Figure 3. In the SYPOR, the Space and Control segments coin- cide with the constellation. dinates should be related to some terrestrial secondary 4- dimensional Newtonian coordinate system. This problem has been solved for a general configuration of the emitters in flat Minkowski spacetime [17] and also for the case of a special configuration of the emitters in a Schwarzschild spacetime [18]. However, in general the known results for the two dimen- sional case are not trivially generalizable for the realistic four dimensional one [16] and much work remains to be done in the future. One of the authors (JFPS) is grateful to Bartolom´e Coll by communications on this subject along several years and to Joan Josep Ferrando and Juan Antonio Morales by some friendly discussions. The authors are currently par- tially supported by the Spanish Ministerio de Educaci´on y Ciencia MEC-FEDER ESP2006-01263 grant. ACKNOWLEDGMENTS REFERENCES [1] Ashby N., 2003, Liv. Rev. Relat. 6, 1 [2] Pascual-S´anchez, J.-F., 2007a, Ann. der Phys. (Leipzig) 16, 258; arXiv:gr-qc/0507121 [3] Pascual-S´anchez, J.-F., 2007b, in: First Colloquium on Scientific and Fundamental Aspects of the Galileo Program ; arXiv:0710.1282 [4] Coll B., 2001, in: Reference Frames and Gravi- tomagnetism , p. 53, eds J.-F. Pascual-S´anchez et al., World Scientific, Singapore [5] Coll B., Morales J. A., 1992, Intern. Jour. Theor. Phys. 31, 1045 [6] Coll B. , 2002, in: Proc. Jour. Sys. Ref. Spat. Temp. , Brussels, 2001, p. 169, ed N. Capitaine, Observatoire de Paris [7] Coll B., et al., 2009, Found. Phys. 39, 1280; arXiv:0707.0248 [8] Trautman A., 1963, C. R. Acad. Sc Paris 257, 617 [9] Rovelli C., 2002, Phys. Rev. D 65, 044017 [10] Blagojevic M., et al., 2002, Phys. Rev. D 65, 044018 [11] Coll B., et al., 2006, Phys. Rev. D 73, 084017 [12] Coll B., et al., 2006, Phys. Rev. D 74, 104003 [13] Graven P., et al., 2008, in: Proc. 31st Ann. AAS Guid. and Control Conf. , AAS 08-054 [14] Tarantola A., et al., 2009; arXiv:0905.3798 [15] Lachi`eze-Rey M., 2006, CQG 23, 3531 [16] Coll B., Pozo J. M., 2006, CQG 23, 7395 [17] Coll B., et al., 2009, in: Physics and Mathematics of Gravitation , p. 225, eds K. E. Kunze et al., AIP Conf. Proc. No. 1122 [18] Bini D. , et al., 2008, CQG 25, 205011