GRUAN, the Global Climate Observing System Reference Upper Air Network, is a worldwide network of currently 30 observing stations designed to determine climate-relevant parameters with the highest accuracy over long periods of time. The most important data are temperature, air pressure and water vapor content of the atmosphere. The aim is to create a very accurate reference data set that can be incorporated into climate models and to which other measurements can refer.

Since 2013, the GFZ has been an official GRUAN analysis center for processing Global Navigation Satellite Systems (GNSS) data for the calculation of atmospheric water vapor content using software developed in-house. On January 22, 2021, the analysis center and GNSS water vapor data product were officially certified by the GRUAN Lead Center. GRUAN originally stems from an initiative of the UNESCO World Meteorological Organization (WMO) Global Climate Observing System in 2007. Very high quality observations of key climate variables in the altitude range from the Earth's surface to the stratosphere were urgently needed for an accurate understanding of global climate change.

Dr. Galina Dick and Prof. Jens Wickert from GFZ are part of the GRUAN expert group for the very precise water vapor measurements with GNSS. Together with international colleagues, they check the accuracy of the GNSS data at the receiving stations. They also determine how the data should be processed so that the derived water vapor data meet the high quality requirements for climate research. Together with other research institutes and meteorological services, the quality of GNSS water vapor products is monitored and regularly compared with data from radiosondes, radiometers, and weather models. All GRUAN measurements are collected via the control center at the German Weather Service (DWD) and made available to all climate researchers.