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Western Greece is one of the seismotectonically most active regions in Europe. The main tectonic structures are the West Hellenic Arc (WHA) and the Kephalonia Fault Zone. In order to monitor and understand the crustal movements in space and time, a continuous GPS network was installed. In this paper we present results of 6 yr (1995-2001) of measurements. To ensure a consistent reference frame, 54 mainly European IGS and EUREF sites were included in the processing. A selected subset was used to estimate an Euler pole for the rotation of Eurasia. In order to obtain coordinate time-series of high precision that are representative for crustal deformation, special emphasis was given to the elimination of non-tectonic effects. Four steps of improvement were pursued, including a reprocessing after exclusion of poor data, the removal of remaining outliers, the correction of unknown phase centre offsets after antenna changes and weighted common-mode filtering. With this procedure, non-tectonic irregularities were reduced significantly, and the precision was improved by an average of 40 per cent. The final time-series are used as a base for depicting trajectories of crustal motion, interpreting the temporal behaviour of the sites and for estimating velocities. For the first time, height changes in the WHA area were detected and quantified by GPS. Sites that are located near the epicentres of the 1997 Strofades (Mw = 6.6) and the 1999 Athens (Mw = 6.0) earthquakes are particularly considered

Tropospheric water vapour is the main limiting factor in using GPS to determine crustal deformation at highest accuracy. On the other hand, it is an important variable to monitor meteorological and climatic processes. This paper discusses both aspects: the modelling of tropospheric water vapour using meteorological data as well as the determination of the integrated amount of water vapour and its spatiotemporal variation using GPS data. Switzerland has been chosen as experiment area. The Swiss continuous GPS (CGPS) network AGNES is used as a reference network, which represents a realistic scenario for GPS-based water vapour determination. Data of the Swiss numerical weather model aLMo are used for systematic comparison and validation. For the first aspect, integrated tropospheric wet refractivity values are determined from meteorological measurements and compared with GPS path delays. An overall agreement of 1 cm of zenith wet path delay was achieved. For the second aspect a tomographic approach has been developed. A total of 6720 GPS-determined profiles are compared with data of the numerical weather model and radio soundings. The results are statistically evaluated and systematically compared with each other. A correlation between the accuracy and the weather situation was found. Overall, an agreement of 5-7 ppm (refractivity unit) was obtained compared to aLMo. The use of GPS-determined path delays from a permanent GPS network is the recommended method to correct GPS measurements. In all other cases, the two methods presented (COITROPA, COMEDIE) are a feasible alternative to determine path delays accurately. Furthermore, GPS is a convenient application to determine the amount of water vapour in the troposphere. It is demonstrated that the vertical distribution of water vapour can be deduced by applying the tomographic approach

The calculation of recent crustal movements and the associated crustal deformation rely on a suitable interpolation of geodetic measurements with repetition cycles of years or decades and modern GPS permanent networks. A common interpolation methods is the least-square collocation (LSC). LSC requires some a priori assumptions about the characteristics of the velocity field, that is, stocasticity in Moritz's definition of LSC. We present a novel approach, called adaptative LSC (ALSC) to the interpolation of non-stochastic fields, which encompass the traditional LSC and the block model as special cases. This modified collocation method is based on the empirical estimation of a anisotropic and inhomogeneous covariance function of the interpolated field. The method has been tested on synthetic data that simulate geodetic measurements over a triple plate junction and with real data from precise levelling measurements over the Swiss Alps. In both cases, ALSC gave better and more stable results, compared to LSC and other interpolation methods, such as smoothed splines