FUTURE DEDICATED SATELLITE GRAVITY MISSIONS

The field of global gravity field determination is enter-
ing a new era as satellite missions dedicated to measuring
the Earth’s gravity field are being developed and flown.
In the next few years, the Gravity Recovery and Climate
Experiment (GRACE) and the Global Ocean Circulation
Explorer (GOCE) will be launched. GRACE will use pre-
cise microwave measurements between two satellites fly-
ing at an altitude of approximately 450 km to precisely
map the Earth’s gravity field. In addition, GRACE will
be able to detect temporal variations of the Earth’s gravity
field, which after tidal variations are removed, are predom-
inantly due to water mass being redistributed on the sur-
face of the Earth (snow, ice, ground water, aquifers, etc.),
in the atmosphere (water vapor), and in the ocean. It is
expected that GRACE will be capable of making monthly
FIGURE 6 Magnitude of various quantities related to Earth global gravity anomalies at different wavelengths.
gr is
the gravity anomaly in mGals, ht is the relative variation of ocean surface height in meters, ε(Nt) is the accuracy of
the geoid height in meters, ε(R) is the accuracy of the radial component of a typical artificial satellite orbit in meters,
and ε(Pt) is the relative position accuracy typically obtained using geodetic techniques.
estimates of the gravity field with a spatial resolution of
∼300–500 km and an accuracy of 1 cm equivalent water
thickness (Dickey et al., 1997). GOCE will consist of a
single satellite carrying a gravity gradiometer, which will
directly measure the gravity gradient (spatial derivative of
gravity) in three axes. While GOCE will likely not have
enough sensitivity to detect temporal gravity variations
at long wavelengths, it will provide a much better deter-
mination of the static gravity field that can be provided
by GRACE alone. The primary objective for the GOCE
mission is to improve our knowledge of the geoid over the
oceans to allow detailed studies of ocean circulation using
satellite altimetry measurements. The expected errors in
our knowledge of the Earth’s gravity derived from each of
these future missions is shown in Fig. 5.