SCHWARZSCHILD SPACETIME - MEASUREMENTS IN ORBITING SPACE-STATIONS

We investigate the mechanical measurements which can be performed within a frame moving on spatially circular orbits in the Schwarzschild spacetime. The aim of this study is to provide an operational unambiguous definition of the inward direction to the centre of symmetry. To this purpose one needs two independent sets of measurements: the thrust necessary to keep the orbit circular and the magnitude of the proper frequency of the orbital revolution. The knowledge of the inward direction to the centre of symmetry allows one to determine the sense of the orbital revolution relative to a local clockwise direction. Once the latter is known, then one can determine the frequency of precession of the local compass of inertia relative to the orbital frequency.