Simulation study of a follow-on gravity mission to GRACE

The Gravity Recovery and Climate Experiment (GRACE) has been providing monthly estimates of the Earth's time variable gravity field since its launch in March 2002. The GRACE gravity estimates are used to study temporal mass variations on global and regional scales, which are largely caused by a redistribution of water mass in the Earth system. The accuracy of the GRACE gravity fields are primarily limited by the satellite-to-satellite range-rate measurement noise, accelerometer errors, and temporal aliasing caused by un-modeled high-frequency variations in the gravity signal. Full numerical simulations are performed for a GRACE Follow-On mission (GFO) to determine if a future satellite gravity recovery mission with improved technologies will provide better estimates of time-variable gravity, thus benefiting many area of Earth systems research.Several GFO configurations are considered for this study. The first case considered is a two-satellite collinear pair similar to GRACE. The best-case two-satellite mission is equipped with an interferometric laser ranging system and a drag-free system in a lower altitude orbit. The laser ranging system improves the satellite-to-satellite range-rate measurement accuracy to &sim1 nm/s as compared to &sim1 micron/s for GRACE K-band microwave ranging, and the drag-free system more accurately removes the nonconservative forces acting on the satellites than the GRACE on-board accelerometers. Two "hybrid" missions are also considered. One hybrid is the GRACE design where the K-band ranging is replaced by the laser and the other is a drag-free, low altitude scenario with K-band ranging. A comparison of simulated gravity estimates is an important design tool in selecting the most important technologies to be considered for a future mission. A method for local time variable gravity recovery through mass concentration blocks (mascons) is used to form simulated gravity estimates for Greenland and the Amazon region for three GFO configurations and GRACE.Simulation results show that only modest improvement is realized for even the best-case two-satellite mission due to temporal aliasing errors. These errors are caused by high-frequency variations in the hydrology signal and imperfections in the atmospheric, oceanographic, and tidal models which are used to remove unwanted signal. The various sources of aliasing errors are investigated separately through a series of numerical simulations, showing that the leading cause of the errors is dependent on the considered region.This work concludes that applying the updated technologies alone, will not immediately advance the accuracy of the gravity estimates. If the scientific objectives of a GFO require more accurate gravity estimates, then future work should focus on improvements in the geophysical models, and ways in which the mission design or data processing could reduce the effects of temporal aliasing.