Finnish permanent GNSS network FinnRef: Evolution towards a versatile positioning service

The National Land Survey of Finland maintains the FinnRef network of continuously operating GNSS reference stations (CORS). FinnRef is the basis of the EUREF-FIN reference frame in Finland. Continuous time series ensure an accurate link between global GNSS-based coordinates and the national reference frame. In a CORS network it is essential that coordinates and coordinate time series of the reference stations are up to date, accurate and free from biases. In this dissertation we introduce the development of FinnRef from a network of 13 GPS stations into a versatile modern positioning service. Both old and new FinnRef stations are explained and it is shown how a high quality CORS station should be established today. We were one of the first groups to show the annual periodicity of GNSS time series. In Finland, land uplift is of high importance since it changes the coordinates continuously. We compared our uplift rates to independent results (tide gauges, precise levelling, GPS results of the BIFROST group). Agreement was very good showing that GPS is a powerful tool for monitoring the land uplift in Finland. Using a baseline in Lithuania we tested an idea to validate GPS processing parameters against metrological ground truth. The length of the baseline is traceable to the definition of the metre with an uncertainty based on our calibrations. The test was successful and showed that for most accurate measurements individually calibrated antennas must be used. After the first renewal phase of FinnRef we showed that our national network of 20 stations can provide NRTK corrections of the same accuracy level as services having five times more stations. The challenge was the reliability of individual coordinate measurements but this can be overcome by proper use of repeated measurements. One of the most important results of this dissertation was that we showed the power of using metrological ground truth for validating GPS. The results of this dissertation will enable the creation of a dense GNSS based velocity field for intra-plate deformation models. This will improve the accuracy of transformations from measured GNSS coordinates to the national reference frame and make possible an accurate, reliable (semi)dynamic reference frame in Finland. It is also noteworthy that we showed how the FINPOS positioning service based on FinnRef data could give citizens direct access to the national EUREF-FIN reference frame. FinnRef could also be used as a backbone for GNSS corrections needed for intelligent traffic applications in Finland.