Pure gravitational orbits of GNSS — The tool for the relativistic space geodesy

Precise orbit determination of the GNSS constellation is essential for its high quality performance. Two new technologies are under development offering considerable improvement of the accuracy of orbits: namely on-board accelerometers and inter-satellite links. Due to the fact that some non-gravitational forces cannot be modeled with the desired precision, the direct measurements on the board of the satellite of those forces may improve the solutions for orbits, positioning and timing. The range and range-rate measurements between satellites open the way to the elimination of errors caused by the ionosphere and station uncertainties. In effect, this will allow us to observe tens of objects (point masses) orbiting in the field of pure gravitational potential, eliminating other factors and effects. Thus, even very subtle distortions from the Newtonian motion could be detected and measured. It means that GNSS can be an excellent tool for relativistic mechanics consistent with the modern understanding of space and time. The efforts to create this tool have recently begun with works on the Galileo second generation system.