Time Rate of Change of Gravity in North America and Greenland due to Post Glacial Rebound and Other Tectonic Movements

In the process to regain isostatic equilibrium following the Last Glacial Maximum, the Earths crust is experiencing continuous uplift and/or subsidence, a phenomenon called Glacial Isostatic Adjustment (GIA). We determine the time rate of change of gravity (g-dot) due to GIA by estimating it directly from a Least-Squares adjustment of an integrated gravity network covering the continent. Observation equations are created based on historical relative gravity measurements and the network is constrained using g-dot values obtained from absolute gravity measurements. Recognizing that gravity variation is also influenced by other significant continuous geophysical processes (tides, tidal load and hydrology), such effects are removed by correcting all gravity measurements at the pre-adjustment stage. Results are presented in the form of a g-dot map and demonstrate that Canadas National Gravity Data Base (NGDB), with its over 50-year-long history, can provide us with useful constraints for the evaluation/verification and refinement of post-glacial rebound models

Performance analysis of multi-GNSS static and RTK techniques in estimating height differences

Establishing reliable elevation differences is imperative for most geoscience and engineering applications. This work has traditionally been accomplished through spirit leveling techniques; however, surveyors have been utilizing satellite positioning systems in measuring height differences for more than a decade. Yet the quality of these heights needs to be evaluated in order to adopt them in different applications. In this article, we present the outcome of an accuracy assessment of height differences obtained with static and RTK surveys. Twenty control points with an average baseline length of 1 km were occupied with dual-frequency GNSS receivers for different time periods. Collected signals were processed using open-source software and verified with an online processing tool. Heights were estimated by processing the GPS and the GLONASS data individually, and combined (i.e. GNSS). Height differences were determined and compared with those measured by spirit levels and corrected through geoid models. Best results were achieved by combining GPS and GLONASS solutions for both static and RTK surveys. Solutions with either GPS or GLONASS satellites were comparable, but in most cases, the GPS solutions performed better. For the static surveys, longer occupation provided much accurate height differences. Inconsistencies among 10 different RTK surveys were minimum for the GPS + GLONASS solutions and worst for the GLONASS solutions. The ANOVA, LSD, F, and χ² statistical tests confirmed our findings at the 95% confidence level