After a call for proposals from the Italian Space Agency (ASI), for small
satellite missions a group of Italian research teams and industries, led by the
Politecnico di Milano conceived SAGE. This mission aims at determining the
gravity field of the Earth by means of high-low SST, in other words the satellite orbit
is detetrmined by GPS, while the non-gravitational perturbations are determined by
a three-axes accelerometer. This is basically the same concept of the CHAMP
mission(Reigber et al., 1996). SAGE underwent a Phase A Study during the year
1998(ASI,1998). In this framework, the task of the Politecnico di Milano group
was to analyze the data by means of the spacewise approach. Besides studying new
simulations of the data expected from SAGE, the complete spacewise approach
requires the inversion of Hill’s equations to be performed, to form average values on
a regular grid over the sphere and to recover the gravity field coefficients. The
simulations are required in order to: assess the accuracy of the data obtained after
the inversion of Hill’s equations, introducing a realistic measurement noise;
formulate the overdetermined boundary value problem to be solved; determine
indices enabling the evaluation of the performances of the solution.