Within General Relativity (GR) gravitational phenomena can be interpreted as geometrical effects of a non-Euclidean four-dimensional space-time due to the local equivalence of gravity and inertia. This view leads to a number of phenomena not being predicted by Newtonian physics, which, however, are very tiny in weak gravitational fields (like that of the Sun) and are thus difficult to detect. In this field of research satellite experiments are important since they may provide an almost undisturbed environment which allows to test some of the most fascinating predictions of GR. Besides GR, other metric theories predict similar though quantitatively different effects and hence satellite experiments may also help to verify competing theories about gravity.

One of the most fundamental predictions of GR concerns the existence of gravitomagnetic phenomena and has not been directly observed to date due to the enormous technical difficulties involved. The project GP-C (Gravity Probe C) aims to investigate whether one specific effect, the so-called gravitomagnetic clock effect which involves the time shift in the period between two counter orbiting satellites may be detectable with today's technologies.