The principal force acting on a satellite is that of the Earth's gravitational field. Ground-based tracking systems provide highly accurate means of monitoring the perturbed motion of satellites. By modeling these measurements, the broad features of the gravity field are determined in the form of spherical harmonic coefficients. When tracking data are combined with satellite altimetry over the oceans, and with surface gravimetry measurements on land, the gravity field is sensed over an extensive spatial bandwidth. The combination of these measurement types has yielded comprehensive models of the Earth's gravity field [Nerem, 1995 this issue; Nerem et al., 1994a; Lerch et al., 1993a; Tapley et al., 1993; Tapley et al., 1994b]. Lerch et al., [1993b] has assessed SLR's contribution to the Goddard Earth Model (GEM) geopotential fields.
Of particular importance in the development of contemporary gravity models are the laser satellites. These satellites are passive targets constructed as solid, dense spheres. By design, their simple form reduces both the magnitude and complexity of their surface forces. Because separation and modeling of conservative and non-conservative forces acting on these satellites is more achievable than with complex satellite forms, they have provided the most important data for geopotential recovery. SLR observations acquired on Starlette, Lageos-1 and -2, Ajisai, and Stella account for the dramatic improvements seen in the long wavelength static and tidal geopotential fields. SLR is the most highly weighted data-type within all of the current standard gravitational models.
Nevertheless, while gravity modeling has significantly advanced, there is evidence on T/P that small errors remain. Part of this geopotential error produces geographically correlated effects [c.f. Rosborough and Tapley, 1987; Schrama, 1992]. The ``reduced-dynamic'' reduction method, which is less sensitive to dynamic force modeling errors, is being used with the continuous GPS tracking of T/P to assess these effects [ Wu et al., 1991; Yunck et al., 1994; Bertiger et al., 1994; Schutz et al., 1994]. Christensen et al., [1994] presented results which have isolated geographically correlated radial errors in the Joint Gravity Model-2 (JGM-2) based SLR/DORIS orbits at the 1-2 cm RMS level. These correlated errors have been reduced to below the centimeter level for the JGM-3 model, which directly incorporated GPS tracking on T/P [ Tapley et al., 1994b].