Scale variation of GPS time series Markku Poutanen, Jorma Jokela, Matti Ollikainen, Hannu Koivula, Mirjam Bilker, Heikki Virtanen Finnish Geodetic Institute Geodeetinrinne 2, FIN-02430 Masala Finland Markku.Poutanen@fgi.fi Abstract. We give an overview of time series analyses of permanent GPS stations using solutions of the IGS and FinnRef ® networks. Lomb periodograms show in most cases a statistically significant annual period both in station coordinates and inter-station distances. In regional networks the scale of the whole network changes periodically, and in some cases there is also a secular trend. There are several possible causes of scale variations, which may not be separable in the data. These include computational artefacts, periodic systematic errors in satellite orbits, signal path delay variations, and geophysical causes like loading and postglacial rebound. We discuss possible reasons, their significance, and their consequences on high-precision GPS observations. Additional constraints, e.g. time series from the superconducting gravimeter, are also discussed. Keywords. GPS time series, periodic variations ________________________________________ 1 Introduction The Finnish permanent GPS network FinnRef ® consists of 13 GPS stations. The network is the backbone of the Finnish realisation of the EUREF frame, referred to as EUREF-FIN. Four stations in the FinnRef network belong to the EUREF permanent GPS-network (EPN), and one station belongs to the network of the International GPS Service (IGS). Through these stations FinnRef ® creates a seamless connection to the global reference frames. The observations obtained by FinnRef ® stations can be used for studying the geodynamics of the Earth’s crust (Fig. 1). All FinnRef ® stations are also used in the computation of the joint Nordic GPS network in the BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea Level and Tectonics) project (Milne et al., 2001, Scherneck et al., 2002). Currently we have a seven year GPS time series. Quite soon after the beginning it became obvious that there were both seasonal and short term variations. Because one of the tasks was to measure the land uplift rate, we had to take the variation into account. In the worst case it could lead to poor results or even misinterpretation, as demonstrated in Fig. 2. The variation is visible both in the vertical component and in inter-station distances. In distances the amplitude of the variation depends on the vector length, thus leading us to consider the phenomenon as a possible scale error. Fig. 1. Finnish permanent GPS network, FinnRef ® . Sub- sets of points belong to the IGS and EPN networks, and all points are used for Nordic geodynamics studies. FinnRef ® is additionally used as a fundamental network for the definition of the new Finnish reference frame EUREF-FIN (Ollikainen et al., 2000).