ALBERT

Description: Analysis of Global Positioning System (GPS) data from two sites separated by horizontal distance of only approximately 2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an approximately 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 vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the vertical coordinate of site position changed by 9.7 plus or minus 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, and demonstrated that the elevation angle cutoff test results and the pattern of the residual 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 source of the elevation-angle-dependent error were multipath reflections and scattering and that the horizontal surface of the pillar, located a distance of approximately 0.2 m beneath the antenna phase center, was the primary reflector. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and analyzed the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main reflector, (2) both the concrete and the metal plate embedded in the pillar are significant reflectors, and (3) the reflection can be reduced to a great degree 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 configuration identical or similar to the one used here (at the Westford WFRD GPS site).

Description: This project involves obtaining GPS measurements in Scandinavia and using the measurements to estimate the viscosity profile of the earth's mantle and to correct tide-gauge measurements for the rebound effect. We report on several aspects of this project. The DSGS was not scheduled to be reoccupied with DOSE receivers during the report period. The permanent network set up by Onsala Space Observatory continues to operate, and the data are being evaluated. An important technical advance we intend for this project is to use the full three dimensional site velocity information for inferring geophysical parameters. During the report period, two papers have been been accepted for publication in the Journal of Geophysical Research and will be published in April. Reprints of these papers are contained in the Appendix.

Description: This project involves obtaining Global Positioning System (GPS) measurements in Scandinavia, and using the measurements to estimate the viscosity profile of the Earth's mantle and to correct tide-gage measurements for the rebound effect. Several aspects of this project are reported.

Description: Work is continuing on the study of atmospheric gradients. We include a preprint entitled 'The effect of turbulence on atmospheric gradient parameters determined from ground-based radiometric and space geodetic measurements'. Work has begun on a study of solid Earth tidal deformations using the VLBI data set. We have examined deformations at the semi-diurnal tidal period using the IRIS data set.

Description: This project involves obtaining GPS measurements in Scandinavia, and using the measurements to estimate the viscosity profile of the Earth's mantle and to correct tide-gauge measurements for the rebound effect. Below, we report on several aspects of this project. The DSGS was occupied in August 1993. This campaign also inaugurated SWEPOS, the Swedish permanent GPS network. Initial results are presented in Johansson et al., a copy of which is contained in Appendix A. An important technical advance we intend for this project is to use the full three dimensional site velocity information for inferring geophysical parameters. To this end, we have investigated using VLBI determined baseline length rates in North America to constraining proposed combinations of ice history and earth rheology, and presented this work in Mitrovica et al., a copy of which is contained in Appendix B.

Description: A straightforward expression for the covariance of a general vector of atmospheric parameters is employed to calculate the effects of estimates of spatial gradients for a frozen Kolmogorov turbulence field moving over a site at constant velocities. Numerical calculations are performed to investigate the implications for three techniques: ground-based microwave radiometry, geodesy with the Global Positioning System, and VLBI. It is found that a prevailing wind with no true static gradients may give rise to apparent azimuthal asymmetries when gradient parameters are estimated. The standard deviation of the estimated north gradient parameter as a function of wind direction, for various wind speeds, is illustrated.