Seite - 15 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

Gravity and Other Signals 3 The GRACE satellites orbit Earth under the influence of a complex interplay of conser- vative and non-conservative forces. The most important force, gravity, originates from many overlapping sources, only some of which are of interest when processing data from GRACE. The following sections give an overview of the acting forces and their origins. They are broadly categorized into conservative forces due to direct gravitational fields (section 3.1) and tidal effects (section 3.2), and non-conservative forces (section 3.3). 3.1 Earth’s Mean and Time-Variable Gravitational Potential Earth’s gravitational field is determined through the distribution of mass in Earth’s volume and immediate vicinity. At time scales for which global gravity observations are available most of Earth’s core and mantle mass can be considered static. Current space geodetic methods have neither the spatial nor temporal resolution to resolve the signal that results from its motion. A notable exception is the effect of glacial isostatic adjustment described below. Along the same lines, horizontal crustal motion along Earth’s surface is similarly too small to be resolved. Together with the mean state of the remaining non-static masses on Earth, all features with minimum sampling periods longer than the time span covered by the available data are lumped together as Earth’s mean gravitational field. Complementary to Earth’s mean gravitational field there exists Earth’s time-variable gravitational field, which encompasses all signals that can be resolved with the current observational record. The time-variable field is described by the superposition of signals at increasingly shorter time spans. At the longer end of this spectrum is the trend, which describes constant-rate change in the gravitational field. Superimposed on the trend are the annual and semi-annual cycle, covering major portions of the inter-annual variability of the gravitational field. All signals at shorter wave lengths make up the gravity variations. These are the target signals for GRACE monthly and sub-monthly solutions. Signals at wavelengths shorter than the GRACE minimum sampling period alias into the monthly and sub-monthly solutions. These signals should be reduced by other observations or models as much as possible. The following paragraphs give an overview of the constituents of the time-variable gravitational field. A more detailed description is given by e.g. Dobslaw et al. (2014). 15