Residual K1 and K2 Oscillations in Precise GPS Solutions: Case Study

The continuous GPS observations from Polish Ground-Based Augmentation System (ASG-EUPOS) with two different strategies were analysed. Within the first one the adopted 3-hour observation window was shifted every hour to obtain hourly geocentric coordinates for investigation of the short-period oscillations. The second was the daily solution using EPN (EUREF Permanent Network) standards and recommended models. The Eterna 3.4 software was used for determination of the variations in the tidal frequencies. The almost three-year course of data allow for investigation even close tidal frequencies, especially in PSK1 and S2K2 groups. The variations of the Up component in K1 and K2 waves were found to be significant. It turned out that K1 frequency involves the greatest amount of energy reaching even 11 mm for the Up component. Spurious oscillation up to 4 mm and the period of about 1 year in GPS height time series at the selected ASG-EUPOS sites were obtained. Long-period GPS satellite orbit modelling errors could be directly responsible for small periodic biases in station position determinations as well as the repeating geometry of the satellite constellation with respect to the tracking stations. But it is very difficult to distinguish whether it comes from artefacts (transition of K1 ad K2 in long periods) or real effects (local hydrology). Comparison with the geophysical fluid loadings needs further investigations.