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
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