The Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) of the World Meteorological Organization (WMO) is an international observing network, designed to meet climate requirements. Upper air observations within the GRUAN network will provide long-term high-quality climate records. The data are used to constrain and validate data from space based remote sensors and for scientific studies of atmospheric processes. A GNSS receiver is part of the GRUAN station equipment with highest priority.

GRUAN GNSS station network

GFZ operates a global network of GNSS receivers installed at GRUAN stations. From a current total of 33 GRUAN stations, 20 stations are equipped with GNSS receivers (as of March 2025) and included in the operational GNSS water vapor determination (Fig. 1, blue dots).

One example of this is the GRUAN station Ny-Alesund at Spitsbergen, Norway (https://www.gruan.org/network/sites/ny-aalesund). In Ny-Ålesund, a GNSS receiving system is operated by GFZ (Fig. 2) as part of the GRUAN station of Alfred Wegener Institute for Polar and Marine Research (AWI) and the French Institut Polaire Paul Emile Victor (IPEV). This is the world’s first measurement station that is certified according to the GRUAN standards.

GRUAN GNSS Data Central Processing Centre

GRUAN observations are intended to provide long-term high-quality data for the reliable determination of climatological trends and to provide further insight into atmospheric processes. Precise GNSS data analysis is a key to reach data quality on the highest level. Due to its long-term experience in GNSS data processing, GFZ was selected as a Central GRUAN GNSS Data Processing Centre for Precipitable Water (https://www.gruan.org/network/processing-centres).

An example for the GRUAN-PW data analysis at Ny-Ålesund is shown in Fig. 3.