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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 detailGPS Measurements of Crustal Uplift near Jakobshavn Isbrae due to Glacial Ice Mass Loss
Khan, S. A.; Liu, L.; Wahr, J. et al

in Journal of Geophysical Research (2010), 115

We analyze 2006–2009 data from four continuous Global Positioning System (GPS) receivers located between 5 and 150 km from the glacier Jakobshavn Isbræ, West Greenland. The GPS stations were established ... [more ▼]

We analyze 2006–2009 data from four continuous Global Positioning System (GPS) receivers located between 5 and 150 km from the glacier Jakobshavn Isbræ, West Greenland. The GPS stations were established on bedrock to determine the vertical crustal motion due to the unloading of ice from Jakobshavn Isbræ. All stations experienced uplift, but the uplift rate at Kangia North, only 5 km from the glacier front, was about 10 mm yr−1 larger than the rate at Ilulissat, located only "45 km further away. This suggests that most of the uplift is due to the unloading of the Earth’s surface as Jakobshavn thins and loses mass. Our estimate of Jakobshavn’s contribution to uplift rates at Kangia North and Ilulissat are 14.6 ± 1.7 mm yr−1 and 4.9 ± 1.1 mm yr−1, respectively. The observed rates are consistent with a glacier thinning model based on repeat altimeter surveys from NASA’s Airborne Topographic Mapper (ATM), which shows that Jakobshavn lost mass at an average rate of 22 ± 2 km3 yr−1 between 2006 and 2009. At Kangia North and Ilulissat, the predicted uplift rates computed using thinning estimates from the ATM laser altimetry are 12.1 ± 0.9 mm yr−1 and 3.2 ± 0.3 mm yr−1, respectively. The observed rates are slightly larger than the predicted rates. The fact that the GPS uplift rates are much larger closer to Jakobshavn than further away, and are consistent with rates inferred using the ATM!based glacier thinning model, shows that GPS measurements of crustal motion are a potentially useful method for assessing ice!mass change models. [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 detailGeodetic measurements of postglacial adjustments in Greenland
Khan, S. A.; Wahr, J.; Leuliette, E. et al

in Journal of Geophysical Research (2008)

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare ... [more ▼]

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare our results with predictions based on the ICE-5G deglaciation model of Peltier (2004). Results from the GPS receiver at Kellyville (-1.2 ± 1.1 mm/a) and from the tide gauge at Nuuk (-2.2 ± 1.3 mm/a), indicate that ICE-5G overestimates the subsidence rates at those locations by 2.1 and 1.1 mm/a, respectively. Kellyville and Nuuk are located along the southwestern margin of the Greenland ice sheet, and the observed negative uplift rates are consistent with independent evidence that the ice margin along the southwestern edge readvanced during the last ~8 ka to its current position. The ICE-5G glaciation-deglaciation history includes a readvance between the latitudes of 62°N and 72°N. The GPS measurements suggest the ICE-5G readvance may be too large or mistimed. Our GPS results at Qaqortoq, located at the southern tip of Greenland, suggest a secular subsidence rate of 􏰀0.3 ± 1.1 mm/a, while ICE-5G predicts an uplift rate of 1.0 mm/a. ICE-5G assumes no ice sheet readvance in south Greenland, including no readvance of the Qassimiut lobe. The difference of 1.3 ± 1.1 mm/a can tentatively be explained as due to a ~33 km readvance of the Qassimiut lobe during the last ~3 ka. For the other GPS sites, the observed/predicted uplift rates are 3.6 ± 1.1/-0.1 mm/a at Thule, 0.0 ± 1.1/2.0 mm/a at Scoresbysund, and -0.4 ± 1.1/-1.7 mm/a at Kulusuk. For Thule, Kulusuk, and Scoresbysund the differences between the observed and predicted rates are on the order of 1.3 – 3.7 mm/a, though with opposite signs, and indicate that ICE-5G does not exactly reproduce the correct rebound signal at those locations. [less ▲]

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See detailA comparison of annual vertical crustal displacements from GPS and GRACE
van Dam, Tonie UL; Wahr, J.; Lavallée, David

in Journal of Geophysical Research (2006), 111

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity ... [more ▼]

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity Recovery and Climate Experiment (GRACE) gravity fields for stations in Europe. An annual signal fit to the residual monthly heights, corrected for atmospheric pressure and barotropic ocean loading effects, should primarily represent surface displacements due to long-wavelength variations in water storage. A comparison of the annual height signal from GPS and GRACE over Europe indicates that at most sites, the annual signals do not agree in amplitude or phase. We find that unlike the annual signal predicted from GRACE, the annual signal in the GPS heights is not coherent over the region, displaying significant variability from site to site. Confidence in the GRACE data and the unlikely possibility of large-amplitude small- scale features in the load field not captured by the GRACE data leads us to conclude that some of the discrepancy between the GPS and GRACE observations is due to technique errors in the GPS data processing. This is evidenced by the fact that the disagreement between GPS and GRACE is largest at coastal sites, where mismodeling of the semidiurnal ocean tidal loading signal can result in spurious annual signals. [less ▲]

