Temporal Characteristics of GPS Coordinate Series Obtained From the Global IGS Tracking Network

We have investigated continuous coordinate time series of more than 300 globally distributed IGS GPS-sites. The time series cover a 4 years period ranging from GPS week 1021 until GPS week 1230 (August 1999 till August 2003). The input data, which stems from the weekly SINEX solutions of eight different IGS Analysis Centres (ACs), has been analysed for signal, noise and outliers. Software has been developed which allows for the detection and proper filtering of position outliers prior to the spectral analysis. Moreover, robust interpolation methods were implemented to deal with gaps in the time series. Afterwards a spectral analysis has been performed for both the horizontal and the vertical component. Although all IGS ACs take into account solid Earth tide effects when processing station coordinate series we still have detected remaining power at the tidal frequencies, predominately in the annual and semi-annual band. Vertical movements, in the order of 5 mm, may stem from atmospheric and oceanic loading or are due to environmental effects like snow or the ground water cycle. In a worst case scenario, they reflect unreported changes of the stations hardware. The poster presents results of the spectral analyses for a number of stations around the globe and gives reasonable estimates for the noise component in the time series. In addition, examples of "apparent" site-movements of a few cm that do not yet have a reasonable explanation are shown.