A simulation study of effects of GRACE orbit decay on the gravity field recovery

The effects of satellite ground track changes of GRACE on monthly gravity field recoveries are investigated. In the case of a gravity field recovery using a relatively short period of a month or so, the variation of ground tracks affects the precision of the gravity field solutions. It is a serious problem when the solutions are employed for detecting temporal gravity changes which are almost at their detection limits. In this study, the recoveries of four-weekly gravity fields are simulated and the relation between the recovery precision and the ground track is investigated. The result shows that the GRACE ground track of the year 2003 was in good condition for four-week gravity field recovery, but it will sometimes appear as worse cases as the orbit altitude decays. In those cases, the global standard deviations of geoid height errors will be about one order worse than the best case. From our simulation, ground tracks of around altitudes of 473, 448, 399, 350 and 337 km give insufficient spatial resolutions, even for gravity field recovery up to degree 30.