Geoscience Reference
In-Depth Information
The gravity field is derived by inverting (in the sense of inverse problems,
cf. the remark on inverse problems at the end of Sect. 1.13) the information
obtained from the satellite orbit
Satellite-to-satellite tracking in low-low mode
The principle is shown in Fig. 7.9. Two LEO satellites are placed in the same
orbit but separated by some hundreds of kilometers (about 220 km in the
case of GRACE). Ranges and range rates between the satellites are measured
to utmost accuracy. Individually, the orbit of each LEO satellite is affected
by perturbing accelerations which correspond to the first derivatives of the
gravitational potential. In combination, differences of accelerations result.
In addition, the position of the LEOs is determined by GPS satellites. This
means that inherently satellite-to-satellite tracking in high-low mode is also
implied. The effect of nongravitational forces on the satellite, e.g., due to air
drag, must either be compensated or measured by an accelerometer.
GPS satellites
LEO satellites
3D accelerometers
earth's surface
Fig. 7.9. Satellite-to-satellite tracking in low-low mode
Satellite gravity gradiometry
Compared to the just decribed low-low mode of satellite-to-satellite tracking
with a long baseline between the two LEOs, the baseline between the ac-
celerometer units tends to zero in case of satellite gravity gradiometry. This
is achieved by placing both units into a single satellite (Fig. 7.10). Therefore,
satellite gradiometry is the measurement of acceleration differences in three
spatial orthogonal directions between the test masses of the six accelerometer
units (two on each of the three axes) inside the satellite. In other words, the
measured signal is the difference in gravitational acceleration at the satellite,
