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Title
International cooperation and technology transfer
Author
Mussio, Luigi

382
a complete solution, however, has not yet been
computed with an accurate enough algorithm. The
influence of coloured noise could further degrade the
results by a factor of about 2 in accuracy. The
differences between the results with the two
approaches are not yet fully understood, and are
certainly worth a deeper theoretical and numerical
investigation; nevertheless, they are not large enough
to result in different evaluation of the scientific merit of
the mission.
The possibility to obtain a solution up to a comparatively
high degree with only six months of data implies the
possibility of detecting time variations. The geopotential
coefficients are not expected to change by 100 % of
their value, however, and the significant result is the
possibility to detect changes of 1 % with periods of one
year and longer, up to and even beyond degree 25.
This should be compared with the present knowledge of
secular trends for the coefficients of degree 2 and 3
only.
Fig. 4 The performance expected over the entire
mission, assuming 5 years at an average altitude of 400
km plus one year at an average altitude of 360 km. The
lower line is the extrapolated performance for such a
data set, the upper lines are the formal standard
deviation for 180 days of data at 400 and 360 km
respectively.
The essential goal is to obtain a gravitational field
complete to a given resolution, and therefore a
reference geoid, which is reliable, with the possibility of
an external and independent check of the absolute
accuracy of the results. The simultaneous
measurements from CHAMP and SAGE for at least two
years should allow a direct comparison of the results,
providing an objective assessment of the reality of the
solution. In this respect the spacewise approach, with
its transformation of the observations into
geographically distributed quantities, can provide an
interesting comparison tool, directly at the satellite
altitude.
4. CONCLUSIONS
The Phase A studies and simulations of the mission
showed that SAGE can meet its targets.
Though SAGE is not developing a new scientific
concept, its scientific targets are new: this mission
would extend the gravity sampling of CHAMP for three
more years and will improve the spatial resolution of
high low SST; in case SAGE would fly polar, the polar
caps will be surveyed and filled with data; moreover, the
accuracy of the measurements and the data analysis in
SAGE will allow for the determination of a much higher
degree than CHAMP.
All the technological requirements of SAGE are based
on existing instrumentation fully developed in Italy. In
particular, the measurement principle of the spring
accelerometer ISA is different from that of any other
instrument employed in other projects, which makes the
mission more valuable.
In case of approval, SAGE would open new
perspectives for the scientific policy of the Italian Space
Agency, while the development of a fully Italian
technology could be the first step for future, more
advanced targets: the Italian Spring Accelerometer,
once qualified for space missions, could become an
Italian Spring Gradiometer.
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