The Velocity Model for South America and the Caribbean (VEMOS2009) was calculated from the SIRGAS95 and SIRGAS2000 station positions, the velocities of the SIRGAS-CON stations provided by the IGS-RNAAC-SIR, and several geodynamics projects performed in the region (Drewes and Heidbach 2009). The accuracy of the time-reduced station positions depends directly on the reliability of the velocity model; therefore, the permanent improvement of this model is also a main component of SIRGAS.
The GNSS data processing requires that the reference station positions are given in the same reference frame as the satellite orbits and at the same observation epoch. The weekly station positions of the SIRGAS Continuously Operating Stations (SIRGAS-CON network) satisfy these requirements and therefore, it is highly recommended to use SIRGAS-CON stations (in combination with their weekly coordinates) as reference points for precise GNSS positioning in the SIRGAS Region. If the reference points in a given GNSS surveying are not SIRGAS-CON stations, the following is needed:
1. Transformation of the station reference positions to the same reference frame, in which the satellite orbits are given. For instance, at present, the SIRGAS95 coordinates shall be transformed from the ITRF94 to the ITRF2005 (IGS05). In the same manner, the SIRGAS2000 coordinates have to be transformed from ITRF2000 to ITRF2005 (IGS05) (see transformation parameters between different ITRFs).
2. Reduction of the reference positions from the realization epoch to the observation epoch; i.e. the coordinates referred to SIRGAS95 shall be reduced from 1995.4 to that day, at the GNSS positioning was performed, for instance, 2010.0. Similarly, the SIRGAS2000 associated coordinates must be reduced from 2000.4 to 2010.0. This reduction is carried out through:
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being X(t), Y(t), Z(t) the station positions at the desired epoch, X(t0), Y(t0), Z(t0) the reference coordinates, (t - t0) the time interval between the reference epoch and the observation epoch, and Vx, Vy, Vz the velocities of the reference stations. 3. It is desirable that the velocities of reference stations are derived from iterative GNSS measurements covering a minimum time span of two years. If this is not possible, and the velocities of the reference stations are unknown, Vx, Vy, Vz can be taken from the VEMOS2009 model. This model corresponds to a 1° x 1° grid with horizontal velocities, which can be interpolated with the VMS2009 program. It is necessary to keep in mind that the mean precision of VEMOS2009 is about 1,5 mm/y. The corresponding bibliographical reference is:
4. Once the reference station positions are related to the observation epoch, the positions of the new GNSS points can be determined. 5. The coordinates of the new points shall be reduced to the reference epoch (see item 2). 6. Finally, the new coordinates must be transformed into the official reference frame, i.e. SIRGAS95 (ITRF94), SIRGAS2000 (ITRF2000), etc. Velocities applied for reducing the new station positions to the reference epoch shall be stored together with those positions. GNSS points including VEMOS2009 velocities should not be classified as reference stations. More details in: Procesamiento de información GPS con relación a marcos de referencia de épocas diferentes (H. Drewes) 
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