Jan M. Johansson

Continuous GPS measurements of postglacial adjustment in Fennoscandia 1.Geodetic results

[1] Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present-day, three-dimensional (3-D) crustal deformation rates in Fennoscandia [Johansson et al., 2002]. A preliminary forward analysis of these estimates [Milne et al., 2001] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3-D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the ( assumed constant) upper and lower mantle viscosity (nu(UM), nu(LM)); specifically, we derive a 95% confidence interval of 5 x 10(20) less than or equal to nu(UM) less than or equal to 10(21) Pa s and 5 x 10(21) less than or equal to nu(LM) less than or equal to 5 x 10(22) Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the ( Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.

Geodesy using the Global Positioning System: The effects of signal scattering on estimates of site position

Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1-m-high concrete pillar, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7±0.8 mm when the minimum elevation angle was increased from 10° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation-angle-dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar-antenna configurations identical or similar to the one used for this study at the Westford WFRD GPS site.

Three months of continuous monitoring of atmospheric water vapor with a network of Global Positioning System receivers

Three months of continuous data from the Global Positioning System (GPS) using 20 sites in Sweden and 5 sites in Finland have been used to estimate the integrated amount of atmospheric water vapor. The quality of the data has been assessed by comparisons with a microwave radiometer (water vapor radiometer (WVR)) at the Onsala Space Observatory and with data from four different radiosonde stations. We found the agreement in integrated water vapor (IWV) between the GPS estimates and the radiometer data to be 1–2 kg/m2 in terms of daily root-mean-square (rms) differences. A major part of these rms differences were caused by a bias between the data sets. This bias (WVR-GPS) varied from day to day between −1.0 and +2.5kg/m2 with a mean value of +1.3kg/m2. Comparisons with radiosonde data showed rms differences around or slightly above 2kg/m2 for each station using the entire 3 month data set. Also here the GPS estimates were, on the average, below the radiosonde results. We show that the radomes used to protect the GPS antennas are likely to cause a large part of the observed bias. Spatial structure functions were calculated by using the GPS and the radiosonde data. An overall consistency between the GPS-based and the radiosonde-based structure functions indicates that the spatial correlations between the GPS estimates are not affected by the estimation process used in the GPS data analysis.

Climate monitoring using GPS

Abstract We present results on long-term trends of integrated precipitable water vapor (IPWV) over the Scandinavian region based on data from the Swedish permanent Global Positioning System (GPS) network, obtained during the period August 1993 to the end of 2000. We assess the magnitude of the effects on the estimated IPWV caused by antenna radome changes by comparisons with other independent techniques, such as microwave radiometry and radiosondes. The agreement between the techniques is at 1 mm level for the IPWV content and at 0.1 mm/yr for the estimated linear trend. Using the IPWV differences between the techniques, we assess the effects of radome changes to be in the interval 0–1.8 mm depending on the type of radome used. The estimated trends of IPWV over Scandinavia show a general increase of 0.1–0.2 mm/yr, and are more pronounced in the south–west region. We also estimate trends based on summer and winter periods. We find them to be larger for the winter periods compared to the summer in the southern parts and the opposite in the northern regions of Scandinavia.

A Comparison of Precipitable Water Vapor Estimates by an NWP Simulation and GPS Observations

Abstract Simulated time series of the total precipitable water (PW) vapor from a limited area numerical weather prediction model are compared to estimates derived from observations done with ground-based Global Positioning System (GPS) receivers. The model data examined are from the delayed-mode High Resolution Limited Area Model (HIRLAM) data assimilation (reanalysis) and the short-range forecasts on double nested grids. The observational data are derived from GPS measurements at 25 sites in Sweden and Finland over a 4-month period, August–November 1995. In general, the HIRLAM reanalysis system demonstrates considerable skill in reproducing the spatial and temporal evolution of the PW as depicted by the GPS estimations. Using a 0.2° horizontal resolution and 31 vertical levels, the HIRLAM reanalysis generates a PW time series that has, in comparison to that of the GPS estimates, an average offset of −0.1 mm and a root-mean-square difference of 2.4 mm. The average correlation between the PW time series fr...

The systematic behavior of water vapor estimates using four years of GPS observations

The authors have used four years of Global Positioning System (GPS) data to study the amount of integrated water vapor (IWV) in the atmosphere. They find that the presence of certain radomes at some sites highly affects the water vapor estimates at other sites. These effects are eliminated by using a technique in which data from each station are processed individually.

Measuring regional atmospheric water vapor using the Swedish Permanent GPS Network

We investigate the application of a geodetic network of ground-based GPS receivers in Sweden to the measurement of atmospheric water vapor. Using data acquired during four days in December 1993, we show that it is possible to study the detailed motions of air mass systems. Estimates of water vapor from GPS data agree with those from radiosonde and microwave radiometer data to within 1 mm RMS.

Geodesy using the Swedish Permanent GPS Network∷ Effects of snow accumulation on estimates of site positions

We have observed variations at the several centimeter level in estimates of the vertical coordinate of site position. The estimates are obtained from our analysis of data acquired from the Swedish permanent Global Positioning System (GPS) network. The observed variations are strongly correlated with changes in the indirectly inferred accumulation of snow, which we assume collects on the radomes and pillars; the GPS sites could not be observed directly due to their remoteness. Numerical simulations which assume a simple geometry for the snow cover are used to study the effects of snow accumulation on GPS phase observables and hence on estimates of the vertical coordinate of site position obtained from these observables. Our results indicate that the variations in the vertical coordinate of site position can be fully explained by reasonable accumulations of snow which retard the GPS signals and enhance signal scattering effects.

Vertical crustal motion observed in the BIFROST project

Abstract 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.

Geodesy using the Swedish permanent GPS network: Effects of signal scattering on estimates of relative site positions

This paper presents results from a study of elevation-angle-dependent systematic effects on estimates of relative site positions within the Swedish permanent Global Positioning System (GPS) network. Two months of data from 16 sites have been analyzed with three different elevation cutoff angles, namely, 10°, 15°, and 20°. We present offsets between these solutions and demonstrate that estimates of the vertical component of several baselines strongly depend on the minimum elevation angle (elevation cutoff angle) of the data analyzed. Offsets of 22.3 ± 1.6 mm in the vertical component are evident when the elevation cutoff angle is changed from 10° to 20°. We investigate these offsets and conclude that a significant part is due to differential phase errors caused by scattering from structures associated with the mounting of the antenna to the pillar and with the pillar itself. The horizontal components of baseline are less affected. We found, however, that the offsets in the horizontal components increase with baseline length. For the longest baselines (∼1500 km) offsets of more than 5 mm are evident in the north component when the elevation cutoff angle is changed from 10° to 20°. These offsets are most likely due to differential phase errors caused by nonuniform antenna phase patterns ; an effect that presumably increases with baseline length and which also might increase because of scattering from the pillars and the antenna mounts. We identify the scattering structure and reduce associated errors in the vertical component of baseline to a significant degree on one of the sites by using microwave-absorbing material. The results presented are of importance for those analyzing data from existing networks and for those who intend to establish permanent GPS geodetic networks.