References of "van Dam, Tonie 50003245"
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See detailTowards combined global monthly gravity field solutions
Jäggi, Adrian; Meyer, Ulrich; Weigelt, Matthias UL et al

Scientific Conference (2014, April)

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See detailTidal gravity measurements in Southeast Asia revisited
Francis, Olivier UL; van Dam, Tonie UL

in Sun, Heping (Ed.) Progress on Geodesy and Geodynamics (2014)

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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 detailOn the capability to derive mass estimates from high-low satellite-to-satellite tracking data
Weigelt, Matthias UL; van Dam, Tonie UL; Tourian, M. J. et al

Poster (2013, December)

Recently it has been shown that it is possible to derive time-variable gravity signals from high-low satellite-to-satellite tracking (hl-SST) missions (Weigelt et al. 2013, JGR:Solid Earth, doi:10.1002 ... [more ▼]

Recently it has been shown that it is possible to derive time-variable gravity signals from high-low satellite-to-satellite tracking (hl-SST) missions (Weigelt et al. 2013, JGR:Solid Earth, doi:10.1002/jgrb.50283). Based on the GPS information only, we will present results derived from the dedicated gravity field missions CHAMP, GRACE and GOCE which allow us to determine mass estimates for various applications. Hydrologically induced mass changes on land cause the strongest mass variations in the gravity field and can be easily identified in the hl-SST data, especially in areas with strong signals such as the Amazon basin. Ice melt in Greenland can be derived from the data and mass estimates compare well to corresponding GRACE estimates. Also, loading time series based on these gravity field solutions agree well with GPS observations for various stations around the globe. We also discuss the limitations of the data, e.g. in detecting signals related to glacial isostatic adjustment or earthquake-induced gravity field changes. Overall, we will demonstrate that the quality of the GPS data is sufficient nowadays and with a proper processing strategy it is possible to derive reasonable mass estimates. As such, this type of observations may allow to bridge a possible gap between GRACE and its successor GRACE Follow-On scheduled for launch in 2017. [less ▲]

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See detailTowards combined global monthly gravity field solutions
Jäggi, A.; Meyer, U.; Beutler, G. et al

Scientific Conference (2013, October)

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See detailAn assessment of degree-2 Stokes coefficients from Earth rotation data
Meyrath, Thierry UL; van Dam, Tonie UL; Weigelt, Matthias UL et al

in Geophysical Journal International (2013), 195((1)), 249-259

Variations in the degree-2 Stokes coefficients C20, C21 and S21 can be used to understand long and short-term climate forcing. Here, we derive changes in these coefficients for the period 2003 ... [more ▼]

Variations in the degree-2 Stokes coefficients C20, C21 and S21 can be used to understand long and short-term climate forcing. Here, we derive changes in these coefficients for the period 2003 January–2012 April using Earth rotation data. Earth rotation data contain contributions from motion terms (the effects of winds and currents) and contributions from the effects of mass redistribution. We remove the effects of tides, atmospheric winds and oceanic currents from our data. We compare two different models of atmospheric and oceanic angular momentum for removing the effects of winds and currents: (1) using products from the National Centers for Environmental Prediction and (2) using data from the European Centre for Medium-range Weather Forecasts (ECMWF). We assess the quality of these motion models by comparing the two resulting sets of degree-2 Stokes coefficients to independent degree-2 estimates from satellite laser ranging (SLR), GRACE and a geophysical loading model. We find a good agreement between the coefficients from Earth rotation and the coefficients from other sources. In general, the agreement is better for the coefficients we obtain by removing winds and currents effects using the ECMWF model. In this case, we find higher correlations with the independent models and smaller scatters in differences. This fact holds in particular for C20 and C21, whereas we cannot observe a significant difference for S21. At the annual and semiannual periods, our Earth rotation derived coefficients agree well with the estimates from the other sources, particularly for C21 and S21. The slight discrepancies we obtain for C20 can probably be explained by errors in the atmospheric models and are most likely the result of an over-/underestimation of the annual and semiannual contributions of atmospheric winds to the length-of-day excitation. [less ▲]

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See detailBoundary elements for modelling gravitational signals observed by inter-satellite ranging
Antoni, M.; Weigelt, Matthias UL; Keller, W. et al

Poster (2013, September)

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See detailOn the capability of non-dedicated GPS-tracked satellite constellations for estimating mass variations: case study SWARM
Reubelt, T.; Baur, O.; Weigelt, Matthias UL et al

Scientific Conference (2013, September)

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See detailTime-variable gravity signal in Greenland revealed by high-low satellite-to-satellite tracking
Weigelt, Matthias UL; van Dam, Tonie UL; Jäggi, Adrian et al

in Journal of Geophysical Research. Solid Earth (2013), 118

In the event of a termination of the GRACE mission before the launch of GRACE Follow-On (due for launch in 2017) high-low satellite-to-satellite tracking (hl-SST) will be the only dedicated observing ... [more ▼]

In the event of a termination of the GRACE mission before the launch of GRACE Follow-On (due for launch in 2017) high-low satellite-to-satellite tracking (hl-SST) will be the only dedicated observing system with global coverage available to measure the time variable gravity field (TVG) on a monthly or even shorter time scale. Until recently, hl-SST TVG observations were of poor quality and hardly improved the performance of Satellite Laser Ranging observations. To date, they have been of only very limited usefulness to geophysical or environmental investigations. In this paper, we apply a thorough reprocessing strategy and a dedicated Kalman filter to CHAMP data to demonstrate that it is possible to derive the very long wavelength TVG features down to spatial scales of approximately 2000 km at the annual frequency and for multi-year trends. The results are validated against GRACE data and surface height changes from long-term GPS ground stations in Greenland. We find that the quality of the CHAMP solutions is sufficient to derive long-term trends and annual amplitudes of mass change over Greenland. We conclude that hl-SST is a viable source of information for TVG and can serve to some extent to bridge a possible gap between the end-of-life of GRACE and the availability of Grace Follow-On. [less ▲]