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See detailGeocenter motions from GPS: A unified observation model
Lavallée, David A.; van Dam, Tonie UL; Blewitt, Geoffrey 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 ▲]

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See detailHydrogeological investigations at the Membach station, Belgium and application to correct long periodic gravity variations
Van Camp, M.; Vanclooster, M.; Crommen, O. et al

in Journal of Geophysical Research (2006), 111

A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the ... [more ▼]

A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of Membach, Belgium. Applying a regional water storage model, the gravity contribution due to the elastic deformation of the Earth was derived. In addition, the Newtonian gravity effect induced by the local water mass variations was calculated, using soil moisture observations taken at the ground surface (about 48 m above the gravimeters). The computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms. The obtained results are compared with the observations of a superconducting gravimeter (SG). We find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local Newtonian effects. Shorter-period effects depend on the local changes in hydrology. This result shows the sensitivity of SG observations to very local water storage changes. [less ▲]

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See detailUncertainty of absolute gravity measurements
Van Camp, Michel; Williams, Simon D. P.; Francis, Olivier UL

in Journal of Geophysical Research (2005), 110(B05406), 1-9

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See detailEffects of atmospheric pressure loading and seven-parameter transformations on estimates of geocenter motion and station heights from space geodetic observations
Tregoning, P.; van Dam, Tonie UL

in Journal of Geophysical Research (2005), 110

Variations in fluid loads such as the oceans and the atmosphere deform the surface of the Earth. The accuracy of station coordinates, in particular, heights, that can be estimated depends on how well one ... [more ▼]

Variations in fluid loads such as the oceans and the atmosphere deform the surface of the Earth. The accuracy of station coordinates, in particular, heights, that can be estimated depends on how well one can separate these surface deformations from the associated translational motion between the center of mass of the solid Earth and the total Earth (CM). We applied simulated atmospheric pressure loading effects to the coordinates of sites in the CM frame to explore to what level of accuracy both geocenter motion and accurate station coordinates can actually be recovered from geodetic analyses. We found that standard seven-parameter transformations (three rotations, three translations, scale) generally recover about 80% of the geocenter motion; however, the inclusion of a scale factor permits the aliasing of surface loading deformation, introducing scale errors of up to 0.3 ppb and daily height errors as large as 4 mm. This limits the geophysical studies that can be performed accurately using the results of geodetic analyses where the magnitudes of the signals are small (e.g., tectonic movement of tide gauges, uplift rates for interpreting glacial isostatic adjustment). The quality of the geodetic results is extremely sensitive to the number and distribution of sites used to estimate the transformations and becomes worse when regional (rather than global) sets of sites are used. If the scale factor parameter is omitted, then the amount of aliasing of surface loading effects is reduced considerably and more accurate site velocities and geocenter motion estimates are achieved. [less ▲]

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See detailComment on "Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium" by A. Demoulin and A. Collignon
Camelbeeck, Thierry; Van Camp, Michel; Jongmans, Denis et al

in Journal of Geophysical Research (2002), 107(B11), 2281-2281

Comment on ‘‘Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium’’ by A. Demoulin and A. Collignon

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See detailGeodetic measurements in Greenland and their implications
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(B8), 16567-16581

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two ... [more ▼]

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two locations along the edge of the ice sheet: Kellyville, located about one third of the way up the western ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crustal uplift rates of -5.8±1.0 mm/yr at Kellyville and -2.1±1.5 mm/yr at Kulusuk. There have not yet been enough absolute gravity occupations to permit useful secular gravity solutions at either location. The negative uplift rate at Kellyville is consistent with independent archeological and historical evidence that the southwestern edge of the continent has been subsiding over the last 3000 years, but it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice during the early Holocene, which predict that Kellyville is likely to be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting -7.8±3.6 mm/yr discrepancy between the observed and predicted uplift rates is too large to be caused by loading from present-day changes in nearby ice. However, it is consistent with independent suggestions that the western ice sheet margin in this region may have advanced by ≈50 km during the past 3000-4000 years. If this advance did occur and if the crustal subsidence it induces is not removed from altimeter measurements of Greenland ice sheet elevations, then the altimeter solutions could underestimate the true snow/ice thickness change by 5-10 mm/yr along portions of the western margin of the ice sheet. [less ▲]

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See detailGPS measurements of vertical crustal motion in Greenland
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(D24), 33755-33759

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component ... [more ▼]

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component gives a negative secular uplift rate of −5.8±1.0 mm/yr. A negative rate (i.e., a subsidence) is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr. However, it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice prior to 4 ka years ago, which predict that Kellyville should be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy is too large to be the result of loading from present-day changes in nearby ice. We show, instead, that it is consistent with independent suggestions that the western ice sheet margin in this region of Greenland may have advanced by ≈50 km during the past 3–4 kyr. [less ▲]

