Lustrumboek "The 5th Element"
The present concept of the GOCE gradiometer has 3 pairs of aligned
a cce I erom eters (figure 7). One pair is pointing along track, one pair perpendicular
to the orbit plane, and one pair pointing towards the Earth. This configuration is
able to recover the three diagonal terms of the gravity gradient tensor. It will also
provide the non-diagonal terms, but with degraded performance, since the proof
masses wil also move a little bit in the directions perpendicular to the sensitive
axis, but the springs in these directions are much stiffen Finallythe linear
acceleration due to surface forces such as atmospheric drag and solar radiation
pressure, and the rotation of the gradiometer frame, i.e., the angular velocity
and the angular accelerations are recovered. The accelerometers are of course
not simple springs but the springs are realized either by electrostatic suspension
or through magnetic levitation with superconducting coils. But even then, the
high accuracy level can only be maintained over a certain time period of less
than about 200 s, corresponding to a spatial resolution of some 800 km, due to
instrument drifts. That means wavelengths above 800 km can only be determined
with reduced accuracy. To build such a gradiometer is an extremely emanding
task, but an even bigger challenge is how such a sensitive instrument can be
isolated from the mechanical, electromagnetic, and thermal environment in the
spacecraft. Therefore, instrument calibration is one of the most delicate issues.
Figure 7: GOCE 3 axes gradiometer
In order to recover the gravity field we have to know where the observations have
been taken, i.e., we have to know the orbit of the GOCE satellite at any time
epoch. This is done by tracking simultaneously the satellites of the NAVSTAR-
GPS (cf. figure 6). The GPS tracking data, however, also have another purpose.
Any GPS satellite and the GOCE satellite can be treated as a pair of moving
proof masses in the total gravitational field. From tracking the position of the low
proof mass (GOCE satellite) with respect to the position of the high proof mass
(GPS satellite) and the known GPS satellite orbits, we may recover the Earth's
gravitational field according to the principle of gradiometry (cf. figure 6).
Compared to gravity gradiometry, the differencing effect is less pronounced since
the distance between the GPS proof mass and the GOCE proof mass is about
20000 km. Consequently, only the long wavelength features of the aeopotential
can be recovered. This is complementary to the characteristics of the gravity
gradiometer measurements, which are strong at the medium and short
wavelengths, but less strong at the long wavelengths and weak at the very long
wavelengths. Therefore, the GOCE mission will make use of both concepts in
order to resolve the entire spectrum up to a maximum resolution.
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