On the periodicity of GPS time series

We have computed GPS time series of different resolutions in the Finnish permanent GPS network FinnRef. In the Lomb periodigrams we can distinguish an annual period (Mäkinen et al. 2000) but also a diurnal period. The annual period is visible in all components, including the computed baseline length between two stations. The amplitude of a period is a function of the baseline length, thus behaving like a scale error.The annual periods, which can be seen in almost all baseline lengths of the FinnRef network, are in phase for each baseline. It may indicate that some modelling error exists in the GPS computation. However, we cannot exclude a physical reason, i.e. an unmodelled crustal deformation that causes a network scale error. Therefore, a geophysical interpretation of annual variations in the height component of GPS time series must be attempted, though with caution.The annual period in the baseline lengths between the FinnRef stations can be found also in the GPS solutions made by the EUREF computing centre.We discuss on the etfect and possible reasons for periods, and tests made for studying and removing the periodicity.