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Testing General Relativity with Galileo satellites in an eccentric orbit

Kolloquium der Abteilung 4

The European GNSS satellites Galileo 5 and 6 launched in August 2014 have not reached their targeted circular orbit and instead have been injected into an eccentric orbit due to a launch mishap. After several correction maneuvers their orbits now still possess an eccentricity of approximately e = 0.16. This accident provides a unique and fortunate opportunity for science, since it allows to perform a test of the gravitational redshift as predicted by Einstein's theory of General Relativity. Both satellites are equipped with passive hydrogen masers which on the eccentric orbit show a periodic relativistic frequency shift of up to δf/f = 10‑10.

In this talk I will present the results from an analysis of three years of clock and orbit data from these satellites, which have been specifically processed for the purpose of a precision test by the ESOC navigation support office. From our analysis we determine an upper limit on the test parameter quantifying a possible deviation from the redshift as predicted by General Relativity. I will show how we deduce a limit that is approximately 4-fold more stringent than the result from the most sensitive test so far done by the Gravity Probe A mission in 1976. In particular, I will discuss the main systematic error sources we have identified, among them systematic errors in the applied orbit solution due to insufficient modeling of solar radiation pressure. Finally, I will also briefly touch on further perspectives for tests of relativity theory that may be done with satellite-based clocks in the future.