Geoscience Reference
In-Depth Information
Earth
Fig. 9
Scheme of CHAMP: one on-board 3-axis accelerometer, tracked by GNSS satellites
Earth
Fig. 10
Scheme of GRACE: twin-satellites with one on-board 3-axis accelerometer and microwave
ranging system, both tracked by GNSS satellites
3.1.2 GRACE: Satellite-to-Satellite-Tracking in Low-Low Mode
Two LEO satellites equipped with three-dimensional accelerometers are placed in
the same orbit but separated by several hundred kilometers. Both satellite orbits
are monitored by GNSS and additionally the range rate between them is measured,
corresponding to differences in gravity acceleration (Fig.
10
).
The GRACE (Gravity Recovery and Climate Experiment, (Tapley et al.
2004
))
mission was launched in March 2002 under the NASA Earth System Science
Pathfinder (ESSP) Program, to accurately map variations in the Earth's gravity
field over its lifetime. The GRACE mission has two identical spacecrafts flying
about 220km apart in a polar orbit initially 500km above the Earth making accu-
rate measurements of the distance between the two satellites, using GPS and a
microwave ranging system. The ranging system is sensitive enough to detect separa-
tion changes as small as some
ยต
m over the distance of 220km between the satellites
(Wahr et al.
1998
).
