Global Surface Mass Variations From Multiple Geodetic Techniques - Comparison and Assessment

As a part of the global change process, horizontal water mass transport in the Earth's surface layer leaves several distinct geodetic signatures. These include translational motion between the Earth system's center-of-mass and the center-of-figure of the solid Earth surface, load-induced crustal deformation, time-variable gravity, and ocean bottom pressure (OBP) changes. By measuring or inferring these signatures, the ever-improving dedicated gravity/altimeter mission series and the global geodetic infrastructure of SLR/VLBI/GNSS/DORIS systems have provided an emerging global monitoring capability for the water transport phenomenon. While the different measurement systems can be combined to offer more complete spatio-temporal coverage and better resolution and accuracy, they also contain certain valuable redundancies that can be used to unravel weaknesses or systematic errors. We will present results of several different combination studies using SLR, re-processed GNSS, GRACE, and data-assimilated ocean bottom pressure models from 2002 to 2014. The combination of GNSS deformation/GRACE/OBP results in excellent agreements in geocenter motion and J2 with direct SLR tracking using some network enhancement from VLBI/GNSS/DORIS. During the entire GRACE period, no significant acceleration is detected in geocenter motion along any coordinate axis. GNSS deformation/OBP inversion is also compared in global spatial domain with GRACE indicating significantly improved quality of reprocessed GNSS data. Other combinations have revealed certain discrepancies and disagreements. The results are analyzed and assessed through isolation, perturbation and covariance studies for possible causes and future improvements.