“…Various concepts and techniques being useful to develop the 1-order RPS have been found in previous papers, among them, we may point out the definition and uses of the world function (Synge, 1931;Bahder, 2001;Bini et al, 2008;San Miguel, 2007) and the time transfer function, the form of this last function in the S-ST (Teyssandier and Le Poncin-Lafitte, 2008), and a method to find the user position coordinates by using the time transfer function (Čadež and Kostić, 2005;Čadež et al, 2010;Delva et al, 2011). Here, this last method is modified by using the analytical formula derived by Coll et al (2010) -instead of numerical iterations-to work with photons moving in M-ST The Earth's center is at rest in the asymptotic M-ST; hence, the S-ST may be considered as a perturbation of the asymptotic M-ST with a static metric g αβ = η αβ +s αβ , where η αβ is the Minkowski metric, and s αβ are perturbation terms depending on GM ⊕ /R, where R is the Schwarzschild radial coordinate.…”