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. 10

Digitale Tijdschriftenarchief Stichting De Hollandse Cirkel en Geo Informatie Nederland

Lustrumboek Snellius | 2000 | | pagina 23