Adjustment and Quality Assessment of the Portuguese National Geodetic Network (RGN) Using Historical and Contemporary GNSS Observations

National geodetic networks increasingly require the integration of legacy campaign observations with continuously operating GNSS infrastructure and contemporary reference frames. This study presents a hierarchical methodology for establishing an updated ETRS89 realization of the Portuguese National Geodetic Network (RGN), integrating historical GPS observations from 1997 to 2005 with new GNSS observations from 2025 and long-term observations from the Portuguese permanent GNSS network (ReNEP). The ReNEP station coordinates were first estimated in ITRF2020 and transformed to ETRF2020, providing the reference infrastructure for transferring the reference-frame realization to the campaign-based RGN. Despite substantial differences in observation quality, systematic quality control and stochastic modelling enabled the historical and contemporary observations to be combined in a consistent national adjustment. Historical observations proved essential for determining updated coordinates of geodetic points not re-observed in 2025, thereby preserving network coverage. Resilience experiments showed that reducing the 42 fixed ReNEP stations to 30 geographically distributed stations produced coordinate differences with standard deviations of 0.4 mm in Easting, 1.0 mm in Northing, and 2.0 mm in ellipsoidal height relative to the reference solution. Removing baselines longer than 75–100 km had a limited effect, whereas removing baselines longer than 50 km increased coordinate dispersion, particularly in height. These findings demonstrate the value of historical GPS observations for preserving network coverage and provide a framework for the modernization and long-term maintenance of national geodetic reference networks.