References of "Altamimi, Z"
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See detailNontidal ocean loading: amplitudes and potential effects in GPS height time series
van Dam, Tonie UL; Collilieux, X.; Wuite, J. et al

in Journal of Geodesy (2012), 86(11), 1043-1057

Ocean bottom pressure (OBP) changes are caused by a redistribution of the ocean’s internal mass that are driven by atmospheric circulation, a change in the mass entering or leaving the ocean, and/or a ... [more ▼]

Ocean bottom pressure (OBP) changes are caused by a redistribution of the ocean’s internal mass that are driven by atmospheric circulation, a change in the mass entering or leaving the ocean, and/or a change in the integrated atmospheric mass over the ocean areas. The only previous global analysis investigating the magnitude of OBP surface displacements used older OBP data sets (van Dam et al. in J Geophys Res 129:507–517, 1997). Since then significant improvements in meteorological forcing models used to predict OBP have been made, augmented by observations from satellite altimetry and expendable bathythermograph profiles. Using more recent OBP estimates from the Estimating the Circulation and Climate of the Ocean (ECCO) project, we reassess the amplitude of the predicted effect of OBP on the height coordinate time series from a global distribution of GPS stations. OBP-predicted loading effects display an RMS scatter in the height of between 0.2 and 3.7 mm, larger than previously reported but still much smaller (by a factor of 2) than the scatter observed due to atmospheric pressure loading. Given the improvement in GPS hardware and data analysis techniques, the OBP signal is similar to the precision of weekly GPS height coordinates. We estimate the effect of OBP on GPS height coordinate time series using the MIT reprocessed solution, mi1. When we compare the predicted OBP height time series with mi1, we find that the scatter is reduced over all stations by 0.1 mm on average with reductions as high as 0.7 mm at some stations. More importantly we are able to reduce the scatter on 65 % of the stations investigated. The annual component of the OBP signal is responsible for 80 % of the reduction in scatter on average.We find that stations located close to semi-enclosed bays or seas are affected by OBP loading to a greater extent than other stations. [less ▲]

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See detailTopographically corrected atmospheric loading effects
van Dam, Tonie UL; Altamimi, Z.; Collileux, X. et al

in Journal of Geophysical Research (2010), (115), 5-6

Atmospheric pressure variations are known to induce vertical displacements of the Earth’s surface with magnitudes large enough to be detected by geodetic observations. Estimates of these loading effects ... [more ▼]

Atmospheric pressure variations are known to induce vertical displacements of the Earth’s surface with magnitudes large enough to be detected by geodetic observations. Estimates of these loading effects are derived using global reanalysis fields of surface pressure as input. The input surface pressure has a minimum spatial sampling, which does not capture true surface pressure variations due to high topographic variability in some regions. In this paper, we investigate the effect that unmodeled topographic variability has on surface pressure estimates and subsequent estimates of vertical surface displacements. We find that the estimated height changes from the topographic surface pressure can be significant (2–4 mm) for sites in regions of high topographic variability. When we compare the estimated height changes to Global Positioning System residuals from the 2005 International Terrestrial Reference Frame Realization, we find that the heights derived from the topographic surface pressure, versus those from the normal surface pressure, perform better at reducing the scatter on the height coordinate time series. [less ▲]

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See detailImpact of loading effects on determination of the International Terrestrial Reference Frame
Collilieux, X.; Altamimi, Z.; Coulot, D. et al

in Advances in Space Research (2010), 45(1), 144-154

The International Terrestrial Reference Frame (ITRF), as a realization of the International Terrestrial Reference System (ITRS), is represented by a set of station positions and linear velocities. They ... [more ▼]

The International Terrestrial Reference Frame (ITRF), as a realization of the International Terrestrial Reference System (ITRS), is represented by a set of station positions and linear velocities. They are intended to be used as regularized coordinates to which some corrections should be added to access instantaneous coordinates. The latest ITRS realization is the ITRF2005, which has integrated time series of station positions to form long-term solutions for the four space geodetic techniques. Currently, a purely linear model is used to parameterize station displacements in the estimation process, plus occasional discontinuities in case of earthquakes or equipment changes. However the input data have been derived without applying surface loading models and so surface loading effects are supposed to be embedded in the coordinates as measured quantities. We evaluate the effect of applying a posteriori loading corrections, which include the effect of atmospheric, non-tidal ocean, and continental water loading, to time series of positions estimated from Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning System (GPS) data. We notice that they reduce about 50% or more of the annual signals in the translation and scale parameter time series of the SLR and VLBI techniques, except in SLR Z translation. In general, the estimated secular frame definition is negligibly affected and estimated positions and velocities are not significantly modified for stations that have accumulated a large number of observations. A multi-technique combination of such derived frames allows concluding that, for some cases, loading model corrections might degrade co-located station coordinates almost as much as they benefit them. However, most significant improvement of the estimated secular coordinates is observed for stations with less than 100 estimated positions as demonstrated with a multi-technique combination. [less ▲]

