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International cooperation and technology transfer
Mussio, Luigi

toma - Italy
- Pisa - Italy
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less at the same altitude for at least two years,
allowing for a reciprocal (indirect) calibration of the
two instruments, which are based on different
The relation to the other two gravity missions, GRACE
and GOCE, is the same as that of CHAMP (namely,
SAGE works on medium-high wavelengths, increasing
the accuracy of the estimated coefficients), but
strengthened by the fact that CHAMP and SAGE
together can provide a highly reliable and dense data
set at an average altitude of 400 km.
In case SAGE should fly polar, it could also reach the
important aim of filling the “polar gaps”, which are
normally not avoided by similar missions.
The main instruments designed to constitute the
payload of SAGE are made after Italian technology.
The ISA accelerometer has been developed at the IFSI
Institute (Istituto di Fisica dello Spazio Interplanetary)
of the CNR (National Research Council) . This
accelerometer has been designed to be the
fundamental element of a space-borne, room
temperature, gravity gradiometer having a sensitivity of
10' 2 EU/VSz. In this case the accelerometers must
have a sensitivity of 5 10‘ 13 g/*JHz . Even if this target
requires further effort, the sensitivity of 10 ■ 9 gI-Jhz.
required for the SAGE experiment, has been reached at
the present state of the art. A prototype of an
accelerometer having sensitivity better than 10' 9
g/Vtfz has been built and is operating in the
underground laboratory of Gran Sasso (L’Aquila). The
fundamental part of this equipment is a mechanical
harmonic oscillator. Typical displacements of the
oscillating proofmass due to accelerations of 10' 9 g are
of the order of 2 10' 11 m. A capacitor transducer in a
bridge configuration, followed by a low noise amplifier,
provides the detection of the signal.
Fig. 2 Prototype of the Italian Spring Accelerometer
Fig. 3 Power spectral density of the seismic noise
measured at the Gran Sasso laboratory by the spring
accelerometer ISA, at the level of 10 ‘ 9 gijHz.
The Lagrange instrument is under development at
Laben S.p.A. . This equipment has been designed as a
GPS/GLONASS receiver to be used not only for
navigation purposes but also for scientific objectives like
Precise Orbit Determination (POD). The error of
Lagrange is less than 1 mm at a sampling rate of 10 Hz.
To describe the overall scientific performance of the
SAGE gravimetry mission we can use the maximum
degree £ for which the error is larger than the signal;
i.e., the RMS value of the error in the £2.£ + 1)
coefficients with the same degree (and different order
m) is larger than the RMS value of the coefficients
themselves. For the error we use the square root of the
formal variance of the determined coefficients, that is
the standard error according to the Gaussian theory.
This formal error, however, must also be compared with
the systematic errors, in particular with those resulting
from dynamic model errors and those resulting from
coloured noise. This check has been performed with
two different methods for the two different approaches
used to treat the SAGE simulated data (timewise and
spacewise) and it turns out that the systematic errors
are not dominant for the timewise approach, while they
can be significant for some noise models in the
spacewise approach.
The largest complete simulation we have performed
has used six months of data with the timewise
approach. The maximum degree £ max for which the
signal exceeds the error has been found to be 62. If this
result is extrapolated to a data span ten times longer,
by using the Gaussian statistics which is well known to
be applicable in this kind of simulations, then £ max can
be estimated to be about 72. Taking also into account
one year of extended mission, when SAGE should be
orbiting at a lower altitude (about 360 km), the value of
£ max could be as high as 74.
The largest simulation with the spacewise approach has
been performed with one year of data. The results of
the spacewise treatment are similar to the ones of the
timewise approach, if the noise is assumed to be white;