Fundamental physics measurements with laser-ranged satellites

The key role that laser-ranged satellites -- such as the two LAGEOS (LAser GEOdynamic Satellite) -- have had in the field of space geodesy, with a paramount of significant applications and results in geophysics, as well in the measurements of tiny relativistic effects in the weak-field and slow-motion (WFSM) limit of Einstein's theory of general relativity (GR), is well known. In this regard, these achievements have been gathered thanks to two very important ingredients: i) the quality of the tracking observations of the orbit of the satellites, guaranteed by the powerful Satellite Laser Ranging (SLR) technique, and ii) the quality of their overall dynamical model implemented in a software code. Moreover, today, GR has to be considered a fundamental pillar of space geodesy and of geophysics in general: the use of atomic clocks on-board a navigation satellite or on-ground, are just two important examples, among many. Indeed, terms like "relativistic metrology", "relativistic geodesy" and "relativistic celestial mechanics" are very frequent in the literature, in fact they are pertinent and represent bridges among fields wrongly considereted separated in the past. The SLR technique is one of the techniques that constitute the Global Geodetic Observing System (GGOS). In the near future it is expected, within the GGOS activities, an improvement of one order of magnitude in global accuracies in the observational as well as the theoretical components of space geodesy. This implies improvements in measuring accuracies, in reference frames realization, in modelling, in the stations network geometry and, consequently, into the accuracies involved in fundamental physics measurements using space geodesy techniques. However, in order to reach these ambitious objectives with a fruitful contribution from the existing satellites, as the two LAGEOS and the more recently launched LARES, several improvements are necessary in their precise orbit determination (POD). For instance, under this point of view, an hot topic is constituted by the knowledge of the so-called center-of-mass correction for these satellites, that directly impact the range determination of the satellites, i.e. the precision of their normal points, and correlates with the so-called range bias of the Earth-bound tracking stations. One more very important aspect that deeply affects the POD of the satellites is represented by a reliable modelling of the subtle thermal effects produced by the visible solar radiation and the infrared radiation emitted by the Earth's surface. These are the Sun Yarkovsky-Schach effect and the Earth-Yarkovsky effect. Their complexity arise from two main aspects: i) the knowledge of the temperature distribution on the satellite surface, and ii) the knowledge of the satellite attitude, i.e of its spin vector evolution. In this talk, the main activities of the LAser Ranged Satellites Experiment (LARASE) will be described. The LARASE research program is funded by the Italian National Institute for Nuclear Physics (INFN) and it is a collaboration between different institutions. The main goal of LARASE is to provide accurate measurements for the gravitational interaction in the WFSM limit of GR by means of the very precise laser tracking of geodetic satellites orbiting around the Earth. In particular, LARASE aims to improve the dynamical model of the current best laser-ranged satellites in order to perform a refined POD of their orbit. This represents a first step towards new refined tests and measurements of GR in the field of the Earth and of a most profitable use of the orbit analysis of the considered satellites for space geodesy and geophysics. The current results of LARASE in terms of development of new models for the non-gravitational perturbations acting on the two LAGEOS and LARES satellites, their POD and new measurements of relativistic effects on their orbit will be shown together with an accurate evaluation of the error budget due to the main systematic sources of error.