Enrico Kurtenbach

ITG-GRACE: Global Static and Temporal Gravity Field Models from GRACE Data

More than 4 years of GRACE data were used to determine the gravity field model ITG-Grace03 s. The solution consists of three parts: a static high resolution model up to a spherical harmonic degree of 180, temporal variations up to degree 40 and the full variance-covariance matrix for the static solution. The temporal gravity field variations are parameterized by continuous basis functions in the time domain. The physical model of the gravity field recovery technique is based on Newton’s equation of motion, formulated as a boundary value problem in the form of a Fredholm type integral equation. The principal characteristic of this method is the use of short arcs of the satellite’s orbit in order to avoid the accumulation of modeling errors and a rigorous consideration of correlations between the range observations in the subsequent adjustment procedure.

Regionally Refined Gravity Field Models from In-Situ Satellite Data

The satellite mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will enable the determination of the Earth’s gravity field with unprecedented accuracy, especially regarding the high-frequency part of the gravity field spectrum. To exploit the full potential of the mission, it is advantageous to develop methods to extract as much information out of the given signal as possible. In the approach presented here a global gravity field represented by a spherical harmonic expansion up to a moderate degree is derived in a first step and then refined by regionally adapted high resolution refinements being parameterized by splines as space localizing basis functions. These radial basis functions are designed to reflect the spectral characteristics of the gravity field to be modeled. Another important aspect in the regional gravity field analysis approach is the downward continuation process. In this context, a regionally adapted regularization will be introduced, which assigns different regularization matrices to geographical areas with varying signal content. Regularization parameters individually determined for each region take into account the varying frequency behavior, allowing to extract additional information out of a given data set. If desired, regional solutions with global coverage can be combined to a global solution using quadrature methods. The approach is demonstrated by a simulation scenario that combines a global GRACE solution as reference field with regional refinements calculated from GOCE observations.

Challenges in Deriving Trends from GRACE

The following contribution addresses some of the problems involved with the determination of long-term gravity field variations from GRACE satellite observations. First of all the choice of the time span plays a very important role, especially since it generally is a hard task to derive secular trends from only a few years of satellite data. Another issue, when one is interested in a single trend phenomenon, is the reduction of all other geophysical effects causing long-term gravity field variations. This paper uses the example of trends in continental hydrological water masses for the case of the High Plains aquifer to demonstrate some of the challenges implicated by trend analysis from GRACE.

Comparison of Daily GRACE Solutions to GPS Station Height Movements

In Kurtenbach (2011) and Kurtenbach et al. (2012) an approach has been introduced that allows to calculate daily gravity field solutions from GRACE data within the framework of a Kalman filter and smoother estimation. The method utilizes spatial and temporal correlations of the expected gravity field signal derived from geophysical models in addition to the daily observations, thus effectively constraining the spatial and temporal evolution of the GRACE solution. Here, we offer an extended validation of these daily solutions by comparing the derived mass variations to vertical displacements at various permanent GPS stations. The comparison confirms the conclusion that the daily solutions contain significant high-frequent temporal gravity field information, especially in higher latitudes.