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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AA.VV. (1998). SAGE Phase A Final Report (Albertella 
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ALBERTELLA A., MIGLIACCIO F„ SANSO F„ SONA 
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