Time-Variable Gravity Seen by Satellite Missions: On its Sampling and its Parametrization

Summary

Temporal variations of the gravity field may act either as a signal or as a source of noise for the current satellite gravity missions. This depends, to some extent, on the parametrization of the gravity field solution. We discuss qualitatively how temporal variations affect satellite gravity products and how their effects may be controlled by an adequate parametrization. We describe a mechanism how unparametrized temporal variations may alias into orbit-parallel spatial patterns of a gravity field solution. While the effect is too small to corrupt static gravity field models like EIGEN-2 or EIGEN-3p it may be a concern for studies on time-variable gravity from consecutive GRACE period solutions. Moreover, time-varying errors in non-gravity parameters such as CHAMP accelerometer corrections may, due to correlations with gravity parameters, cause similar effects as geophysical variations. These issues suggest that an adequate parametrization of the gravity field as a function of space and time needs further study. Eigenvalue analyses of solution normal matrices may be a useful tool for these studies.