On non-linear motion of IGS station

With the daily SINEX files of IGS, the time series of IGS stations are obtained by independently developed software under generalized network adjustment model with coordinate pattern. From the time series, non-linear motion are found. With spectral analysis method, the variation frequency of site velocity are found. Moreover, the empirical model of the velocity variation of the station has been established by regressive analysis method based on week solution coordinate series of the station. With respect to the velocity of IGS tracking station, it had better to model the variation periodically or to give a velocity periodically using piece-wise linear function rather than using a linear variable to estimate its bias.