Aorde 4.3 The design Scientific mission objectives and mission concept have to be converted into a mission design (e.g. satellite orbit) and system design (e.g. payload, attitude and drag controf, satellite system, ground segment, launcher). Our contribution to find an 'optimal' design, i.e., a design that meets the scientific goals and is feasible from a technical and financial point of view, is mainly devoted to performance analysis by means of simulation and error analysis studies. The goal of such studies is to quantify for any given mission and system design and observational error characterstic, the expected accuracy of recovered potential coefficients and gravity field functional. Every time the design has been changed, a new simulation study has to be done. Error analysis is mostly based on a covariance propagation using a more or less adequate linear observation model connecting observations and gravity field parameters. This allows on a case-by- case basis, without simulating any observations, to study the effects on gravity field functionals like geoid heights and gravity anomalies of, e.g. satellite altitude and orbit, stochastic model, observation type. Let us give two examples of mission and system design aspects for which error propagation studies are being done. The first example illustrates the role of satellite altitude, one of the important mission design parameters. From a scientific point of view low altitudes are preferred in order to counteract the attenuation effect. On the other hand, at low altitudes aerodynamic forces and torques are also higher. This requires higher thrust levels to compensate for atmospheric drag, i.e., higher electric power making the mission much more expensive. For GOCE a mean orbit of 250 km has been chosen, mainly from spacecraft constraints. It is the altitude that can be maintained by ion propulsion with a power demand of the order of 500 W; altitudes below 200-250 km are not allowed because of the requirement for the spacecraft not to re-enter before 7 days in case of failure. The task is to investigate what the relation is between satellite altitude and scientific mission requirements. This relation depends on many parameters, among them the assumed measurement noise level and the type of observation. Figure 8 shows the result of an error propagation study. It indicates the expected geoid commission error as a function of the satellite altitude for (i) various observation types (i.e., full tensor, diagonal, cross-track component) and (ii) various measurement noise levels (white noise over the entire measurement frequency band). Diagonal altitude [km] altitude [km] j-p Cross-track 0.0001 altitude [km] Figure 8: Geoid commission error over 1 x 1 degree blocks as function of satellite altitude and grodiometer measurement noise 11

Digitale Tijdschriftenarchief Stichting De Hollandse Cirkel en Geo Informatie Nederland

Lustrumboek Snellius | 2000 | | pagina 24