GOCE Gravity Gradient Performance: Evolution during Mission Lifetime
Siemes, Christian1; Fehringer, Michael1; Floberghagen, Rune2; Frommknecht, Bjorn2; Haagmans, Roger1

This presentation is concerned with the evolution of the performance of gravity gradients provided by the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission during mission lifetime. The performance is analyzed in a concise way in the frequency, time and spatial domain in order to highlight peculiarities that are of interest to users of GOCE gravity gradients.
GOCE was launched on 17 March 2009 when the solar cycle was close to a minimum. Since then, the solar cycle advances towards an expected maximum in early 2013, while accordingly the drag environment becomes harsher. In addition, beginning in August 2012, GOCE's orbit has been lowered several times from an altitude of 256 km to, at the time of writing, 235 km, which leads the satellite into an even harsher drag environment. Since the gravity gradiometer requires a "quiet" environment for providing ultimate performance, the GOCE satellite is equipped with a drag-free control system that compensates drag in flight direction. Drag perpendicular to the flight direction, however, is not compensated.
Therefore, we oppose the evolution of gravity gradient performance to the evolution of drag measured by the accelerometers, in particular for the across-track direction where drag attacks the largest cross-section of the satellite. Through combining frequency and spatial domain analyses, we show that the performance evolution in regions around the geomagnetic poles is different from the one outside these regions. Furthermore, special attention is paid to gravity gradients acquired before, after and also during orbit lowering maneuvers. We show that, due to lessons learnt from the orbit lowering in August 2012, the performance of gravity gradients acquired during subsequent orbit lowering maneuvers is remarkably good considering that the satellite experiences increased drag.