Martin Lidberg

A GPS velocity field for Fennoscandia and a consistent comparison to glacial isostatic adjustment models

In Fennoscandia, the process of Glacial Isostatic Adjustment (GIA) drives ongoing crustal deformation. Crustal velocities from GPS observations have proved to be a useful tool in constraining GIA models. However, reference frame uncertainties, plate tectonics, intraplate deformations as well as other geophysical processes contaminate the results. Former studies have shown that different International Terrestrial Reference Frames have large discrepancies, especially in the vertical component, which hamper geophysical interpretation. We present new velocity estimates for the Fennoscandian and North European GPS network. Our GPS velocity field is directly realized in a GIA reference frame. Using this method (named the GIA frame approach) we are able to constrain GIA models with minimal influence of errors in the reference frame or biasing signals from plate tectonics. The drawbacks are more degrees of freedom that might mask real but unmodeled signals. Monte Carlo tests suggest that our approach is robust at the 97% level in terms of correctly separating different models of ice history but, depending on deformation patterns, the identified Earth model may be slightly biased in up to 39% of cases. We compare our results to different one- and three-dimensional GIA models employing different global ice-load histories. The GIA models generally provide good fit to the data but there are still significant discrepancies in some areas. We suggest that these differences are mainly related to inaccuracies in the ice models and/or lateral inhomogeneities in the Earth structure under Fennoscandia. Thus, GIA models still need to be improved, but the GIA frame approach provides a base for further improvements.

Combination of geodetic observations and models for glacial isostatic adjustment fields in Fennoscandia

We demonstrate a new technique for using geodetic data to update a priori predictions for Glacial Isostatic Adjustment (GIA) in the Fennoscandia region. Global Positioning System (GPS), tide gauge, and Gravity Recovery and Climate Experiment (GRACE) gravity rates are assimilated into our model. The technique allows us to investigate the individual contributions from these data sets to the output GIA model in a self‐consistent manner. Another benefit of the technique is that we are able to estimate uncertainties for the output model. These are reduced with each data set assimilated. Any uncertainties in the GPS reference frame are absorbed by reference frame adjustments that are estimated as part of the assimilation. Our updated model shows a spatial pattern and magnitude of peak uplift that is consistent with previous models, but our location of peak uplift is slightly to the east of many of these. We also simultaneously estimate a spatially averaged rate of local sea level rise. This regional rate (∼1.5 mm/yr) is consistent for all solutions, regardless of which data sets are assimilated or the magnitude of a priori GPS reference frame constraints. However, this is only the case if a uniform regional gravity rate, probably representing errors in, or unmodeled contributions to, the low‐degree harmonic terms from GRACE, is also estimated for the assimilated GRACE data. Our estimated sea level rate is consistent with estimates obtained using a more traditional approach of direct “correction” using collocated GPS and tide gauge sites.

BIFROST: Noise properties of GPS time series

The BIFROST project uses GPS to observe the intra-continental deformation of Fennoscandia caused predominantly by Glacial Isostatic Adjustment (GIA). The noise in GPS position time series has been proven correlated, so we investigate a fractal model in order to obtain a parameter that can gauge our network stations true velocity uncertainties and utilize an empirical orthogonal function (EOF) to remove the inherent common mode. We employ a Kaiser window to reduce the power spectrum variance and retain independent power estimates based on the window’s main lobe width ratio compared to that of a boxcar. As power spectra lack Gaussian distribution properties, we devise a transform that normalizes the power spectrum and subsequently iterate a fractional power law noise model.

EUREF’s Contribution to National, European and Global Geodetic Infrastructures

The EUREF key infrastructures are the EUREF Permanent GNSS Network (EPN) and the Unified European Levelling Network (UELN). The EPN runs almost 250 Global Navigation Satellite System (GNSS) stations in a well organized environment and serves as the backbone of the realization of and access to the European Terrestrial Reference System (ETRS89) and as contribution to the densification of the International Terrestrial Reference Frame (ITRF2008). The upcoming European navigation system Galileo will be a big challenge for the EPN in sense of upgrading the station network.

Implementation of the ETRS89 in Europe: Current Status and Challenges

The EUREF (Reference Frame Sub-Commission for Europe) Permanent GNSS Network (EPN) serves as the backbone for the realization of, and access to, the European Terrestrial Reference System (ETRS89). The cumulative site positions and velocities for the EPN stations are used for national ETRS89 densifications and geo-information applications. EUREF has developed specific guidelines through which European countries ask validation of their national ETRS89 densification campaigns. Today, the majority of the European countries has passed this process and a large part of European National Mapping and Cartographic Agencies have officially adopted ETRS89. In addition, ETRS89 plays a fundamental role in INSPIRE (Infrastructure for Spatial Information in the European Community).