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See detailDerivation of angular velocities of the GRACE satellite formation from the star camera data
Bandikova, T.; Weigelt, Matthias UL; van Dam, Tonie UL et al

Scientific Conference (2013, June)

The presentation discusses possibilites to derive common frame angular velocities from the star cameras in the GRACE system. Generally the star cameras measure the rotation of each satellite independently ... [more ▼]

The presentation discusses possibilites to derive common frame angular velocities from the star cameras in the GRACE system. Generally the star cameras measure the rotation of each satellite independently from the other satellite. However, common frame rotations are also needed for various applications. Specifically in this presentation we discuss the application to a variant of the differential gravimetry approach. In this, angular rates around the crosstrack component of the so-called instantaneous relative reference frame are needed. The latter is a common reference frame formed by the inter-satellite connection of the two GRACE satellites. We will quantify the accuracy demands on the derivation of the angular velocities and discuss the possibilites to meet these demands with existing star camera observations. We will also give an outlook on new possibilites in the determination of the orientation and the derivation of angular velocities offered by the laser ranging instrument which will be used in the GRACE Follow-On mission. [less ▲]

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See detailA new variant of the differential gravimetry approach
Weigelt, Matthias UL; van Dam, Tonie UL; Bandikova, T.

Scientific Conference (2013, June)

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See detailOn the numerical stability in the derivation of Slepian base functions
Weigelt, Matthias UL; van Dam, Tonie UL; Wang, Lin UL

Scientific Conference (2013, June)

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See detailMonthly Crustal Loading Corrections for Satellite Altimetry
Ray, Richard; Luthcke, Scott; van Dam, Tonie UL

in Journal of Atmospheric & Oceanic Technology (2013), 30(5), 999-1005

Satellite altimeter measurements of sea surface height include a small contribution from vertical motion of the seafloor caused by crustal loading. Loading by ocean tides is routinely allowed for in ... [more ▼]

Satellite altimeter measurements of sea surface height include a small contribution from vertical motion of the seafloor caused by crustal loading. Loading by ocean tides is routinely allowed for in altimeter data processing. Here, loading by nontidal fluids of the atmosphere, ocean, and terrestrial hydrosphere is examined. The crustal deformation can be computed from either geophysical models or from Gravity Recovery and Climate Experiment (GRACE) gravity inversions of mass variability. The loading corrections are found to be very small, rarely exceeding a few millimeters. Nonetheless, they form a significant correction to altimetric determinations of global mean sea level. The correction is most important at the annual cycle and should be accounted for when attempting to balance the global sea level budget. [less ▲]

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See detailHydrological mass changes inferred from high-low satellite-to-satellite tracking data
van Dam, Tonie UL; Weigelt, Matthias UL; Tourian, M. J. et al

Scientific Conference (2013, April)

The technique of deriving time variable gravity (TVG) field observations from high-low satellite-to-satellite tracking (hl-SST) is beginning to establish itself as a valuable and supplementary source for ... [more ▼]

The technique of deriving time variable gravity (TVG) field observations from high-low satellite-to-satellite tracking (hl-SST) is beginning to establish itself as a valuable and supplementary source for the determination and description of long wavelength geophysical phenomena. Recent developments in data processing techniques have pushed the limits of the accuracy of these types of observations and now allows for realistic determinations of long-term trends and annual amplitudes of hydrological signals. We use CHAMP data and a dedicated signal processing to derive annual and inter-annual variations in the largest catchments of the Earth system, e.g. Amazon, Ob and Lena. Results are validated by computing the correlation of aggregated water storage changes from CHAMP (and GRACE) with the hydro-meteorological storage changes. High noise levels demand a stronger filtering, e.g. larger filtering radius (1000-1400km), than usually applied in case of GRACE. We therefore also investigate the effect of filtering on the consistency with the hydrological mass changes and estimate the signal to noise ratio and the spatial and temporal dependency of the noise. We will show that hl-SST observations are a viable source of information for TVG which can even serve as a reliable substitute in the event of the impending end of GRACE's active lifetime. [less ▲]

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See detailLong-term mass changes over Greenland derived from high-low satellite-to-satellite tracking
Weigelt, Matthias UL; van Dam, Tonie UL; Jäggi, Adrian et al

Poster (2013, April)

In the last decade, temporal variations of the global gravity field have become an ubiquitous and invaluable source of information for geophysical and environmental studies. It is important that the time ... [more ▼]

In the last decade, temporal variations of the global gravity field have become an ubiquitous and invaluable source of information for geophysical and environmental studies. It is important that the time series of observations is not interrupted as some geophysical phenomena, e.g. postglacial rebound or long term ice mass trends, are only beginning to be observable. To date, the most valuable source for time variable gravity (TVG) is the GRACE mission which has already exceeded its nominal lifetime. It can cease operations any time now and then only high-low satellite-to-satellite (hl-SST) observations will be available. These observations have, however, only demonstrated limited application for TVG. In this presentation, we show that by using CHAMP data, a thorough reprocessing strategy and a dedicated Kalman filter it is possible to derive the very long wavelength features of the time variable gravity field. The results are validated against GRACE data and height coordinates from long-term GPS ground stations in Greenland. We find that the quality of the CHAMP solutions is sufficient to derive realistic long-term trends and annual amplitudes of mass changes of Greenland. We conclude that hl-SST would be a viable substitute (although at lower spatial resolution) for TVG in the event of a profund operational breakdown of GRACE. [less ▲]

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