Fig. 2
From: PPP-RTK considering the ionosphere uncertainty with cross-validation
The EPN [blue] and SONEL [red] networks in Europe provide PPP-RTK service in (a). In this study, they are mainly used to generate the ionosphere precision map and regional troposphere model. The red stars denote for assessing the PPP-RTK performance and the green triangles denote for the nearby reference stations in (b) which provide the interpolated ionosphere for the user stations