Relativistic GPS in 3-dimensions
We extend to three dimensions the proposal of a completely relativistic positioning system (rPS). The system does not rely on approximations, in fact, it works at a few Schwarzschild radii from a black hole, and it does not rely on Newtonian physics or special relativity. Since general relativity (GR) claims to be our fundamental framework to describe classical physics, it must provide tools to bootstrap physics within the theory itself, without relying on previous approximated frameworks. The rPS is able to self-diagnose, that is, it detects deviations from assumptions about the gravitational field and consequently stops operations; in addition it is robust, i.e., it is able to autonomously restore operations when assumptions are restored. From a more general viewpoint, the rPS is equivalent to geodesy in spacetime, which establishes a (conventional) coordinate system on a surface by means of measurements within the surface itself, as well as allowing it to extract information about the intrinsic geometry of the same surface. In other words, the positioning system is potentially able to extract information about the gravitational field (which in fact is identified with the geometry of spacetime) in addition to the gravitational theory, which describes its dynamics. Thus, it becomes a framework within which one can operationally distinguish different theories of gravitation.