An integrated framework for assessing the vertical component of fiducial suitability in continuous GNSS networks

Abstract Reliable fiducial stations form the foundation of regional and national geodetic reference frames, requiring long-term coordinate stability and well-characterized stochastic behaviour. Although continuous Global Navigation Satellite System (GNSS) networks are increasingly used for geodetic applications, objective methodologies for evaluating the suitability of individual stations as fiducial references remain limited. This study presents an integrated framework for assessing the vertical-component fiducial suitability using multi-year GNSS position time series. Daily Precise Point Positioning (PPP) solutions were generated with GipsyX software [Bertiger W, Bar-Sever Y, Dorsey A, Haines B, Harvey N, Hemberger D, et al. GipsyX/RTGx, a new tool set for space geodetic operations and research. Adv Space Res 2020;66:469–89] and analysed for 66 continuously operating reference stations in Uttar Pradesh, India, spanning March 2022 to December 2025. Of the total of 66 continuous GNSS stations initially considered, one station (KASH) had insufficient data for time-series assessment, leaving 65 stations for classification. Only data quality, residual variability, deterministic model fit, and stochastic uncertainty contributed to the classification. Other estimated parameters are presented for completeness and discussion but were not used as classification criteria. Using thresholds defined in this study, for vertical-component fiducial suitability, stations were classified into suitability categories of Good, Moderate, Questionable, Not recommended, and Excluded. The results demonstrate substantial variability in station quality despite comparable observation durations, with differences reflected in data continuity, seasonal vertical behaviour, and stochastic noise characteristics. Seasonal vertical signals also varied substantially among stations but were treated as complementary characteristics rather than direct classification criteria. Stations exhibiting low residual scatter, high deterministic model fit, and small velocity uncertainties consistently showed the greatest potential for vertical-component fiducial applications, whereas stations affected by large uncertainties, poor model fit, or incomplete observations showed lower vertical-component fiducial suitability. The proposed framework provides a practical and reproducible methodology for assessing vertical-component fiducial suitability of GNSS stations and can support the maintenance of regional and national geodetic reference frames.