Lustrumboek "The 5th Element" model for various instrumental errors, e.g. various misalignment errors, scaling errors, and non-perfect drag and attitude control. The behaviour of the coupled system of proof masses is modelled by a system of differential equations ("forward step"). From the solution of the equations of motion and after adding read-out errors linear accelerations, Euler accelerations, and gravity gradients as measured are computed ("backward step"). The linear accelerations are due to non- gravitational forces mainly atmospheric drag and solar radiation pressure. They are fed into the drag free control system (DFC). Then, a pair of on thrusters corrects for these disturbances such that the orbit is reconstituted. From the measured Euler accelerations and the observations of a star camera any attitude motion of the satellite can be computed. This information is fed into the attitude control system (ACS), causing a set of cold-gas proportional thrusters to control this attitude motion. However, DFC and ACS cause control forces and moments, respectively, which in turn affect for instance the satellite orbit, the satellite dynamics, and the gradiometer signal. They are fed into the gradiometer forward model, which closes the loop ('closed-loop simulation'). ^non-gravitational Fcontrol modelling Figure 1 1GOCE end-to-end closed-loop simulator The result of such a closed-loop simulation is a time series of output gravity gradients. We compare them with the input gravity gradients, and from the error powerspectral densities (error PSD's) is computed. Figure 12 shows as an example th error PSD derived from a simulation with all known types of errors switched on. For comparison, the mission requirements of 5 mE/ÖHz and 1 /f behaviour below 27 cpr (5 x 1 0"3 Hz) are also shown in this figure. In a second step we propagate 14

Digitale Tijdschriftenarchief Stichting De Hollandse Cirkel en Geo Informatie Nederland

Lustrumboek Snellius | 2000 | | pagina 27