Proposals for Two Satellite-Borne Experiments to Test Relativity of   Simultaneity in Special Relativity
  An orbiting `photon clock' is proposed to test directly the relativity of simultaneity effect of special relativity. This is done by exchanging microwave signals between two satellites in low Earth orbit carrying clocks that have previously been synchronised in the Earth-centered inertial system. A similar experiment using synchronised signals from two GPS satellites with the receiver on a single satellite in low Earth orbit, is also proposed. 
J.H.Field