The fixed stations of a geodetic network are continuously changing, due to the change of its tectonic plate. When the instantaneous coordinates of the stations at any epoch are needed, the velocity of the stations should be known. The velocity of any station is determined by two techniques, firstly by using the time series of GPS observation and secondly by using the created velocity model of plate which obtained from the plate Cartesian rotation vector. In this research, using the available number of the permanent GPS stations made by the Egyptian National Research Institute of Astronomy and Geophysics (ENRIAG) to obtain the two velocity models of Nubian-plate ITRF2008 for year 2011 and 2013. Using the High Accuracy Reference Network (HARN) stations after transforming them from ITRF94 epoch 1996.0 into ITRF08 epoch 1996.0 to check the two local models by determining the velocities at all stations. The maximum difference in velocities between these models is found within ±0.2 cm. Therefore we took the mean of the two local models to create the best fit local velocity model. Additionally we used the HARN network and permanent GPS stations to assess the local velocity model with the velocity model of Nubian-plate ITRF08 which created by the IGS stations, the maximum difference between them is found within ±0.14cm. From the results, the velocities obtained at all permanent stations by processing the time series of GPS observations are changing yearly while the velocity model gives constant velocity for each station yearly. Nevertheless, the velocity vector length which obtained by the local velocity model is closest to the velocity vector length which obtained by GPS processing when increasing the time difference between the GPS sessions , this is appear in HARN station (0Z17) where the difference in velocity vector length is 0.5 mm at 17.14 years. Finally we created a computer program to compute the HARN stations at any epoch on ITRF08 by using the local velocity model and the transformation parameters between ITRF94 and ITRF08. |
