Measurement in the field of gravitation of the Galileo for science project

The Galileo for Science (G4S 2.0) project, funded by the Italian Space Agency (ASI), aims to perform a set of gravitational measurements with the two Galileo satellites GSAT-0201 (Doresa) and GSAT-0202 (Milena) exploiting the relatively high eccentricity of their orbits with respect to that of the other satellites of the Full Operational Capability (FOC) constellation. These two satellites have been already used in 2018 by both ZARM and SYRTE for a new measurement of the gravitational redshift (GRS) that has improved the 1976 measurement of Gravity Probe A by a factor between 4 and 6 respectively. In fact, from an accurate analysis of the orbits and clocks of these two Galileo satellites, a set of relativistic tests can be performed with the objectives of comparing the predictions of Einstein’s theory of General Relativity with those of other gravitational theories concerning, mainly, the motion of a test particle along a geodesic of space-time and the time dilation of the on-board clocks. Three Italian research institutes are involved in G4S 2.0: Center for Space Geodesy (ASI-CGS) in Matera, Istituto di Astrofisica e Planetologia Spaziali (IAPS-INAF) in Roma and Politecnico (POLITO) in Torino. We will present some of the ongoing activities at IAPS-INAF in the field of tests and measurements of gravitational interaction. Among these, the possibility of measuring the relativistic precessions of the orbits of the satellites, the constraints on a possible long-range force at a scale comparable to the semi-major axis of the satellite orbit and, consequently, on the validity of the Newtonian inverse square law and, finally, on the validity of the Local Position Invariance (i.e., a measurement of the GRS), that is one of the three ingredients that constitute, in its modern conception, Einstein’s Principle of Equivalence. A key aspect, to perform such measurements in the field of fundamental physics, is to improve the dynamic model for the non-conservative forces acting on the Galileo FOC satellites, starting from that of the solar radiation pressure, the largest non-gravitational perturbation on navigation satellites