10. Comparison of Spacewise and Timewise Methods for GRACE Gravity Field Recovery

Historically, there have been two fundamental views, timewise and spacewise, to recover the Earth’s gravity field using satellite observations. This has resulted in different temporal gravity field solutions using the Gravity Recovery and Climate Experiment (GRACE) observations. In this chapter, we compare timewise batch processor algorithm for solving variational equations with spacewise energy balance approach using simulated GRACE observations. When using error free simulated observations, both approaches perform similarly well. Energy balance approach has the advantage of using less data storage and less computational time. With error contaminated observations, energy balance approach performs worse than variational equations. Because the noise in orbital velocity corrupts the potential difference observables, and respectively the estimate of the gravity field. Although, variational equations perform better, it is important that both positions and range rates are combined and they are properly weighted in solving normal equations.