5 Scientific issues
Lustrumboek "The 5th Element"
For instance, when measuring the three diagonal elements of the tensor at an
altitude of 250 km a measurement noise level of about 5 x 10 4 E is required in
order to keep the averaged geoid commission error over 1 x 1 degree blocks
below 2 cm. When increasing the altitude by only 40 km, the gradiometer
performance has to be improved by a factor of 5.
These results are based on the assumption that the measurement noise is white
over the entire measurement spectrum. The problem was already studied in the
context of a prototype mission called Aristoteles which is remarkably similar to
that of GOCE. The system studies indicated that it is more realistic that this
specification can only be met over a subband above 27 cycles per revolution
(cpr), which corresponds to a frequency of 5 x 10 Hz. Below 27 cpr the exact
noise characteristic is still unknown, but one currently expects that the noise
increases somehow proportional to 1/f, where f denotes the frequency. Below 4
cpr (8 x 1 04 Hz) the measurements are likely to contain no useful information at
all aue to accelerometer drift. It is therefore useful to look into the effect of any
band-limitation of the instrument. Figure 9 shows an extreme example of the
effect on the estimated potential coefficients using gravity gradient observations
only. If no information is available below 27 cpr, no improvement of the long
wavelengths and only little improvements of the short wavelengths would be
achieved compared to the current situation. If information is available above 4
cpr the situation becomes more favorable. Of course, this is still worse compared
with the most favorate situation of a white noise error spectrum over the entire
measurement spectrum. This also indicates the importance of the measurement
bandwidth ana the stochastic model of the observations for proper error
propagation studies.
Fiaure 9; Effect of band-limitation of the GOCE gradiometer on degree-order variances
of the gravitational potential
In the autumn of the year 2000, phase B of the GOCE mission will start. It is
likely that during this phase preparations for setting up a data center for scientific
data analysis will start (figure 10). Such a data center could be thought of as
consisting of five task units: the sensor unit, the data processor, the end product
unit, the quality assessment unit, and the simulator. The sensor unit will provide
the instrument readouts (raw data), calibration data and various corrections.
Moreover, it computes the satellite orbit, calibrated and corrected gravity gradients
and GPS observations, and provides information about linear accelerations,
angular velocity and angular accelerations including a stochastic model for the
various types of observations. The data processor unit, which forms the heart of
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