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See detailImproved Constraints on Models of Glacial Isostatic Adjustment: A Review of the Contribution of Ground-based Geodetic Observations
King, M.; Altamimi, Z.; Boehm, J. et al

in Surveys in Geophysics (2010), 31(5), 465-507

The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE ... [more ▼]

The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA isuncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth’s interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/ accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates. [less ▲]

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See detailEffect of the satellite laser ranging network distribution on geocenter motion estimation
Collilieux, X.; Altamimi, Z.; Ray, J. et al

in Journal of Geophysical Research (2009), 114

SLR network translations estimated between a quasi-instantaneous station position set, theoretically expressed with respect to the center of mass of the Earth (CM), and a secular reference frame are the ... [more ▼]

SLR network translations estimated between a quasi-instantaneous station position set, theoretically expressed with respect to the center of mass of the Earth (CM), and a secular reference frame are the signature of the motion of the CM with respect to the Earth crust. Geocenter motion is defined here to be the motion of the CM with respect to the geometric center of the solid Earth surface (CF). SLR translational variations cannot be rigorously interpreted as identical to geocenter motion due to the sparse and nonuniform distribution of the SLR network. Their difference is called the network effect, which should be dominated at subdecadal timescales by loading signals.We have computed translation time series of the SLR network using two independent geophysically based loading models. One is a displacement model estimated from surface fluid data (Green’s function approach), called forward model, and the other is a displacement model estimated from GPS and ocean bottom pressure (OBP) data, called inverse model. The translation models have been subtracted from their respective geocenter motion models computed from degree-1 mass load coefficients in order to evaluate their network effect biases. Scatter due to the SLR network effect is at the level of 1.5 mm RMS. It could slightly shift the phase of the annual SLR geocenter motion estimate by less than 1 month and could affect X and Z annual geocenter motion amplitudes at the 1-mm level, which is about one third of the expected signal. Two distinct methods are suggested to account for network effect when comparing SLR translations to geocenter motion models. The first is to add the network effect term predicted by a displacement model to the geocenter motion loading model. The second relies on an adequate combination of SLR and GPS products to estimate SLR translation that could be better compared with geocenter motion. [less ▲]

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See detailAnomalous harmonics in the spectra of GPS position estimates
Ray, R.; Altamimi, Z.; Collilieux, X. et al

in GPS Solutions (2008), 12(1), 55-64

Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background of flicker noise plus white noise. Dong et al. (2002) estimated that ... [more ▼]

Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background of flicker noise plus white noise. Dong et al. (2002) estimated that less than half the observed GPS seasonal power can be explained by redistributions of geophysical fluid mass loads. Much of the residual variation is probably caused by unidentified GPS technique errors and analysis artifacts. Among possible mechanisms, Penna and Stewart (2003) have shown how unmodeled analysis errors at tidal frequencies (near 12- and 24-hour periods) can be aliased to longer periods very efficiently. Signals near fortnightly, semiannual, and annual periods are expected to be most seriously affected. We have examined spectra for the 167 sites of the International GNSS (Global Navigation Satellite Systems) Service (IGS) network having more than 200 weekly measurements during 1996.0–2006.0. The non-linear residuals of the weekly IGS solutions that were included in ITRF2005, the latest version of the International Terrestrial Reference Frame(ITRF), have been used. To improve the detection of common-mode signals, the normalized spectra of all sites have been stacked, then boxcar smoothed for each local north (N), east (E), and height (H) component. The stacked, smoothed spectra are very similar for all three components. Peaks are evident at harmonics of about 1 cycle per year (cpy) up to at least 6 cpy, but the peaks are not all at strictly 1.0 cpy intervals. Based on the 6th harmonic of the N spectrum, which is among the sharpest and largest, and assuming a linear overtone model, then a common fundamental of 1.040 ± 0.008 cpy can explain all peaks well, together with the expected annual and semiannual signals. A flicker noise power-law continuum describes the background spectrum down to periods of a few months, after which the residuals become whiter. Similar sub-seasonal tones are not apparent in the residuals of available satellite laser ranging (SLR) and very long baseline interferometry (VLBI) sites, which are both an order of magnitude less numerous and dominated by white noise. There is weak evidence for a few isolated peaks near 1 cpy harmonics in the spectra of geophysical loadings, but these are much noisier than for GPS positions. Alternative explanations related to the GPS technique are suggested by the close coincidence of the period of the 1.040 cpy frequency, about 351.2 days, to the ‘‘GPS year’’; i.e., the interval required for the constellation to repeat its inertial orientation with respect to the sun. This could indicate that the harmonics are a type of systematic error related to the satellite orbits. Mechanisms could involve orbit modeling defects or aliasing of site-dependent positioning biases modulated by the varying satellite geometry. [less ▲]

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