The aim of the project RELAGAL is to use the data obtained by the errant GALILEO navigation satellites GSAT0201 and GSAT0202 for a precision test of relativity theory. In August 2014 the two satellites GSAT0201 and GSAT0202 have accidentally been injected into an orbit of high eccentricity. The onboard atomic clocks thus are now subject to a daily modulation of the gravitational potential, with a height difference of approximately 8000 km between apogaeum and perigaeum. This provides us with an excellent opportunity to determine the modulated gravitational time dilation these clocks experience and to perform an accurate test of this fundamental prediction of general relativity.

Within this project our main focus lies on the improvement of the modelling of solar radiation pressure effects. Our ansatz includes

i) the detailed geometry of the satellite's surface,

ii) the physical properties of the surfaces (e.g. absorptivity, reflectivity), and

iii) the degradation of the considered materials.

By using this approach we will work on a discrimination of one of the major systematic effects from the actual gravitational redshift signal.

Acknowledgements

The contribution of ZARM is supported and funded by the German Space Agency DLR under grant number 50 WM 1547.