B. O. Ronnang

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.

Precision Geodesy Using the Mark-III Very-Long-Baseline Interferometer System

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between easternl-California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 ± 1 cm/year where the quoted standard deviation includes the estimated effects of systematic atic errors

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 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.

Space geodetic activities at the Onsala Space Observatory: 25 years in the service of plate tectonics

Abstract We review the development of the space geodetic techniques of Very-Long Baseline Interferometry (VLBI) and the Global Positioning System (GPS) at the Onsala Space Observatory. This facility has been participating already in the first transatlantic VLBI measurement. Throughout the period from 1980 to date Onsala has been involved in more than 1000 individual baseline determinations. If we include the first technical developent phase we can look back at 25 years of history during which the observatory has contributed to refining our understanding of plate tectonics. As a main theme of research Onsala Space Observatory has been striving for improved models for the propagation of radio waves through the atmosphere. Related perturbations imply critical limitations for the accuracy that can be achieved on a wide range of distance and time scales. We report on developments for the measurement of water vapour content in the atmosphere, in its importance to derive improved strategies and models for space geodetic observations and data analysis. While VLBI surveys for geodynamics are continuing, shifting their emphasis more to large scale control of reference systems, GPS provides a precise technique suitable to monitor crustal deformation on the regional scale. We relate recent developments, particularly the establishment of a continuous network of permanent sites in Sweden (SWEPOS) and experience from the first three years of operation.

The distance to the center of the galaxy

We have estimated the distance to the center of the galaxy from the measurements of the proper motions of 24 H2O maser spots in the source Sgr B2‐North. A comparison of the transverse angular velocities measured from VLBI observations and the line of sight Doppler velocities gives a distance estimate of 7.1±1.5 kpc.