The artificial satellites orbit determination is fundamental to Geodesy. For realization of this process it is needed, first, a precise modeling of the many forces and perturbations acting on the satellite. These forces include the gravitational and non-gravitational ones that considered in the resulting acceleration vector, must be integrated by a numerical process in the inertial reference system, once to obtain the velocity and twice for the satellite positions. In the cases of the gravitational force the perturbations are due to the gravitational attraction of the Earth, Moon and Sun, besides the oceanic and terrestrial tides, while in the non-gravitational ones the surface forces are included (drag of the air, solar radiation pressure and infrared radiation), besides others. In the Faculdade de Ciências e Tecnologia - FCT/UNESP, is available the GAS (GPS Analysis Software) software (Monico, 1995 and Stewart et al., 1994), that has the GPSORBIT module for orbit integration, besides the software GPSATORB for evaluation of the orbits quality. The GPSORBIT is in phase of updating, in which were already implemented the resolution IAU 2000 (International Astronomical Union 2000) that has relationship with the transformation between celestial and terrestrial reference frame, the gravity global Earth model WGS84-EGM 96 (World Geodetic System Earth Gravitational Model 96) and the solar radiation pressure model ROCK (Fliegel, 1995). In the current stage of this project, the aim is to improve the implemented mathematical models and to add the models with terrestrial and oceanic tides, based on the IERS Conventions definitions (International Earth Rotation Service and Reference System - Conventions) (McCarthy, 2003).
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