temporal–spatial measurements. Nowadays, space geodetic techniques, particularly GNSS, VLBI, SLR, DORIS, InSAR, satellite gravimetry and altimetry provide a unique opportunity to monitor and, therefore, understand the processes and feedback mechanisms of the Earth system with high resolution and precision. In this paper, the status of current space geodetic techniques, some recent observations, and interpretations of those observations in terms of the Earth system are presented. These results include the role of space geodetic techniques, atmospheric–ionospheric sounding, ocean monitoring, hydrologic sensing, cryosphere mapping, crustal deformation and loading displacements, gravity field, geocenter motion, Earth's oblateness variations, Earth rotation and atmospheric-solid earth coupling, etc. The remaining questions and challenges regarding observing and understanding the Earth system are discussed. [less ▲]

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See detailGeocenter motion and its geodetic and geophysical implications
Wu, X.; Ray, J.; van Dam, Tonie UL

in Journal of Geodynamics (2012), 58

The horizontal transport of water in Earth’s surface layer, including sea level change, deglaciation, and surface runoff, is a manifestation of many geophysical processes. These processes entail ocean and ... [more ▼]

The horizontal transport of water in Earth’s surface layer, including sea level change, deglaciation, and surface runoff, is a manifestation of many geophysical processes. These processes entail ocean and atmosphere circulation and tidal attraction, global climate change, and the hydrological cycle, all having a broad range of spatiotemporal scales. The largest atmospheric mass variations occur mostly at synoptic wavelengths and at seasonal time scales. The longest wavelength component of surface mass transport, the spherical harmonic degree-1, involves the exchange of mass between the northern and southern hemispheres. These degree-1 mass loads deform the solid Earth, including its surface, and induce geocenter motion between the center-of-mass of the total Earth system (CM) and the center-of-figure (CF) of the solid Earth surface. Because geocenter motion also depends on the mechanical properties of the solid Earth, monitoring geocenter motion thus provides an additional opportunity to probe deep into Earth’s interior. Most modern geodetic measurement systems rely on tracking data between ground stations and satellites that orbit around CM. Consequently, geocenter motion is intimately related to the realization of the International Terrestrial Reference Frame (ITRF) origin, and, in various ways, affects many of our measurement objectives for global change monitoring. In the last 15 years, there have been vast improvements in geophysical fluid modeling and in the global coverage, densification, and accuracy of geodetic observations. As a result of these developments, tremendous progress has been made in the study of geocenter motion over the same period. This paper reviews both the theoretical and measurement aspects of geocenter motion and its implications. [less ▲]

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See detailGravity Tide and seasonal gravity variation at Ny-Alesund, Svalbard In Arctic
Sato, Tadahiro; Boy, Jean-Paul; Tamura, Yoshiaki et al

in Journal of Geodynamics (2006), 41(1-3), 234-241

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See detailDevelopment of a European Combined Geodetic Network (ECGN)
Ihde, Johannes; Baker, Trevor; Bruyninx, Carine et al

in Journal of Geodynamics (2005), 40(4-5), 450-460

To ensure the long-time stability of the terrestrial reference system with an accuracy of 10‑9 in the global and continental scale, the interactions between different time-dependent influences of the ... [more ▼]

To ensure the long-time stability of the terrestrial reference system with an accuracy of 10‑9 in the global and continental scale, the interactions between different time-dependent influences of the system Earth to the terrestrial reference system and the related observation has to be considered in the evaluation models. It is proposed to establish a kinematic European Combined Geodetic Network (ECGN) and to integrate the spatial and height reference system into the Earth gravity field parameter estimation. This plan is in agreement with the foreseen IAG project of an Integrated Global Geodetic Observation System (IGGOS). In selected European stations ECGN will establish the combination of time series of spatial/geometric GNSS observations, precise levelling and tide gauge records with gravity field related observations (gravity, Earth tides). Observations are complemented with meteorological parameters, surrounding information of the stations, e.g. eccentricities and ground water level. A first call for participation in the project was directed to the implementation of the ECGN stations. These stations include the standard observation techniques GNSS (GPS/GLONASS—permanent), gravity (super conducting gravimeter and/or absolute gravimeter—permanent or repeated), levelling connections to nodal points of the European levelling network (UELN) (repeated) and meteorological parameters (permanent). A basic constituent of the ECGN stations is a local network for controlling the eccentricities at the 1 mm accuracy level in all three spatial components. For the contributing observation techniques, guidelines have been agreed upon to ensure equal observation principles [less ▲]

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See detailVertical crustal motion observed in the BIFROST project
Scherneck, Hans-Georg; Johansson, Jan M.; Koivula, Hannu et al

in Journal of Geodynamics (2003), 35(4-5), 425-441

This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations ... [more ▼]

This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations, Sealevel and Tectonics). We study loading effects due to ocean, atmosphere and hydrology and their impact on estimated rate parameters. We regularly find the admittance of a modelled perturbation at less than fifty percent of the full effect. We think that the finding relates to a difficult noise situation at all periods, and that a satisfying model for the dominating noise source has not been found yet. An additional reason for low admittance is found in the mapping process of the no-fiducial network solution into a conventional reference frame. [less ▲]

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