1. Department of Navigation and Positioning, Finnish Geospatial Research Institute, National Land Survey of Finland, Espoo, Finland
2. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
3. German Aerospace Center, Institute for Solar-Terrestrial Physics, Neustrelitz, Germany
4. European Commission-Joint Research Centre, Ispra, Italy