SAGE: AN ITALIAN PROJECT OF SATELLITE ACCELEROMETRY 
F. Sanso’ (1) , A. Albertella 0) , G. Bianco (2) , A. Della Torre (3) , M. Fermi (4) , 
V. lafolla (5) , A. Lenti (6) , F. Migliaccio 0 A. Milani 00 , A. Rossi (8) 
(1) DIIAR Politecnico di Milano -Italy (2) ASI - Centro di Geodesia Spaziale di Matera - Italy 
(3) Carlo Gavazzi Space S.p.A. - Milano Italy (4) Telespazio S.p.A. - Roma - Italy (5) IFSI/CNR- Roma - Italy 
(6) Laben S.p.A. - Vimodrone - Italy (7) Dip. Matematica - Università di Pisa - Italy (8) CNUCE/CNR - Pisa - Italy 
KEY WORDS: Gravity field determination, Satellite accelerometry. 
ABSTRACT: 
SAGE is a project for a satellite accelerometric mission aimed at determining with high precision the low-medium 
coefficients of the earth gravity field and their time variations. 
In the international panorama, SAGE would be a prosecution of CHAMP, both in time (in case of approval, SAGE launch 
date would be within the year 2001) and in space (SAGE would fly polar, filling in the polar gaps). 
SAGE has gone through a Phase A study commissioned by the Italian Space Agency (ASI), which lasted from 
December 1997 to October 1998. 
1. THE PROJECT OF THE MISSION SAGE 
AND ITS RELATION TO THE OTHER 
GRAVITY FIELD MISSIONS 
SAGE is a fully Italian project aiming at determining the 
gravity field of the Earth by exploiting the concept of 
high-low satellite-to-satellite tracking (SST). This 
means that the satellite orbit is determined by GPS, 
while the non-gravitational perturbations are determined 
by a three-axes accelerometer which, integrated back 
two times, gives the “virtual” orbit followed under 
gravitational effects only. 
The outline of the mission 
requirements are as follows: 
SAGE and the general 
Launch date 
Lifetime 
Altitude 
Inclination 
Nominal eccentricity 
Bus 
Payload 
Italian Spring Accelerometer (ISA) 
GPS receiver Lagrange 
Laser Retroreflector Array (by GFZ Potsdam) 
2001 
5 years (minimum) 
between 450 and 350 km 
87° (possibly polar) 
e = 0 
MITA (C. Gavazzi Space) 
Several Italian research institutions and industries 
participated to the SAGE Phase A study. Namely, the 
following research centers were involved: 
• ASI/CGS Matera, for telemetry and GPS analysis; 
• IFSI/CNR Roma, for the development of the ISA 
accelerometer; 
• Politecnico di Milano - IGeS, for the study of the 
spacewise reduction strategy of the data; 
• Università di Padova, for the GPS data analysis; 
• Università di Pisa - CNUCE, for the sudy of the 
timewise reduction strategy of the data and the 
orbital analysis; 
• Università di Trento, for complementary studies on 
the spacewise reduction startegy. 
The industrial partners were: 
• Carlo Gavazzi Space S.p.A., developing the study 
of the MITA bus and of the navigation and data 
communication segment; 
• Laben S.p.A., involved in the accelerometer 
engineering and in the development of the 
Lagrange GPS receiver; 
• Telespazio S.p.A., for the GPS data analysis, data 
downloading and telemetry. 
Right Solar 
Panel 
Braces 
Fig. 1 SAGE structure and internal accommodation, 
showing the position of the ISA accelerometer next to 
the centre of mass of the spacecraft. 
It is interesting to remark that SAGE would be logically 
and temporarily interwoven with CHAMP, as the two 
missions are based on the same scientific principle. 
Specifically, SAGE would aim at: 
• improving the CHAMP goals aiming at the useful 
determination of coefficients up to degree 70; 
• extending in time the CHAMP mission allowing for 
a high-low SST sampling of the gravity field at an 
altitude of 400 km with an overall time span of 7 to 
8 years, which could give more reliable information 
on time variations of the field; 
• cross-checking the two missions one with respect 
to the other, because they will fly together more or