The precise determination of in situ geopotential differences at satellite altitude was demonstrated by simultaneously adjusting the orbits of the coorbiting GRACE satellites with intersatellite low-low range rate measurements. The results agree well (correlation coefficients 0.5--0.8) with the monthly solutions of the spherical harmonic coefficients estimated using contemporary methods of precise orbit determination and parameter recovery. Higher correlation (>0.8 in general) was found for the in situ estimates over the continents than over the oceans. The analysis of 4 months of GRACE satellite-to-satellite tracking data from July 2003 to October 2003 indicated a significant migration of mass over the Amazon basin (hydrological effect), which agreed well with the predictions from the published monthly mean global gravity field solutions. Our results demonstrate the validity and utility of this alternative technique for global and especially regional modeling of the gravity field and its variation with temporal and spatial resolutions finer than monthly and 1000 km, respectively.