Cause analysis of the non-linear variation of the IGS reference station coordinate time series inside China

GPS position time series exhibit dramatic seasonal variation. It is generally considered that atmospheric pressure loading, nontidal ocean loading, snow depth and soil moisture loading account for the main contribution of this seasonal change. Focusing on the 11 IGS reference stations inside China, this paper calculates the above four kinds of surface mass loadings induced station displacement to correct the coordinate time series of these 11 stations during period of 1995-2010 under ITRF2005. We establish preliminary numerical link between certain geophysical phenomenon, seasonal variation and noise characteristics, finding that the calculated four kinds of surface mass loading could produce variation in station's noise feature, mainly exhibiting characteristics of band pass noise and random walk noise, and could reduce the annual amplitude of station's Up component. However, it could not well interpret station's semi-annual and horizontal annual movement. We deeply investigate the possible factors that could cause non-linear variations of IGS stations, take emphasis on the contributions of diurnal (Si) and semi-diurnal (S2) atmospheric tides to the annual amplitude of the 11 stations, and propose that the S1-S2 atmospheric tides could be one of the main reasons that cause annual motion of IGS stations inside China, especially for the vertical annual motion of stations in central and southern regions.