In this project, we use the data obtained by the errant GALILEO navigation satellites GSAT-0201 and GSAT-0202 for a precision test of relativity theory. In August 2014 the two satellites GSAT-0201 and GSAT-0202 have accidentally been injected into an orbit of high eccentricity. Therefore, the onboard atomic clocks are now subject to a daily modulation of the gravitational potential, with a height difference of approximately 8700 km between apogaeum and perigaeum. This provides us with an excellent opportunity to determine the modulated gravitational redshift these clocks experience and to perform an accurate test of this fundamental prediction of general relativity.