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See detailAccuracy assessment of recent ocean tide models
Shum, C. K.; Woodworth, P. L.; Andersen, O. B. et al

in Journal of Geophysical Research (1997), 102(C11), 25173-25194

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See detailDetection of atmospheric pressure loading using the Global Positioning System
van Dam, Tonie UL; Blewitt, Geoffrey; Heflin, M.

in Journal of Geophysical Research (1994), 99(B12), 23939-23950

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in ... [more ▼]

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in atmospheric pressure account for up to 24% of the total variance in the GPS height estimates. The detected loading signals are larger at higher latitudes where pressure variations are greatest; the largest effect is observed at Fairbanks, Alaska (latitude 65°), with a signal RMS of 5 mm. Out of 19 continuously operating GPS sites (with a mean of 281 daily solutions per site), 18 show a positive correlation between the GPS vertical estimates and the modeled loading displacements. Accounting for loading reduces the variance of the vertical station positions on 12 of the 19 sites investigated. Removing the modeled pressure loading from GPS determinations of baseline length for baselines longer than 6000 km reduces the variance on 73 of the 117 baselines investigated. The slight increase in variance for some of the sites and baselines is consistent with expected statistical fluctuations. The results from most stations are consistent with ∼65% of the modeled pressure load being found in the GPS vertical position measurements. Removing an annual signal from both the measured heights and the modeled load time series leaves this value unchanged. The source of the remaining discrepancy between the modeled and observed loading signal may be the result of (1) anisotropic effects in the Earth's loading response, (2) errors in GPS estimates of tropospheric delay, (3) errors in the surface pressure data, or (4) annual signals in the time series of loading and station heights. In addition, we find that using site dependent coefficients, determined by fitting local pressure to the modeled radial displacements, reduces the variance of the measured station heights as well as or better than using the global convolution sum. [less ▲]

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See detailDetection of atmospheric pressure loading using Very Long Baseline Interferometry measurements
van Dam, Tonie UL; Herring, T. A.

in Journal of Geophysical Research (1994), 99(B3), 4505-4518

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates ... [more ▼]

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates of these displacements are determined by convolving National Meteorological Center (NMC) global values of atmospheric surface pressure with Farrell's elastic Green's functions. An analysis of the distances between radio telescopes determined by very long baseline interferometry (VLBI) between 1984 and 1992 reveals that in many of the cases studied there is a significant contribution to baseline length change due to atmospheric pressure loading. Our analysis covers intersite distances of between 1000 and 10,000 km and is restricted to those baselines measured more than 100 times. Accounting for the load effects (after first removing a best fit slope) reduces the weighted root-mean-square (WRMS) scatter of the baseline length residuals on 11 of the 22 baselines investigated. The slight degradation observed in the WRMS scatter on the remaining baselines is largely consistent with the expected statistical fluctuations when a small correction is applied to a data set having a much larger random noise. The results from all baselines are consistent with ∼60% of the computed pressure contribution being present in the VLBI length determinations. Site dependent coefficients determined by fitting local pressure to the theoretical radial displacement are found to reproduce the deformation caused by the regional pressure to within 25% for most inland sites. The coefficients are less reliable at near coastal and island stations. [less ▲]

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See detailSome Results of Heterogeneous Data Inversions for Oceanic Tides
Jourdin, F.; Francis, Olivier UL; Vincent, P. et al

in Journal of Geophysical Research (1991), 96(B12), 20267-20288

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See detailGlobal Charts of Ocean Tide Loading Effects
Francis, Olivier UL; Mazzega, P.

in Journal of Geophysical Research (1990), 95(C7), 11411-11424

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See detailDisplacements of the Earth's surface due to atmospheric loading: Effects on Gravity and Baseline Measurements
van Dam, Tonie UL; Wahr, John

in Journal of Geophysical Research (1987), 92

Atmospheric mass loads and deforms the Earth's crust. By performing a convolution sum between daily global barometric pressure data and mass loading Green's functions, we estimate the time dependent ... [more ▼]

Atmospheric mass loads and deforms the Earth's crust. By performing a convolution sum between daily global barometric pressure data and mass loading Green's functions, we estimate the time dependent effects of atmospheric loading, including those associated with short-term synoptic storms, on surface point positioning measurements and surface gravity observations. We calculate the response of both an oceanless Earth and an Earth with an inverted barometer ocean. Peak to peak vertical displacements are frequently 15-20 mm with accompanying gravity perturbations of 3-6 microgal. Baseline changes can be as large as 20 mm or more. The perturbations are largest at higher latitudes and during winter months. These amplitudes are consistent with the results f Rabbel and Zschau (1985) who modeled synoptic pressure disturbances as Gaussian functions of radius about a central point. Deformation can be adequately computes using real pressure data from points within about 1000 km of the station. Knowledge of local pressure alone, is not sufficient. Rabbel and Zschau's hypothesized corrections for these displacements, which use local pressure and regionally averaged pressure, prove accurate at points well inland but are, in general, inadequate within a few hundred kilometers of the coast. [less ▲]

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