Relativistic positioning: errors due to uncertainties in the satellite   world lines
  Global navigation satellite systems use appropriate satellite constellations to get the coordinates of an user -close to Earth- in an almost inertial reference system. We have simulated both GPS and GALILEO constellations. Uncertainties in the satellite world lines lead to dominant positioning errors. In this paper, a detailed analysis of these errors is developed inside a great region surrounding Earth. This analysis is performed in the framework of the so-called relativistic positioning systems. Our study is based on the Jacobian, J, of the transformation giving the emission coordinates in terms of the inertial ones. Around points of vanishing J, positioning errors are too large. We show that, for any 4-tuple of satellites, the points with J=0 are located at distances, D, from the Earth centre greater than about 2R/3, where R is the radius of the satellite orbits which are assumed to be circumferences. Our results strongly suggest that, for D-distances greater than 2R/3 and smaller than 100000 km, a rather good positioning may be achieved by using appropriate satellite 4-tuples without J=0 points located in the user vicinity. The way to find these 4-tuples is discussed for arbitrary users with D<100000 km and, then, preliminary considerations about satellite navigation at D<100000 km are presented. Future work on the subject of space navigation -based on appropriate simulations- is in progress. 
Neus Puchades and Diego S\'aez
gr-qc