Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 66

Fig. 2.22 Schematic
diagram of SST-hl
tracking mode
determine the medium- and long-wavelength static part and the temporal variations
of the gravity field.
GRACE (Gravity Recovery and Climate Experiment) is a joint project of the
USA and Germany. It employs the combination of two tracking modes: SST-hl and
SST-ll, as shown in Fig. 2.23 . The GRACE satellite was successfully launched into
orbit in March, 2002 on a 5-year mission. One of the scientific missions of GRACE
was to determine precisely the medium- and long-wavelength static part of the
gravity field and to analyze and determine the variations in the Earth's gravity field
every 2-4 weeks (see Klees et al. 2008).
Determining the Gravitational Acceleration Differences in the Earth's
Gravity Field by Satellite Gravity Gradiometry
Satellite gravity gradiometry (SGG) allows determination of the differences in
gravitational acceleration in three mutually orthogonal directions by using the
differential accelerometer on one or more fixed baselines (about 70 cm) inside
the satellite. The signals observed indicate gradients of the gravitational accelera-
tion component, i.e., the second derivative of the gravitational potential.
Non-gravitational accelerations (e.g., air resistance), in the same way, exert some
effects on all the accelerometers inside the satellite, and the difference can be
removed perfectly when differentiating. One of the missions of SGG is to detect
the Earth's gravity field and its variations with higher temporal and spatial
resolution.
The European Space Agency (ESA) launched a third gravity satellite in March,
2009, the GOCE (Gravity and Ocean Circular Exploration) with SGG mode
(Fig. 2.24 ) (see Bouman et al. 2004).