Coordinate Reference Systems and Gravity

Abstract

Reference systems and gravity are topics relevant not only to Geodesy, but also to several other fields of research. The Theory of General Relativity is based on a geometric view of gravity, and therefore coordinates and gravity become inseparable. As the precision of the geodetic measurements with satellite techniques increases, the classical measurement modeling progressively gives way to a treatment in which concepts of Relativity take on an ever greater importance. In this paper, the differences between the classical and relativistic concepts of space-time in the presence of gravity, and the corresponding different role of the coordinates are first analyzed. Then, we move on to the application plan, facing the three major - for the moment - examples of the presence of General Relativity in the representation of geodetic measurements: secular and periodic modification of the time delivered by on-board atomic clocks in GNSS satellites; deviation of the trajectory of light rays in a gravitational field, and corresponding delay in arrival time; and finally dragging of the inertial systems due to the rotation of the central body (Lense Thirring effect).