Temporal variations of the Earth's gravity field are caused by a variety of complex phenomena including lunar-solar tides, atmospheric and oceanic mass redistribution, variations in groundwater storage and snow cover/ice thickness, earthquakes, post-glacial rebound in the Earth's mantle, long-term mantle convection and core activities, and other geophysical phenomena [ Chao, 1993]. It is important to understand these variations because of the implications they have for understanding and monitoring global climatic and geophysical changes, Earth rotation, and synoptic sea level changes. There have been a number of geophysical studies of the response of the Earth to loading and the secular changes this induces in the gravity field [ Ivins et al., 1993; Trupin et al., 1992; Trupin, 1993; Mitrovica et al., 1994a; 1994b] which would provide important constraints on mantle viscosity and sea level if reliable independent satellite estimates of the changes in the gravity field were available. Redistribution of the Earth's mass will also cause changes in the location of its center of mass, which have been measured using Lageos by Watkins and Eanes [1993].
Temporal variations in gravity caused by ocean and solid Earth tides
are relatively well determined because they occur at well known
astronomical frequencies, but non-tidal variations in gravity are
more difficult to detect. Recent progress has been made in both
measuring and modeling temporal gravity variations. Chao and
Au [1991] provided estimates of the temporal variations of the low
degree zonal coefficients from an analysis of surface atmospheric
pressure fields. For J
, the degree 2 zonal spherical
harmonic coefficient, these results were corroborated by the measurement
of similar variations using SLR tracking data from Lageos by Nerem
et al. [1993b], Tapley et al. [1993], and Chao and
Eanes [1995]. Schutz et al. [1993] have done related studies
with Starlette. These studies concluded that much of the observed
variation of J
is forced by mass redistribution in the
atmosphere, with substantial year-to-year variations. These studies
also showed that the J
variations measured using Lageos SLR data
did not compare favorably with the models of atmospheric
mass redistribution. This is partially caused by the contamination of
the Lageos J
results by other unmodeled effects (whether these
effects are gravitational or not is still unknown), but there is
also undoubtedly significant variations in J
and many other
individual harmonics that are caused by phenomena other than
the atmosphere. Undoubtedly, ocean mass redistribution is a
significant source of non-tidal temporal gravity variations [
Marshall and Pavlis, 1993].