A Study on Reference Frame Variations Based on Long-term GPS Measurements

According to the tectonic theory, the Earth’s surface can be divided into several rigid plates which move with respect to each other with velocities from millimeters to centimeters per year. Consequently, the coordinates of ground control points defining a terrestrial reference frame will also vary with time. In the past, detecting the tiny surface movements using traditional surveying technique is not an easy task. In recent years, with the advance of space geodesy techniques and the application of the Global Positioning System ( GPS ), the dynamical behavior of the Earth’s surface can be continuously observed and precisely determined. As soon as the time-variant coordinates and velocities of ground control points can be estimated, the geometric relations between terrestrial reference frames at different epochs can be established using a time-variant reference frame transformation model. he purpose of this study is to develop a complete procedure for establishing a time-variant reference frame transformation model. First, the high quality information for the coordinates and velocities of ground control stations are obtained by an integrated GPS data processing and time-series improvement procedure. Then a rigorous parameter estimation approach ( including a general and a unified least-squares techniques ) is proposed for estimating the parameters of different time-variant transformation models. The estimated parameters are tested for their statistical significance and the actual geometric relations between time-variant reference frames can thus be identified. n the numerical analysis, the RINEX data from 2001 to 2007 of seven GPS tracking stations in Taiwan area has been processed and analyzed. The results show that the quality of the velocity estimations can be significantly improved by applying the proposed time-series improvement approach. Furthermore, due to the non-uniform deformations in this area, a time-variant similarity transformation model does not provide an acceptable result. On the other hand, a time-variant affine transformation model can produce a coordinate and velocity transformation of a higher quality due to its capability on modeling non-uniform deformations. Consequently, a time-variant affine transformation model can serve as an eligible candidate model especially in establishing a time-variant reference system in a deforming area.