A new test of gravitational redshift using Galileo satellites: The GREAT experiment

International audience; We present the result of the analysis of the GREAT (Galileo gravitational Redshift test with Eccentric sATellites) experiment. An elliptic orbit induces a periodic modulation of the fractional frequency difference between a ground clock and the satellite clock, partly due to the gravitational redshift, while the good stability of Galileo clocks allows one to test this periodic modulation to a high level of accuracy. GSAT0201 and GSAT0202, with their large eccentricity and on-board H-maser clocks, are perfect candidates to perform this test. Satellite laser ranging data allows us to partly decorrelate the orbit perturbations from the clock errors. By analyzing several years of Galileo tracking data, we have been able to improve the Gravity probe A test (1976) of the gravitational redshift by a factor of 5.6, providing, to our knowledge, the first reported improvement since more than 40 years.