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The LAser RElativity Satellite (LARES) has been successfully launched on the 13th of February 2012 with the VEGA maiden flight. Its main objective is to test an aspect predicted in Einstein General Relativity known as dragging of inertial frames, the Earth rotation by dragging the spacetime will also drag the orbital plane of LARES by a tiny but measurable amount, this is also known as Lense-Thirring effect. The International Laser Ranging Service is providing the ground tracking of the satellite by sending laser pulses and by measuring the relevant return time of flight. Due to the presence of sensible periodical perturbations, it is necessary to analyze several years of data before an accurate test of the Lense-Thirring effect could be performed. Furthermore to eliminate some huge effects of classical gravitational perturbations on the orbital node of LARES, data have to be analyzed together with those of the LAGEOS and LAGEOS 2 satellites, provided also that an accurate determination of the gravitational field of Earth is inserted in the orbital determination programs used in the orbital estimation. In this paper the physics underlying the experiment will be recalled and the ground operations and all the experimental activities performed to acquire the orbital data of the satellite will be described.

The LARES mission was conceived to put an almost perfect test particle into an orbit around Earth that, when the known non-gravitational perturbations are removed, will approximate a geodesic of pacetime. Accurate orbit determination along with the accurate modeling of classical perturbation effects on the orbital dynamics of the satellite are key factors for the success of the mission. According to the theory of General Relativity a current of mass-energy, such as a rotating mass, induces an additional deformation to the spacetime. Thus the Earth, with its rotation, produces a very small perturbation on the node of the orbit. This phenomenon is caused by the gravitomagnetic field and in the case of an orbiting satellite is known as frame dragging or Lense-Thirring effect. To measure this effect with a reasonable accuracy, analysis of LAGEOS and LAGEOS 2 data was already performed back in 2004. For a very accurate test of the Lense-Thirring effect, a third specifically designed satellite was required. Many years after the proposal was submitted, in the year 2008 the Italian Space Agency supported the mission and the European Space Agency Launcher Programme Board approved LARES (LAser RElativity Satellite) as the primary payload to be accommodated for the VEGA Launcher qualification flight. Several university satellites were selected to be launched as secondary payload passengers. The launch, on the 13th of February 2012, was very successful for both the VEGA and LARES teams, the satellite being released with a very high accuracy into the nominal orbit. In this paper it will be shown that LARES, once the known non-gravitational perturbations are removed, behaves as the best test particle available in the solar system. So it turns out to be the ideal instrument for testing not only fundamental physics, but also for carrying out studies on geodesy and geodynamics. Accurate measurement of the Lense-Thirring effect requires several years of data acquisition because of the presence of some periodical perturbations, but some improvements are expected in the next years over the 2004 measurements obtained with only the two LAGEOS satellites.

LARES, Laser Relativity Satellite, is a spherical laser-ranged satellite, passive and covered with retroreflectors. It will be launched with ESA’s new launch vehicle VEGA (ESA-ELV-ASI-AVIO) in early 2012. Its orbital elements will be: inclination 70 ◦ ± 1 ◦ , semi-major axis 7830 km and near zero eccentricity. Its weight is about 387 kg and its radius 18.2 cm. It will be the single known most dense body orbiting Earth in the solar system, and the non-gravitational perturbations will be minimized by its very small ’cross-section-to-mass’ ratio. The main objective of the LARES satellite is a test of the frame-dragging effect, a consequence of the gravitomagnetic field predicted by Einstein’s theory of General Relativity. Together with the orbital data from LAGEOS and LAGEOS 2, it will allow a measurement of frame-dragging with an accuracy of a few percent.

Dragging of Inertial Frames and gravitomagnetism are predictions of Einstein’s theory of General Relativity. Here, after a brief introduction to these phenomena of Einstein’s gravitational theory, we describe the method we have used to measure the Earth’s gravitomagnetic field using the satellites LAGEOS (LAser GEOdynamics Satellite), LAGEOS 2 and the Earth’s gravity models obtained by the spacecraft GRACE. We then report the results of our analysis with LAGEOS and LAGEOS 2, and with a number of GRACE (Gravity Recovery and Climate Experiment) models, that have confirmed this prediction of Einstein General Relativity and measured the Earth’s gravitomagnetic field with an accuracy of approximately 10%. We finally discuss the error sources in our measurement of gravitomagnetism and, in particular, the error induced by the uncertainties in the GRACE Earth gravity models. Here we both analyze the errors due to the static and time-varying Earth gravity field, and in particular we discuss the accuracy of the GRACE-only gravity models used in our measurement. We also provide a detailed analysis of the errors due to atmospheric refraction mis-modelling and to the uncertainties in measuring the orbital inclination. In the appendix, we report the complete error analysis and the total error budget in the measurement of gravitomagnetism with the LAGEOS satellites.

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